Questions: 1) What is the effect of functional type on speciesâ€™ distribution model performance, and which plant speciesâ€™ traits explain the most variation? 2) Does model performance vary more as a result of functional type, individual species characteristics, or modeling method?; Location: Californiaâ€™s Southwest Ecoregion, USA; Methods: We developed species distribution models (SDMs) for 45 plant species using four modeling methods (GLMs, GAMs, classification trees, and Random Forests). Using AUC as a performance measure of prediction accuracy, we carried out regression analyses to compare the effects of functional type, longevity, dispersal mechanism, range size, cover, species prevalence, and model type. Results: Functional type explained more variation in model performance than any other variable, but other species traits and biogeographical factors were also significant. Differences in prediction accuracy reflected variation in speciesâ€™ life history, disturbance response, and rarity. AUC was significantly higher for species that are longer-lived, found at intermediate levels of abundance, and have smaller range sizes. Models also performed better for shrubs than for subshrubs and perennial herbs. The disturbance response functional type with the highest SDM accuracy was obligate seeding shrubs with ballistic dispersal that regenerate via fire-cued germination from a dormant seed bank. Conclusions: The effect of species characteristics on model performance overrides any differences in modeling technique. Prediction accuracy may be related to the way a suite of species characteristics covary along environmental gradients, and including disturbance response was important because SDMs predict the realized niche. Classification of plant species into functional types, particularly in disturbance-prone ecosystems, may provide a strong framework for evaluating performance of SDMs.
Questions: How did an initial tree harvest in 1894 influence the spatial and temporal patterns of Pinus ponderosa recruitment? How do these patterns compare to our understanding of P. ponderosa stand dynamics prior to Euro‐American settlement? How might spatial pattern information, particularly with respect to patch characteristics, inform current restoration and management practices?
Location: A 2.59‐ha permanent sample plot in the Fort Valley Experimental Forest, Flagstaff, Arizona. The plot was selectively harvested in 1894 and measured in 1909 and 2002.
Methods: We used historical stem‐map and ledger data, contemporary data, and dendrochronological techniques to reconstruct stand structure (tree size, age, location) in three scenarios: (1) unharvested (1909), (2) harvested (1909), and (3) contemporary (2002). We used Clark and Evans' R, Ripley's K ( t ) univariate analysis, and correlogram analysis to assess the spatial pattern in each scenario. We also used Ripley's K 12 ( t ) bivariate analysis and tree age data to examine spatial and temporal recruitment patterns as observed in the contemporary scenario.
Results and Conclusions: The unharvested stand was aggregated at scales up to 28 m. The selective harvest accentuated the spatial patchiness of the stand in 1909 and changed spatial patterns by homogenizing tree size within patches. By 2002, the stand was a single patch dominated by small trees. Post‐harvest recruitment patterns were not spatially random; Pinus seedlings initially established in natural grass openings and then proceeded to fill‐in stump patches created by harvesting. Knowledge of spatial pattern should be explicitly incorporated into restoration activities in these forests.
The first broad‐based, paleoecological analysis of a sedimentary sequence on the British chalk, dating to the terminal Pleistocene, reveals a history of climatic, vegetational and faunal change. The past co‐occurrence of currently allopatric species among molluscs, beetles and plants supports hypotheses of the impermanence of communities. Modern pollen rain data are utilized to refine the ecological interpretation of the fossil pollen data.
The presence of the Windermere Interstadial (Allerod) and the Loch Lomond re‐advance (Younger Dryas) are represented by decreased abundances of arboreal taxa and increased representation of cold grassland elements. Open grassland habitats appear to have been a continuous landscape element, at least locally, since the late‐glacial period on the northern British chal Mands, although their species composition has changed greatly in the last 11400 yr.
