bluuebunny/crossref_metadata_2025 huggingface dataset:
Interaction between a lichen and a fungal parasite in a successional community: Implications for conservation
Authors: Hedenås H. , Lundin K. , Ericson L.
Published: February 2009

Question:

Populus tremula(Aspen) is a post‐fire successional tree with a large number of host‐specific lichens. One,Ramalina sinensis, will colonize very young aspen stands. The apothecia of this species are attacked by a parasitic fungus,Abrothallus suecicus.The incidence and severity of disease caused by this parasite was studied in nine stands between 22 and 180 years old.

Location:

Central Sweden.

Method:

Thalli ofR. sinensiswere randomly sampled from nine stands. Each thallus was weighed and the number of diseased and non‐diseased apothecia were scored for each thallus.

Results:

The data show (1) an increase in both disease incidence and disease severity with increasing stand age; (2) that smaller thalli show an increasing probability of being diseased in older stands; (3) that high disease levels prevail in older aspen stands; and (4) a broad variation in disease severity for thalli of similar size in the four older stands.

Conclusions:

The studied chronosequence is too short to elucidate whether theAbrothallus‐Ramalinasystem is driven by disease escape in space and time, or co‐existence. However, the high disease levels in older stands suggest that conservation programs aiming to sustain this system cannot only focus on preservation of isolated old‐growth stands, but also need to incorporate continuous creation of young stands. If succession is ignored, conservation of organisms originally adapted to, now vanishing, large‐scale disturbance regimes is likely to fail. We point out the need to assess the role of disease in conservation.
Environment, disturbance history and rain forest composition across the islands of Tonga, Western Polynesia
Authors: Franklin Janet , Wiser Susan K. , Drake Donald R. , Burrows Larry E. , Sykes William R.
Published: February 2009

Questions:

How do forest types differ in their distinctiveness among islands in relation to environmental and anthropogenic disturbance gradients? Are biogeographic factors also involved?

Location:

Tonga, ca. 170 oceanic islands totalling 700 km2spread across 8° of latitude in Western Polynesia.

Method

Relative basal area was analysed for 134 species of woody plants in 187 plots. We used clustering, indirect gradient analysis, and indicator species analysis to identify continuous and discontinuous variation in species composition across geographical, environmental and disturbance gradients. Partial DCA related environmental to compositional gradients for each major forest type after accounting for locality. CCA and partial CCA partitioned observed compositional variation into components explained by environment/disturbance, locality and covariation between them.

Results:

Differences among forest types are related to environment and degree of anthropogenic disturbance. After accounting for inter‐island differences, compositional variation (1) in coastal forest types is related to substrate, steepness and proximity to coast; (2) in early‐successional, lowland rain forest to proximity to the coast, steepness and cultivation disturbance; (3) in late‐successional, lowland forest types to elevation. For coastal/littoral forests, most of the compositional variation (71%) is explained by disturbance and environmental variables that do not covary with island while for both early and late‐successional forests there is a higher degree of compositional variation reflecting covariation between disturbance/environment and island.

Conclusions:

There are regional similarities, across islands, among littoral/coastal forest types dominated by widespread seawater‐dispersed species. The early‐successional species that dominate secondary forests are distributed broadly across islands and environmental gradients, consistent with the gradient‐in‐time model of succession. Among‐island differences in early‐successional forest may reflect differences in land‐use practices rather than environmental differences or biogeographical history. In late‐successional forests, variation in composition among islands can be partly explained by differences among islands and hypothesized tight links between species and environment. Disentangling the effects of anthropogenic disturbance history versus biogeographic history on late‐successional forest in this region awaits further study.
Trait convergence and trait divergence in herbaceous plant communities: Mechanisms and consequences
Authors: Grime J. Philip
Published: February 2009

In landscapes subject to intensive agriculture, both soil fertility and vegetation disturbance are capable of impacting strongly, evenly and simultaneously on the herbaceous plant cover and each tends to impose uniformity on the traits of constituent species. In more natural and ancient grasslands greater spatial and temporal variation in both productivity and disturbance occurs and both factors have been implicated in the maintenance of species‐richness in herbaceous communities. However, empirical data suggest that disturbance is the more potent driver of trait differentiation and species co‐existence at a local scale. This may arise from the great diversity in opportunities for establishment, growth or reproduction that arise when the intensity of competition is reduced by damage to the vegetation.

In contrast to the diversifying effects of local disturbances, productivity‐related plant traits (growth rate, leaf longevity, leaf chemistry, leaf toughness, decomposition rate) appear to be less variable on a local scale. This difference in the effects of the productivity and disturbance filters arises from the relative constancy of productivity within the community and the diversity in agency and in spatial and temporal scales exhibited by disturbance events. Also, evolutionary responses to disturbances involve minor adaptive shifts in phenological and regenerative traits and are more likely to occur as micro‐evolutionary steps than the shifts in linked traits in the core physiology associated with the capacity to exploit productive and unproductive habitats.

