the observed positive response in vegetation productivity (up to 20% increase) to increased
nutrient availability is in line with many fertilization experiments that were carried out in subarctic
tundra (Haugwitz & Michelsen, 2011, Lamb et al., 2011, Parsons et al., 1994, Shaver et al., 2001).
In carbon currency, 20% higher permafrost peatland vegetation productivity would mean an
increased C-uptake of 5 g C m⁻<fnref n="2" /> yr⁻¹ (18 Mt C yr⁻¹ for all permafrost peatlands) (Box 1 & 2).

## 2. Permafrost peatlands and climate change: comparison of sink and source strengths

### 2a. Direct effects

The values for increased C-uptake (carbon sink function) through increased vegetation productivity in response to spring- and summer warming (yearly 70 Mt C for vascular plants and 535 Mt C for bryophytes), easily offset the predicted increase in carbon (CO₂) emissions based on measurements in the same experiment, for which the maximum given value is an annual increase of 100 Mt C per year (Dorrepaal et al., 2009)(Box 2). No direct effect of warming on methane (CH₄) emission is expected in permafrost peatlands without a groundwater table and with relatively mild redox conditions<fnref n="3" /> (Dorrepaal et al., 2009, Roulet et al., 1992). In wetter (permafrost) peatlands, temperature is one of the primary factors influencing CH₄ emission (Moore et al., 2011) and can have a direct positive impact of up to 40% (based on Q₁₀ ranging between 2 and 3) (Svensson & Rosswall, 1984, Williams & Crawford, 1984), although large variation exists in observed Q₁₀ for the direct effect of warming on methane emission (Segers, 1998, van Hulzen et al., 1999). Even so, estimations based on four warming scenarios (part of the IPCC fifth assessment report) show that most of the released carbon will be in the form of CO₂, with only about 2.7% in the form of CH₄ (Schuur et al., 2011). In line with this, Lee et al. (2012) conclude that the direct potential effect on climate from permafrost C release is greater under aerobic (dry) conditions than under anaerobic (wet) conditions, even when taking the higher greenhouse warming potential<fnref n="4" /> of CH₄ into account. Hence, the net direct effect of warming on C-uptake and release suggests that climatic warming will increase the carbon sink-strength of permafrost peatlands.

<footnote n="2">² An exact value can not be provided here because the fertilizer was added in the form of ‘slow release grains’, of which the exact release rate is not known. The inserted amount was equivalent to 8 g N m⁻².</footnote>

<footnote n="3">³ The permafrost peatlands studied in this thesis are ombrotrophic peatlands, which, by definition, do not have a groundwater table. Unfortunately, the available global area coverage data which are generally used for upscaling results of permafrost peatlands (Walker 2004; Tarnocai 2009; Gorham 1991) do not distinguish between ombrotrophic and non-ombrotrophic peatlands, nor is the dominant type of peatmoss specified. This is a major uncertainty in the calculations presented in this chapter. Fortunately, for the response to warming specific C-source data were available from the same experimental site (Dorrepaal 2009). However, for the C-source strength in response to permafrost thawing, I chose to use an educated guess provided by the permafrost community in Schuur (2004), which is not ombrotrophic peatland specific. I could have used 19% of this educated guess number, to represent the percentage of permafrost area that is covered by peatlands (Tarnocai 2009), but as most greenhouse gas emissions are expected to come from soils with high organic carbon contents, this correction would most likely give an underestimation. Hence, under the assumption that the main part of the C-source strength of permafrost will come from permafrost peatlands, I used the number provided by Schuur (2011).</footnote>

<footnote n="4">⁴ CH4 has a greater global warming potential (GWP CH4 = 25) than CO2 (GWP = 1) over a 100-year time period (IPCC, 2007).</footnote>
## 2b. *Indirect effects*

The value for C-uptake by vegetation as a result of increased N-availability in response to permafrost thawing (18 Mt C per year, Box 2) is approximately 50 to 100 times smaller than the estimate for the increase in total C-emissions as a result of permafrost thaw (faster release of CO₂, CH₄ and DOC) based on an educated guess from the permafrost community (Schuur *et al.*, 2011). This estimation of carbon release from permafrost degradation was 30 billion to 63 billion ton of carbon by 2040 (equal to approximately 1000-2000 Mt C yr⁻¹) (Box 2). Hence, apart from being one order of magnitude smaller than the direct effect of warming, the effects of increased N-availability through permafrost thawing on the sink strength of permafrost peatlands would, according to these simple calculations, most likely not counteract a shift from sink to source function of permafrost peatlands as a result of permafrost thawing.

## 2c. *Factors potentially affecting the calculations*

The carbon calculations presented here are fairly crude. Firstly, vegetation changes were not observed within the scope of the performed experiments, and thus not taken into account in the calculations in Box 1 (purely based on the work performed in this thesis). However, in response to warming for example, vegetation changes may occur on the longer term in permafrost peatlands with a more dense vascular cover, where the here observed bryophyte-driven stability in the vascular community (Chapter 2) does not take place (Elmendorf *et al.*, 2012). Unfortunately, detailed information about vegetation density and bryophyte cover is not yet available for all area covered by permafrost peatlands (Walker *et al.*, 2005). For this reason, it remains challenging to upscale the experimental findings or estimate the longer-term implications for C-uptake of either vegetation changes or the observed bryophyte-vascular community interaction (Chapter 2) for permafrost peatlands. Another potential driver of longer-term vegetation shifts on the longer term which was beyond the scope of this thesis, is degrading palsa structure due to disappearance of the physical support provided by permafrost, and associated changes in hydrology. Accompanying shifts to a graminoid-dominated vascular community could lead to mildly increased C-sequestration by the vegetation of up to 4 g C m⁻² yr⁻¹ (7.3 %) (Malmer *et al.*, 2005). However, such changes in hydrology are also associated with faster CO₂ and CH₄ release and increased outflow of dissolved organic C from permafrost peatlands (Christensen *et al.*, 2004, von Deimling *et al.*, 2012). Moreover, such changes in hydrology could increase outflow of plant available N from permafrost peatlands (Frey & McClelland, 2009, McClelland *et al.*, 2007) and thus negatively affect our estimations of potential increased C-uptake by vegetation as a result of increased N-availability through thawing of permafrost soil.

A second simplification in our calculations is the omission of potential changes in belowground biomass. Since approximately 80% of vascular plant biomass in northern tundra ecosystems resides below-ground (Chapin & Ruess, 2001), not taking into account potential changes in this carbon pool is a major oversight (van Noordwijk *et al.*, 1998). This is most likely especially true for permafrost affected ecosystems, where plant root growth is restricted by the depth of the active layer (Billings *et al.*, 1976) that will increase as a result of permafrost thawing (Akerman & Johansson, 2008). If above-ground-below-ground allometry remains stable (or decreases), increased below-ground biomass (Chapter 5) and concomitant C-uptake can be expected. Hence,
not taking potential changes in belowground biomass into account may lead to a conservative
estimate of C-uptake by the vegetation in response to climatic changes by permafrost peatlands.

Overall, the discussed interactions between different elements of the permafrost peatland
system (plant-plant; plant-microbe; plant-permafrost soil) can be affected by climatic changes
in ways that were not addressed in this thesis. Further, unforeseen feedback mechanisms may
occur on the longer term and either increase or decrease C-sequestration. Yet, the calculations
presented in this chapter are, to our knowledge, the first calculations specifically about the
vegetation productivity component of the carbon balance of permafrost peatlands. Moreover,
the straightforward comparison between C-uptake in response to specific experimental
manipulations relevant to current climate change scenarios is, to our knowledge, unique. Hence,
the insight these calculations provide in the sink-strength of these climate sensitive ecosystems
is, albeit crude, highly relevant in order to direct future research.

My recommendations for future research include a stronger focus on belowground processes, both plant-root as well as microbe related. Moreover, ongoing research in long-term experiments is essential for detecting longer-term changes in vegetation community and microbial activity as a result of climatic changes. And lastly, since according to the calculations provided here (Box 1 & 2) the peatmoss *Sphagnum fuscum* can play a crucial role in the carbon-sink function of permafrost peatlands, more research should be performed on total land area coverage and on factors affecting the productivity of this 'living soil'.

**3. Conclusions**

In summary, the work presented here suggests that the direct effect of warming, and the indirect
effect of thawing of permafrost soil via a release of plant available nutrients, will have a greater
impact on carbon-uptake (sink-function) of permafrost peatland vegetation than an increase in
summer precipitation. The sum of the here discussed direct effects (623 Mt C yr⁻¹) and indirect
effects (18 Mt C yr⁻¹) of climate change on the C-sink value of permafrost peatlands is large
enough to offset approximately half the predicted release of carbon from thawing permafrost
(1000-2000 Mt C yr⁻¹, Schuur 2011). Hence, in depth understanding of the underlying processes
of changes in carbon uptake as a result of climate change, as provided in this thesis<fnref n="5" />, is of major
importance to estimate future impacts of climatic changes on the terrestrial carbon balance.

<footnote n="5">⁵ Note: according to the latest IPCC report (2007), the anthropogenic C-source strength is approximately 13.4 x 10⁵ Mt C yr⁻¹. This is at minimum one order of magnitude larger than the projected climate driven release of greenhouse gases from permafrost soils and at least two orders of magnitude larger than the in this chapter presented maximum value for increased C-uptake by peatland vegetation in response to climatic changes. Hence, “despite the massive amount of carbon stored in permafrost soils, emissions from these soils are unlikely to overshadow those from the burning of fossil fuels, which will continue to be the main source of climate forcing” (Schuur, 2011). Increased C-uptake by permafrost peatland vegetation will not easily offset this.</footnote>
References

ACIA (2004) *Impacts of a Warming Arctic: Arctic Climate Impact Assessment*. Cambridge University Press, Cambridge.

