Exploring the functional significance of forest diversity: A new long-term experiment with temperate tree species (BIOTREE)

Effects of biodiversity on ecosystem functioning have been mainly studied in experiments that artificially create gradients in grassland plant diversity. Woody species were largely excluded from these early experiments, despite the ecological and socioeconomic importance of forest ecosystems. We discuss conceptual aspects of mechanistically driven research on the biodiversity–ecosystem functioning relationship in forests, including the comparison of scientific approaches like ‘observational studies’, ‘removal experiments’, and ‘synthetic-assemblage experiments’. We give a short overview on the differences between herbaceous and forest ecosystems, focusing on canopy characteristics, and the possibilities for individual versus population-based investigations.We present detailed information about the first large-scale, multisite and long-term biodiversity–ecosystem functioning experiment with tree species of temperate forests (BIOTREE – BIOdiversity and ecosystem processes in experimental TREE stands). At three sites of differing geology and local climate, we planted 200,000 saplings on a total area of 70 ha. At two sites, diversity gradients were established by varying the number of tree species (BIOTREE-SPECIES). At a third site, only functional diversity at a constant level of tree species richness was manipulated by selecting mixtures that differ in the functional trait values of the corresponding species (BIOTREE-FD). Additional experimental treatments at the subplot level include silvicultural management options, the addition of subdominant species, and the reduction of genetic diversity. Response variables focus on productivity, biogeochemical cycles and carbon sequestration, and resource use complementarity.We explore the use of different measures of functional diversity for a posteriori classifications of functional richness and their use in the analysis of our tree diversity experiment. The experiment is thought to provide a long-term research platform for a variety of scientific questions related to forest biodiversity and ecosystem processes.     

Impacts of tree species diversity on litter decomposition in northern temperate forests of Wisconsin, USA: a multi-site experiment along a latitudinal gradient

Over the past decade an increasing amount of research has sought to understand how the diversity of species in an ecosystem can influence fluxes of biologically important materials, such as the decomposition of organic matter and recycling of nutrients. Generalities among studies have remained elusive, perhaps because experimental manipulations have been performed at relatively small spatial scales where site-specific variation generates patterns that appear idiosyncratic. One approach for seeking generality is to perform parallel experiments at different sites using an identical species pool. Here we report results from a study where we manipulated the diversity of leaf litter from the same six dominant tree species in the litter layer of three forested ecosystems. These ecosystems spanned a 300 km latitudinal transect in Wisconsin, USA, and were characterized by a large gradient in temperature and moisture, and thus, rates of decomposition. After allowing combinations of one, two, four, and six species of leaf litter to decompose for 1 year, we found that increasing leaf litter richness led to slower rates of decomposition and higher fractions of nitrogen lost from litter. Across all sites, climate and initial litter chemistry explained more of the variation in decomposition rates than did litter richness. Effects of leaf litter diversity were non-additive, meaning they were greater than expected from the impacts of individual species, and appeared to be strongly influenced by the presence/absence of just 1–2 species (Tilia americana and Acer saccharum). The rate of decomposition of these two species was highly site-specific, which led to strong negative effects of litter richness only being observed at the southernmost sites where T. americana and A. saccharum decomposed more quickly. In contrast, litter diversity increased nitrogen loss at the northernmost sites where decomposition of T. americana was notably slowed. Our study shows that species diversity affected at least one of the two litter processes at each site along this 300-km gradient, but the exact nature of these effects were spatially variable because the performance of individual species changed across the heterogeneous landscape.     