During the assembly of a community and over its subsequent lifespan filters with diversifying and convergent effects may operate simultaneously on recruitment from the local species pool and impose contrasted effects on the similarity of the trait values exhibited by co‐existing species. Moreover, as a consequence of the frequent association of productivity with the convergence filter, an additional difference is predicted in terms of the effects of the two filters on ecosystem functioning. Convergence in traits selected by the productivity filter will exert effects on both the plant community and the ecosystem while divergent effects of the disturbance filter will be restricted to the plant community.
Soil heterogeneity and community composition jointly influence grassland biomass
Authors: Maestre Fernando T. , Bradford Mark A. , Reynolds James F.
Published: February 2009

Question:Does the spatial pattern of nutrient supply modify community biomass responses to changes in both species composition and richness?

Location:Duke University Phytotron (Durham, North Carolina, USA).

Methods:We conducted a microcosm experiment to evaluate individual plant and whole community responses to species richness, species composition and soil nutrient heterogeneity. The experiment consisted of seven levels of species composition (all possible combinations ofLolium perenne, Poa pratensisandPlantago lanceolata) crossed with three levels of soil nutrient distribution (homogeneous, heterogeneous‐up, and heterogeneous‐down, where up and down indicates the location of a nutrient patch in either the upper or the lower half of the soil column, respectively).

Results:Communities containingPlantagoandLoliumresponded to nutrient heterogeneity by increasing above‐ and below‐ground biomass. Nutrient heterogeneity also increased size inequalities among individuals of these species. Significant species composition X nutrient heterogeneity interactions on community biomass and individual size inequality were observed when nutrient patches were located in the upper 10 cm of the soil columns. However, root proliferation in nutrient patches was equivalent regardless of the vertical placement of the patch.

Conclusions:Our results suggest that nutrient heterogeneity may interact with plant species composition to determine community biomass, and that small‐scale vertical differences in the location of nutrient patches affect individual and community responses to this heterogeneity.
Tornado damage ofQuercus stellataandQuercus marilandicain the Cross Timbers, Oklahoma, USA
Authors: Shirakura Fumiko , Sasaki Kiyoshi , Arévalo José Ramón , Palmer Michael W.
Published: February 2009

Questions:The Cross Timbers are a mosaic of savannas, grasslands and upland forests, occupying a significant portion of south‐central North America. Our questions here were (1) how does a severe tornado affect the two most dominant tree species of the areaQuercus marilandicaandQ. stellatawith respect to damage and mortality; (2) how do such patterns vary as a function of tree size? What are the implications of disturbance for codominance in species‐poor systems?

Location:The Cross Timbers in Oklahoma, USA.

Methods:We established a 14.48‐ha permanent plot following a severe tornado in 2003. We identified, numbered and tagged each tree and recorded its diameter at breast height (DBH), spatial coordinates, status (dead or alive), and damage type. We examined (1) relative abundance before and after the tornado; (2) differences in damage and mortality, and (3) the influence of tree diameter on the probability of damage and mortality for each species.

Results:Differences in species identity and tree characteristics were significantly related to tree mortality following the tornado, after accounting for spatial locations. The odds of mortality were 12.0 times greater forQ. marilandicathan forQ. stellata.Such greater vulnerability ofQ. marilandicaversusQ. stellatawas also reflected in changes in density and basal area. Tree diameter clearly influenced the damage and mortality pattern inQ. stellata; larger trees sustained more damage and mortality. However,Q. marilandicadid not exhibit size‐dependent mortality.

Conclusion:The tornado affected the two dominant species differently. The intra‐ and inter‐specific differences in windstorm susceptibility may allow coexistence of the two species and are potentially important in the dynamics of the Cross Timbers. Species more damaged might finally benefit from the wind disturbance due to their resprouting ability.
Effects of seed predation by ants on Mediterranean grassland related to seed size
Authors: Azcárate Francisco M. , Peco Begoña
Published: February 2009

Questions:1. Do harvester ants (Messor barbarus) promote seed mortality in Mediterranean grassland?; 2. Is this effect greater in large‐seeded species?

Location:Central Spain.

Methods:We established an ant‐exclusion experiment of five circular (1.5 m diameter) plots from where ants were excluded during one year, along with ten control plots. We recorded the seed bank of all species in the plots both before and after the treatment. The effect of seed length and weight was analysed after transforming data into phylogenetically independent contrasts, and alternatively by dividing the species data set into morphological groups.

Results:Longer and heavier seeded species significantly increased in the seed banks under the exclusion treatment, although ants did not significantly modify overall seed densities.

ConclusionsAlthough the ants do not collect large numbers of seeds, they differentially affect the composition of the seed banks by selecting the longest or heaviest seeds, or both. The persistence of this short‐term effect in the seed bank may result, over a number of years, in the system evolving towards a predominance of small‐seeded annuals, congruent with the species composition actually observed in Mediterranean grasslands.
Association of ecotones with relative elevation and fire in an upland Florida landscape
Authors: Boughton E.A. , Quintana‐Ascencio Ascencio F. , Menges E.S. , Boughton R.K.
Published: February 2009

Question: What are the importance of elevation and fire in maintaining ecotones of Florida scrub assemblages along a gradual topographic gradient?