We studied the effects of windthrow on the understory plant species composition of a pine forest (dominated by Pinus strobus) and an oak forest (dominated by Quercus ellipsoidalis). We recorded the presence of vascular plant species in randomly located quadrats in the two forests, and in three microsite types associated with tipup mounds (pit, old soil and new soil) in the pine forest at irregular intervals over the course of 14 years. The understories of the two forests remained distinct throughout the study. The frequency of occurrence of a number of forest floor species considerably increased; few species decreased. The disturbance specialists Rubus idaeus and Polygonum cilinode increased in frequency throughout the study in the pine forest, but are beginning to decline in the less disturbed oak forest. Annuals and biennials preferentially colonized the disturbed soil of microsites on tipups, and declined in frequency after about 7 yr. Both forests have increased in understory species richness, but have not changed substantially in the distribution of growth forms. Despite early differences in species composition, microsite types associated with tipup mounds became more similar through time. Although small in magnitude, there was a directional change in understory composition at both forests, with no apparent sign of a return to pre-disturbance conditions.
We measured tree damage and mortality following a catastrophic windthrow in permanent plots in an oak forest and a pine forest in central Minnesota. We monitored changes in forest structure and composition over the next 14 years. Prior to the storm, the oak forest was dominated by Quercus ellipsoidalis, and the pine forest by Pinus strobus. The immediate impacts of the storm were to differentially damage and kill large, early-successional hardwoods and pines. Subsequent recovery was characterized by the growth of late-successional hardwoods. In both forests the disturbance acted to accelerate succession. Ordination of tree species composition confirmed the trend of accelerated succession, and suggested a convergence of composition between the two forests.
Question: Are seed size and plant size linked to species responses to inter-annual variations in rainfall and rainfall distribution during the growing season in annual grasslands?
Location: A 16-year data set on species abundance in permanent plots 15 km north of Madrid in a Quercus ilex subsp. ballota dehesa.
Methods: At species level, a GLM was used to analyse the effects of various rainfall indices (total autumn rainfall, early autumn rainfall and spring drought) on species abundance residuals with respect to time and topography. We also assessed the importance of seed size and plant size in the species responses at community level using species as data points. Seed mass and maximum stem length were used as surrogates for seed size and plant size, respectively.
Results: Seed mass and plant size may explain some of the fluctuations in the floristic composition of annual species associated with autumn rainfall patterns. Species that are more abundant in dry autumns have greater seed mass than those species that are more abundant in wet autumns. Early autumn rainfall seems to favour larger plants.
Conclusions: Our empirical results support the hypothesis that autumn rainfall patterns affect the relative establishment capacity of small and large seedlings in annual species.
Aim: Concentration of species occurrences in groups of classified sites can be quantified with statistical measures of fidelity, which can be used for the determination of diagnostic species. However, for most available measures fidelity depends on the number of sites within individual groups. As the classified data sets typically contain site groups of unequal size, such measures do not enable a comparison of numerical fidelity values of species between different site groups. We therefore propose a new method of measuring fidelity with presence/absence data after equalization of the size of the site groups. We compare the properties of this new method with other measures of statistical fidelity, in particular with the Dufrêne‐Legendre Indicator Value ( IndVal ) index.
Methods: The size of site groups in the data set is equalized, while relative frequencies of species occurrence within and outside of these groups are kept constant. Then fidelity is calculated using the phi coefficient of association.
Results: Fidelity values after equalization are independent of site group size, but their numerical values vary independently of the statistical significance of fidelity. By changing the size of the target site group relative to the size of the entire data set, the fidelity measure can be made more sensitive to either common or rare species. We show that there are two modifications of the IndVal index for presence/absence data, one of which is also independent of the size of site groups.
Conclusion: The phi coefficient applied to site groups of equalized size has advantages over other statistical measures of fidelity based on presence/absence data. Its properties are close to an intuitive understanding of fidelity and diagnostic species in vegetation science. Statistical significance can be checked by calculation of another fidelity measure that is a function of statistical significance, or by direct calculation of the probability of observed species concentrations by Fisher's exact test. An advantage of the new method over IndVal is its ability to distinguish between positive and negative fidelity. One can also weight the relative importance of common and rare species by changing the equalized size of the site groups.
