DOI: 10.5061/dryad.612jm649d
title: Higher vascular plant abundance associated with decreased ecosystem respiration after 20 years of warming in the forest-tundra -ecotone

description: The ongoing climate warming is promoting shrub abundance in high
 latitudes, but the effect of this phenomenon on ecosystem functioning is
 expected to depend on whether deciduous or evergreen species increase in
 response to warming. To explore effects of long-term warming on shrubs and
 further on ecosystem functioning, we analyzed vegetation and ecosystem CO2
 exchange after 20 years of warming in the forest-tundra ecotone in
 sub-arctic Sweden. A previous study conducted nine years earlier had found
 increased evergreen Empetrum nigrum ssp. hermaphroditum in the forest and
 increased deciduous Betula nana in the tundra. Following current
 understanding, we expected a continued increase in shrub abundance that
 would be stronger in tundra than in forest. We expected warming to
 increase ecosystem respiration (Re) and gross primary
 productivity (GPP), with a greater increase in Re in
 tundra due to increased deciduous shrub abundance, leading to a less
 negative net ecosystem exchange (NEE) and reduced ecosystem C sink
 strength. As predicted, vascular plant abundances were higher in the
 warmed plots with a stronger response in tundra than in forest. However,
 whereas B. nana had increased in abundance since the last survey, E.
 hermaphroditum abundance had declined due to several moth and rodent
 outbreaks during the past decade. In contrast to
 predictions, Re was significantly lower in the warmed
 plots irrespective of habitat, and GPP increased marginally only in the
 forest. The lower Re and a higher GPP under warming in the forest together
 led to increased net C sink. Re was negatively
 associated with the total vascular plant abundance. Our results highlight
 the importance of disturbance regimes for vegetation responses to warming.
 Climate warming may promote species with both a high capacity to grow
 under warmer conditions and a resilience towards herbivore outbreaks.
 Negative correlation between Re and total vascular plant abundance further
 indicates that the indirect impacts of increased plants on soil
 microclimate may become increasingly important for ecosystem CO2 exchange
 in the long run, which adds to the different mechanisms that link warming
 and CO2 fluxes in northern ecosystems.
description: <em>Vegetation
 analyses</em> The plant community
 composition was earlier recorded in 1999 and 2009 in five control plots
 and five OTCs in both habitats (Kaarlejärvi et al. 2012). We used the same
 plots during the present investigation and analyzed the composition of
 vegetation in July 2018 with the point intercept method: in OTCs, a total
 of 87 pins was systematically distributed among three diagonals of the
 hexagons, 29 pins per diagonal. For each pin, the total number of hits as
 well as the height of the highest hit were recorded for each plant. Only
 one hit for each species was counted at the ground layer for each pin. The
 same method was applied to control plots. Later the total number of hits
 was normalized to hits per 100 pins. Data from one forest plot was
 discarded because of poor plot condition. <em>Ecosystem
 carbon flux analyses</em> For the ecosystem
 carbon flux analyses, we included a few additional plots to have seven
 plots per treatment in the forest and eight plots per treatment in the
 tundra. The fluxes were analyzed at two-week intervals throughout the
 growing season 2018 (from 5<sup>th</sup> of June to
 19<sup>th</sup> of August) using a closed system composed of a
 custom-built acrylic chamber (diameter 146 cm, height 60 cm) coupled to a
 Vaisala Carbon Dioxide Probe GMP343, Vaisala Humidity and Temperature
 Probe HMP75 and Vaisala Measurement Indicator MI70. Measurements included
 four consecutive measures of gradually changing light intensity: ambient
 light, 35% and 60% shading, and darkness to reveal ecosystem respiration,
 R<sub>e</sub>. Shading was implemented using hoods made of
 single- and double-layer white mosquito nets while darkness was obtained
 by covering the chamber with an opaque white hood. The chamber was vented
 before each measurement and placed carefully on top of the study plot so
 that the leakage of air from beneath the chamber was minimized.