Location: Archbold Biological Station (ABS), 12 km south of Lake Placid, Florida, USA.

Methods: Vegetation cover of upland Florida shrublands was quantified using the line‐intercept method along 20 transects traversing similar elevation gradients, stratified by time since fire (TSF). We objectively identified shrubland ecotones using a split moving windows boundary analysis (SMW) with three different window widths. Non‐metric multidimensional scaling ordination was used to determine relationships among plant assemblages defined by SMW.

Results: We located up to four ecotones per transect, the majority of which were wide, highly heterogeneous zones. Relative elevation controlled the distribution of plant assemblages in upland Florida shrublands. Ecotones in shrublands > 30 years TSF had relatively low dissimilarity values in SMW, indicating that previously discrete plant assemblages with longer TSF were becoming more similar with time.

Conclusions Split Moving Windows (SMW) analysis identified ecotones relatively well although patches generated by oak clonal growth were sometimes identified as ecotones. Fire suppression caused ecotones to become more diffuse, suggesting that without fire at least every 30 years, discrete plant assemblages within upland Florida shrublands will be more continuous.
Tree growth in an African woodland savanna affected by disturbance
Authors: Holdo R.M.
Published: February 2009

Questions: How does tree growth in a tropical woodland savanna vary as a function of size, and how is it affected by competition from neighbours, site attributes, and damage caused by disturbance?

Location: western Zimbabwe.

Methods: Trees of common species were tagged, mapped, and measured annually between 2001 and 2003 in a Kalahari sand woodland savanna. Diameter increments were analysed with mixed model regressions for the largest ramet in each genet. Stem diameter and damage, soil texture, and indices of competition at multiple spatial scales were used as covariates.

Results: Stem diameter increased initially and then declined as a function of size in undamaged trees, which grew faster than damaged trees. Growth in damaged trees declined with size. No site differences were detected, and there was evidence for between‐tree competition on growth only in the fastest‐growing species, Brachystegia spiciformis. In several species the growth rate of the largest ramet increased as a function of the basal area of secondary ramets, contrary to expectations. For many species, the growth models showed poor explanatory power.

Conclusions: Growth in Kalahari sand savanna trees varies as a function of size and changes in tree architecture caused by disturbance agents such as fire, frost, and elephant browsing. Disturbance may thus play an important role on vegetation dynamics through its effects on growth in the post‐disturbance phase. Growth is highly stochastic for some species in this system, and more deterministic in others. It is hypothesized that this dichotomy may be driven by differences in rooting depth among species.
Long‐term mortality patterns of the deep‐rooted Acacia erioloba: The middle class shall die!
Authors: Moustakas Aristides , Guenther Matthias , Wiegand Kerstin , Mueller Karl‐Heinz , Ward David , Meyer Katrin M. , Jeltsch Florian
Published: February 2009

Question: Is there a relationship between size and death in the long‐lived, deep‐rooted tree, Acacia erioloba, in a semi‐arid savanna? What is the size‐class distribution of A. erioloba mortality? Does the mortality distribution differ from total tree size distribution? Does A. erioloba mortality distribution match the mortality distributions recorded thus far in other environments?

Location: Dronfield Ranch, near Kimberley, Kalahari, South Africa.

Methods: A combination of aerial photographs and a satellite image covering 61 year was used to provide long‐term spatial data on mortality. We used aerial photographs of the study area from 1940, 1964, 1984, 1993 and a satellite image from 2001 to follow three plots covering 510 ha. We were able to identify and individually follow ca. 3000 individual trees from 1940 till 2001.

Results: The total number of trees increased over time. No relationship between total number of trees and mean tree size was detected. There were no trends over time in total number of deaths per plot or in size distributions of dead trees. Kolmogorov‐Smirnov tests showed no differences in size class distributions for living trees through time. The size distribution of dead trees was significantly different from the size distribution of all trees present on the plots. Overall, the number of dead trees was low in small size classes, reached a peak value when canopy area was 20 ‐ 30 m2, and declined in larger size‐classes. Mortality as a ratio of dead vs. total trees peaked at intermediate canopy sizes too.

Conclusion: A. erioloba mortality was size‐dependent, peaking at intermediate sizes. The mortality distribution differs from all other tree mortality distributions recorded thus far. We suggest that a possible mechanism for this unusual mortality distribution is intraspecific competition for water in this semi‐arid environment.
Elephant herbivory, frost damage and topkill in Kalahari sand woodland savanna trees
Authors: Holdo Ricardo M.
Published: February 2009

Questions: Which factors best predict the probability of elephant and frost damage in Kalahari sand woodland savanna? What is the association between tree mortality and the disturbance regime?

