Tree species traits but not diversity mitigate stem breakage in a subtropical forest following a rare and extreme ice storm

Peer reviewedPublisher PD

Effects of tree sapling diversity and nutrient addition on herb-layer invasibility in communities of subtropical species

Exotic species are assumed to alter ecosystem functioning. However, little is known of the relationships within vertically structured plant communities such as forests, where tree saplings interact with herbaceous species, especially in the early phases of succession. This relationship was tested in a common garden experiment which assessed the impacts on tree saplings and herbaceous species following nutrient addition and the introduction of exotic herb species. The experiment was established in South- East China using four broad-leaved tree species (Elaeocarpus decipiens, Schima superba, Castanea henryi and Quercus serrata) to study the relationships between tree sapling diversity, herb-layer productivity and invasibility. Tree saplings were planted in monoculture and in mixtures of two and four species. A full factorial design was applied, within which species composition was crossed with nutrient and exotic seed-addition treatments. The seed-addition treatment included mixtures of seeds from eight exotic herb species, and herb community attributes were assessed after a four month growing season. Results indicate that certain tree species negatively affect native as well as exotic herbs; however, the high productivity of native herbs had a stronger negative impact on exotic species than tree saplings. Nutrient addition increased the productivity of exotic herbs but had no effect on native herbs. Remarkably, exotic species introduction had a negative feedback effect on the growth of tree saplings, which highlights the potential of exotic herbs to diminish tree recruitment. Although tree saplings reduced invasive effects on the herb-layer during the earliest phase of forest succession, nutrient addition had a more profound and opposite effect on these invaders

Globally, functional traits are weak predictors of juvenile tree growth, and we do not know why

1. Plant functional traits, in particular specific leaf area (SLA), wood density and seed mass, are often good predictors of individual tree growth rates within communities. Individuals and species with high SLA, low wood density and small seeds tend to have faster growth rates. 2. If community-level relationships between traits and growth have general predictive value, then similar relationships should also be observed in analyses that integrate across taxa, biogeographic regions and environments. Such global consistency would imply that traits could serve as valuable proxies for the complex suite of factors that determine growth rate, and, therefore, could underpin a new generation of robust dynamic vegetation models. Alternatively, growth rates may depend more strongly on the local environment or growth-trait relationships may vary along environmental gradients. 3. We tested these alternative hypotheses using data on 27352 juvenile trees, representing 278 species from 27 sites on all forested continents, and extensive functional trait data, 38% of which were obtained at the same sites at which growth was assessed. Data on potential evapotranspiration (PET), which summarizes the joint ecological effects of temperature and precipitation, were obtained from a global data base. 4. We estimated size-standardized relative height growth rates (SGR) for all species, then related them to functional traits and PET using mixed-effect models for the fastest growing species and for all species together. 5. Both the mean and 95th percentile SGR were more strongly associated with functional traits than with PET. PET was unrelated to SGR at the global scale. SGR increased with increasing SLA and decreased with increasing wood density and seed mass, but these traits explained only 3.1% of the variation in SGR. SGR-trait relationships were consistently weak across families and biogeographic zones, and over a range of tree statures. Thus, the most widely studied functional traits in plant ecology were poor predictors of tree growth over large scales. 6. Synthesis. We conclude that these functional traits alone may be unsuitable for predicting growth of trees over broad scales. Determining the functional traits that predict vital rates under specific environmental conditions may generate more insight than a monolithic global relationship can offer.Additional co-authors: Hervé Jactel, Xuefei Li, Kaoru Kitajima, Julia Koricheva, Cristina Martínez-Garza, Christian Messier, Alain Paquette, Christopher Philipson, Daniel Piotto, Lourens Poorter, Juan M. Posada, Catherine Potvin, Kalle Rainio, Sabrina E. Russo, Mariacarmen Ruiz-Jaen, Michael Scherer-Lorenzen, Campbell O. Webb, S. Joseph Wright, Rakan A. Zahawi, and Andy Hecto

Globally, functional traits are weak predictors of juvenile tree growth, and we do not know why