Aerts R. (1997) Climate, leaf litter chemistry and leaf litter decomposition in terrestrial ecosystems: A triangular relationship. Oikos, 79, 439-449.

Aerts R. (2006) The freezer defrosting: global warming and litter decomposition rates in cold biomes. Journal of Ecology, 94, 713-724.

Aerts R. (2009) Nitrogen-dependent recovery of subarctic tundra vegetation after simulation of extreme winter warming damage to Empetrum hermaphroditum. Global Change Biology, 16, 1071-1081.

Aerts R., Cornelissen J. H. C., Dorrepaal E. (2006a) Plant performance in a warmer world: General responses of plants from cold, northern biomes and the importance of winter and spring events. Plant Ecology, 182, 65-77.

Aerts R., van Logtestijn R., Karlsson P. S. (2006b) Nitrogen supply differentially affects litter decomposition rates and nitrogen dynamics of sub-arctic bog species. Oecologia, 146, 652-658.

Aerts R., Wallen B., Malmer N. (1992) Growth-limiting nutrients in Sphagnum-dominated bogs subject to low and high atmospheric nitrogen supply. Journal of Ecology, 80, 131-140.

Aerts R., Wallen B., Malmer N., de Caluwe H. (2001) Nutritional constraints on Sphagnum-growth and potential decay in northern peatlands. Journal of Ecology, 89, 292-299.

Akerman H. J., Johansson M. (2008) Thawing permafrost and thicker active layers in sub-arctic Sweden. Permafrost and Periglacial Processes, 19, 279-292.

Allard M., Rousseau L. (1999) The internal structure of a palsa and a peat plateau in the Riviere Boniface region, Quebec: Inferences on the formation of ice segregation mounds. Geographie Physique Et Quaternaire, 53, 373-387.

Batjes N. H. (1996) Total carbon and nitrogen in the soils of the world. European Journal of Soil Science, 47, 151-163.

Beilman D. W., Robinson S. D. (2003) Peatland permafrost thaw and landform type along a climatic gradient. In: the 8th International Conference on Permafrost. pp Page, Zurich, Switserland, Balkema Publishers.

Bell J. N. B., Tallis J. H. (1973) Biological Flora of British-Isles - Empetrum Nigrum L. Journal of Ecology, 61, 289-305.

Bengtsson L., Hodges K. I., Koumoutsaris S., Zahn M., Keenlyside N. (2011) The changing atmospheric water cycle in Polar Regions in a warmer climate. Tellus Series a-Dynamic Meteorology and Oceanography, 63, 907-920.

Berendse F. (1982) Competition between plant-populations with different rooting depths. Oecologia, 53, 50-55.

Berendse F., Jonasson S. (1992) Nutrient use and nutrient cycling in northern ecosystems. In: Arctic ecosystems in a changing climate, an ecophysiological perspective. (eds Chapin FS, Jefferies RL, Reynolds JF, Shaver GS, Svoboda J) pp Page. San Diego, Academic Press.
Beringer J., Lynch A. H., Chapin F. S., Mack M., Bonan G. B. (2001) The representation of arctic soils in the land surface model: The importance of mosses. Journal of Climate, 14, 3324-3335.

Billings W. D., Shaver G. R., Trent A. W. (1976) Measurement of root growth in simulated and natural temperature gradients over permafrost. Arctic and Alpine Research, 8, 247-250.

Bliss L. C. (1956) A Comparison of Plant Development in Microenvironments of Arctic and Alpine Tundras. Ecological Monographs, 26, 303-337.

Bliss L. C., Henry G. H. R., Svoboda J., Bliss D. I. (1994) Patterns of Plant-Distribution within 2 Polar Desert Landscapes. Arctic and Alpine Research, 26, 46-55.

Bliss L. C., Svoboda J., Bliss D. I. (1984) Polar Deserts, Their Plant Cover and Plant-Production in the Canadian High Arctic. Holarctic Ecology, 7, 305-324.

Blok D., Sass-Klaassen U., Schaepman-Strub G., Heijmans M. M. P. D., Sauren P., Berendse F. (2011) What are the main climate drivers for shrub growth in Northeastern Siberian tundra? Biogeosciences, 8, 1169-1179.

Borner A. P., Kielland K., Walker M. D. (2008) Effects of simulated climate change on plant phenology and nitrogen mineralization in Alaskan arctic Tundra. Arctic Antarctic and Alpine Research, 40, 27-38.

Bragazza L., Freeman C., Jones T., Rydin H., Limpens J., Fenner N., ... Toberman H. (2006) Atmospheric nitrogen deposition promotes carbon loss from peat bogs. Proceedings of the National Academy of Sciences of the United States of America, 103, 19386-19389.

Bret-Harte M. S., Shaver G. R., Chapin F. S. (2002) Primary and secondary stem growth in arctic shrubs: implications for community response to environmental change. Journal of Ecology, 90, 251-267.

Bret-Harte M. S., Shaver G. R., Zoerner J. P., Johnstone J. F., Wagner J. L., Chavez A. S., ... Laundre J. A. (2001) Developmental plasticity allows Betula nana to dominate tundra subjected to an altered environment. Ecology, 82, 18-32.

Brown J., Ferrians O. J. J., Heginbottom J. A., Melnikov E. S. (1998) Circum-arctic map of permafrost and ground ice conditions (revised February 2001). pp Page, Boulder, CO: National Snow and Ice Data Center/World Data Center for Glaciology.

Cabrera M. L., Beare M. H. (1993) Alkaline persulfate oxidation for determining total nitrogen in microbial biomass extracts. Soil Science Society of America Journal, 57, 1007-1012.

Cadish G., Giller K. E. (eds) (1997) Driven by nature: plant litter quality and decomposition, Wallingford, UK, CAB International.

Callaghan T. V., Bergholm F., Christensen T. R., Jonasson C., Kokfelt U., Johansson M. (2010) A new climate era in the sub-Arctic: Accelerating climate changes and multiple impacts. Geophysical Research Letters, 37, 6.

Callaghan T. V., Bjorn L. O., Chernov Y., Chapin T., Christensen T. R., Huntley B., ... Sitch S. (2004) Effects of changes in climate on landscape and regional processes, and feedbacks to the climate system. Ambio, 33, 459-468.

Callaghan T. V., Headley A. D., Lee J. A. (1991) Root function related to the morphology, life history and ecology of tundra plants. In: Plant root growth, an ecological perspective. (ed Atkinson D) pp Page, 311-340, Blackwell Scientific Publications.

Camill P., Lynch J. A., Clark J. S., Adams J. B., Jordan B. (2001) Changes in biomass, aboveground net primary production, and peat accumulation following permafrost thaw in the boreal peatlands of Manitoba, Canada. Ecosystems, 4, 461-478.

Casper B. B., Schenk H. J., Jackson R. B. (2003) Defining plant's belowground zone of influence. Ecology, 84, 2313-2321.

Chapin F. S., Bret-Harte M. S., Hobbie S. E., Zhong H. L. (1996) Plant functional types as predictors of transient responses of arctic vegetation to global change. Journal of Vegetation Science, 7, 347-358.

Chapin F. S., Ruess R. W. (2001) Carbon cycle - The roots of the matter. Nature, 411, 749-752.
Chapin F. S., Schulze E.-D., Mooney H. A. (1990) The ecology and economy of storage in plants. Annual Review of Ecology and Systematics, 21, 423-447.

Chapin F. S., Shaver G. R. (1985) Individualistic growth-response of tundra plant-species to environmental manipulations in the field. Ecology, 66, 564-576.

Chapin I., Stuart F., Shaver G. R., Giblin A. E., Nadelhoffer K. J., Laundre J. A. (1995) Responses of Arctic Tundra to Experimental and Observed Changes in Climate. Ecology, 76, 694-711.

Christensen J. H., Hewitson B., Busuioc A., Chen A., Gao X., Held I., . . . Whetton P. (2007) Regional Climate Projections. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. (ed Solomon S, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller) pp Page. Cambridge, United Kingdom and New York, NY, USA, Cambridge University Press.

Christensen T. R., Johansson T. R., Akerman H. J., Mastepanov M., Malmer N., Friborg T., . . . Svensson B. H. (2004) Thawing sub-arctic permafrost: Effects on vegetation and methane emissions. Geophysical Research Letters, 31.

Clements F. E., Goldsmith G. W. (1924) The phytometer method in ecology. Carnegie Institute Washington publications, 356.

Clymo R. S. (1970) Growth of Sphagnum - Methods of measurement. Journal of Ecology, 58, 13-17.

Cornelissen J. H. C., Callaghan T. V., Alatalo J. M., Michelsen A., Graglia E., Hartley A. E., . . . Aerts R. (2001) Global change and arctic ecosystems: is lichen decline a function of increases in vascular plant biomass? Journal of Ecology, 89, 984-994.

Cornelissen J. H. C., Lang S. I., Soudzilovskaja N. A., During H. J. (2007) Comparative cryptogam ecology: A review of bryophyte and lichen traits that drive biogeochemistry. Annals of Botany, 99, 987-1001.

Cornelissen J. H. C., Lavorel S., Garnier E., Diaz S., Buchmann N., Gurvich D. E., . . . Poorter H. (2003) A handbook of protocols for standardised and easy measurement of plant functional traits worldwide. Australian Journal of Botany, 51, 335-380.

Cornwell W. K., Ackerly D. D. (2009) Community assembly and shifts in plant trait distributions across an environmental gradient in coastal California. Ecological Monographs, 79, 109-126.