Questions: What are the relationships between weed species traits and their change in distribution over a 30-year period? What does it tell us about factors that have driven shifts in the composition of weed communities?
Methods: We analysed the links between change in the status of weed species in sunflower crops (decreasing or increasing) and a set of 17 traits using data sets collected in the 1970s and the 2000s, respectively. We analysed the contribution of traits to explain changes in the status of species both individually and in a multivariate way by mean of a clustering of species into functional groups.
Results: 69% of the most widespread species had significantly changed their frequency rank status over the last 30 years. Nearly two thirds of the increasing species belonged to a single functional group, out of the five groups identified in this analysis. Overall, the weed flora occurring in sunflower crops has specialised since the 1970s in favour of ‘sunflower mimicking’ functional groups: increasing species were more nitrophilous, more heliophilous, less sensitive to sunflower herbicides and shared a rapid summer life cycle.
Conclusions: The individual trait approach gave some indication as to the environmental factors likely to have caused the shift in sunflower weed communities. The functional group approach seemed to outperform direct trait comparisons as it accounted for major traits combinations i.e. cases where a species has a number of favourable traits, but is severely disadvantaged by the possession of one or a few deleterious traits.
Patterns of plant succession were studied in areas of scorched and blown‐down forest resulting from the 1980 eruption of Mount St. Helens, Washington. Changes in species abundance were observed for 7 years in permanent sample plots representing four post‐disturbance habitats, or site types. Total plant cover and species richness increased with time on all site types. In blown‐down forests supporting snowpack at the time of eruption, understory recovery was dominated by the vegetative regeneration of species persisting through disturbance. In forests without snowpacks, plant survival was poorer. Increases in cover and diversity were dominated first by introduced grasses, then by colonizing forbs characteristic of early successional sites. Epilo‐bium angustifolium and Anaphalis margaritacea showed widespread recruitment and clonal expansion throughout the devastated area. As a result, species composition on previously forested sites converged toward that on formerly clearcut sites, where early serai forbs resprouted vigorously from beneath the tephra.
Total plant cover and species diversity were poorly correlated with post‐disturbance habitat and general site characteristics (e.g. distance from the crater, elevation, slope, and aspect). However, distributions of several life‐forms (e.g. low sub‐shrubs and tall shrubs) were strongly correlated with depth of burial by tephra and with cover of tree rootwads. Thus, early community recovery may reflect microsite variation or chance survival and recruitment rather than broad‐scale gradients in environment or disturbance.
Recovery of pre‐disturbance composition and structure will undoubtedly be much slower than after other types of catastrophic disturbance. The rate and direction of community recovery will largely depend on the degree to which original understory species survived the eruption.
Understorey vegetation changes in a South Norwegian old‐growth coniferous forest were studied between 1988 and 1993 in 200 1‐m ² vegetation plots. Our aims were to quantify the amount of between‐year compositional change, and to elaborate the environmental basis for long‐term vegetation change, including the previously identified gradient structure with a major gradient related to topography (and soil nutrient status and soil depth) and a minor gradient reflecting paludification and canopy coverage.
Species richness (yearly mean and cumulative species number) and change in species richness differed between vascular plants and cryptogams, and between forest types. The number of vascular plant species decreased in pine forest in dry years; bryophyte species number increased in spruce forest.
Statistically significant vegetation change, as tested by constrained ordination (CCA) with time as the constraining variable, is demonstrated for most one‐year periods and for the five‐year period in most forest types. Vegetation change along identified gradients, measured as plot displacement along DCA ordination axes, also occurred. The magnitude of year‐to‐year vegetation change was related neither to forest type nor to one‐year period; different responses to climatic and environmental change were observed in each forest type. The largest average displacement observed, from medium‐rich spruce forest towards poor spruce forest, was interpreted as a long‐term trend. Humus‐layer pH decreased by ca. 0.25 units from 1988 to 1993, most strongly in medium‐rich spruce forest where exchangeable Ca decreased and Al and Mn increased strongly.