1. Plant functional traits, in particular specific leaf area (SLA), wood density and seed mass, are often good predictors of individual tree growth rates within communities. Individuals and species with high SLA, low wood density and small seeds tend to have faster growth rates. 2. If community-level relationships between traits and growth have general predictive value, then similar relationships should also be observed in analyses that integrate across taxa, biogeographic regions and environments. Such global consistency would imply that traits could serve as valuable proxies for the complex suite of factors that determine growth rate, and, therefore, could underpin a new generation of robust dynamic vegetation models. Alternatively, growth rates may depend more strongly on the local environment or growth–trait relationships may vary along environmental gradients. 3. We tested these alternative hypotheses using data on 27 352 juvenile trees, representing 278 species from 27 sites on all forested continents, and extensive functional trait data, 38% of which were obtained at the same sites at which growth was assessed. Data on potential evapotranspiration (PET), which summarizes the joint ecological effects of temperature and precipitation, were obtained from a global data base. 4. We estimated size-standardized relative height growth rates (SGR) for all species, then related them to functional traits and PET using mixed-effect models for the fastest growing species and for all species together. 5. Both the mean and 95th percentile SGR were more strongly associated with functional traits than with PET. PET was unrelated to SGR at the global scale. SGR increased with increasing SLA and decreased with increasing wood density and seed mass, but these traits explained only 3.1% of the variation in SGR. SGR–trait relationships were consistently weak across families and biogeographic zones, and over a range of tree statures. Thus, the most widely studied functional traits in plant ecology were poor predictors of tree growth over large scales. 6. Synthesis. We conclude that these functional traits alone may be unsuitable for predicting growth of trees over broad scales. Determining the functional traits that predict vital rates under specific environmental conditions may generate more insight than a monolithic global relationship can offer

Aboveground primary productivity in forest ecosystems as function of species richness and composition