Davidson E. A., Janssens I. A., Luo Y. Q. (2006) On the variability of respiration in terrestrial ecosystems: moving beyond Q(10). Global Change Biology, 12, 154-164.

de Kroon H., Hendriks M., van Ruijven J., Ravenek J., Padilla F. M., Jongejans E., . . . Mommer L. (2012) Root responses to nutrients and soil biota: drivers of species coexistence and ecosystem productivity. Journal of Ecology, 100, 6-15.

Dentener F., Drevet J., Lamarque J. F., Bey I., Eickhout B., Fiore A. M., . . . Wild O. (2006) Nitrogen and sulfur deposition on regional and global scales: A multimodel evaluation. Global Biogeochemical Cycles, 20.

Dormann C. F., Woodin S. J. (2002) Climate change in the Arctic: using plant functional types in a meta-analysis of field experiments. Functional Ecology, 6, 4-17.

Dorrepaal E., Aerts R., Cornelissen J. H. C., Callaghan T. V., van Logtestijn R. S. P. (2004) Summer warming and increased winter snow cover affect Sphagnum fuscum growth, structure and production in a sub-arctic bog. Global Change Biology, 10, 93-104.

Dorrepaal E., Aerts R., Cornelissen J. H. C., Van Logtestijn R. S. P., Callaghan T. V. (2006) Sphagnum modifies climate-change impacts on subarctic vascular bog plants. Functional Ecology, 20, 31-41.

Dorrepaal E., Cornelissen J. H. C., Aerts R. (2007) Changing leaf litter feedbacks on plant production across contrasting sub-arctic peatland species and growth forms. Oecologia, 151, 251-261.
Dorrepaal E., Cornelissen J. H. C., Aerts R., Wallen B., Van Logtestijn R. S. P. (2005) Are growth forms consistent predictors of leaf litter quality and decomposability across peatlands along a latitudinal gradient? Journal of Ecology, 93, 817-828.

Dorrepaal E., Toet S., van Logtestijn R. S. P., Swart E., van de Weg M. J., Callaghan T. V., Aerts R. (2009) Carbon respiration from subsurface peat accelerated by climate warming in the subarctic. Nature, 460, 616-617.

Drew M. C. (1975) Comparison of effects of a localized supply of phosphate, nitrate, ammonium and potassium on growth of seminal root system, and shoot, in Barley. New Phytologist, 75, 479-490.

Elmendorf S. C., Henry G. H. R., Hollister R. D., Björk R. G., Bjorkman A. D., Callaghan T. V., ... Wookey P. A. (2012) Global assessment of experimental climate warming on tundra vegetation: heterogeneity over space and time. Ecology Letters, 15, 164-175.

Freschet G. T., Cornelissen J. H. C., van Logtestijn R. S. P., Aerts R. (2010) Evidence of the 'plant economics spectrum' in a subarctic flora. Journal of Ecology, 98, 362-373.

Frey K. E., McClelland J. W. (2009) Impacts of permafrost degradation on arctic river biogeochemistry. Hydrological Processes, 23, 169-182.

Frey K. E., McClelland J. W., Holmes R. M., Smith L. C. (2007) Impacts of climate warming and permafrost thaw on the riverine transport of nitrogen and phosphorus to the Kara Sea. Journal of Geophysical Research-Biogeosciences, 112.

Gavazov K. S., Soudzilovskaia N. A., Logtestijn R. S. P. v., Braster M., Cornelissen J. H. C. (2010) Isotopic analysis of cyanobacterial nitrogen fixation associated with subarctic lichen and bryophyte species. Plant and Soil, 333, 507-517.

Gorham E. (1991) Northern peatlands - Role in the carbon-cycle and probable responses to climatic warming. Ecological Applications, 1, 182-195.

Gornall J. L., Jonsdottir I. S., Woodin S. J., Van der Wal R. (2007) Arctic mosses govern below-ground environment and ecosystem processes. Oecologia, 153, 931-941.

Granath G., Strengbom J., Rydin H. (2010) Rapid ecosystem shifts in peatlands: linking plant physiology and succession. ESA preprint.

Grime J. P., Brown V. K., Thompson K., Masters G. J., Hillier S. H., Clarke I. P., ... Kielty J. P. (2000) The response of two contrasting limestone grasslands to simulated climate change. Science, 289, 762-765.

Haugwitz M. S., Michelsen A. (2011) Long-term addition of fertilizer, labile carbon, and fungicide alters the biomass of plant functional groups in a subarctic-alpine community. Plant Ecology, 212, 715-726.

Henry G. H. R., Freedman B., Svoboda J. (1986) Effects of fertilization on 3 tundra plant-communities of a polar desert oasis. Canadian Journal of Botany-Revue Canadienne De Botanique, 64, 2502-2507.

Hobbie S. E., Chapin F. S. (1996) Winter regulation of tundra litter carbon and nitrogen dynamics. Biogeochemistry, 35, 327-338.

Hobbie S. E., Nadelhoffer K. J., Hogberg P. (2002) A synthesis: The role of nutrients as constraints on carbon balances in boreal and arctic regions. Plant and Soil, 242, 163-170.

Hobbie S. E., Shevtsova A., Chapin F. S. (1999) Plant responses to species removal and experimental warming in Alaskan tussock tundra. Oikos, 84, 417-434.

Hodge A. (2004) The plastic plant: root responses to heterogeneous supplies of nutrients. New Phytologist, 162, 9-24.

Hodkinson I. D., Webb N. R., Bale J. S., Block W. (1999) Hydrology, water availability and tundra ecosystem function in a changing climate: the need for a closer integration of ideas? Global Change Biology, 5, 359-369.

Hudson J. M. C., Henry G. H. R. (2010) High Arctic plant community resists 15 years of experimental warming. Journal of Ecology, 98, 1035-1041.
Hungate B. A., Dukes J. S., Shaw M. R., Luo Y. Q., Field C. B. (2003) Nitrogen and climate change. Science, 302, 1512-1513.

IPCC (2001) Third Assessment Report - Climate Change 2001. Intergovernmental Panel for Climate Change. In: Report by Working Group II: Impacts Adaptation and Vulnerability Chapter 16, Polar Regions. pp Page.

IPCC (2007) Climate Change 2007 - The Physical Science Basis. In: Contribution of Working Group I to the fourth assessment report of the Intergovernmental Panel on Climate Change. pp Page, Cambridge, Cambridge University Press.

Johansson M., Åkerman J., Keuper F., Christensen T. R., Lantuit H., Callaghan T. V. (2011) Past and present permafrost temperatures in the Abisko area: Redrilling of boreholes. Ambio, 558-565.

Johansson M., Callaghan T. V., Akerman H. J., Jackowicz-Korczynsky M., Christensen, T.R. (2009) Rapid response of active layer thickness and vegetation in sub-arctic Sweden to experimentally increased snow cover. In: In: Changing lowland permafrost in northern Sweden: multiple drivers of past and future trends (PhD thesis). pp Page. Lund.

Johansson T., Malmer N., Crill P. M., Friborg T., Akerman J. H., Mastepanov M., Christensen T. R. (2006) Decadal vegetation changes in a northern peatland, greenhouse gas fluxes and net radiative forcing. Global Change Biology, 12, 2352-2369.

Jonasson S. (1988) Evaluation of the point intercept method for the estimation of plant biomass. Oikos, 52, 101-106.

Jonasson S., Michelsen A., Schmidt I. K., Nielsen E. V., Callaghan T. V. (1996) Microbial biomass C, N and P in two arctic soils and responses to addition of NPK fertilizer and sugar: Implications for plant nutrient uptake. Oecologia, 106, 507-515.

Jones D. L., Kielland K., Sinclair F. L., Dahlgren R. A., Newsham K. K., Farrar J. F., Murphy D. V. (2009) Soil organic nitrogen mineralization across a global latitudinal gradient. Global Biogeochemical Cycles, 23.

Kade A., Walker D. A., Raynolds M. K. (2005) Plant communities and soils in cryoturbated tundra along a bioclimate gradient in the Low Arctic, Alaska. Phytocoenologia, 35, 761-820.

Karlsson P. S. (1985) Effects of water and mineral nutrient supply on a deciduous and an evergreen dwarf shrub -Vaccinium uliginosum L. and V. vistis-idaea L. Holarctic Ecology, 8, 1-8.

Karlsson P. S. (1996) Plant ecology in the subarctic Swedish Lapland. In: Ecol. Bull. (eds Karlsson PS, Callaghan TV) pp Page. Copenhagen, Munksgaard International Publishers.

Keuper F., Dorrepaal E., Van Bodegom P. M., Aerts R., Van Logtestijn R. S. P., Callaghan T. V., Cornelissen J. H. C. (2011) A Race for Space? How Sphagnum fuscum stabilizes vegetation composition during long-term climate manipulations. Global Change Biology, 17, 2162-2171.

Keuper F., van Bodegom P. M., Dorrepaal E., Weedon J. T., van Hal J., van Logtestijn R. S. P., Aerts R. (2012) A frozen feast: thawing permafrost increases plant-available nitrogen in subarctic peatlands. Global Change Biology, 18, 1998-2007.

Kohler J., Brandt O., Johansson M., Callaghan T. (2008) A long-term Arctic snow depth record from Abisko, northern Sweden, 1913-2004 (vol 25, pg 91, 2006). Polar Research, 27, 94-95.

Kokelj S. V., Burn C. R. (2003) Ground ice and soluble cations in near-surface permafrost, Inuvik, Northwest Territories, Canada. Permafrost and Periglacial Processes, 14, 275-289.

Kuhry P. (1998) Late Holocene permafrost dynamics in two subarctic peatlands of the Hudson Bay Lowlands (Manitoba, Canada). Eurasian Soil Science, 31, 529-534.