Our study supports the hypothesis that vascular plants show a long‐term and broad‐scale response to soil acidification. Change in bryophyte composition is linked to some very long growing‐seasons. Detailed analysis of short‐term vegetation dynamics enhances the interpretation of long‐term changes and stresses the complementarity of univariate and multivariate methods in the analysis of vegetation change.
Question: How strong are climate warming-driven changes within mid-elevation forest communities? Observations of plant community change within temperate mountain forest ecosystems in response to recent warming are scarce in comparison to high-elevation alpine and nival ecosystems, perhaps reflecting the confounding influence of forest stand dynamics.Location: Jura Mountains (France and Switzerland).Methods: We assessed changes in plant community composition by surveying 154 Abies alba forest vegetation relevés (550-1,350 m a.s.l.) in 1989 and 2007. Over this period, temperatures increased while precipitation did not change. Correspondence analysis (CA) and ecological indicator values were used to measure changes in plant community composition. Relevés in even- and uneven-aged stands were analysed separately to determine the influence of forest stand dynamics. We also analysed changes in species distribution to detect shifts along the elevation gradient by focusing on the lowest, central and highest positions of lowland and mountain species altitudinal ranges.Results: We found significant shifts along the first CA axis, which reflected a change in plant community composition towards a greater frequency of lowland species. Analyses of ecological indicator values indicated increases in temperature and light availability in A. alba stands, particularly in even-aged stands. However, no major changes in overall species distribution were found.Conclusions: The community-level changes are consistent with effects of climate warming and local stand dynamics. Changes in species distribution were small in comparison to observed local temperature increases, perhaps reflecting dispersal limitation, phenotypic plasticity or microclimatic buffering by the tree canopy. Causality cannot rigorously be inferred from such a descriptive study; however, we suggest that recent warming is now driving plant community change in the climatically more moderate mid-elevation forest setting.
Question: What are the trends and patterns in the application of ordination techniques in vegetation science since 1990?
Location: Worldwide literature analysis.
Methods: Evaluation of five major journals of vegetation science; search of all ISI‐listed ecological journals. Data were analysed with ANCOVAs, Spearman rank correlations, GLMs, biodiversity indices and simple graphs.
Results: The ISI search retrieved fewer papers that used ordinations than the manual evaluation of five selected journals. Both retrieval methods revealed a clear trend in increasing frequency of ordination applications from 1990 to the present. Canonical Correspondence Analysis was far more frequently detected by the ISI search than any other method. Applications such as Correspondence Analysis/Reciprocal Averaging and Detrended Correspondence Analysis have increasingly been used in studies published in “applied” journals, while Canonical Correspondence Analysis, Redundancy Analysis and Non‐Metric Multidimensional Scaling were more frequently used in journals focusing on more “basic” research. Overall, Detrended Correspondence Analysis was the most commonly applied method within the five major journals, although the number of publications slightly decreased over time. Use of Non‐Metric Multidimensional Scaling has increased over the last 10 years.
Conclusion: The availability of suitable software packages has facilitated the application of certain techniques such as Non‐Metric Multidimensional Scaling. However, choices of ordination techniques are currently less driven by the constraints imposed by the software; there is also limited evidence that the choice of methods follows social considerations such as the need to use fashionable methods. Methodological diversity has been maintained or has even increased over time and reflects the researcher's need for diverse analytical tools suitable to address a wide range of questions.