Resistance to rust fungi in Lolium perenne depends on within-species variation and performance of the host species in grasslands of different plant diversity

The hypothesis that plant species diversity and genetic variation of the host species decrease the severity of plant diseases is supported by studies of agricultural systems, but experimental evidence from more complex systems is scarce. In an experiment with grassland communities of varying species richness (1, 2, 4, 8, 16, and 60 species) and functional group richness (1, 2, 3, and 4 functional groups), we used different cultivars of Lolium perenne (perennial ryegrass) to study effects of biodiversity and cultivar identity on the occurrence and severity of foliar fungal diseases caused by Puccinia coronata (crown rust) and P. graminis (stem rust). Cultivar monocultures of perennial ryegrass revealed strong differences in pathogen susceptibility among these cultivars. Disease intensity caused by both rust fungi decreased significantly with growing species richness of species mixtures. The response to the diversity gradient was related to the decreased density and size of the host individuals with increasing species richness. The occurrence of other grass species known to be possible hosts of the pathogens in the experimental mixtures did not promote disease intensity in L. perenne, indicating that there was a high host specificity of pathogen strains. Differences in pathogen susceptibility among perennial ryegrass cultivars persisted independent of diversity treatment, host density and host individual size, but resulted in a cultivar-specific pattern of changes in pathogen infestation across the species-richness gradient. Our study provided evidence that within-species variation in pathogen susceptibility and competitive interactions of the host species with the environment, as caused by species diversity treatments, are key determinants of the occurrence and severity of fungal diseases. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Landscape-level tree diversity assessment in tropical forests of southern Eastern Ghats,India

This large-scale, landscape-level study aims to assess tree species diversity, stem density and stand structure of six major tropical hill forests of southern Eastern Ghats, India, namely, Bodamalai (BM), Chitteri (CH), Kalrayan (KA), Kolli hills (KO), Pachaimalai (PM) and Shervarayan hills (SH). The Eastern Ghats of India is relatively under-studied compared with the Western Ghats biodiversity hotspot. The entire stretch of southern Eastern Ghats was divided into smaller grids of 6.25 km × 6.25 km, totaling to 120 grids. Within each grid, a belt transect of 0.5 ha (5 m × 1000 m) area was laid and all trees ≥30 cm girth at breast height (gbh) were enumerated. A total of 272 tree species (≥30 cm gbh) that belonged to 181 genera and 62 families were recorded in the total 60 ha area inventoried. Diversity indices such as Shannon, Simpson and Fisher's alpha indices were 2.44, 0.03 and 42.1, respectively, for the whole 60 ha area. One way ANOVA revealed that the species richness varied significantly across the six sites (F_(5,823) = 4.854, p < 0.0002). Also, the contribution of tree species to total species richness classified by three plant types viz. evergreen, brevi-deciduous and deciduous species varied significantly across the sites (One way ANOVA: F_(2,15) = 10.05, p < 0.002). Similarity indices such as Jaccard and Sørensen showed that sites CH and KA are more similar in terms of species composition. The total stand density and basal area for the total 60 ha area were 27,412 stems (457 stems ha⁻¹) and 1012.12 m² (16.9 m² ha⁻¹), respectively. The stand density and basal area for the six sites ranged from 290 (in site BM) to 527 stems ha⁻¹ (in site KA) and from 5.6 (in site BM) to 24.4 m² ha⁻¹ (in site KO), respectively. Stand density and basal area of tree species varied significantly across the six hill complexes (F_(5,823) = 4.85, p < 0.0002 and F_(5,823) = 2.71, p < 0.02, respectively). A positive correlation was obtained between stand density and species richness in sites PM (r_s = 0.65, p < 0.05) and SH (r_s = 0.67, p < 0.05), but not in other sites. The predominant tree species in the tropical forests of southern Eastern Ghats include Albizia amara, Euphorbia antiquorum, Canthium dicoccum var. dicoccum, Memecylon edule, Chloroxylon swietenia and Nothopegia heyneana. Taxonomically, Euphorbiaceae constituted the most diverse family with 25 species. Whereas, by tree abundance the Mimosaceae with 4126 stems enumerated from the 60 ha area formed the dominant family. Bray–Curtis cluster analysis, based on tree species composition and abundance revealed that the low-diverse site BM formed a separate entity from other hill complexes. This large-scale tree diversity inventory provides a baseline data for a variety of investigations and is expected to be useful for effective forest management and biodiversity conservation of southern Eastern Ghats region.     