Aktuell erleben wir eines der grössten Artensterben der Weltgeschichte. Die Bedeutung der Biodiversität für die Ökosysteme der Erde und für deren Funktionen ist daher eines der Wichtigsten Themen in der ökologischen Forschung. Biodiversitätsexperimente mit Wiesenpflanzen zeigen einen positiven Zusammenhang zwischen Artenvielfalt und mehreren Ökosystemfunktionen, wie zum Beispiel der Produktivität. Waldökosysteme, hingegen, wurden aufgrund logistischer Schwierigkeiten viel weniger untersucht, trotz ihrer globalen Relevanz in Bezug auf Biodiversität und Kohlenstoffassimilation. In diesem Projekt untersuchten wir den Zusammenhang zwischen der oberirdischen Produktivität und der Biodiversität von Baumarten in südtropischen Wäldern. In Kapitel 1 berichte ich über die Ergebnisse einer vergleichenden Studie in der wir erstens die Biomasse und das Baumwachstum als Funktion der Artenzahl, zweitens die funktionelle und phylogenetische Vielfalt und drittens die Gleichmässigkeit der Artenverteilung („Evenness“) in natürlichen südtropischen Wäldern mit verschiedenen Sukzessionstadien über eine Wachstumsperiode von 2008-2010 untersuchten. Ich mass zwei Kohorten von Bäumen: alle Bäume mit einem Brusthöhendurchmesser (BHD) von ≥ 10 cm und alle Bäume mit einem BHD von 3 cm ≤ BHD < 10 cm. Die Basalfläche und das Wachstum steigen mit grösserer Artenzahl, sowie mit der funktionellen und phylogenetischen Diversität, während das Wachstum mit zunehmendem Sukzessionsstadium abnimmt. Die Artenzahl und die Gelichmässigkeit der Artenverteilung korrelieren negativ. Der Biodiversitätseffekt wurde von grösseren Stammdurchmessern und höheren Baumdichten in artenreicheren Versuchsflächen bestimmt. In Kapitel 2 untersuchte ich den Effekt von Artenzahl und Baumdichte auf die Biomasseallokation, die Baumkronenarchitektur und die Zweigdemographie von Schösslingen. Ich pflanzte einen Versuch mit vier Baumarten in vier Monokulturen, sechs 2-Arten-Mischungen und einer 4-Arten-Mischung. Zusätzlich pflanzte ich eine niedrige, eine intermediäre und eine hohe Pflanzendichte pro Plot. Die Artenzahl erhöhte den Astumsatz, aber das Wachstum wurde vor allem von der Identität einzelner Arten bestimmt. Anscheinend wird die Baumkronenarchitektur und die Zweigdemographie vom Wettbewerb um Licht bestimmt. Weiter fand ich eine stärkere intraspezifische als interspezifische Konkurrenz bei zwei von vier Arten. In Kapitel 3 wendete ich einen experimentellen Ansatz an, um die Biomasse von Jungbeständen als Funktion der Baumartenzahl unter zwei Lichtbedingungen zu messen. Ich pflanzte einen Versuch mit drei Artenpools, jeder bestehend aus vier Baumarten. Innerhalb jedes Artenpools pflanzte ich vier Monokulturen, sechs 2-Arten-Mischkulturen und eine 4-Arten-Mischkultur, sowohl in direktem Sonnenlicht wie auch unter Beschattung. Die Versuchsflächen wurden in vier Reihen à vier Pflanzen gepflanzt, das heisst 16 Individuen pro Quadratmeter (1 m2). Achtzehn Monate später erntete ich die oberirdischen Pflanzenteile der vier im Zentrum positionierten Pflanzen und ermittelte deren Trockengewicht. Auch hier fand ich einen positiven Effekt der Artenzahl auf die Biomasse unter beiden Lichtbedingungen. Der Biodiversitätseffekt war dabei die Folge der Wahrscheinlichkeit, dass Pflanzen von dominanten Arten jedes Artenpools öfters in den artenreicheren Mischungen gefunden werden (Selektionseffekt). Die Biomasse in den schattigen Mischungen war niedriger als die Biomasse bei direkter Sonneneinstrahlung, dies aufgrund artspezifischer Unterschiede im Überleben und individuellen Wachstum. In Kapitel 4 analysierte ich die 2-Arten-Mischkulturen aus Kapitel 3 noch genauer, um den Effekt der Artenidentität, der Artenkombination und der funktionellen Distanz zwischen den Arten auf die oberirdische Biomasse, sowie deren Beziehung zu Selektions- und Komplementaritätseffekten zu bestimmen. Ich untersuchte die achtzehn 2-Arten Mischungen unter beiden Lichtbedingungen. Dabei fand ich einen starken Einfluss der Artenidentität, und insbesondere der funktionellen Distanz zwischen den Arten innerhalb der Mischungen auf die gemessenen Biodiversitätseffekte. Ich verwendete eine mechanistische Diallel-Analyse, um die Auswirkung der spezifischen Arten in Mischkulturen zu bewerten und, um die Korrelation der funktionellen Distanz mit der spezifischen Kombinationsfähigkeit der Arten zu analysieren. Ich fand einen positiven Zusammenhang von Diversität und Produktivität in Gemeinschaften von Baumarten, unter natürlichen und experimentellen Bedingungen. Meine Ergebnisse unter experimentellen Bedingungen zeigen die Relevanz der Artzusammensetzung: Dominante Arten und die funktionelle Distanz zwischen Arten in der Gemeinschaft erklären die Unterschiede in der Gesamtbiomasse und den Biodiversitätseffekten zwischen verschiedenen Baumgemeinschaften. Der Zusammenhang zwischen Diversität und Produktivität war gleichbleibend unter verschiedenen Lichtbedingungen. Zusammenfassend unterstreichen diese Resultate die grosse Bedeutung der Erhaltung artenreicher Wälder für die Biomasseproduktion und damit die Kohlenstoffspeicherung. We are living through one of the largest species extinction events in world history. The relevance of diversity for world ecosystems and their functioning is therefore one of the major topics in current ecological research. Evidence in grassland experiments demonstrates a positive diversity−productivity relationship. Due to logistic difficulties, forest ecosystems have been much less studied, even though their global relevance in terms of diversity and carbon assimilation is particularly high. In this project we investigate the aboveground diversity−aboveground productivity relationship of tree species communities in subtropical forests. In Chapter 1 I report results from a comparative study, in which we assessed standing biomass and growth as a function of species richness, functional and phylogenetic diversity and evenness in natural subtropical forests of different successional stages over two growing seasons (2008–2010). I measured two cohorts of individuals: all individuals ≥ 10 cm DBH (diameter at breast height) and all individuals 3 cm ≤ DBH < 10 cm. Tree basal area and tree basal area increment at plot level were positively related with species richness and functional and phylogenetic diversity, whereas growth was negatively related to successional stage. Species richness and evenness showed a negative correlation, thus basal area and basal area increment were negatively correlated with evenness. The diversity effect was determined by larger mean individual sizes and higher densities in more diverse plots. In Chapter 2 I investigated the effect of species richness and density on sapling biomass allocation, crown architecture and branch demography. I planted an experiment with a pool of four tree species. The four species were planted in monoculture, in all six 2-species mixtures and in 4-species mixture. I had in addition a low-, an intermediate- and a high-density level of individuals per plot. Species richness enhanced pruning and branch turnover, but growth was mainly determined by individual species identity. I conclude that crown architecture and branch demography were mainly controlled by light competition. I found intraspecific competition to be stronger than interspecific competition in two species. In Chapter 3 I used an experimental approach to assess biomass of sapling communities as a function of species richness in two light conditions. I planted an experiment with three species pools, each of four species. Within each species pool I planted four monocultures, six 2-species mixtures and the 4-species mixture in direct sunlight and in a shade cage. Plots were planted in a four-by-four array of sixteen individuals per square meter. Eighteen months after planting the aboveground section of the four central individuals was harvested, dried and weighed. I found a positive effect of species richness on mixture biomass in the two light treatments. The species diversity effect was caused by an increased likelihood of finding individuals of the dominant species of each species pool in the more diverse mixtures (so-called sampling effect of biodiversity). Biomass in the shaded mixtures was lower than in direct sunlight, due to lowered individual growth and increased mortality. In Chapter 4, I analyzed in depth the 2-species mixtures presented in Chapter 3 by exploring the effect of species identity, species composition and functional distance between species on aboveground biomass and on biodiversity effects, in particular selection and complementarity effects. I found a strong effect of species identity and of functional distance between the species on the aboveground biomass of the mixtures, net biodiversity and selection effects. I used a mechanistic diallel analysis to assess the effect of general and specific combining ability of species on mixture performance and found a positive correlation between functional distance and specific combining ability. Overall, my work shows that a positive diversity−productivity relationship also occurs in communities of tree species, both in mature natural stands and in experimental communities of samplings. The results from the sapling experiment furthermore demonstrate the relevance of species identity and species differences for mixture performance. The diversity−productivity relationship was consistent under different light conditions, demonstrating its generality

