Changes in biodiversity and vegetation composition in the central Swiss Alps during the transition from pristine forest to first farming

Aim: We investigate the response of vegetation composition and plant diversity to increasing land clearance, burning and agriculture at the Mesolithic–Neolithic transition (c. 6400–5000 bc) when first farming was introduced. Location: The Valais, a dry alpine valley in Switzerland. Methods: We combine high-resolution pollen, microscopic charcoal and sedimentological data to reconstruct past vegetation, fire and land use. Pollen evenness, rarefaction-based and accumulation-based palynological richness analyses were used to reconstruct past trends in plant diversity. Results: Our results show that from c. 5500 cal. yr bc, slash-and-burn activities created a more open landscape for agriculture, at the expense of Pinus and Betula forests. Land clearance by slash-and-burn promoted diverse grassland ecosystems, while on the long term it reduced woodland and forest diversity, affecting important tree species such as Ulmus and Tilia. Main conclusions: Understanding the resilience of Alpine ecosystems to past disturbance variability is relevant for future nature conservation plans. Our study suggests that forecasted land abandonment in the Alps will lead to pre-Neolithic conditions, with significant biodiversity losses in abandoned grassland ecosystems. Thus, management measures for biodiversity, such as ecological compensation areas, are needed in agricultural landscapes with a millennial history of human impact, such as the non-boreal European lowlands. Our study supports the hypothesis that species coexistence is maximized at an intermediate level of disturbances. For instance, species richness decreased when fire exceeded the quasi-natural variability observed during the Mesolithic times. Under a more natural disturbance regime, rather closed Pinus sylvestris and mixed oak forests would prevail.

Tree species and diversity effects on soil water seepage in a tropical plantation

Plant diversity has been shown to influence the water cycle of forest ecosystems by differences in water consumption and the associated effects on groundwater recharge. However, the effects of biodiversity on soil water fluxes remain poorly understood for native tree species plantations in the tropics. Therefore, we estimated soil water fluxes and assessed the effects of tree species and diversity on these fluxes in an experimental native tree species plantation in Sardinilla (Panama). The study was conducted during the wet season 2008 on plots of monocultures and mixtures of three or six tree species. Rainfall and soil water content were measured and evapotranspiration was estimated with the Penman-Monteith equation. Soil water fluxes were estimated using a simple soil water budget model considering water input, output, and soil water and groundwater storage changes and in addition, were simulated using the physically based one-dimensional water flow model Hydrus-1D. In general, the Hydrus simulation did not reflect the observed pressure heads, in that modeled pressure heads were higher compared to measured ones. On the other hand, the results of the water balance equation (WBE) reproduced observed water use patterns well. In monocultures, the downward fluxes through the 200 cm-depth plane were highest below Hura crepitans (6.13 mm day−1) and lowest below Luehea seemannii (5.18 mm day−1). The average seepage rate in monocultures (±SE) was 5.66 ± 0.18 mm day−1, and therefore, significantly higher than below six-species mixtures (5.49 ± 0.04 mm day−1) according to overyielding analyses. The three-species mixtures had an average seepage rate of 5.63 ± 0.12 mm day−1 and their values did not differ significantly from the average values of the corresponding species in monocultures. Seepage rates were driven by the transpiration of the varying biomass among the plots (r = 0.61, p = 0.017). Thus, a mixture of trees with different growth rates resulted in moderate seepage rates compared to monocultures of either fast growing or slow growing tree species. Our results demonstrate that tree-species specific biomass production and tree diversity are important controls of seepage rates in the Sardinilla plantation during the wet season.