Location: Western Zimbabwe.

Methods: Elephant and frost damage, topkill, and whole‐plant mortality were quantified in ten common tree species in a Kalahari sand savanna in Zimbabwe. Individual trees were tagged in 20 plots and monitored over a two‐year period. A model selection approach was used to test the association between the probability of damage and size, prior damage, and neighbourhood effects, and to investigate the effect of damage on mortality.

Results: Elephant damage differed strongly among species, and was not influenced by neighbourhood effects or prior disturbance. Frost damage also varied across species, and declined as a function of stem size and neighbourhood tree cover, and ‐ against expectations ‐ prior disturbance. Topkill increased as a function of elephant and frost damage, but was lower in previously damaged than in undamaged trees.

Conclusions: Frost and elephant damage are influenced by community composition, and frost damage is also correlated with community structure and prior disturbance. Frost is an important and generally overlooked disturbance agent in southern African woodlands, where it may play a key role in association with other disturbance factors ‐ such as elephant herbivory ‐ that reduce woodland canopy cover.
Geothermal bryophyte habitats in the South Sandwich Islands, maritime Antarctic
Authors: Convey P. , Lewis Smith R.I.
Published: February 2009

Question: How does geothermal activity influence terrestrial plant colonization, species composition and community development in the Antarctic?

Location: South Sandwich Islands, maritime Antarctic.

Methods: Bryophytes were documented during a biological survey of the archipelago in January and February 1997. Particular attention was given to sites under current or recent influence of geothermal activity. Temperature profiles obtained across defined areas of activity on several islands were linked with the presence of specific bryophytes.

Results: Greatest bryophyte richness was associated with geothermally influenced ground. Of 35 moss and nine liverwort species recorded, only four mosses were never associated with heated ground, while eight of the liverworts and 50% of the mosses were found only on actively or recently heated ground. Some species occur in unheated sites elsewhere in the maritime Antarctic, but were absent from such habitats on the South Sandwich Islands. Several species occurred in distinct zones around fumaroles. Maximum temperatures recorded within the upper 0.5 cm of the vegetation surface were 40 ‐ 47 °C, with only Campylopus introflexus tolerating such temperatures. Maximum temperatures 2.5 or 5 cm below the vegetation surface of this moss reached 75 °C. Other bryophytes regularly present in zoned vegetation included the mosses Dicranella hookeri, Sanionia georgico‐uncinata, Pohlia nutans and Notoligotrichum trichodon, and the liverworts Cryptochila grandiflora and Marchantia berteroana. Surface temperatures of 25 ‐ 35 °C and subsurface temperatures of 50 ‐ 60 °C were recorded in these species.

Conclusions: These exceptional plant communities illustrate the transport of viable propagules into the Antarctic. Individually ephemeral in nature, the longer term existence of geothermal habitats on islands along the Scotia Arc may have provided refugia during periods of glacial expansion, facilitating subsequent recolonization of Antarctic terrestrial habitats.
Spontaneous vegetation succession in disused gravel‐sand pits: Role of local site and landscape factors
Authors: Rehounková Klára , Prach Karel
Published: February 2009

Questions: What is the variability of succession over a large geographical area? What is the relative importance of (1) local site factors and (2) landscape factors in determining spontaneous vegetation succession?

Location: Various regions of the Czech Republic, Central Europe. The regions represent two categories characterized by agrarian lowlands, with a relatively warm and dry climate, and predominant woodland uplands with a relatively cold and wet climate.

Methods: Gravel‐sand pits ranged in age from 1–75 years since abandonment. Three types of sites were distinguished: dry, wet and hydric in shallow flooded sites. Vegetation relevés were recorded with species cover (%) visually estimated using the space‐for‐time substitution approach. Local site factors, such as water table and soil characteristics, and landscape characteristics, namely climatic parameters, presence of nearby (semi‐) natural plant communities and main land cover categories in the wider surroundings, were evaluated.

Results: Ordination analyses showed that water table was the most important local site factor influencing the course of spontaneous vegetation succession. Succession was further significantly influenced by soil texture, pH, macroclimate, the presence of some nearby (semi‐) natural communities and some land cover categories in the wider surroundings. Spontaneous vegetation succession led to the formation of either shrubby grassland, deciduous woodland, alder and willow carrs, and tall sedge or reed and Typha beds in later stages depending predominantly on the site moisture conditions. Conclusions: Although the water table was the most influential on the course of vegetation succession, the landscape factors together explained more vegetation variability (44%) than local site factors (23%).
Changes in spatial pattern of trees and snags during structural development in Picea mariana boreal forests
Authors: Harper Karen A. , Bergeron Yves , Drapeau Pierre , Gauthier Sylvie , De Grandpré Louis
Published: February 2009

Questions: How do gap abundance and the spatial pattern of trees and snags change throughout stand development in Picea mariana forests? Does spatial pattern differ among site types and structural components of a forest?

