Publications

Biodiversity and stability of grassland ecosystem functioning

No abstract is provided for this article.

Contrasting effects of plant inter‐ and intraspecific variation on community‐level trait measures along an environmental gradient

Summary Despite widespread focus on interspecific variation in trait‐based ecology, there is growing evidence that intraspecific trait variability can play a fundamental role in plant community responses to environmental change and community assembly. Here, we quantify the strength and direction of inter‐ and intraspecific plant community trait responses along a 900 m elevation gradient spanning alpine and subalpine plant communities in southern New Zealand. We measured five commonly used leaf traits (i.e. dry matter content, N and P concentrations, leaf area and specific leaf area) on all 31 dominant and subordinate species recorded along the gradient, and examined their species‐specific and community‐level responses to elevation using both abundance‐weighted and nonweighted averages of trait values. By decomposing the variance of community‐level measures of these traits across the gradient, we showed that the contribution of interspecific variation to the response of plant assemblages to elevation was stronger than that of intraspecific variation, for all traits except specific leaf area. Further, the relative contributions of interspecific effects were greater when abundance‐weighted rather than nonweighted measures were used. We also observed contrasting intraspecific trait responses to the gradient among species (particularly for leaf N and P concentrations), and found both positive and negative covariation between inter‐ and intraspecific effects on community‐level trait values. The weak community‐average trait responses to elevation, as found for specific leaf area ( SLA ) and leaf N and P concentrations, resulted from strong but opposing responses among vs. within species, which are not typically accounted for in species‐based measures of plant community responses. For instance, increasing elevation (and associated factors such as a decrease in soil nutrient availability) favoured the dominance of species with relatively high leaf nutrient concentrations while simultaneously triggering an intraspecific decrease in the leaf nutrient concentrations of these species. The context dependency of positive and negative covariation between inter‐ and intraspecific trait variability, and the species‐specific nature of intraspecific shifts in functional trait values, reveal highly complex plastic responses of plants to environmental changes, and highlights the need for greater consideration of the role that intraspecific variation plays in community‐level processes.

Effects of Plant Species Diversity and Composition on Nitrogen Cycling and the Trace Gas Balance of Soils

Experiments addressing the role of plant species diversity for ecosystem functioning have recently proliferated. Most studies have focused on plant biomass

Litter isopod mouthpart traits respond to a tropical forest recovery gradient

Functional trait ecology has the potential to provide generalizable and mechanistic predictions of ecosystem function from data of species distributions and traits. The traits that are selected should both respond to environmental factors and influence ecosystem functioning. Invertebrate mouthpart traits fulfil these criteria, but are seldom collected, lack standardized measurement protocols, and have infrequently been investigated in response to environmental factors. We surveyed isopod species that consume plant detritus, and tree communities in 58 plots across primary and secondary forests in Singapore. We measured body dimensions (body size traits), pereopod and antennae lengths (locomotory traits), dimensions of mandible structures (morphological mouthpart traits), and mechanical advantages generated by mandible shape (mechanical mouthpart traits) for six isopod species found in these plots and investigated if these traits respond to changes in tree community composition, tree diversity, and forest structure. Morphological mouthpart traits responded to a tree compositional gradient reflecting forest recovery degree. Mouthpart features associated with greater consumption of litter (broader but less serrated/rugose lacinia mobilis [an important cutting and chewing structure on the mandible]) were most prevalent in abandoned plantation and young secondary forests containing disturbance-associated tree species. Feeding strategies associated with fungi grazing (narrower and more serrated/rugose lacinia mobilis) were most prevalent in late secondary forests containing later successional tree species. Since morphological mouthpart traits likely also predict consumption and excretion rates of isopods, these traits advance our understanding of environment–trait–ecosystem functioning relationships across contrasting tropical forest plots that vary in composition, disturbance history and post-disturbance recovery.

Urbanisation shapes microbial community composition and functional attributes more so than vegetation type in urban greenspaces across climatic zones

Urbanisation, as a global driver of change, modifies the natural environment with well-known consequences to biological communities. Under natural conditions, vegetation drives soil processes in concert with the soil microbial community in their rhizosphere. It remains unclear whether and how vegetation influences these communities in heavily disturbed urban systems where many ecosystem services are also strictly linked to soils and their biota. Here, we used amplicon sequencing and GeoChip arrays to study soil microbiota responses to urbanisation and tree functional types across climatic zones. Our data show that soil microbial communities vary widely across biomes, yet urban parks have compositionally unique microbial communities that are distinct from semi-natural forests. Neither functional trait richness nor functional gene relative abundances responded clearly to urbanization or vegetation type. Despite functional redundancy, vegetation type did affect soil communities compositionally. Soils under trees producing recalcitrant litter had a higher richness of fungal species than the labile ones, whereas lawns, despite of their structural simplicity, had an unexpectedly high diversity of bacteria and fungi. In summary, despite distinct differences in the soil microbiota across biomes, urbanisation and vegetation type have similar effects on structuring microbial communities within biomes. However, the urban soil microbiota, irrespective of the plant functional type they associate with, are functionally comparable to those in semi-natural forests, suggesting functional redundancy within this unique microbiota.