 Photosynthetically active radiation (PAR) within the chamber was recorded
 using an HD 9021 Quantum-Photo-Radiometer. The CO<sub>2</sub>
 concentration, temperature, and humidity within chambers were logged at
 5-s intervals for 90 s. The CO<sub>2</sub> flux was calculated
 using CO<sub>2</sub> and the chamber microclimate data and
 corrected for changes in temperature and water vapor pressure (Hooper et
 al., 2002). The net CO<sub>2</sub> flux with light intensity
 above zero was regarded as NEE. For NEE, negative fluxes indicate a net
 uptake of CO<sub>2</sub> from the atmosphere, whereas positive
 fluxes indicate a net release of CO<sub>2</sub> into the
 atmosphere.  For the comparison of
 daily CO<sub>2</sub> flux measurements between the treatments
 and control plots we normalized GPP to the PAR level of 600 µmol
 m<sup>-2</sup> s<sup>-1</sup>. The GPP was
 calculated from the NEE and R<sub>e</sub>
 as: GPP = NEE -
 R<sub>e</sub> Daily plot-specific GPP values
 were fitted to their corresponding PAR levels using the nonlinear least
 squares (nls) function from stats package in R software environment 
 as: GPPij =
 A<sub>max</sub>PAR / k + PAR where i stands for
 i<sup>th</sup> plot and j for j<sup>th</sup> date,
 A<sub>max</sub> is the maximum GPP rate when saturated to
 light (mg CO<sub>2</sub> m<sup>−2</sup>
 h<sup>−1</sup>) and k is the half-saturation light constant
 (μmol m<sup>−2</sup> s<sup>−1</sup>).
 Subsequently, the GPP<sub>600</sub> was calculated for each
 plot and day at the light level of 600 μmol m<sup>−2</sup>
 s<sup>−1</sup> using Eq. 2 with PAR set to
 600.
description: # Higher vascular plant abundance associated with decreased ecosystem
 respiration after 20 years of warming in the forest-tundra -ecotone
 [https://doi.org/10.5061/dryad.612jm649d](https://doi.org/10.5061/dryad.612jm649d) Dataset comprises of four different excel sheets, one read\_me sheet and three data sheets. First sheet: read\_me. Description of the data and units for the next three sheets containing the actual data. Second sheet: 2018 vascular vegetation data and ground layer data. Also seasonal averages of CO<sub>2</sub> fluxes measured in the same plots during the growing season 2018 (net ecosystem exchange NEE<sub>600</sub>, gross primary productivity GPP<sub>600</sub> and ecosystem respiration R<sub>e</sub>). Vegetation data hits / 100 pins, CO<sub>2 </sub>data mg CO<sub>2</sub> m<sup>−2</sup> h<sup>−1</sup> Third sheet: Vegetation data from the years 1999, 2009 and 2018 which is used to test the temporal change of plant abundances between years. Values represent hits / 100 pins Forth sheet: CO<sub>2</sub> data from year 2018. All values standardized to par value 600, unit mg CO<sub>2</sub> m<sup>−2</sup> h<sup>−1</sup> Context: we examine whether longer-term changes in vegetation differ from shorter-term changes, and whether the effects of warming on CO<sub>2</sub> exchange differ between tundra and forest due to the different response of deciduous and evergreen dwarf shrubs to warming. For this, we report the responses of plant species composition and ecosystem CO<sub>2</sub> fluxes to 20 years of simulated climate warming, carried out using open top chambers at tundra and mountain birch forest sites located in the forest-tundra ecotone in northernmost Sweden. Results: Vascular plant abundances were higher in the warmed plots with a stronger response in tundra than in forest. However, whereas *B. nana* had increased in abundance since the last survey, *E. hermaphroditum* abundance had declined due to several moth and rodent outbreaks during the past decade. In contrast to predictions, R<sub>e</sub> was significantly lower in the warmed plots irrespective of habitat, and GPP increased marginally only in the forest. The lower R<sub>e</sub> and a higher GPP under warming in the forest together led to increased net C sink. R<sub>e</sub> was negatively associated with the total vascular plant abundance. Our results highlight the importance of disturbance regimes for vegetation responses to warming. Climate warming may promote species with both a high capacity to grow under warmer conditions and a resilience towards herbivore outbreaks. Negative correlation between R<sub>e</sub> and total vascular plant abundance further indicate that the indirect impacts of increased plants on soil microclimate may become increasingly important for ecosystem CO<sub>2</sub><sup> </sup>exchange in the long run, which adds to the different mechanisms that link warming and CO<sub>2</sub> fluxes in northern ecosystems. ## Description of the data and file structure Data is structured as written above. All the data is in one excel file comprising of four sheets. 1. Read me. 2. Vegetation and CO2 data in 2018 3.Vegetation 1999-2018 4. CO2data\_2018. First sheet "Read\_me contains the detailed information about the names and metrics used in the data. Note that 3. Vegetation 1999-2018 contains less plots than 2. Vegetation and CO2 in 2018. This is simply due to the fact that researchers in 1999 and 2009 used less plots for their study than we in 2018. Therefore temporal vegetation development is tested using less plots and year 2018 with NMDS ordination and correlations and CO<sub>2</sub> fluxes use data from seven OTCs and controls in forest and eight in the tundra. ## Sharing/Access information There are no other ways at the moment to access the data except dryad depository or contacting the researchers in person.