Kuhry P., Dorrepaal E., Hugelius G., Schuur E. A. G., Tarnocai C. (2010) Potential Remobilization of Belowground Permafrost Carbon under Future Global Warming. Permafrost and Periglacial Processes, 21, 208-214.

Lamb E. G., Han S., Lanoil B. D., Henry G. H. R., Brummell M. E., Banerjee S., Siciliano S. D. (2011) A High Arctic soil ecosystem resists long-term environmental manipulations. Global Change Biology, 17, 3187-3194.
Lambers H., Stuart Chapin III F., Pons T. L. (1998) Plant Physiological Ecology, New York, Springer-Verlag.

Lang S. I., Cornelissen J. H. C., Holzer A., ter Braak C. J. F., Ahrens M., Callaghan T. V., Aerts R. (2009) Determinants of cryptogam composition and diversity in Sphagnum-dominated peatlands: the importance of temporal, spatial and functional scales. Journal of Ecology, 97, 299-310.

Larsen K. S., Grogan P., Jonasson S., Michelsen A. (2007) Dynamics and microbial dynamics in two subarctic ecosystems during winter and spring thaw: Effects of increased snow depth. Arctic Antarctic and Alpine Research, 39, 268-276.

Lawton J. H. (1994) What do species do in ecosystems. Oikos, 71, 367-374.

Lee H., Schuur E. A. G., Inglett K. S., Lavoie M., Chanton J. P. (2012) The rate of permafrost carbon release under aerobic and anaerobic conditions and its potential effects on climate. Global Change Biology, 18, 515-527.

Limpens J., Berendse F., Blodau C., Canadell J. G., Freeman C., Holden J., ... Schaepman-Strub G. (2008) Peatlands and the carbon cycle: from local processes to global implications - a synthesis. Biogeosciences, 5, 1475-1491.

Limpens J., Granath G., Gunnarsson U., Aerts R., Bayley S., Bragazza L., ... Xu B. (2011) Climatic modifiers of the response to nitrogen deposition in peat-forming Sphagnum mosses: a meta-analysis. New Phytologist, 191, 496-507.

Limpens J., Heijmans M., Berendse F. (2006) The Nitrogen Cycle in Boreal Peatlands. In: Boreal Peatland Ecosystems. (eds Wieder RK, Vitt DH) pp Page. New York, Springer Berlin.

Luoto M., Heikkinen R. K., Carter T. R. (2004) Loss of palsa mires in Europe and biological consequences. Environmental Conservation, 31, 30-37.

Mack M. C., Finlay J. C., DeMarco J., Chapin F., Schuur E. A. G., Neff J. C., Zimov S. A. (2010) Nitrogen and phosphorus in Yedoma soils of Northeast Siberia: stocks, fluxes and the ecosystem consequences of nutrient release from permafrost thaw, Abstract GC52A-05. presented at 2010 Fall Meeting AGU, San Francisco, Calif., 13-17 Dec.

Mack M. C., Schuur E. A. G., Bret-Harte M. S., Shaver G. R., Chapin F. S. (2004) Ecosystem carbon storage in arctic tundra reduced by long-term nutrient fertilization. Nature, 431, 440-443.

Mackay J. R. (1983) Downward water-movement into frozen ground, Western Arctic coast, Canada. Canadian Journal of Earth Sciences, 20, 120-134.

Madsen I.-L., Widell S. (eds) (1974) A vegetation map of the Stordalen site.

Malmer N., Johansson T., Olsrud M., Christensen T. R. (2005) Vegetation, climatic changes and net carbon sequestration in a North-Scandinavian subarctic mire over 30 years. Global Change Biology, 11, 1895-1909.

Marion G. M., Henry G. H. R., Freckman D. W., Johnstone J., Jones G., Jones M. H., ... Virginia R. A. (1997) Open-top designs for manipulating field temperature in high-latitude ecosystems. Global Change Biology, 3, 20-32.

Matthes-Sears U., Matthes-Sears W. C., Hastings S. J., Oechel W. C. (1988) The effects of topography and nutrient status on the biomass, vegetative characteristics, and gas exchange of two deciduous shrubs on an Arctic tundra slope. Arctic and Alpine Research, 20, 342-351.

McClelland J. W., Stieglitz M., Pan F., Holmes R. M., Peterson B. J. (2007) Recent changes in nitrate and dissolved organic carbon export from the upper Kuparuk River, North Slope, Alaska. Journal of Geophysical Research-Biogeosciences, 112.

McGraw J. B. (1985) Experimental ecology of Dryas-octopetala ecotypes. 3. environmental factors and plant-growth. Arctic and Alpine Research, 17, 229-239.

McKane R. B., Johnson L. C., Shaver G. R., Nadelhoffer K. J., Rastetter E. B., Fry B., ... Murray G. (2002) Resource-based niches provide a basis for plant species diversity and dominance in arctic tundra. Nature, 415, 68-71.
McNickle G. G., St Clair C. C., Cahill J. F., Jr. (2009) Focusing the metaphor: plant root foraging behaviour. *Trends in Ecology & Evolution*, 24, 419-426.

Michelsen A., Schmidt I. K., Jonasson S., Quarmby C., Sleep D. (1996) Leaf N-15 abundance of subarctic plants provides field evidence that ericoid, ectomycorrhizal and non- and arbuscular mycorrhizal species access different sources of soil nitrogen. *Oecologia*, 105, 53-63.

Moore T. R., De Young A., Bubier J. L., Humphreys E. R., Lafleur P. M., Roulet N. T. (2011) A Multi-Year Record of Methane Flux at the Mer Bleue Bog, Southern Canada. *Ecosystems*, 14, 646-657.

Mossberg B., Stenberg L., Ericsson S. (1992) Den Nordiska Floran, Stockholm, Wahlstrom and Widestrand.

Myers-Smith I. H., Forbes B. C., Wilmking M., Hallinger M., Lantz T., Blok D., ... Hik D. S. (2012) Shrub expansion in tundra ecosystems: dynamics, impacts and research priorities. *Environmental Research Letters*, 6.

Nasholm T., Kielland K., Ganeteg U. (2009) Uptake of organic nitrogen by plants. *New Phytologist*, 182, 31-48.

Oberbauer S., Miller P. C. (1982) Growth of Alaskan Tundra Plants in Relation to Water Potential. *Holarctic Ecology*, 5, 194-199.

Olefeldt D., Roulet N. T. (2012) Effects of permafrost and hydrology on the composition and transport of dissolved organic carbon in a subarctic peatland complex. *Journal of Geophysical Research-Biogeosciences*, 117, 15.

Olefeldt D., Roulet N. T., Bergeron O., Crill P., Backstrand K., Christensen T. R. (2012) Net carbon accumulation of a high-latitude permafrost peatland similar to permafrost-free peatlands. *Geophysical Research Letters*, 39.

Olofsson J., Oksanen L., Callaghan T., Hulme P. E., Oksanen T., Suominen O. (2009) Herbivores inhibit climate-driven shrub expansion on the tundra. *Global Change Biology*, 15, 2681-2693.

Ostendorf B., Reynolds J. F. (1998) A model of arctic tundra vegetation derived from topographic gradients. *Landscape Ecology*, 13, 187-201.

Panikov N. S., Flanagan P. W., Oechel W. C., Mastepanov M. A., Christensen T. R. (2006) Microbial activity in soils frozen to below -39 degrees C. *Soil Biology & Biochemistry*, 38, 3520-3520.

Parmentier F. J. W., van der Molen M. K., van Huissteden J., Karsanaev S. A., Kononov A. V., Suzdalov D. A., . . . Dolman A. J. (2011) Longer growing seasons do not increase net carbon uptake in the northeastern Siberian tundra. *Journal of Geophysical Research-Biogeosciences*, 116.

Parsons A. N., Press M. C., Wookey P. A., Welker J. M., Robinson C. H., Callaghan T. V., Lee J. A. (1995) Growth-Responses of Calamagrostis-Lapponica to Simulated Environmental-Change in the Sub-Arctic. *Oikos*, 72, 61-66.

Parsons A. N., Welker J. M., Wookey P. A., Press M. C., Callaghan T. V., Lee J. A. (1994) Growth-responses of four sub-arctic dwarf shrubs to simulated environmental-change. *Journal of Ecology*, 82, 307-318.

Phoenix G. K., Gwynn-Jones D., Callaghan T. V., Sleep D., Lee J. A. (2001) Effects of global change on a sub-Arctic heath: effects of enhanced UV-B radiation and increased summer precipitation. *Journal of Ecology*, 89, 256-267.

Piao S., Friedlingstein P., Ciais P., Zhou L., Chen A. (2006) Effect of climate and CO(2) changes on the greening of the Northern Hemisphere over the past two decades. *Geophysical Research Letters*, 33.

Potter J. A., Press M. C., Callaghan T. V., Lee J. A. (1995) Growth responses of Polytrichum commune and Hylocomium splendens to simulated environmental change in the sub-arctic. *New Phytologist*, 131, 533-541.

Press M. C., Callaghan T. V., Lee J. A. (1998a) How will European Arctic ecosystems respond to projected global environmental change? *Ambio*, 27, 306-311.

Press M. C., Potter J. A., Burke M. J. W., Callaghan T. V., Lee J. A. (1998b) Responses of a subarctic dwarf shrub heath community to simulated environmental change. *Journal of Ecology*, 86, 315-327.
Qian H., Joseph R., Zeng N. (2010) Enhanced terrestrial carbon uptake in the Northern High Latitudes in the 21st century from the Coupled Carbon Cycle Climate Model Intercomparison Project model projections. Global Change Biology, 16, 641-656.

Quested H. M., Press M. C., Callaghan T. V. (2003) Litter of the hemiparasite Bartsia alpina enhances plant growth: evidence for a functional role in nutrient cycling. Oecologia, 135, 606-614.