Species diversity and functional assessment of macroinvertebrate communities in Austrian rivers

We applied an extensive data set from 211 locations along Austrian rivers to assess community structure and the ratios of functional feeding groups of benthic macroinvertebrates. A total of 569 taxa have been identified. At the catchment scale, the Enns, Salzach, and Traun Rivers exhibited the highest taxa richness whereas the Inn River showed the lowest richness. Beta-diversity was highest along the impounded and fragmented Enns and Drau Rivers. Consequently, high corridor diversity corresponded to a low degree of nestedness. Overall, scrapers and gathering-collectors dominated the benthic community. Further, the relationship between habitat conditions and metrics based on functional feeding groups were statistically analyzed to validate the potential of these metrics as indicators of ecosystem attributes. We examined four major ecosystem attributes: species diversity, material cycling, longitudinal material transport, and lateral material input. Multiple regression analyses for midorder rivers demonstrated that metrics were significantly related to habitat conditions. For example, the metric set indicating primary production was positively correlated with periphyton cover, dissolved oxygen, dominant sediment size, and average annual discharge. Overall, most metrics exhibited unique responses to habitat conditions, implying that they are useful proxies of ecosystem attributes. Thus, a function-based approach based on macroinvertebrates has the potential to become an effective tool for the assessment of river ecosystems.     

Structure,composition and species diversity in an altitude-substrate matrix of rain forest tree communities on Mount Kinabalu,Borneo

We studied forest structure, composition and tree species diversity of eight plots in an environmental matrix of four altitudes (700, 1700, 2700 and 3100 m) and two types of geological substrates (ultrabasic and non-ultrabasic rocks) on Mount Kinabalu, Borneo. On both substrate series, forest stature, mean leaf area and tree species diversity (both 4.8 cm and 10 cm diameter at breast height [dbh]) decreased with altitude. The two forests on the different substrate series were similar at 700 m in structure, generic and familial composition and tree species diversity, but became dissimilar with increasing altitude. The decline in stature with altitude was steeper on the ultrabasic substrates than on the non-ultrabasic substrates, and tree species diversity was generally lower on ultrabasic substrates than on non-ultrabasic substrates at 1700 m. The forests on non-ultrabasic substrates at higher altitudes and those on ultrabasic substrates at the lower altitudes were similar in dbh versus tree height allometry, mean leaf area, and generic and familial composition at 1700 m. These contrasting patterns in forest structure and composition between the two substrate series suggested that altitudinal change was compressed on the ultrabasic substrates compared to the non-ultrabasic substrates. Tree species diversity was correlated with maximum tree height and estimated aboveground biomass, but was not with basal area, among the eight study sites. We suggest that forests with higher tree species diversity are characterized by greater biomass allocation to height growth relative to trunk diameter growth under more productive environment than forests with lower tree species diversity.     

Effects of nonindigenous species on diversity and community functioning in the eastern Gulf of Finland (Baltic Sea)

An increase of xenodiversity in plankton and benthos in the eastern Gulf of Finland was observed from 1998 to 2004. Nonindigenous species account for 4.8% of all species found and up to 96% of total biomass. Invasive benthic omnivores, the alien amphipods Gmelinoides fasciatus and Pontogammarus robustoides and the predaceous fish Perccottus glenii with their versatile diets strongly affect the community structure. Invasive sessile seston-feeders that directly (through grazing and water clearance) and indirectly (through recycling of nutrients) interact with other ecosystem components, are mainly represented by the zebra mussel Dreissena polymorpha, which affect the structure of benthic and planktonic communities as well as benthic-pelagic coupling. The invasive predatory cladocerans Cercopagis pengoi and Evadne anonyx and larvae of D. polymorpha are only temporary components in the zooplankton, which is limiting their overall effect. Alien benthic bioturbators, the polychaetes Marenzelleria neglecta and the oligochaete Tubificoides pseudogaster account for a high proportion of total abundance and biomass but their effects on native species need further research.     