Location: Boreal forests dominated by Picea mariana, northern Quebec and Ontario, Canada.

Methods: Data on the abundance, characteristics and spatial location of trees, snags and gaps were collected along 200 m transects at 91 sites along a chronosequence. Spatial analyses included 3TLQV, NLV and autocorrelation analysis. Non‐parametric analyses were used to analyse trends with time and differences among structural components and site types.

Results: Gaps became more abundant, numerous and more evenly distributed with time. At distances of 1–4 m, tree cover, sapling density and snag density became more heterogeneous with time. Tree cover appeared to be more uniform for the 10–33 m interval, although this was not significant. Patch size and variance at 1 m were greater for overstorey than for understorey tree cover. Snags were less spatially variable than trees at 1 m, but more so at intermediate distances (4–8 m). Few significant differences were found among site types.

Conclusions: During stand development in P. mariana forest, gaps formed by tree mortality are filled in slowly due to poor regeneration and growth, leading to greater gap abundance and clumping of trees and snags at fine scales. At broader scales, patchy regeneration is followed by homogenization of forest stands as trees become smaller with low productivity due to paludification.
Post‐fire bryophyte establishment in a continental bog
Authors: Benscoter Brian W.
Published: February 2009

Questions: What is the mechanism of bog ground layer colonization post‐fire? Is species colonization stochastic or does facilitation occur?

Location: Boreal bog peatland near Crow Lake, Alberta, Canada.

Methods: Diaspore‐addition treatments were applied in 2003 to autoclaved peat samples from high and low microtopographic positions within a recently burned bog. Colonization was assessed within the plots in 2005 and compared to control plots to determine treatment success and patterns of colonization.

Results: A significant degree of ground layer colonization was found two years after fire, with Polytrichum strictum dominating the site. Colonization was greater in low (wet) plots, although only P. strictum and Sphagnum angustifolium had significant colonization. No effect of diaspore addition was observed and Sphagnum was only found in conjunction with P. strictum.

Conclusions: Environmental conditions and species life history strategy are more important than diaspore availability for post‐fire colonization. True mosses (e.g. P. strictum) appearto facilitate Sphagnum colonization.
Structure and composition of vegetation along an elevational gradient in Puerto Rico
Authors: Gould W.A. , González G. , Carrero Rivera G.
Published: February 2009

Question: What are the composition, conservation status, and structural and environmental characteristics of eight mature tropical forest plant communities that occur along an elevational gradient.

Location: Northeastern Puerto Rico.

Methods: We quantified the species composition, diversity, conservation status, and ecological attributes of eight mature tropical forest plant communities in replicated plots located to sample representative components of important forest types occurring along an elevational gradient. A suite of environmental and vegetation characteristics were sampled at each plot and summarized to characterize communities and analyse trends along the elevational gradient.

Results: The set of communities included 374 species; 92% were native, 14% endemic, and 4% critical elements (locally endangered) to the island. All communities, occurring within a wide range of patch sizes and degree of conservation protection, showed a high percentage of native species (> 89% per plot). The lowland moist forest communities, occurring within a matrix of urbanization, agriculture, and disturbance, had the highest degree of invasion by exotics. Community descriptions were nested within a variety of hierarchies to facilitate extrapolation of community characteristics to larger ecosystem units. Basal area, above‐ground biomass, canopy heights, and mean species richness peaked at mid elevations.

Conclusions: It is significant that all of these forest communities continue to be dominated by native species while existing in a matrix of human and natural disturbance, species invasion, and forest regeneration from widespread agriculture. The lowland moist and dry forest types represent a minority of the protected forested areas in Puerto Rico, serve as unique genetic reservoirs, and should be protected.
Long‐term rain forest succession and landscape change in Hawai'i: The ‘Maui Forest Trouble’ revisited
Authors: Mueller‐Dombois D.
Published: February 2009

Because of their isolation and geographical position, and in contrast to the multi‐species tree canopies of tropical rain forests on the continents, the Hawaiian Islands have only two native dominant canopy species in their rain forests, Acacia koa and Metrosideros polymorpha. The wetter forest ecosystems are dominated by only the latter. In 1905, a dieback of lowland tropical Metrosideros rain forest was observed over a 35 km stretch on the lower east slope of Haleakala Mountain on Maui Island. This was dubbed ‘The Maui Forest Trouble.’ Although the synchronous decline of so many trees was initially believed to be caused by an epidemic disease, a decade of research yielded no pathogen. The conclusion was that the Hawaiian flora consisted primarily of colonizer species that were unable to continue growing on aging soils. Although this made ecological sense at that time, it was a rather limited and thereby unfortunate conclusion. Further research has shown that the Maui Forest Trouble was a ‘bog‐formation dieback’, a process of vegetation dynamics not only related to soil aging but more broadly to geomorphic aging and fundamental landscape change. This process is clearly a marginal‐site syndrome, but a natural process of profound consequence for biological conservation. This will be further explained as a paradigm for vegetation ecology.
Ecological effects of changes in fire regimes in Pinus ponderosa ecosystems in the Colorado Front Range
Authors: Sherriff Rosemary L. , Veblen Thomas T.
Published: February 2009

Question: What is the relative importance of low‐ and high‐severity fires in shaping forest structure across the range of Pinus ponderosa in northern Colorado?