A guide to analyzing biodiversity experiments

Aims: The aim of this guide is to provide practical help for ecologists who analyze data from biodiversity–ecosystem functioning experiments. Our approach differs from others in the use of least squares-based linear models (LMs) together with restricted maximum likelihood-based mixed models (MMs) for the analysis of hierarchical data. An original data set containing diameter and height of young trees grown in monocultures, 2- or 4-species mixtures under ambient light or shade is used as an example. Methods: Starting with a simple LM, basic features of model fitting and the subsequent analysis of variance (ANOVA) for significance tests are summarized. From this, more complex models are developed. We use the statistical software R for model fitting and to demonstrate similarities and complementarities between LMs and MMs. The formation of contrasts and the use of error (LMs) or random-effects (MMs) terms to account for hierarchical data structure in ANOVAs are explained. Important Findings: Data from biodiversity experiments can be analyzed at the level of entire plant communities (plots) and plant individuals. The basic explanatory term is species composition, which can be divided into contrasts in many ways depending on specific biological hypotheses. Typically, these contrasts code for aspects of species richness or the presence of particular species. For significance tests in ANOVAs, contrast terms generally are compared with remaining variation of the explanatory terms from which they have been ‘carved out’. Once a final model has been selected, parameters (e.g. means or slopes for fixed-effects terms and variance components for error or random-effects terms) can be estimated to indicate the direction and size of effects

Can niche plasticity promote biodiversity-productivity relationships through increased complementarity?