Rapp K., Steenberg K. (1977) Studies of phosphorus uptake from different depths in cloudberry mires using P32-labelled fertilizer. Acta Agriculturae Scandinavica, 27, 319-325.

Reich P. B., Hobbie S. E., Lee T., Ellsworth D. S., West J. B., Tilman D., ... Trost J. (2006) Nitrogen limitation constrains sustainability of ecosystem response to CO2. Nature, 440, 922-925.

Remer (2009) Temperature and Precipitation Graphs. NASA Earth Observatory. pp Page.

Robinson C. H. (2002) Controls on decomposition and soil nitrogen availability at high latitudes. Plant and Soil, 242, 65-81.

Robinson C. H., Saunders P. W., Madan N. J., Pryce-Miller E. J., Pentecost A. (2004) Does nitrogen deposition affect soil microfungal diversity and soil N and P dynamics in a high Arctic ecosystem? Global Change Biology, 10, 1065-1079.

Robinson C. H., Wookey P. A., Lee J. A., Callaghan T. V., Press M. C. (1998) Plant community responses to simulated environmental change at a high arctic polar semi-desert. Ecology, 79, 856-866.

Roswall T., Granhall U. (1980) Nitrogen cycling in a subarctic ombrotrophic mire. In: Ecol Bull. (ed Sonesson M) pp Page. Stockholm, Swedish National Science Research Council.

Roulet N., Moore T., Bubier J., Lafleur P. (1992) Northern fens: methane flux and climatic change. Tellus Series B-Chemical and Physical Meteorology, 44, 100-105.

Rustad L. E., Campbell J. L., Marion G. M., Norby R. J., Mitchell M. J., Hartley A. E., ... Gurevitch J. (2001) A meta-analysis of the response of soil respiration, net nitrogen mineralization, and aboveground plant growth to experimental ecosystem warming. Oecologia, 126, 543-562.

Rydén B. E. (1976) Water availability to some Arctic Ecosystems. Nordic Hydrology, 7, 73-80.

Rydin H., Jeglum J. (2006) The Biology of Peatlands, New York, Oxford university Press.

Sælthun N. R., Barkved L. (2003) Climate Change Scenarios for the SCANNET Region. (ed Research NIOW) pp Page, Oslo, Norwegian Institute of Water Research.

Sannel A. B. K., Kuhry P. (2008) Long-term stability of permafrost in subarctic peat plateaus, west-central Canada. Holocene, 18, 589-601.

Schmidt I. K., Jonasson S., Michelsen A. (1999) Mineralization and microbial immobilization of N and P in arctic soils in relation to season, temperature and nutrient amendment. Applied Soil Ecology, 11, 147-160.

Schuur E. A. G., Abbott B., Permafrost Carbon N. (2011) High risk of permafrost thaw. Nature, 480, 32-33.

Schuur E. A. G., Bockheim J., Canadell J. G., Euskirchen E., Field C. B., Goryachkin S. V., ... Zimov S. A. (2008) Vulnerability of permafrost carbon to climate change: Implications for the global carbon cycle. Bioscience, 58, 701-714.

Schuur E. A. G., Crummer K. G., Vogel J. G., Mack M. C. (2007) Plant species composition and productivity following permafrost thaw and thermokarst in alaskan tundra. Ecosystems, 10, 280-292.

Segers R. (1998) Methane production and methane consumption: a review of processes underlying wetland methane fluxes. Biogeochemistry, 41, 23-51.

Shaver G. R., Bret-Harte S. M., Jones M. H., Johnstone J., Gough L., Laundre J., Chapin F. S. (2001) Species composition interacts with fertilizer to control long-term change in tundra productivity. Ecology, 82, 3163-3181.
Shevtsova A., Haukioja E., Ojala A. (1997) Growth response of subarctic dwarf shrubs, Empetrum nigrum and Vaccinium vitis-idaea, to manipulated environmental conditions and species removal. Oikos, 78, 440-458.

Small E. (1973) Xeromorphy in Plants as a Possible Basis for Migration between Arid and Nutritionally-Deficient Environments. Botaniska Notiser, 126, 534-539.

Sonesson M. (1974) Late quaternary forest development of Tornetrask area, North Sweden, 2. Pollen analytical evidence. Oikos, 25, 288-&.

Sonesson M. (1980) Ecology of a Subarctic Mire, Stockholm, Ecological Bulletins 30.

Sonesson M., Carlsson B. A., Callaghan T. V., Halling S., Bjorn L. O., Bertgren M., Johanson U. (2002) Growth of two peat-forming mosses in subarctic mires: species interactions and effects of simulated climate change. Oikos, 99, 151-160.

Street L. E., Stoy P. C., Sommerkorn M., Fletcher B. J., Sloan V. L., Hill T. C., Williams M. (2012) Seasonal bryophyte productivity in the sub-Arctic: a comparison with vascular plants. Functional Ecology, 26, 365-378.

Sturm M., McFadden J. P., Liston G. E., Chapin F. S., Racine C. H., Holmgren J. (2001) Snow-shrub interactions in Arctic tundra: A hypothesis with climatic implications. Journal of Climate, 14, 336-344.

Sturm M., Schimel J., Michaelson G., Welker J. M., Oberbauer S. F., Liston G. E., ... Romanovsky V. E. (2005) Winter biological processes could help convert arctic tundra to shrubland. Bioscience, 55, 17-26.

Svensson B. H., Rosswall T. (1984) In situ methane production from acid peat in plant-communities with different moisture regimes in a subarctic mire. Oikos, 43, 341-350.

Tank A., Wijngaard J. B., Konnen G. P., Bohm R., Demaree G., Gocheva A., ... Petrovic P. (2002) Daily dataset of 20th-century surface air temperature and precipitation series for the European Climate Assessment. International Journal of Climatology, 22, 1441-1453.

Tape K., Sturm M., Racine C. (2006) The evidence for shrub expansion in Northern Alaska and the Pan-Arctic. Global Change Biology, 12, 686-702.

Tarnocai C., Canadell J. G., Schuur E. A. G., Kuhry P., Mazhitova G., Zimov S. (2009) Soil organic carbon pools in the northern circumpolar permafrost region. Global Biogeochemical Cycles, 23, 11.

Taylor K. (1971) Biological Flora of British-Isles: Rubus Chamaemorus L. Journal of Ecology, 59, 293-&.

Turetsky M. R. (2004) Decomposition and organic matter quality in continental peatlands: The ghost of permafrost past. Ecosystems, 7, 740-750.

Turetsky M. R., Wieder R. K., Williams C. J., Vitt D. H. (2000) Organic matter accumulation, peat chemistry, and permafrost melting in peatlands of boreal Alberta. Ecoscience, 7, 379-392.

Tybirk K., Nilsson M. C., Michelson A., Kristensen H. L., Shevtsova A., Strandberg M. T., ... Johnsen I. (2000) Nordic Empetrum dominated ecosystems: Function and susceptibility to environmental changes. Ambio, 29, 90-97.

Uhlirova E., Santruckova H., Davidov S. P. (2007) Quality and potential biodegradability of soil organic matter preserved in permafrost of Siberian tussock tundra. Soil Biology & Biochemistry, 39, 1978-1989.

Van Breemen N. (1995) How Sphagnum bogs down other plants. Trends in Ecology & Evolution, 10, 270-275.

van Huissteden J., Maximov T. C., Dolman A. J. (2005) High methane flux from an arctic floodplain (Indigirka lowlands, eastern Siberia). Journal of Geophysical Research-Biogeosciences, 110.

van Hulzen J. B., Segers R., van Bodegom P. M., Leffelaar P. A. (1999) Temperature effects on soil methane production: an explanation for observed variability. Soil Biology & Biochemistry, 31, 1919-1929.

van Noordwijk M., Martikainen P., Bottner P., Cuevas E., Rouland C., Dhillon S. S. (1998) Global change and root function. Global Change Biology, 4, 759-772.
Van Wijk M. T., Clemmensen K. E., Shaver G. R., Williams M., Callaghan T. V., Chapin F. S., ... Rueth H. (2004) Long-term ecosystem level experiments at Toolik Lake, Alaska, and at Abisko, Northern Sweden: generalizations and differences in ecosystem and plant type responses to global change. Global Change Biology, 10, 105-123.

Venhuizen G. (2009) The influence of rooting depth and thaw-front released nitrogen on the vegetation composition of a subarctic peatbog. MSc. thesis, VU University Amsterdam, Amsterdam.

von Deimling T. S., Meinshausen M., Levermann A., Huber V., Frieler K., Lawrence D. M., Brovkin V. (2012) Estimating the near-surface permafrost-carbon feedback on global warming. Biogeosciences, 9, 649-665.

Wahren C. H. A., Walker M. D., Bret-Harte M. S. (2005) Vegetation responses in Alaskan arctic tundra after 8 years of a summer warming and winter snow manipulation experiment. Global Change Biology, 11, 537-552.

Walker D. A., Raynolds M. K., Daniels F. J. A., Einarsson E., Elvebakk A., Gould W. A., ... Yurtsev B. A. (2005) The Circumpolar Arctic vegetation map. Journal of Vegetation Science, 16, 267-282.

Walker M. D., Wahren C. H., Hollister R. D., Henry G. H. R., Ahlquist L. E., Alatalo J. M., ... Wookey P. A. (2006) Plant community responses to experimental warming across the tundra biome. Proceedings of the National Academy of Sciences of the United States of America, 103, 1342-1346.

Wallenstein M. D., McMahon S. K., Schimel J. P. (2009) Seasonal variation in enzyme activities and temperature sensitivities in Arctic tundra soils. Global Change Biology, 15, 1631-1639.