Location: Colorado Front Range, USA.

Methods: To assess severities of historic fires, 24 sites were sampled across an elevation range of 1800 to 2800 m for fire scars, tree establishment dates, tree mortality, and changes in tree‐ring growth.

Results: Below 1950 m, the high number of fire scars, scarcity of large post‐fire cohorts, and lack of synchronous tree mortality or growth releases, indicate that historic fires were of low severity. In contrast, above 2200 m, fire severity was greater but frequency of widespread fires was substantially less. At 18 sites above 1950 m, 34 to 80% of the live trees date from establishment associated with the last moderate‐ to high‐severity fire. In these 18 sites, only 2 to 52% of the living trees pre‐date these fires suggesting that fire severities prior to any effects of fire suppression were sufficient to kill many trees.

Conclusions: These findings for the P. ponderosa zone above ca. 2200 m (i.e. most of the zone) contradict the widespread perception that fire exclusion, at least at the stand scale of tens to hundreds of hectares, has resulted in unnaturally high stand densities or in an atypical abundance of shade‐tolerant species. At relatively mesic sites (e.g. higher elevation, north‐facing), the historic fire regime consisted of a variable‐severity regime, but forest structure was shaped primarily by severe fires rather than by surface fires.
When does dead wood turn into a substrate for spruce replacement?
Authors: Zielonka Tomasz
Published: February 2009

Question: How many years must elapse for freshly fallen Picea abies stems to be transformed into a substrate for P. abies recruitment?

Location: Natural sub‐alpine spruce forest, 1200–1300 m a.s.l., western Carpathians, Poland.

Methods: Coarse woody debris (CWD) was measured on nine plots with a total area of 4.3 ha. All individuals of P. abies regeneration growing on dead wood were counted and their age was estimated. Decay rate of logs was determined using dendrochronological cross‐dating of samples from logs in different decay stages.

Results: Although CWD covered only 4% of the forest floor, 43% of the saplings were growing on decaying logs and stumps. The highest abundance of P. abies recruitment occurs on logs 30–60 years after tree death, when wood is in decay stages no. 4–7 (on an 8 degree decay scale). However, much earlier colonization is possible. The first seedlings may germinate on a log during the second decade after tree death and survive for decades. Their slow growth is possibly due to the gradual progressive decomposition of wood.

Conclusions: This study confirms the importance of decaying wood for P. abies recruitment. The decaying logs exhibit continuous and favourable conditions for the germination of P. abies seeds throughout their decay process. Logs, irrespective of their decay stage and age, are colonized by young seedlings. This recruitment bank is constantly renewed.
Taxonomic, morphological and structural surrogates for the rapid assessment of vegetation
Authors: Ramsay Paul M. , Kent Martin , Reid Catherine L. , Duckworth Jennifer C.
Published: February 2009

Question: To what extent do surrogate approaches to vegetation description provide equivalent data and estimates of the patterns of variability in species composition to that which would be obtained using species‐level abundance data?

Locations: Picos de Europa in Northern Spain and South Uist, Outer Hebrides, Scotland, UK.

Methods: Percentage cover data were collected from 117 plots in montane and sub‐alpine grassland and woodland mosaics in the Picos de Europa and 217 quadrats across a gradient from temperate coastal dunes to upland grassland and heath on South Uist. From these data, 14 surrogate datasets were derived using supra‐species (genus, family) levels of description, presence‐absence, life form, growth form and plant functional types (PFTs; derived from seven easily measurable traits). The descriptive power of each of the 14 surrogate datasets in each area, relative to the original cover abundance dataset, was assessed by ‘second stage’ analysis, based on Spearman's rank correlation coefficient calculated between pairs of similarity matrices.

Results: In both areas, presence‐absence data showed the closest similarities to the original cover abundance data, followed by data generalised at supra‐species level. PFT, life form, growth form and leaf trait data were less similar.

Conclusions: Many situations exist where it may be more efficient or desirable to collect neither species nor abundance data and to use appropriate surrogates. The results demonstrate clearly that the information content of surrogate datasets in our study may be an acceptable substitute. These ideas merit further investigation in a wider range of habitats and contexts.
Effects of repeated burning on Mediterranean communities of the northeastern Iberian Peninsula
Authors: Eugenio Màrcia , Lloret Francisco
Published: February 2009

Question:In relation to a single fire, do repeated wildfires in Mediterranean type ecosystems (1) reduce plant species richness or diversity; (2) modify patterns of abundance or dominance of plant species or (3) alter plant composition?