Plant species diversity as a driver of early succession in abandoned fields: a multi-site approach

Succession is one of the most studied processes in ecology and succession theory provides strong predictability. However, few attempts have been made to influence the course of succession thereby testing the hypothesis that passing through one stage is essential before entering the next one. At each stage of succession ecosystem processes may be affected by the diversity of species present, but there is little empirical evidence showing that plant species diversity may affect succession. On ex-arable land, a major constraint of vegetation succession is the dominance of perennial early-successional (arable weed) species. Our aim was to change the initial vegetation succession by the direct sowing of later-successional plant species. The hypothesis was tested that a diverse plant species mixture would be more successful in weed suppression than species-poor mixtures. In order to provide a robust test including a wide range of environmental conditions and plant species, experiments were carried out at five sites across Europe. At each site, an identical experiment was set up, albeit that the plant species composition of the sown mixtures differed from site to site. Results of the 2-year study showed that diverse plant species mixtures were more effective at reducing the number of natural colonisers (mainly weeds from the seed bank) than the average low-diversity treatment. However, the effect of the low-diversity treatment depended on the composition of the species mixture. Thus, the effect of enhanced species diversity strongly depended on the species composition of the low-diversity treatments used for comparison. The effects of high-diversity plant species mixtures on weed suppression differed between sites. Low-productivity sites gave the weakest response to the diversity treatments. These differences among sites did not change the general pattern. The present results have implications for understanding biological invasions. It has been hypothesised that alien species are more likely to invade species-poor communities than communities with high diversity. However, our results show that the identity of the local species matters. This may explain, at least partly, controversial results of studies on the relation between local diversity and the probability of being invaded by aliens. Received: 13 July 1999 / Accepted: 4 February 2000     

Environmental heterogeneity and disturbance by humans control much of the tree species diversity of Atlantic montane forest fragments in SE Brazil

The effects of human impact and environmental heterogeneity on the tree species diversity were assessed in 20 fragments of tropical montane seasonal forest in southeastern Brazil. Previous surveys of the tree community, soils and topography of the fragments provided the bulk of the data. The diversity parameters used were the means of species richness, Shannon diversity (H′), and Pielou evenness (J′) obtained from “bootstrap” sub-samplings of 1,000 trees. Morphometric variables obtained for the fragments included total, edge, and inner areas. Investigation forms were used to survey the history of human interventions and prepare an impacts matrix containing scores assigned to assess the extent, severity and duration of selected impacts. Scores for overall environmental impacts were obtained from the ordination scores produced by a multivariate analysis of the impacts matrix. A multivariate analysis of the standard deviations of soil variables was used to identify the variable which contributed most to soil heterogeneity. The same procedure was repeated for the variables related to topography and ground-water regime. The three species diversity parameters were related to the proportions of edges, the overall impacts scores, and the standard deviations of two selected soil and topographic variables. The species diversity in the fragments increased with increasing heterogeneity of both soil chemical properties and topographic features, and decreased with increasing proportion of forest edges. The evenness component of species diversity also increased with increasing severity of overall environmental impacts. This probably occurred because the 20 fragments did not include highly disturbed forests in the range and the intermediate disturbance effect on species diversity was therefore detected.     

Influence of sampling effort on saproxylic beetle diversity assessment: implications for insect monitoring studies in European temperate forests

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Ecological effects of forest roads on plant species diversity in Caspian forests of Iran

Current research includes the effects of asphalt forest roads on changes of plant cover and tree regeneration from asphalt forest roads edges towards its inner parts in two compartments of Nave Asalem forests located in the north of Iran. For this reason, in each side of road, 6 sample plots (20 m × 20 m) were established for measuring plant species diversity. In each sample plot, ground vegetation and tree regeneration were assessed within nine 2 × 2 m micro plots. In total, 12 sample plots and 108 μ plots were established. Results indicated that the road positions were effective on plant species diversity. The highest diversity and evenness indices value were observed down of the road compared to the up of the road position for herbal and tree regeneration layers. The same results were found also for herbal richness indices. Up of road position had the greatest value of richness indices in comparison to the other road position for tree regeneration layer. Also, the results showed that diversity, richness, and evenness indices were decreased with the increasing of distance from the road side for herbs and tree regeneration layers. This study indicated that roads can increase plant biodiversity; that is, tree regeneration density.     