Wania R., Ross I., Prentice I. C. (2009) Integrating peatlands and permafrost into a dynamic global vegetation model: 2. Evaluation and sensitivity of vegetation and carbon cycle processes. Global Biogeochemical Cycles, 23, 15.

Weedon J. T., Kowalchuk G. A., Aerts R., van Hal J., van Logtestijn R., Taá N., ... van Bodegom P. M. (2012) Summer warming accelerates sub-arctic peatland nitrogen cycling without changing enzyme pools or microbial community structure. Global Change Biology, 18, 138-150.

Wein R. W. (1973) Biological Flora of British-Isles: Eriophorum Vaginatum L. Journal of Ecology, 61, 601-615.

Weintraub M. N., Schimel J. P. (2003) Interactions between carbon and nitrogen mineralization and soil organic matter chemistry in arctic tundra soils. Ecosystems, 6, 129-143.

Welker J. M., Wookey P. A., Parsons A. N., Press M. C., Callaghan T. V., Lee J. A. (1993) Leaf Carbon-Isotope Discrimination and Vegetative Responses of Dryas-Octopetala to Temperature and Water Manipulations in a High Arctic Polar Semidesert, Svalbard. Oecologia, 95, 463-469.

Whittaker R. H., Marks P. L. (1975) Methods of assessing terrestrial productivity. In: Primary productivity of the biosphere. (eds Lieth H, Whittaker RH) pp Page. New York, Springer.

Williams R. T., Crawford R. L. (1984) Methane production in Minnesota peatlands. Applied and Environmental Microbiology, 47, 1266-1271.

Wipf S., Rixen C. (2010) A review of snow manipulation experiments in Arctic and alpine tundra ecosystems. Polar Research, 29, 95-109.

Wookey P. A., Aerts R., Bardgett R. D., Baptist F., Brathen K. A., Cornelissen J. H. C., ... Shaver G. R. (2009) Ecosystem feedbacks and cascade processes: understanding their role in the responses of Arctic and alpine ecosystems to environmental change. Global Change Biology, 15, 1153-1172.

Wookey P. A., Parsons A. N., Welker J. M., Potter J. A., Callaghan T. V., Lee J. A., Press M. C. (1993) Comparative Responses of Phenology and Reproductive Development to Simulated Environmental-Change in Sub-Arctic and High Arctic Plants. Oikos, 67, 490-502.

Wookey P. A., Robinson C. H., Parsons A. N., Welker J. M., Press M. C., Callaghan T. V., Lee J. A. (1995) Environmental Constraints on the Growth, Photosynthesis and Reproductive Development of Dryas-Octopetala at a High Arctic Polar Semidesert, Svalbard. Oecologia, 102, 478-489.
Wright I. J., Reich P. B., Cornelissen J. H. C., Falster D. S., Groom P. K., Hikosaka K., . . . Westoby M. (2005) Modulation of leaf economic traits and trait relationships by climate. Global Ecology and Biogeography, 14, 411-421.

Wullschleger S. D., Strahl M. (2010) Climate change: a controlled experiment. Scientific American, 302, 78-83.

Zar Z. H. (1999) Biostatistical Analysis, Upper Saddle River, NJ, USA, Prentice Hall.
Summary

Strong climate warming and altered precipitation regimes are predicted to occur at higher latitudes this century, with potentially major consequences for vegetation productivity and carbon sequestration. Although northern permafrost peatlands contain one-third of the world's soil organic carbon, little is known about responses of their vascular plant communities to climatic changes. Both direct effects, of warming and altered precipitation, as well as indirect effects, through permafrost thawing and increased nutrient-availability, are expected. In this thesis I aimed to investigate how short- to long-term experimental climate manipulations, relevant to different predicted future climate scenarios, affect vascular plant productivity and species composition in permafrost peatlands. Specifically, answers were sought to the following research questions: (1) How do spring- and summer warming, and increased snow cover affect species-specific growth responses and species composition in northern permafrost peatlands; (2) How does increased summer precipitation affect species-specific growth responses and species composition in northern permafrost peatlands; and (3) Can permafrost thawing affect species-specific growth responses and species composition in northern permafrost peatlands through a release of plant-available N? To answer these questions three experiments were performed, investigating vegetation responses to: (a) spring- and summer warming and a thicker snow cover (Chapter 2); (b) increased summer precipitation (Chapter 3); and (c) increased nutrient availability at the permafrost thaw front (Chapter 5). Moreover, the amount of plant-available nitrogen (N) that can be released from thawing permafrost into these peatlands in the near-future was quantified (Chapter 4). All experiments were performed on *Sphagnum fuscum*-dominated permafrost peatlands the Abisko area, northern Sweden. The increased summer precipitation experiment (Chapter 3) was complemented by a sister-experiment in the Kytalyk Reserve in north-eastern Siberia.

I found that in response to 8 years of experimental spring- and summer warming, and (moderately) thicker snow cover, vascular plant species composition of *Sphagnum fuscum*-dominated permafrost peatlands was more resistant than is typically observed in (sub)arctic experiments: neither changes in total vascular plant abundance, nor in individual species abundances, Shannon's diversity or evenness were found in response to the climate manipulations. For three key species (*Empetrum hermaphroditum*, *Betula nana* and *S. fuscum*) it was also determined whether the treatments had a sustained effect on plant length growth responses and how these responses interacted. Contrasting with the stability at the community level, both key shrubs and the peatmoss showed sustained positive growth responses to the climate treatments. However, a higher percentage of moss-encroached *E. hermaphroditum* shoots and a lack of change in *B. nana* net shrub height indicated encroachment by *S. fuscum*, resulting in long-term stability of the vascular community composition. These findings show that in a warmer world, vascular species of subarctic peat
bogs appear to just keep pace with growing *Sphagnum* in their 'race for space'. They contribute to general ecological theory by demonstrating that community resistance to environmental changes does not necessarily mean inertia in vegetation response (Chapter 2).

Secondly, vegetation responses to three years of experimentally increased summer precipitation in two tundra types were investigated, in (a) *B. nana*-dominated shrub tundra (northeast Siberia) and (b) a dry *S. fuscum*-dominated bog (northern Sweden). These tundra types were not previously addressed in increased precipitation studies. Positive responses to approximately doubled ambient precipitation (an increase of 200 mm yr⁻¹) were observed at the Siberian site, for *B. nana* (30% larger length increments), *Salix pulchra* (leaf size and length increments) and *Arctagrostis latifolia* (leaf size and specific leaf area), but none were observed at the Swedish site. Total biomass production did not increase at either of the sites. This work corroborates studies in other tundra vegetation types and shows that despite regional differences at the plant-level, total tundra plant productivity is, at least at the short or medium-term, largely irresponsive to experimentally increased summer precipitation (Chapter 3).

Thirdly, the amount of plant-available nitrogen that can be released by near-surface permafrost soil of permafrost peatlands was quantified (Chapter 4), and how such a release can affect species-specific growth responses and species composition in northern permafrost peatlands was investigated (Chapter 5). Plant-available N-pools and -fluxes in near-surface permafrost soil samples (taken 0-10 cm below the thaw front) were compared to those taken from a current rooting zone layer (5-15 cm depth) across five representative permafrost peatlands in subarctic Sweden. A range of complementary methods was used: extractions of inorganic and organic N, inorganic and organic N-release measurements at 0.5 and 11 °C (over 120 days, relevant to different thaw-development scenarios) and a bioassay with *Poa alpina* test-plants. All extraction methods, across all peatlands, consistently showed up to seven times more plant-available N in near-surface permafrost soil compared to the current rooting zone layer. These results were supported by the bioassay experiment, with an eight-fold larger plant N-uptake from permafrost soil than from other N-sources such as current rooting zone soil or fresh litter substrates. Moreover, net mineralisation rates were much higher in permafrost soils compared to soils from the current rooting zone layer (273 mg N m⁻² and 1348 mg N m⁻² per growing season for near-surface permafrost at 0.5 °C and 11 °C respectively, compared to ~30 mg N m⁻² for current rooting zone soil at 11 °C). Hence, these results demonstrate that near-surface permafrost soil of subarctic peatlands can release a biologically relevant amount of plant available nitrogen, both directly upon thawing as well as over the course of a growing season through continued microbial mineralisation of organically bound N (Chapter 4). However, being plant-available does not necessarily imply that peatland plant species will be able to take up this N at the thaw front. Therefore, I identified the potential impact of increased N-availability due to thawing permafrost on subarctic peatland plant production and species composition. This impact was compared to the effect of increased nutrient availability in shallower layers (e.g. through enhanced N-mineralization due to climatic warming). To achieve this, ¹⁵N-labeling of the thaw front was applied and a full-factorial belowground fertilization experiment (deep-fertilization at the thaw front at 45 cm depth and shallow-fertilization at 10 cm depth) was performed. I found that only particular species (e.g. *Rubus chamaemorus*) have active roots at the thaw front. Further, once supplied with nitrogen at the thaw front, these species had higher aboveground biomass and N-content, whereas this was not the case for shallower-rooting species (e.g. *E. hermaphroditum* and *Andromeda polifolia*). Moreover, the effects of increased nutrient availability
at the thaw front on total aboveground biomass production were similar in magnitude to the effects of increased nutrient availability in shallower layers. Additionally, nutrient limitation of plant growth in subarctic peatlands appeared to be strong enough for the effects of increased deep and shallow nutrient availability on biomass production to be additive. Altogether, these results show that plant-available N released from thawing permafrost can be considered a true 'new' N source for deep-rooting sub-arctic plant species, which will increase their biomass production. As this is not the case for shallow-rooting species, the release of plant-available N from thawing permafrost has the potential to alter species composition on the long-term by benefitting specific deep-rooting species only (Chapter 5).