Most experimental biodiversity–ecosystem functioning research to date has addressed herbaceous plant communities. Comparably little is known about how forest communities will respond to species losses, despite their importance for global biogeochemical cycling. We studied tree species interactions in experimental subtropical tree communities with 33 distinct tree species mixtures and one, two, or four species. Plots were either exposed to natural light levels or shaded. Trees grew rapidly and were intensely competing above ground after 1.5 growing seasons when plots were thinned and the vertical distribution of leaves and wood determined by separating the biomass of harvested trees into 50 cm height increments. Our aim was to analyze effects of species richness in relation to the vertical allocation of leaf biomass and wood, with an emphasis on bipartite competitive interactions among species. Aboveground productivity increased with species richness. The community-level vertical leaf and wood distribution depended on the species composition of communities. Mean height and breadth of species-level vertical leaf and wood distributions did not change with species richness. However, the extra biomass produced by mixtures compared to monocultures of the component species increased when vertical leaf distributions of monocultures were more different. Decomposition of biodiversity effects with the additive partitioning scheme indicated positive complementarity effects that were higher in light than in shade. Selection effects did not deviate from zero, irrespective of light levels. Vertical leaf distributions shifted apart in mixed stands as consequence of competition-driven phenotypic plasticity, promoting realized complementarity. Structural equation models showed that this effect was larger for species that differed more in growth strategies that were characterized by functional traits. In 13 of the 18 investigated two-species mixtures, both species benefitted relative to intraspecific competition in monoculture. In the remaining five pairwise mixtures, the relative yield gain of one species exceeded the relative yield loss of the other species, resulting in a relative yield total (RYT) exceeding 1. Overall, our analysis indicates that richness–productivity relationships are promoted by interspecific niche complementarity at early stages of stand development, and that this effect is enhanced by architectural plasticity

Can niche plasticity promote biodiversity-productivity relationships through increased complementarity?

Most experimental biodiversity-ecosystem functioning research to date has addressed herbaceous plant communities. Comparably little is known about how forest communities will respond to species losses, despite their importance for global biogeochemical cycling. We studied tree species interactions in experimental subtropical tree communities with 33 distinct tree species mixtures and one, two, or four species. Plots were either exposed to natural light levels or shaded. Trees grew rapidly and were intensely competing above ground after 1.5 growing seasons when plots were thinned and the vertical distribution of leaves and wood determined by separating the biomass of harvested trees into 50 cm height increments. Our aim was to analyze effects of species richness in relation to the vertical allocation of leaf biomass and wood, with an emphasis on bipartite competitive interactions among species. Aboveground productivity increased with species richness. The community-level vertical leaf and wood distribution depended on the species composition of communities. Mean height and breadth of species-level vertical leaf and wood distributions did not change with species richness. However, the extra biomass produced by mixtures compared to monocultures of the component species increased when vertical leaf distributions of monocultures were more different. Decomposition of biodiversity effects with the additive partitioning scheme indicated positive complementarity effects that were higher in light than in shade. Selection effects did not deviate from zero, irrespective of light levels. Vertical leaf distributions shifted apart in mixed stands as consequence of competition-driven phenotypic plasticity, promoting realized complementarity. Structural equation models showed that this effect was larger for species that differed more in growth strategies that were characterized by functional traits. In 13 of the 18 investigated two-species mixtures, both species benefitted relative to intraspecific competition in monoculture. In the remaining five pairwise mixtures, the relative yield gain of one species exceeded the relative yield loss of the other species, resulting in a relative yield total (RYT) exceeding 1. Overall, our analysis indicates that richness-productivity relationships are promoted by interspecific niche complementarity at early stages of stand development, and that this effect is enhanced by architectural plasticity

Effects of tree sapling diversity and nutrient addition on herb-layer invasibility in communities of subtropical species

Exotic species are assumed to alter ecosystem functioning. However, little is known of the relationships within vertically structured plant communities such as forests, where tree saplings interact with herbaceous species, especially in the early phases of succession. This relationship was tested in a common garden experiment which assessed the impacts on tree saplings and herbaceous species following nutrient addition and the introduction of exotic herb species. The experiment was established in South- East China using four broad-leaved tree species (Elaeocarpus decipiens, Schima superba, Castanea henryi and Quercus serrata) to study the relationships between tree sapling diversity, herb-layer productivity and invasibility. Tree saplings were planted in monoculture and in mixtures of two and four species. A full factorial design was applied, within which species composition was crossed with nutrient and exotic seed-addition treatments. The seed-addition treatment included mixtures of seeds from eight exotic herb species, and herb community attributes were assessed after a four month growing season. Results indicate that certain tree species negatively affect native as well as exotic herbs; however, the high productivity of native herbs had a stronger negative impact on exotic species than tree saplings. Nutrient addition increased the productivity of exotic herbs but had no effect on native herbs. Remarkably, exotic species introduction had a negative feedback effect on the growth of tree saplings, which highlights the potential of exotic herbs to diminish tree recruitment. Although tree saplings reduced invasive effects on the herb-layer during the earliest phase of forest succession, nutrient addition had a more profound and opposite effect on these invaders