Designing forest biodiversity experiments: general considerations illustrated by a new large experiment in subtropical China

1. Biodiversity-ecosystem functioning (BEF) experiments address ecosystem-level consequences of species loss by comparing communities of high species richness with communities from which species have been gradually eliminated. BEF experiments originally started with microcosms in the laboratory and with grassland ecosystems. A new frontier in experimental BEF research is manipulating tree diversity in forest ecosystems, compelling researchers to think big and comprehensively. 2. We present and discuss some of the major issues to be considered in the design of BEF experiments with trees and illustrate these with a new forest biodiversity experiment established in subtropical China (Xingangshan, Jiangxi Province) in 2009/2010. Using a pool of 40 tree species, extinction scenarios were simulated with tree richness levels of 1, 2, 4, 8 and 16 species on a total of 566 plots of 25.8x25.8m each. 3. The goal of this experiment is to estimate effects of tree and shrub species richness on carbon storage and soil erosion; therefore, the experiment was established on sloped terrain. The following important design choices were made: (i) establishing many small rather than fewer larger plots, (ii) using high planting density and random mixing of species rather than lower planting density and patchwise mixing of species, (iii) establishing a map of the initial ecoscape' to characterize site heterogeneity before the onset of biodiversity effects and (iv) manipulating tree species richness not only in random but also in trait-oriented extinction scenarios. 4. Data management and analysis are particularly challenging in BEF experiments with their hierarchical designs nesting individuals within-species populations within plots within-species compositions. Statistical analysis best proceeds by partitioning these random terms into fixed-term contrasts, for example, species composition into contrasts for species richness and the presence of particular functional groups, which can then be tested against the remaining random variation among compositions. 5. We conclude that forest BEF experiments provide exciting and timely research options. They especially require careful thinking to allow multiple disciplines to measure and analyse data jointly and effectively. Achieving specific research goals and synergy with previous experiments involves trade-offs between different designs and requires manifold design decisions.

A novel comparative research platform designed to determine the functional significance of tree species diversity in European forests

One of the current advances in functional biodiversity research is the move away from short-lived test systems towards the exploration of diversity-ecosystem functioning relationships in structurally more complex ecosystems. In forests, assumptions about the functional significance of tree species diversity have only recently produced a new generation of research on ecosystem processes and services. Novel experimental designs have now replaced traditional forestry trials, but these comparatively young experimental plots suffer from specific difficulties that are mainly related to the tree size and longevity. Tree species diversity experiments therefore need to be complemented with comparative observational studies in existing forests. Here we present the design and implementation of a new network of forest plots along tree species diversity gradients in six major European forest types: the FunDivEUROPE Exploratory Platform. Based on a review of the deficiencies of existing observational approaches and of unresolved research questions and hypotheses, we discuss the fundamental criteria that shaped the design of our platform. Key features include the extent of the species diversity gradient with mixtures up to five species, strict avoidance of a dilution gradient, special attention to community evenness and minimal covariation with other environmental factors. The new European research platform permits the most comprehensive assessment of tree species diversity effects on forest ecosystem functioning to date since it offers a common set of research plots to groups of researchers from very different disciplines and uses the same methodological approach in contrasting forest types along an extensive environmental gradient. (C) 2013 Elsevier GmbH. All rights reserved.

Current and future variations of nutrient depositions and influences on tree growth

Current nutrient deposition shows episodic variations which likely may impact the local nutrient cycle at the RBSF. Comparing analyses of deposition data during present-day atmospheric circulation and phases of high biomass burning in the Amazon, characteristic relationships between remote emissions and local deposition are determined. By using projections drawn from the special report on emission scenarios (SRES) in combination with a trajectory modeling tool, future nutrient deposition conditions of the mountain ecosystem are assessed. Observations of relations between climatic variables, current time series of nutrient deposition, and tree growth point to an impact of the remote fertilization effect of atmospheric matters, emitted primarily by human activities like biomass burning and agricultural and industrial sources. The increasing emissions in the future may have adverse effects on the ecosystem in the long run.

Neutral biodiversity theory can explain the imbalance of phylogenetic trees but not the tempo of their diversification

Numerous evolutionary studies have sought to explain the distribution of diversity across the limbs of the tree of life. At the same time, ecological studies have sought to explain differences in diversity and relative abundance within and among ecological communities. Traditionally, these patterns have been considered separately, but models that consider processes operating at the level of individuals, such as neutral biodiversity theory (NBT), can provide a link between them. Here, we compare evolutionary dynamics across a suite of NBT models. We show that NBT can yield phylogenetic tree topologies with imbalance closely resembling empirical observations. In general, metacommunities that exhibit greater disparity in abundance are characterized by more imbalanced phylogenetic trees. However, NBT fails to capture the tempo of diversification as represented by the distribution of branching events through time. We suggest that population-level processes might therefore help explain the asymmetry of phylogenetic trees, but that tree shape might mislead estimates of evolutionary rates unless the diversification process is modeled explicitly.