We followed plant cover and soil seed density of shrubs and herbs in two markedly contrasting years with regard to annual rainfall in two opposite-facing slopes (mesic vs xeric) and an intervening, relatively wet, ravine of a typical creek in semi-arid Chile. During the ENSO year 1997 cover of ephemerals increased in all three sites; 43% vs 8% on the xeric slope 75% vs 26% on the mesic slope and 57% vs 32% in the ravine. The number of species was almost three times higher on the xeric slope (27 vs 10 species), increased by 47% on the mesic slope (28 vs 19) and by 14% in the ravine (24 vs 21). Cover of shrubs plus perennial grasses increased from 52% in 1996 to 59% in 1997 on the xeric slope, but no significant cover changes were found on the mesic slope or the ravine. On the xeric slope peak density of seeds was 4500/m2 in 1996, while during the ENSO year it was 24000/m2. On the mesic slope equivalent values were 3000 and 17000 seeds/m2 while in the ravine figures were 8500 and 27000 seeds/m2. The strong responses of ephemerals and seed bank to the ENSO-driven wet year of 1997 demonstrate the importance of this phenomenon in replenishing, recovering and probably maintaining ephemeral vegetation in this region.
Question: How do aggregate trait values and functional diversity of leaf traits linked to palatability and decomposability change during a woody post-cultural succession spanning 200 years?Location: Coastal Marlborough, South Island, New Zealand.Methods: The biomass of all woody species was determined in 32 20-m × 20-m plots ranging from 10 to 200 years in time since last disturbance. Species abundances were combined with data on leaf nutrient, secondary metabolite and structural carbohydrate content to calculate biomass-weighted trait means (i.e. aggregate trait values) and functional diversity index values for each plot. Aggregate trait values and functional diversity were regressed on successional age and total live above-ground carbon content to examine functional shifts with succession and one consequence of succession – increasing above-ground carbon.Results: Almost all significant regressions between aggregate trait values and both successional age and above-ground carbon indicated a shift toward increased leaf palatability and decomposability during succession. The relationships were all non-linear, with aggregate trait value shifts occurring relatively early in the successional sequence. There was weak evidence for an increase in functional richness with succession, but this was a secondary effect relative to the shifts in aggregate trait values.Conclusions: These results are in direct contrast with studies of the early stages of herbaceous post-cultural successions from grasslands to shrublands, which have found a shift towards communities of decreasing palatability and decomposability, suggesting that functional shifts in woody succession may be fundamentally different.
Personal computers of ever-increasing speed have motivated programmers of multivariate software to adapt their programs to be run in Microsoft Windows and Macintosh platforms. Updated versions of these multivariate programs appear more and more frequently and are marketed intensively. In this review we provide a comparative analysis of the most recent versions of three analytical software packages –Canoco for Windows 4.5, PC-ORD version 4 and SYN-TAX 2000. The three packages share two characteristics. First, the most recent versions are now compatible with the most recent Windows platforms and should therefore be accessible for use by virtually all vegetation scientists. Second, they have capabilities for numerous multivariate techniques, although each package has some unique techniques. Thus, any one of the packages will have much to offer the user.
Question: Following a volcanic eruption of ca. 232 AD, known as the Taupo eruption, the emergent conifer Libocedrus bidwillii expanded on Mt. Hauhungatahi, upwards above the current tree-line, and downwards into the mixed montane forest. We ask: (1) if current age-structures at different altitudes support the patterns predicted by the temporal stand replacement model, with cohort senescence and progressively depleting recruitment at ca. 600 year intervals (average cohort age) since the eruption: and (2) if the case history of the population sheds light on the persistence of mixed conifer-hardwood forests in general.
Location: Mt. Hauhungatahi, Tongariro National Park, New Zealand.
Methods: The species composition and structure of seven stands covering the altitudinal range of Libocedrus bidwillii, were quantified. Libocedrus trees were cored, and regression equations used to predict ages. Cohorts were identified.
Results: Libocedrus densities and basal areas, and the abundance of seedlings and saplings, peaked at different altitudes. At the species’lower limits there has been no recruitment for ca. 550 years, and the angiosperm Weinmannia racemosa has gained dominance. In the tree line and sub-alpine forest stands, a low level of continuous regeneration has been boosted by periodic cohort recruitment following exogenous disturbances.