Plurality of tree species responses to drought perturbation in Bornean tropical rain forest

Drought perturbation driven by the El Niño Southern Oscillation (ENSO) is a principal stochastic variable determining the dynamics of lowland rain forest in S.E. Asia. Mortality, recruitment and stem growth rates at Danum in Sabah (Malaysian Borneo) were recorded in two 4-ha plots (trees ≥ 10 cm gbh) for two periods, 1986–1996 and 1996–2001. Mortality and growth were also recorded in a sample of subplots for small trees (10 to <50 cm gbh) in two sub-periods, 1996–1999 and 1999–2001. Dynamics variables were employed to build indices of drought response for each of the 34 most abundant plot-level species (22 at the subplot level), these being interval-weighted percentage changes between periods and sub-periods. A significant yet complex effect of the strong 1997/1998 drought at the forest community level was shown by randomization procedures followed by multiple hypothesis testing. Despite a general resistance of the forest to drought, large and significant differences in short-term responses were apparent for several species. Using a diagrammatic form of stability analysis, different species showed immediate or lagged effects, high or low degrees of resilience or even oscillatory dynamics. In the context of the local topographic gradient, species’ responses define the newly termed perturbation response niche. The largest responses, particularly for recruitment and growth, were among the small trees, many of which are members of understorey taxa. The results bring with them a novel approach to understanding community dynamics: the kaleidoscopic complexity of idiosyncratic responses to stochastic perturbations suggests that plurality, rather than neutrality, of responses may be essential to understanding these tropical forests. The basis to the various responses lies with the mechanisms of tree-soil water relations which are physiologically predictable: the timing and intensity of the next drought, however, is not. To date, environmental stochasticity has been insufficiently incorporated into models of tropical forest dynamics, a step that might considerably improve the reality of theories about these globally important ecosystems.     

Changes in tree species diversity after successive clearcuts in the Southern Appalachians

A 16 ha watershed in the Coweeta Basin was clearcut in 1939 and 1962. Vegetation was inventoried in 1934 and at about 7-year intervals to 1991. After the first clearcut, tree diversity remained high until after the second cut. Diversity based on density and basal area decreased significantly 14 years after the second clearcut and remained low through 1991. Diversity was highest in the early establishment stage of stand development, then declined at the intermediate stage with canopy closure. Evenness based on basal area declined more than evenness based on density because basal area of Liriodendron tulipifera increased substantially from 1977 to 1991. Trends in diversity were due to changes in evenness rather than changes in species richness.     

Simulation of tree species composition and organic matter accumulation in Finnish boreal forests under changing climatic conditions

A model simulating the regeneration, growth and death of trees and the consequent carbon and nitrogen dynamics of the forest ecosystem was applied to determine the effect of expected temperature rise on tree species composition and the accumulation of organic matter in the boreal forest ecosystem in Finland (between latitudes 60°–70° N). In the southern and middle boreal zones a temperature rise of 2–3° C (temperature for 2 x CO₂) over a period of one hundred years increased the competitive capacity of Scots pine (Pinus sylvestris) and birch species (Betula pendula and B. pubescens), and slowed down the invasion by Norway spruce (Picea abies). In the northern boreal zone a corresponding rise in temperature promoted the invasion of sites by Norway spruce. The accumulation of organic matter was promoted only slightly compared to that taking place in the current climatic conditions.A further doubling of temperature (temperature for 4 x CO₂) over an additional period of two hundred years led to the replacement of coniferous stands with deciduous onesin the southern and middle boreal zones. In the northern boreal zone an admixture of coniferous and deciduous species replaced pure coniferous stands with the latter taking over sites formerly classified as tundra woodland. In the southern and middle boreal zones the replacement of coniferous species induced a substantial decrease in the amount of organic matter; this returned to its former level following the establishment of deciduous species. In the northern boreal zone there was no major change in the amount of organic matter such as occurred in the case of the tundra woodland where the amount of organic matter accumulated was nearly as high as in the northern boreal zone.     