Location:Pinus halepensisdominated communities of Catalonia, northeastern Iberian Peninsula, western Mediterranean Basin.

Methods:Regional, paired design with 14 study sites, each consisting of a once burnt area (1994) and a twice burnt area (1975–1993 and 1994). Ten years after the last fire, we recorded all vascular plant species present in nested plots and quantified their relative abundances on transects. We compared species richness, diversity, dominance and relative abundance and species‐area correlations between paired once and twice burnt areas and assessed their floristic composition similarity.

Results:No statistically significant differences were found in species richness or diversity. Slopes of species‐area correlations were higher in once burnt areas. In twice burnt areas, dominance by one or two species was higher.P. halepensisshowed lower relative abundance and nanophanerophytes showed higher relative abundance. No differences were found for resprouter, seeder or resprouter‐seeder species. Floristic composition similarity between paired areas tended to be higher in less productive sites.

Conclusions:Fire recurrence had contrasting effects on species richness at different spatial scales. Repeated burning reduced the relative abundance of the dominant tree species, which resulted in a higher relative abundance of shrubs. It also promoted the dominance of herbs, particularlyBrachypodium retusum.However, it did not change the relative abundance of regenerative groups. Paired areas were more similar as they were more Mediterranean in terms of climatic conditions.
Plant functional group responses to fire frequency and tree canopy cover gradients in oak savannas and woodlands
Authors: Peterson David W. , Reich Peter B. , Wrage Keith J.
Published: February 2009

Questions: How do fire frequency, tree canopy cover, and their interactions influence cover of grasses, forbs and understorey woody plants in oak savannas and woodlands?

Location: Minnesota, USA.

Methods: We measured plant functional group cover and tree canopy cover on permanent plots within a long‐term prescribed fire frequency experiment and used hierarchical linear modeling to assess plant functional group responses to fire frequency and tree canopy cover.

Results: Understorey woody plant cover was highest in unburned woodlands and was negatively correlated with fire frequency. C4‐grass cover was positively correlated with fire frequency and negatively correlated with tree canopy cover. C3‐grass cover was highest at 40% tree canopy cover on unburned sites and at 60% tree canopy cover on frequently burned sites. Total forb cover was maximized at fire frequencies of 4–7 fires per decade, but was not significantly influenced by tree canopy cover. Cover of N‐fixing forbs was highest in shaded areas, particularly on frequently burned sites, while combined cover of all other forbs was negatively correlated with tree canopy cover.

Conclusions: The relative influences of fire frequency and tree canopy cover on understorey plant functional group cover vary among plant functional groups, but both play a significant role in structuring savanna and woodland understorey vegetation. When restoring degraded savannas, direct manipulation of overstorey tree canopy cover should be considered to rapidly reduce shading from fire‐resistant overstorey trees. Prescribed fires can then be used to suppress understorey woody plants and promote establishment of light‐demanding grasses and forbs.
Environmental and neighbourhood effects on tree fern distributions in a neotropical lowland rain forest
Authors: Jones Mirkka M. , Olivas Rojas Paulo , Tuomisto Hanna , Clark David B.
Published: February 2009

Questions:To what extent are the distributions of tropical rain forest tree ferns (Cyatheaceae) related to environmental variation, and is habitat specialization likely to play a role in their local coexistence?

Location:Lowland rain forest at La Selva Biological Station, Costa Rica.

Methods:Generalized linear (GLM) and generalized additive (GAM) logistic regression were used to model the incidence of four tree fern species in relation to environmental and neighbourhood variables in 1154 inventory plots regularly distributed across 6 km2of old‐growth forest. Small and large size classes of the two most abundant species were modelled separately to see whether habitat associations change with ontogeny.

Results:GLM and GAM model results were similar. All species had significant distributional biases with respect to micro‐habitat. Environmental variables describing soil variation were included in the models most often, followed by topographic and forest structural variables. The distributions of small individuals were more strongly related to environmental variation than those of larger individuals. Significant neighbourhood effects (spatial autocorrelation in intraspecific distributions and non‐random overlaps in the distributions of certain species pairs) were also identified. Overlaps between congeners did not differ from random, but there was a highly significant overlap in the distributions of the two most common species.

Conclusions:Our results support the view that habitat specialization is an important determinant of where on the rain forest landscape tree ferns grow, especially for juvenile plants. However, other factors, such as dispersal limitation, may also contribute to their local coexistence.
Regional assessment of plant invasions across different habitat types
Authors: Vilà Montserrat , Pino Joan , Font Xavier
Published: February 2009

Questions: 1. Which habitats have the highest degree of invasion? 2. Do native species‐rich communities have also a high degree of invasion? 3. Do the patterns of association between native and alien species richness vary between habitats.

Location: Catalonia region (NE Spain).

Methods: We conducted a large regional analysis of 15655 phytosociological relevés to detect differences in the degree of invasion between European Nature Information System (EUNIS) habitats representative of temperate and Mediterranean European areas.