Species richness, but not phylogenetic diversity, influences community biomass production and temporal stability in a re-examination of 16 grassland biodiversity studies

* Hundreds of experiments have now manipulated species richness (SR) of various groups of organisms and examined how this aspect of biological diversity influences ecosystem functioning. Ecologists have recently expanded this field to look at whether phylogenetic diversity (PD) among species, often quantified as the sum of branch lengths on a molecular phylogeny leading to all species in a community, also predicts ecological function. Some have hypothesized that phylogenetic divergence should be a superior predictor of ecological function than SR because evolutionary relatedness represents the degree of ecological and functional differentiation among species. But studies to date have provided mixed support for this hypothesis. * Here, we reanalyse data from 16 experiments that have manipulated plant SR in grassland ecosystems and examined the impact on above-ground biomass production over multiple time points. Using a new molecular phylogeny of the plant species used in these experiments, we quantified how the PD of plants impacts average community biomass production as well as the stability of community biomass production through time. * Using four complementary analyses, we show that, after statistically controlling for variation in SR, PD (the sum of branches in a molecular phylogenetic tree connecting all species in a community) is neither related to mean community biomass nor to the temporal stability of biomass. These results run counter to past claims. However, after controlling for SR, PD was positively related to variation in community biomass over time due to an increase in the variances of individual species, but this relationship was not strong enough to influence community stability. * In contrast to the non-significant relationships between PD, biomass and stability, our analyses show that SR per se tends to increase the mean biomass production of plant communities, after controlling for PD. The relationship between SR and temporal variation in community biomass was either positive, non-significant or negative depending on which analysis was used. However, the increases in community biomass with SR, independently of PD, always led to increased stability. These results suggest that PD is no better as a predictor of ecosystem functioning than SR. * Synthesis. Our study on grasslands offers a cautionary tale when trying to relate PD to ecosystem functioning suggesting that there may be ecologically important trait and functional variation among species that is not explained by phylogenetic relatedness. Our results fail to support the hypothesis that the conservation of evolutionarily distinct species would be more effective than the conservation of SR as a way to maintain productive and stable communities under changing environmental conditions.

Direct and indirect effects of invasion by the alien tree Ailanthus altissima on riparian plant communities and ecosystem multifunctionality

Most existing studies addressing the effects of invasive species on biodiversity focus on species richness ignoring better indicators of biodiversity and better predictors of ecosystem functioning such as the diversity of evolutionary histories (phylodiversity). Moreover, no previous study has separated the direct effect of alien plants on multiple ecosystem functions simultaneously (multifunctionality) from those indirect ones mediated by the decrease on biodiversity caused by alien plants. We aimed to analyze direct and indirect effects, mediated or not by its effect on biodiversity, of the invasive tree Ailanthus altissima on ecosystem multifunctionality of riparian habitats under Mediterranean climate. We measured vegetation attributes (species richness and phylodiversity) and several surrogates of ecosystem functioning (understory plant biomass, soil enzyme activities, available phosphorous and organic matter) in plots infested by A. altissima and in control (non-invaded) ones. We used structural equation modelling to tease apart the direct and indirect effects of A. altissima on ecosystem multifunctionality. Our results suggest that lower plant species richness, phylodiversity and multifunctionality were associated to the presence of A. altissima. When analyzing each function separately, we found that biodiversity has the opposite effect of the alien plant on all the different functions measured, therefore reducing the strength of the effect (either positive or negative) of A. altissima on them. This is one of the few existing studies addressing the effect of invasive species on phylodiversity and also studying the effect of invasive species on multiple ecosystem functions simultaneously.