Conclusions: In the montane zone, the Libocedrus age structure, and its replacement by Weinmannia, are consistent with a model of depleting cohorts separated by ca. 600 years since the Taupo eruption. At higher altitudes more frequent disturbances and reduced competition have allowed Libocedrus persistence. Comparison with other studies suggests long-term relationships between gymnosperms and angiosperms are mediated by the scale and frequency of disturbance.
Question: Habitats are characterized by their plant species composition. Therefore, climate change impacts on habitats can be assessed by two complementary statistical approaches: either directly by modelling the climate envelope of the habitat, or indirectly by modelling the habitat in terms of its plant species. How do these approaches differ in their projected habitat distribution? What are the consequences for nature conservation?
Methods: Potential climate change impacts on the distribution of European protected Natura 2000 sites were modelled for five natural and semi-natural grassland habitat types, defined by the EU Habitats Directive, using data from the Atlas Florae Europaeae and reports on Natura 2000 sites. We used random forests (RF) and logistic regression (GLM) to model current and potential future distributions for 2050.
Results: All habitats are projected to lose between 22% and 93% of their range in the ‘no dispersal’ scenario. In the ‘unrestricted dispersal’ scenario, almost all habitats gain suitable climate space, between 5% and 100% of their current range. In the direct habitat approach, both model algorithms have high discriminatory performance on test data and are well calibrated. In the indirect species approach, only GLM shows high model performance; RF models are overfitted. Projections of occurrence probabilities differ more strongly between model approaches (‘direct’ versus ‘indirect’) than between model algorithms (GLM versus RF).
Conclusions: Habitats are complex entities. Because of their dynamic nature, particularly in the face of climate change, we suggest modelling the future distribution of habitat types not exclusively based on their current definitions and mapped distributions, but also based on their constituent elements, and in particular their characteristic plant species.
We document post-fire succession on xeric sites in the southern Appalachian Mountains, USA and assess effects of 20th century reduction in fire frequency on vegetation structure and composition. Successional studies over 18 yr on permanent plots that had burned in 1976–1977 indicate that tree mortality and vegetation response varied with fuel load and fire season. In the first three years after fire, hardwood sprouts dominated tree regeneration. On sites where summer and autumn fires reduced litter depth to less than 1 cm, densities of shade-intolerant Pinus seedlings increased steadily over this period. 4 to 8 yr after fire, large numbers of newly established seedlings and sprouts had grown to 1 – 10 cm DBH. By year 18 growth of these saplings led to canopy closure on most sites. Herbaceous cover and richness peaked in the first decade after fire, then declined. On similar sites that had not burned in more than 50 yr, regeneration of shade-intolerant Pinus spp. and mean cover and richness of herbs were considerably lower than those observed on recently burned plots. Reconstructions of landscape conditions based on observed post-fire succession and 20th century changes in fire regime suggest that reductions in fire frequency circa 1940 led to substantial changes in forest structure and decreases in cover and richness of herbaceous species.
We present results from repeated analyses (1962, 1993) of a permanent plot established in 1947, combined with retrospective stand age structure data, in an old Pinus sylvestris stand in Muddus National Park, northern Sweden. The study points towards a successional pathway governed by concurrent disturbance effects of climate variability, reindeer grazing and fire. This is intermediate to the two often advocated ideas on dynamics in boreal forests, that is, one of disturbance‐related tree regeneration/mortality and one of continuous regeneration.
When the plot was established in 1947 the tree layer (> 1.3 m) consisted of 300 individuals/ha of P. sylvestris and 62/ha of Betula pubescens. Subsequently the stand has become more dense and the species dominance has shifted. In 1993, 362 P. sylvestris and 62 Picea abies individuals were present per ha, while no Betula individuals were found. The number of dead trees increased from zero in 1947 to 200/ha ( Pinus ) in 1993. Pinus was also the most common species in the sapling layer (< 1.3 m) throughout the study period, though the number dropped from 8912/ha in 1947 to 51% in 1993. Dead saplings decreased from 2650/ha in 1947 to ca. 50% in 1962, and only 9% in 1993. Temporal variations in mortality and sapling mean height coincided with variations in snow depth, indicating a critical period in sapling development when saplings are exposed at the snow/atmosphere interface. The number of living Picea saplings increased slowly until 1993; no dead saplings were found.