Results: Alien species were present in less than 17 % of the relevés and represented less than 2% of the total number of species per habitat. The EUNIS habitats with the highest alien species richness were arable land and gardens followed by anthropogenic forb‐rich habitats, riverine and lakeshore scrubs, southern riparian galleries and thickets and trampled areas. In contrast, the following habitats had never any alien species: surface running waters, raised and blanket bogs, valley mires, poor fens and transition mires, base‐rich fens, alpine and sub‐alpine grasslands, sub‐alpine moist or wet tall‐herb and fern habitats, alpine and sub‐alpine scrub habitats and spiny Mediterranean heaths. There was a unimodal relationship between the mean native and mean alien species richness per EUNIS habitat with a high number of aliens in habitats with intermediate number of native species and a low number of aliens at both extremes of the native species gradient. Within EUNIS habitats, the relationship was positive, negative or non‐significant depending on the habitat type without any clear pattern related to the number of native species. Alien species richness was not related to plot size, neither between habitats nor within habitats.

Conclusions: The analysis emphasised that the habitats with a higher degree of invasion were the most disturbed ones and that in general habitats rich in native species did not harbour less invaders than habitats poor in native species.
Gradient analysis of reversed treelines and grasslands of the Valles Caldera, New Mexico
Authors: Coop Jonathan D. , Givnish Thomas J.
Published: February 2009

Objective: : Treeless meadows and parks are widespread but poorly understood features of the montane vegetation of the western USA. These communities frequently form reversed treelines where grassy valleys occur below forested slopes above. Our purpose was to assess the environmental correlates of such treelines, as well as patterns in the composition and diversity of grasslands and forest margins in the Valles Caldera National Preserve.

Location: Valles Caldera National Preserve (35°50′‐36°00’ N, 106°24′‐106°37’ W, 2175–3150 m), Jemez Mountains, New Mexico, USA.

Methods: We conducted a gradient analysis based on 200 nested quadrats on transects crossing reversed treelines and spanning the compositional heterogeneity of grasslands. We used cluster analysis and non‐metric multidimensional scaling to assess relationships between compositional variation and environmental variables.

Results: We found strong, highly significant relationships of the vegetation to gradients in slope inclination, soil texture, moisture, nutrient availability, and nighttime minimum temperatures. Reversed treelines are most strongly associated with shifts in the thermal regime, exhibit weaker relationships with soil texture and nutrient content, and show no relationship with gravimetric soil moisture. Gradients in aspect, soil moisture, and annual mean temperature are associated with compositional variation within grasslands and forest margins.

Conclusions: Lower nightly minimum temperatures and fewer consecutive frost‐free days resulting from cold‐air drainage may prevent tree seedling establishment in valley bottoms via photo‐inhibition, tissue damage, or frost heaving. Fine‐textured soils may also impede tree seedling establishment in valley bottoms. These findings lay the groundwork for experimental and physiological tests of these potential causes of these reversed treelines.
Drought‐related tree death of savanna eucalypts: Species susceptibility, soil conditions and root architecture
Authors: Fensham R.J. , Fairfax R.J.
Published: February 2009

Questions: For eucalypt savanna in northeast Australia subject to multi‐year rainfall deficits this paper asks whether (1) dominant tree species (Ironbarks, Boxes) are more drought susceptible than the sub‐dominant Bloodwoods; (2) whether soil moisture is beyond wilting point in surface soil layers but available at depth; (3) soil conditions (moisture availability and texture) are related to tree death during drought; (4) the root systems of the Boxes and Ironbarks are shallower than the Bloodwoods; and the survivors of drought within species have deeper root systems than those that died.

Location: Central Queensland, Australia.

Methods: Patterns of tree death between eucalypt species were compared from field data collected after drought. Soil conditions during drought were described and compared with patterns of tree death for the Ironbark Eucalyptus melanophloia. The basal area and orientation of coarse roots were measured on upturned trees after broad‐scale tree clearing, and compared between species, and between live and dead trees with tree size as a covariate.

Results: Drought‐induced tree death was higher for dominant Ironbark‐Box than for sub‐dominant Bloodwoods. During a moderate to severe drought in 2004, 41% of 100 cm deep subsoils had soil matric potential less than‐5600 kPa. The drought hardy Bloodwoods had a greater root basal area and particularly so for vertical roots compared to the drought sensitive Ironbark‐Box. Within species there was no significant difference in root basal area characteristics between trees that were recently killed by drought and those that remained relatively healthy. Surface soil moisture availability was lower where tree densities were high, and tree death increased as surface soil moisture became less available. Tree death was also greater as the clay content of sub‐soils increased.

Discussion: The study suggests species with roots confined to upper soil layers will suffer severe water stress. The results strongly indicate that root architecture, and the way it facilitates water use during drought, is important for the relative dominance of the tree species. Patchiness in drought‐induced tree death seems to be at least partially a product of heterogeneity in sub‐soil conditions and competition for soil moisture.