Biotic and abiotic plant–soil feedback depends on nitrogen‐acquisition strategy and shifts during long‐term ecosystem development

Feedback between plants and soil is an important driver of plant community structure, but it remains unclear whether plant–soil feedback (PSF): (i) reflects changes in biotic or abiotic properties, (ii) depends on environmental context in terms of soil nutrient availability, and (iii) varies among plant functional groups. As soil nutrient availability strongly affects plant distribution and performance, soil chemical properties and plant nutrient‐acquisition strategies might serve as important drivers of PSF. We used soils from young and old stages of a long‐term soil chronosequence to represent sites where productivity is limited by nitrogen (N) and phosphorus (P) availability, respectively. We grew three N‐fixing and three non‐N‐fixing plant species in soils conditioned by co‐occurring conspecific or heterospecific species from each of these two stages. In addition, three soil treatments were used to distinguish biotic and abiotic effects on plant performance, allowing measurements of overall, biotic, and abiotic PSF. In young, N‐poor soils, non‐N‐fixing plants grew better in soils from N‐fixing plants than in their own soils (i.e., negative PSF). However, this difference was not only associated with improved abiotic conditions in soils from N‐fixing plants but also with changes in soil biota. By contrast, no significant PSF was observed for N‐fixing plants grown in young soils. Moreover, we did not observe any significant PSF for either N‐fixing or non‐N‐fixing plants growing in old, P‐impoverished soils. Synthesis . The direction and strength of plant‐soil feedback (PSF) varied among N‐acquisition strategies and soils differing in nutrient availability, with stronger plant‐soil feedback in younger, N‐poor soils compared to older, P‐impoverished soils. Our results highlight the importance of considering soil nutrient availability, plant‐mediated abiotic and biotic soil properties, and plant nutrient‐acquisition strategies when studying plant‐soil feedback, thereby advancing our mechanistic understanding of plant‐soil feedback during long‐term ecosystem development.

Explanations for nitrogen decline

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Climate and multiple dimensions of plant diversity regulate ecosystem carbon exchange along an elevational gradient

The impacts of warming on communities and ecosystems are predicted to be significant in mountain ecosystems because physiological processes, including rates of carbon (C) cycling, are often more temperature‐sensitive in colder environments. Plant biodiversity can also influence C exchange, yet few studies integrate how biotic and abiotic factors may directly or interactively impact ecosystem C flux. Here, we examine the link between simultaneous changes in multiple dimensions of plant diversity and peak growing season ecosystem C uptake across a climatic gradient in the Rocky Mountains, Colorado, USA. We found that taxonomic diversity (species richness), functional diversity (functional evenness), and phylogenetic diversity (mean pairwise distance) were significantly and positively related to peak growing season ecosystem C uptake (i.e., net ecosystem exchange) when considered independently. However, when abiotic and biotic factors were integrated in a structural equation model, only plant phylogenetic diversity was significantly related to C uptake. In addition, we found that actual evapotranspiration (AET—a measure that integrates precipitation and temperature) affected ecosystem C exchange indirectly via its impact on the three dimensions of plant diversity that we examined. These findings highlight complex relationships among key measures of biodiversity and ecosystem C uptake in a rapidly warming ecosystem, and the possible mechanisms that underlie relationships between biodiversity and ecosystem functioning. They also point to the need for integrating multiple dimensions of biodiversity into studies of community and ecosystem ecology.

Soluble carbon production by honeydew scale insects in a New Zealand beech forest

We estimated the annual production of honeydew per unit land area of beech (Nothofagus spp.) forest by measuring the amount of honeydew produced in 24 h by scale insects (Ultracoelostoma spp.) (Hemiptera: Margarodidae) every month for 2 years. We used exclosures to prevent animals (notably Vespula wasps) removing honeydew, and we compared the standing crop of honeydew inside permanently closed exclosures with that outside exclosures. Honeydew production and the number of honeydew droplets was highly variable between individual trees, tree type, position on tree, and, exclosure type, and within and between years. The amount of honeydew available outside exclosures was significantly reduced in year 2, predominantly by Vespula wasps, even though wasp density was relatively low. Sugar composition also varied between tree type and between years. Up to 5% of the sugar was glucose, with varying proportions of fructose, sucrose and oligosaccharides. The surface area of trees infested with scale insects was estimated using allometric regression relationships between tree diameter and total surface area of tree trunk and branch material. These estimates were combined with measurements of tree diameter in 10-m radius circular plots to give a production estimate of between 3500 and 4500 kg dry weight honeydew ha -1 year -1 . Using this data, combined with previously published estimates of carbon uptake, it was estimated that between 6 and 8% of net primary productivity was released as honeydew. Honeydew scale insects provide large amounts of biologically available carbon in the form of soluble sugar. It is a crucial resource for the above-ground system, and probably also for the below-ground system. We conclude that scale insects have the potential to function as keystone species in these forests.