Comparing tree diversity and composition in coffee farms and sacred forests in the Western Ghats of India

Expansion of coffee cultivation is one of the causes of deforestation and biodiversity loss. However, shade grown coffee has been promoted as a means for preserving biodiversity in the tropics. In this study we compared tree diversity in two types of coffee management regimes with the sacred groves in the Western Ghats of India. We computed species accumulation curves, species diversity indices and evenness indices to compare the different management regimes. Results of diversity indices showed that shade coffee had less diversity compared to sacred groves. Exotic species dominated the tree diversity in lands where the tree harvesting rights are with the growers. Native trees dominated the tree diversity when growers had no ownership rights on trees. A species accumulation curve suggested that the sacred grove had higher species richness compared to other two habitats. Lack of incentive to preserve endemic species as shade trees is forcing growers to plant more exotic species in shade grown coffee plots. If encouraged, shade grown coffee can preserve some biodiversity, but cannot provide all ecological benefits of a natural forest.     

Resistance of European tree species to drought stress in mixed versus pure forests: evidence of stress release by inter‐specific facilitation

While previous studies focused on tree growth in pure stands, we reveal that tree resistance and resilience to drought stress can be modified distinctly through species mixing. Our study is based on tree ring measurement on cores from increment boring of 559 trees of Norway spruce (Picea abies [L.] Karst.), European beech (Fagus sylvatica [L.]) and sessile oak (Quercus petraea (Matt.) Liebl.) in South Germany, with half sampled in pure, respectively, mixed stands. Indices for resistance, recovery and resilience were applied for quantifying the tree growth reaction on the episodic drought stress in 1976 and 2003. The following general reaction patterns were found. (i) In pure stands, spruce has the lowest resistance, but the quickest recovery; oak and beech were more resistant, but recover was much slower and they are less resilient. (ii) In mixture, spruce and oak perform as in pure stands, but beech was significantly more resistant and resilient than in monoculture. (iii) Especially when mixed with oak, beech is facilitated. We hypothesise that the revealed water stress release of beech emerges in mixture because of the asynchronous stress reaction pattern of beech and oak and a facilitation of beech by hydraulic lift of water by oak. This facilitation of beech in mixture with oak means a contribution to the frequently reported overyield of beech in mixed versus pure stands. We discuss the far‐reaching implications that these differences in stress response under intra‐ and inter‐specific environments have for forest ecosystem dynamics and management under climate change.     

Genetic diversity of Ranunculus acris L. (Ranunculaceae) populations in relation to species diversity and habitat type in grassland communities

Correlates between genetic diversity at intra- and interpopulation levels and the species diversity in plant communities are rarely investigated. Such correlates may give insights into the effect of local selective forces across different communities on the genetic diversity of local plant populations. This study has employed amplified fragment length polymorphism to assess the genetic diversity within and between 10 populations of Ranunculus acris in relation to the species diversity (richness and evenness) of grassland communities of two different habitat types, 'seminatural' and 'agriculturally improved', located in central Germany. Within-population genetic diversity estimated by Nei's unbiased gene diversity (HE) was high (0.258-0.334), and was not correlated with species richness (Pearson's r = -0.17; P = 0.64) or species evenness (Pearson's r = 0.15; P = 0.68) of the plant communities. However, the genetic differentiation between R. acris populations was significantly correlated with the difference in species evenness (Mantel's r = 0.62, P = 0.02), but not with difference in species richness of plant communities (r = -0.17, P = 0.22). Moreover, we also found that populations of R. acris from the 'seminatural' habitat were genetically different (amova, P < 0.05) from those in 'agriculturally improved' habitats, suggesting that gene flow between these habitat types is limited. The results reported in this study may indicate that habitat characteristics influence the genetic diversity of plant species.