Most Pinus recruited shortly after the 1774‐fire, and during the second half of the 1900s. The major part of the spruce regeneration took place during the later half of the 1900s. No successful Betula recruitment has occurred after the 1930s, and no live Betula were present in 1993, which might be explained as an effect of increased reindeer browsing – the reindeer stock has grown by 50% since 1961.
Although subjected to high mortality, Pinus regenerated and maintained a seedling/sapling bank. In this way Pinus remained dominant in the tree layer after more than 200 post‐fire years. The importance of the shade‐tolerant Picea has slowly increased, while Betula has died off. Thus, even after 219 yr since fire there is an early successional trend in the stand. This suggests that an increased chronic disturbance (grazing/browsing by reindeer) has partly succeeded earlier discrete fire‐disturbance events, and maintained a continuous seedbed favouring the shade‐intolerant pine recruitment.
Questions: Most modern fire-prone landscapes have experienced disruptions of their historic fire regimes. Are the primary tallgrass prairies of the Flint Hills reflective of a history of continuous fire occurrence? Did fire frequency, severity, size and seasonality change in connection with changes in land use? Has fire occurrence been related to drought conditions?
Location: Edges of Cross Timbers forest stands at the Tallgrass Prairie Preserve (TGPP) in the Flint Hills of Osage County, Oklahoma, USA.
Methods: Cross-sections of 76 Quercus stellata were collected from Cross Timbers stands at or near the grassland edge in the TGPP. Dendrochronological methods were used to identify years of formation for tree rings and fire scars. Superposed epoch analysis was used to evaluate the effect of drought conditions on fire occurrence.
Results: Fires were recorded in 46.6% of the years between 1729 and 2005. In 41 cross-sections at one site, the mean fire interval between 1759 and 2003 was 2.59 years, with fire interval decreasing from a mean fire interval of 3.76 years in the early part of the record to 2.13 years in modern times. No extended periods without fire were recorded in the study area. Drought conditions had no significant effect on fire occurrence.
Conclusions: In contrast with many fire-prone landscapes worldwide, the prairies of the Flint Hills have experienced no recent fire suppression or exclusion. Changes in fire frequency mark transitions in land use, primarily from being traditionally used by Native Americans to being managed for cattle production.
Questions: (1) Is climate a strong driver of vegetation dynamics, including interannual variation, in a range margin steppic community? (2) Are there long-term trends in cover and species richness in this community, and are these consistent across species groups and species within groups? (3) Can long-term trends in plant community data be related to variation in local climate over the last three decades?
Location: A range margin steppic grassland community in central Germany.
Methods: Cover, number and size of all individuals of all plant species present in three permanent 1-m2 plots were recorded in spring for 26 years (1980–2005). Climatic data for the study area were used to determine the best climatic predictor for each plant community, functional group and species variable (annual data and interannual variation) using best subsets regression.
Results: April and autumn temperature showed the highest correlation with total cover and species richness and with interannual variations of cover and richness. However, key climate drivers differed between the five most abundant species. Similarly, total cover and number and cover of perennials significantly decreased over time, while no trend was found for the cover and number of annuals. However, within functional groups there were also contrasting species-specific responses. Long-term temperature increases and high interannual variability in both temperature and precipitation were strongly related to long-term trends and interannual variations in plant community data.
Conclusions: Temporal trends in vegetation were strongly associated with temporal trends in climate at the study site, with key roles for autumn and spring temperature and precipitation. Dynamics of functional groups and species within groups and their relationships to changes in temperature and precipitation reveal complex long-term and interannual patterns that cannot be inferred from short-term studies with only one or a few individual species. Our results also highlight that responses detected at the functional group level may mask contrasting responses within functional groups. We discuss the implications of these findings for attempts to predict the future response of biodiversity to climate change.