This thesis was concluded by a comparison of response sizes to the different investigated direct and indirect effects of climate change on permafrost peatlands. The comparison was based on potential responses, expressed using carbon-uptake as a unifying currency. The largest response in carbon-uptake by permafrost peatland vegetation is to be expected from warming (approximately 170 g C m⁻² yr⁻¹), mainly through increased productivity of the peatmoss *Sphagnum fuscum* (approximately 150 g C m⁻² yr⁻¹), followed by the vascular vegetation responses to increased nutrient availability through thawing permafrost (5 g C m⁻² yr⁻¹). No effects of increased precipitation are expected on vegetation performance of permafrost peatlands. Based on an estimated permafrost peatland area of 3.5 x 10¹² m² (Tarnocai 2009), the potential increase in C-sink strength as a result of increased vegetation productivity calculated on the findings presented in this thesis would be at approximately 600 Mt C yr⁻¹ in response to warming, and 18 Mt C yr⁻¹ in response to increased nutrient availability as a result of permafrost thawing. Although these increases in C-sink strength are about halve the total global expected C-source strength as a result of permafrost thawing (1000-2000 Mt C yr⁻¹, Schuur et al. 2011), they are about 6 times higher than the projected increased C-source strength of permafrost peatlands in response to warming, for which the maximum given value is an annual increase of 100 Mt C yr⁻¹ (Dorrepaal et al., 2009). Therefore, further in depth understanding of the underlying processes of changes in carbon uptake as a result of climate change, as provided in this thesis, is of major importance for estimating future impacts of climatic changes on the terrestrial carbon balance.
## Synopsis

Voor noordelijke streken worden nog binnen deze eeuw sterke klimaatsveranderingen voorspeld, zoals hogere temperaturen en veranderende neerslagpatronen. Deze veranderingen kunnen grote gevolgen hebben voor vegetatieproductiviteit en koolstofopslag. Een derde van de organische koolstof die mondiaal opgeslagen is in bodems bevindt zich in noordelijke permafrostbodems, met name in veengebieden. Er is nog weinig bekend over de reactie van plantengemeenschappen in deze veengebieden op klimaatsveranderingen. Men verwacht directe effecten van klimaatsverandering op deze plantengemeenschappen, bijvoorbeeld de effecten van opwarming en veranderende neerslagpatronen, maar ook indirecte effecten door het ontdooien van permafrostbodems en door toenemende nutriëntenbeschikbaarheid.

Voor dit proefschrift getiteld 'Directe en indirecte effecten van klimaatsverandering op vegetatieproductiviteit en soortensamenstelling van permafrost veengebieden' heb ik onderzocht hoe experimentele manipulaties van het klimaat, zowel op de lange als op de korte termijn en relevant voor verschillende klimaatscenarios de productiviteit en samenstelling van plantengemeenschappen in permafrost veengebieden beïnvloeden. In het bijzonder heb ik getracht de volgende onderzoeksvragen te beantwoorden: (1) Hoe beïnvloeden voorjaars- en zomeropwarming en een dikkere sneeuwlaag de soortspecifieke groeiresponsen en soortensamenstelling in noordelijke permafrost gebieden; (2) Hoe beïnvloedt meer zomerneerslag (regen) de soortspecifieke groeiresponsen en soortensamenstelling in noordelijke permafrost veengebieden en (3) kan het ontdooien van permafrost een effect hebben op soortspecifieke groeiresponsen en soortensamenstelling in noordelijke permafrost veengebieden via het vrijkomen van plantbeschikbaar stikstof (N)?

Om deze vragen te beantwoorden zijn er drie experimenten opgezet, om vegetatie responsen op de volgende factoren te onderzoeken: (a) voorjaars- en zomeropwarming en een dikkere sneeuwlaag (Hoofdstuk 2); (b) verhoogde zomerneerslag (Hoofdstuk 3) en (c) verhoogde nutriëntenbeschikbaarheid aan het permafrost dooifront (Hoofdstuk 5). Daarnaast heb ik gemeten hoeveel plantbeschikbaar N er door het ontdooien van de permafrost in noordelijke veengebieden vrij kan komen in de nabije toekomst (Hoofdstuk 4). Alle experimenten werden uitgevoerd in door *Sphagnum fuscum* (bruin veenmos) gedomineerde veengebieden nabij Abisko in Zweeds Lapland. Het verhoogde neerslag experiment werd daarnaast gecomplementeerd met een zusterexperiment in het Kytalyk Reservaat in noordoost Siberië.

Uit het eerste experiment is gebleken dat de soortensamenstelling van noordelijke *Sphagnum fuscum* veengebieden met permafrost beter bestand is tegen experimentele voorjaars- en zomeropwarming en een dikkere sneeuwlaag dan over het algemeen geobserveerd wordt in (sub)arctische klimaatveranderingsexperimenten: er waren geen veranderingen in abundantie
van alle vaatplanten samen, noch in de individuele abundances van de specifieke soorten, Shannon's Diversity en Evenness in reactie op de klimaatmanipulaties. Voor drie sleutelsoorten (*Empetrum hermaphroditum*, *Betula nana* en *S. fuscum*) werd bovendien bepaald of de behandelingen effect hadden op de lengtegroeiresponsen en of deze responsen elkaar beïnvloedden. In tegenstelling tot de stabiliteit op het plantengemeenschapsniveau bleek de lengtegroei zowel van de twee struiksoorten als van het veenmos positief te reageren op de behandelingen. Echter, uit een hoger percentage door mos overwoekerde *E. hermaphroditum* (kraaiheide) takjes en het ontbreken van een toename in netto struikhoogte van *B. nana* (dwergberk), bleek overgroeiing door *S. fuscum*. Deze overgroeiing verklaart de stabiliteit van de vaatplantsamenstelling op de lange termijn. Onze bevindingen laten zien dat in een opwarmende wereld de vaatplantsoorten in subarctische veengebieden gelijk op gaan met het oprukkende veenmos in hun 'Race for Space'. De bevindingen dragen bij aan de algemene ecologische theorie door te laten zien dat weerstand van een plantengemeenschap tegen milieuveranderingen niet perse via inertie in vegetatieresponsen plaatsvindt.

In het tweede experiment werden de vegetatieresponsen op drie jaar experimenteel verhoogde zomerneerslag in twee typen toendra onderzocht, te weten (a) in *B. nana* gedomineerde struiktoendra (noordoost Siberie) en (b) in een droog *S. fuscum* gedomineerd veengebied (noord Zweden). Deze typen toendra werden niet eerder onderzocht in het kader van de invloed van een toename in neerslag. Op de Siberische onderzoekslocatie werden positieve plantresponsen op een experimentele verdubbeling (een toename van 200 mm per jaar) van de natuurlijke neerslag waargenomen, voor zowel *B. nana* (30 % meer lengtegroei) als voor Salix pulchra (toegenomen bladgrootte en lengtegroei) en Artagrostis latifolia (grotere bladgrootte en 'Specific Leaf Area'). Op de Zweedse onderzoekslocatie was geen meetbare respons op de bewatering waarneembaar. De totale biomassaproductie bleef op beide locaties onveranderd. Dit werk ondersteunt studies in andere toendravegetatietypen en toont aan dat ongeacht regionale verschillen op het plantniveau, de totale productiviteit van toendraplanten, in ieder geval op de korte tot middellange termijn, grotendeels ongevoelig blijkt voor experimenteel verhoogde zomerneerslag (Hoofdstuk 3).

Verder werd de hoeveelheid plantbeschikbare stikstof die vrij kan komen uit permafrostbodem van noordzweedse veengebieden gemeten (Hoofdstuk 4). Hiertoe werden plantbeschikbare stikstof 'pools' en 'fluxes' in oppervlakkige permafrostbodemmonsters (genomen op 0-10 cm onder het dooifront) vergeleken met bodemmonsters uit de huidige bewortelde bodemlaag (op 5-15 cm bodemdiepte). Deze vergelijking werd gedaan in vijf representatieve permafrost veengebieden in subarctisch Zweden. Een scala aan complementaire technieken werd toegepast: extracties van inorganische en organische stikstof, meting van het vrijkomen van inorganische en organische stikstof zowel bij 0.5 °C als bij 11 °C (relevant voor verschillende dooi-scenarios) gedurende 120 dagen, en een bioassay gebruikmakend van Poa alpina testplanten. Alle extractietechnieken gaven, in alle bemonsterde veengebieden, een gelijkoortig beeld weer: tot zeven maal hogere plantbeschikbare stikstofwaarden in de permafrostbodem dan in de huidige bewortelde bodemlaag. Dit beeld werd bevestigd door de resultaten van het bioassay experiment, waar acht maal grotere stikstofopname door planten werd waargenomen op permafrostbodem vergeleken met opname van andere stikstofbronnen zoals bodem van de huidige bewortelde laag of vers bladstrooisel. Bovendien waren de netto stikstofmineralisaties opmerkelijk hoger in permafrostbodem vergeleken met de bodem uit de bewortelde laag (273 mg N m⁻² en 1348 mg N m⁻² per groeiseizoen voor permafrostbodem bij respectievelijk 0.5 °C en 11 °C, vergeleken met -30 mg N m⁻² voor bodem uit de bewortelde laag bij 11 °C). Deze resultaten laten
zien dat er een biologisch relevante hoeveelheid plantbeschikbare stikstof kan vrijkomen uit permafrostbodems van noordelijke veengebieden, zowel direct na het ontdooien als gedurende de daaropvolgende groeiseizoenen door een voortdurende microbiele mineralisatie van organisch gebonden N (Hoofstuk 4).