Plant macrofossils extracted from fossil woodrat (Neotoma spp.) middens at a single locale in the northwestern Great Basin were used to examine vegetation dynamics during the last 30 000 yr. Although the modern assemblage of xeric species at the study site is a recent occurrence, a large proportion of the modern plant taxa near the study locale were also found 12 000 - 30 000 yr BP. The persistence of extant species through time was likely facilitated by within-species genetic diversity and the formation of coenospecies. The diverse topographic and microhabitat features in the northwestern Great Basin also allowed different species to coexist during glacial periods. Changes in species composition occurred during two time intervals: 20 000 - 30 000 and 10 000 - 12 000 yr BP. Vegetation changes during 20 000 - 30 000 yr BP were cyclic; community composition oscillated between two groups of taxa. Vegetation changes between 10 000 - 12 000 yr BP occurred during the Pleistocene-Holocene transition and were largely directional from the Pleistocene assemblages through two transition assemblages to a Holocene assemblage. These changes in species composition generally reflect changes in climate. The presence of relatively mesic species during 10 000 - 30 000 yr BP is consistent with the regional late-Pleistocene climate, and the gradual loss of relatively mesic species during the Holocene parallels the change to a more xeric climate. Contrasted with other areas of North America and Europe, the magnitude of vegetation changes at our study area were relatively small. Furthermore, the persistence of many species through time at this site in the northwestern Great Basin also differs from results at other study sites in North America and Europe. These differences are probably related to land form characteristics and genetic diversity within species.
Of 34 species of vascular plants recorded in an unimproved calcareous grassland community in 1960 and subsequently buried for 32 yr, just six spp. were recovered as viable seeds from the original land surface, following excavation of an experimental earthwork. Five of the six are known or suspected to form persistent seed banks; the status of Carex flacca as a persistent seed is now established. The exceptionally low density of viable seeds at 35 seeds/m2 and the paucity of other plant remains confirm that these calcareous soils have supported rapid decomposition. The complete absence of viable seeds known to be short-lived indicated that the macrofauna has played no significant role in the introduction of viable seeds from the modern-day surface down to depths in excess of 1.6 m, at least not in recent years. Because of the precise dates of burial and excavation, the results offer a rare opportunity to confirm laboratory and other indirect observations of long-term seed persistence following burial in temperate climate calcareous soils.
We investigated patterns of disturbance and recovery in Fiby urskog, a primeval spruce (Picea abies) forest, situated south of the border between the Boreo‐nemoral and Boreal regions in East‐central Sweden. The main types of disturbances are storm damage, fungal infection and insect attacks. The response of the different tree species varied and the mode of tree‐fall depended on the different combinations of disturbance agents. The DBH distributions of gap creators and gap‐border trees were almost the same. There was a high age diversity (100–240 yr) among the fallen trees. We concluded that all canopy trees (DBH > 20 cm) had the same probability of being felled by storms, irrespective of their age and DBH.
According to an estimate along transect lines, gaps made up 31% of the spruce forest area. Individual gap sizes ranged from 9 m ² to 360 m ² , but 83% of the gaps were < 150 m ² . The varied age structure of logs in individual gaps indicated that gap enlargements were common. 96 tree‐falls were observed on four days with an hourly mean wind speed > 12.0 m/s; all trees fell in the direction of the wind. However, when we consult the 30‐yrrecord(l 959–1989)ofthemeanhourly wind speed >12.0 m/s, it is clear that the pattern of storm‐directions does not match the pattern of orientation of fallen logs.
The present disturbance regime and the predominance of small gaps were more favourable for the regeneration oí Picea abies than of light‐demanding tree species. In one large, 2900 m ² gap, not crossed by the transects, all the major tree species had established within 7 yr, suggesting that classical succession in the sense of complete species replacement or ‘relay floristics’ didnot occur. Our observations seem rather to fit the ‘initial floristic’ model. Estimates of turnover time ranged from 170 to 228 yr, depending on the method used.