In het derde experiment werd bestudeerd hoe het vrijkomen van stikstof uit ontdooiende permafrost de soortspecifieke groeiresponsen en plantensamenstelling in permafrost veengebieden kan beinvloeden (Hoofdstuk 5). Plantbeschikbare stikstof betekent binnen de context van een door permafrost aangedaan ecosysteem niet perse dat de plantensoorten deze stikstof ook daadwerkelijk kunnen opnemen, aan het permafrost dooifront. Daarom heb ik ook specifiek naar de mogelijke invloed van een toename in stikstofbeschikbaarheid aan het dooifront op subarctische veenplantproductie en soortensamenstelling gekeken. Deze mogelijke invloed heb ik vergeleken met het effect van een toename in stikstofbeschikbaarheid in ondiepe bodemlagen (zoals bijvoorbeeld kan ontstaan als gevolg van sterkere stikstofmineralisatie als gevolg van een warmer klimaat). Hiertoe heb ik ¹⁵N-labeling van het dooifront toegepast en een full-factorial ondergronds bemestingsexperiment uitgevoerd (met diepe bemesting aan het dooifront op 45 cm diepte, en ondiepe bemesting op 10 cm bodemdiepte). Hieruit bleek dat slechts bepaalde soorten (bv. Rubus chamaemorus) actieve wortels aan het permafrost dooifront hebben. Verder bleek dat wanneer deze soorten stikstof aan het dooifront toegediend kregen, dit een positief effect had op de bovengrondse biomassa productie en op het bladstikstofgehalte. Dit was niet het geval voor de ondieper wortelende soorten (bv. *E. hermaphroditum* en Andromeda polifolia). Ook bleek dat de effecten op de totale biomassa van toegenomen nutrientenbeschikbaarheid aan het dooifront van gelijke grootte waren als de effecten van toegenomen nutrientenbeschikbaarheid in ondiepere lagen. Bovendien bleek de nutrientlimitatie van plantengroei in subarctische veengebieden dermate sterk te zijn dat de effecten van diepe en ondiepe bemesting op biomassa additief waren. Bij elkaar tonen deze resultaten aan dat de plantbeschikbare stikstof vrijkomend uit ontdooiende permafrost daadwerkelijk beschouwd kan worden als een ‘nieuwe’ stikstofbron voor diepwortelende subarctische plantensoorten, en hun biomassaproductie kan vergroten. Aangezien dit niet geldt voor ondiep-wortelende soorten kan het vrijkomen van stikstof uit ontdooiende permafrost op de lange termijn mogelijk de soortensamenstelling veranderen door slechts diepwortelende soorten te bevoordelen.

Ik sluit dit proefschrift af met een vergelijking van de respons-groottes in reactie op de verschillende onderzochte directe en indirecte effecten van klimaatsverandering op permafrost veengebieden. Deze vergelijking is gebaseerd op potentiele responsen, uitgedrukt in koolstof eenheden. De grootste respons in koolstofopname door vegetatie in permafrost veengebieden kan verwacht worden als gevolg van opwarming (ongeveer 170 g C m⁻² per jaar), voornamelijk door verhoogde productiviteit van het veenmos *S. fuscum* (ongeveer 150 g C m⁻² per jaar), gevolgd door vaatplantresponsen op toegenomen nutrientenbeschikbaarheid door ontdooiende permafrost (5 g C m⁻² per jaar). Verhoogde neerslag heeft waarschijnlijk geen effect op de opname van koolstof door vegetatie in permafrost veengebieden. Gebaseerd op een geschatte mondiale oppervlakte van permafrost veengebieden van 3.5 x 10¹² m² (Tarnocai 2009) zou de potentiele toename in koolstof-‘sink’ sterkte als gevolg van grotere vegetatieproductiviteit berekend op basis van de in dit proefschrift beschreven bevindingen ongeveer 600 Mt C per jaar zijn als gevolg van opwarming, en 18 Mt C per jaar als gevolg van grotere nutrientbeschikbaarheid door ontdooiende permafrost. Al zijn deze toenames in koolstof-‘sink’ sterkte slechts half zo groot als de totale verwachte koolstof- ‘source’ sterkte als gevolg van het ontdooien van permafrost
(1000-2000 Mt C per jaar, Schuur et al 2011), ze zijn zes keer hoger dan de voorspelde toename in koolstof-‘source’ sterkte van permafrost veengebieden in reactie op opwarming (100 Mt C per jaar, Dorrepaal 2009). Een grondiger begrip van de onderliggende processen van veranderingen in koolstofopname als gevolg van klimaatverandering, zoals dit proefschrift biedt, is dus van groot belang bij het inschatten van toekomstige effecten van klimaatsverandering op de terrestrische koolstofbalans.
<figure><img src="image_29.png" /></figure>

© FOTOGRAF EIVON CARLSON LULEÅ SWEDEN
Publications

1. Keuper F., Parmentier F.J.W., Blok D., van Bodegom P.M., Dorrepaal E., Hal J.R., van Logtestijn R.S.P., Aerts R. (2012) Tundra in the Rain: Differential Vegetation Responses to Three Years of Experimentally Doubled Summer Precipitation in Siberian Shrub and Swedish Bog Tundra. Ambio, 41 (Supplement 3), 269-280.

2. Keuper F., van Bodegom P.M., Dorrepaal E., Weedon J.T., van Hal J.R., van Logtestijn R.S.P., Aerts R. (2012) A frozen feast: thawing permafrost increases plant-available nitrogen in subarctic peatlands. Global Change Biology, 18, 1998-2007.

*A Frozen Feast' featured as Research Highlight in Nature Climate Change: Brown A (2012) Release from the cold. Nature Climate Change 2, 313*

3. Keuper F., Dorrepaal E., van Bodegom P.M., Aerts R., Van Logtestijn R.S.P., Callaghan T.V., Cornelissen J.H.C. (2011) A Race for Space? How *Sphagnum fuscum* stabilizes vegetation composition during long-term climate manipulations. Global Change Biology, 17, 2162-2171.

(Co-authored)

4. Tsyganov A.N., Keuper F., Aerts R., Beyens L. (in press) Flourish or Flush: Effects of Simulated Extreme Rainfall Events on *Sphagnum*-dwelling Testate Amoebae in a Subarctic Bog (Abisko, Sweden). Microbial Ecology, Online First™, 6 September 2012.

5. Elmendorf S.C., Henry G.H.R., Hollister R.D., et al. (2011) Global assessment of experimental climate warming on tundra vegetation: heterogeneity over space and time. Ecology Letters 15: 164-175.

6. Johansson M., Åkerman J., Keuper F., Christensen T.R., Lantuit H., Callaghan T.V. (2011) Past and present permafrost temperatures in the Abisko area: Redrilling of boreholes. Ambio, 558-565.

7. Callaghan T.V., Tweedie C.E., Åkerman J., et al. (2011) Multi-decadal changes in tundra environments and ecosystems: synthesis of the International Polar Year-Back to the Future project (IPY-BTF). Ambio, 40, 705-716.

8. Hidding B., Bakker E.S., Keuper F., de Boer T., de Vries P.P., Nolet B.A. (2010) Differences in tolerance of pondweeds and charophytes to vertebrate herbivores in a shallow Baltic estuary. Aquatic Botany, 93, 123-128.
Caminante, no hay camino,
se hace camino al andar.

Al andar se hace camino,
y al volver la vista atrás
se ve la senda que nunca
se ha de volver a pisar.

Caminante, no hay camino,
sino estelas en la mar.

ANTONIO MACHADO, 1912
# Abknowledgements

## A-F

**A** is voor Rien **Aerts**, die dit onderzoek initieerde, chocolade op Stordalen verstopte, en het sferisch perspectief in mijn leven bracht. Rien, dankjewel.

**B** is voor Peter van **Bodegom**, wiens open en bruut eerlijke aanpak van wetenschap en management ik bewonder. Peter, ontzettend bedankt dat je me wilde begeleiden bij het voltooien van dit project. Ik heb het ervaren als de wind in de rug, en er veel van geleerd op precies die fronten waar ik daar behoefte aan had.

**C** is for my dear co-authors Terry **Callaghan** and Hans **Cornelissen**. Thank you both for bringing twinkles in this work and in my eyes!

**D** en **E** zijn voor Ellen **Dorrepaal**, van wie niet alleen de initialen het dichtst bij de mijne staan. Ellen, al is het een dooddoener: het is onmogelijk te verwoorden wat jij voor me betekent. *Festina lente*. De precisie, het plezier en de rust waarmee jij je werk doet contrasteren met de snelheid waarop je je doelen bereikt. Ik weet nog steeds niet hoe je het doet, maar het fascineert en inspireert me. Ik voel me gelukkig dat ik de komende jaren met je zal samenwerken. Tot gauw in Abisko!

**F** is for **friends** and **family**. Please swap the page and enjoy the good memories :) Thank you all!

## G-Z

I had in mind to express verbal thanks only to my formal supervisors (A-E). Yet, this work would not have been possible without the professional help of many others. I would like to express special thanks to: My Mate Marika Makkonen, Matti Kummu, Richard van Logtestijn, Jurgen van Hal, James Weedon, Margareta Johansson, the Staff of the Abisko Vetenskapliga Station, Janny Akkerman, Wessel Keuper, Frans-Jan Parmentier, Daan Blok, Rob Broekman, Karin Uyldert, Michel Groen, Jemima Mensah, Gemma Venhuizen, Sviatlana Marozava and **Jonathan Ramsay**, and to my paranymphs Diana Edo Matas and Eric Massey. Thank you en superbedankt!
<figure><img src="image_30.png" /></figure>

Max Merha Makkonen

Department of Forestry P.O. Box 36
The Netherlands

The Netherlands

Fjordland Islands
<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /></figure>