The interactive effects of surface-burn severity and canopy cover on conifer and broadleaf tree seedling ecophysiology

Fire has an important role for regeneration of many boreal forest tree species, and this includes both wildfire and prescribed burning following clear-cutting. Depending on the severity, fire can have a variety of effects on above- and below-ground properties that impact tree seedling establishment. Very little is known about the impacts of ground fire severity on post-fire seedling performance, or how the effects of fire severity interact with those of canopy structure. We conducted a full-factorial experiment that manipulated surface-burn severity (no burn; light, medium, or heavy burn; or scarification) and canopy (closed forest or open clear-cut) to reveal their interactive effects on ecophysiological traits of establishing broadleaf and conifer seedlings in a Swedish boreal forest. Medium and heavy surface burns increased seedling growth, photosynthesis, respiration, and foliar N and P concentrations, and these effects were most apparent in open clear-cuts. Growth rates of all species responded similarly to surface-burn treatments, although photosynthesis, foliar P, and specific leaf area were more responsive to burning treatments for broadleaf species than for conifers. Our study demonstrates that the positive impacts of fire on tree seedling physiology are dependent on a minimum severity threshold and are more effective when combined with clear-cutting.

Context dependency of litter-mixing effects on decomposition and nutrient release across a long-term chronosequence

No abstract is provided for this article.

Effects of Plant Litter Species Composition and Diversity on the Boreal Forest Plant-Soil System

Although most plant products eventually enter the below-ground subsystem as plant litter, relatively little is known about the effects of plant litter diversity or composition on ecosystem processes and no study has considered the responses of plant growth to these factors. We conducted an experiment in which humus substrate was collected from three field sites in the boreal forest of northern Sweden. Litter was then placed on the humus surface, and the litter used consisted of monocultures of Empetrum hermaphroditum (dwarf shrub), Betula pendula (tree), and Pleurozium schreberi (feather moss), as well as mixtures containing all the possible (two-way and three-way) combinations of these species: the experiment was maintained in out-door conditions. Although decomposition rates of this surface-placed litter differed between species few effects of litter mixing on litter mass loss were apparent. Added litter of Pinus sylvestris litter broke down most slowly when placed in E. hermaphrodnum litter but sometimes showed elevated decomposition rates when placed in some of the multiple species litter mixes. Soil microbial biomass and activity was lowest when plant litter was absent, but as long as plant litter was present on the humus surface the species composition and diversity of the litter was irrelevant. There were few effects of litter treatments on growth of seedlings of either B. pendula or P. sylvestris planted into the humus. However, for one site there were significant effects of mixing litter of P. sclueberi and E. hermaphroditum in reducing growth of both seedling species. Litter treatments generally did not alter the competitive balance between B. pendula and P. sylvestris seedlings when grown together but for all sites litter treatments had significant effects on the overall intensity of competition, and mixing of litter of B. pendula and P. sehreheri had significant non-additive effects on competition intensity for two of the three sites. The abundance of mycorrhizae on seedlings was only weakly related to litter treatment but there were some positive effects of litter mixing on one of the most abundant mycorrhizal morphotypes on both species of seedlings for one of the sites. Our results suggest that litter presence was important in influencing a range of above ground and below ground properties and processes. In some instances individual species effects and litter mixing effects were also important but few general patterns emerged, and the nature of significant effects tended to be idiosyneratic. Ultimately our results show that plant litter has important afterlife effects which need to be considered in order to develop a more complete understanding of how biodiversity affects ecosystems.

Author response for "Root traits and soil microorganisms as drivers of plant‐soil feedbacks within the sub‐arctic tundra meadow"

No abstract is provided for this article.

Interactions between microclimatic variables and the soil microbial biomass

Soil moisture, temperature, microbial substrate-induced respiration and basal respiration were monitored in two plots in an agricultural field from April 3

Fate of 14 C from glucose and the herbicide metsulfuron–methyl in a plant–soil microcosm system

The effects of weeds and soil nutrient additions on the fate of 14C-labelled glucose and metsulfuron–methyl (metsulfuron) herbicide were determined in a growth-chamber soil–plant microcosm study. Nodding thistle (Carduus nutans) was used as a test weed species to which these compounds were applied. The experiment consisted of a full factorial design of N addition, P addition, planting with C. nutans, and addition of 14C-labelled compounds (glucose and herbicide metsulfuron). The experiment ran for 131 days following addition of the 14C-labelled compounds. There was a significant positive effect of C. nutans plants on soil microbial biomass-C and basal respiration. The addition of nutrients did not consistently affect any of the parameters measured in this study. The fate of 14C-labelled compounds was determined by quantifying the relative amounts of 14C absorbed by plants, incorporated into soil, taken up into microbial biomass and respired by soil organisms. Two days after application of the 14C compounds, 7–8% of the 14C was absorbed by the C. nutans (weed) canopy. By day 131, approximately 2–3% of the applied 14C was still detected in the tissues of dead thistle plants. The amounts and pattern of decomposition differed considerably between metsulfuron and glucose. Almost all (99%) of the applied 14C glucose was either respired by the microbes or incorporated into microbial biomass by day 131 while only 38–42% of the applied metsulfuron was respired or incorporated into microbes by this time. Greater amounts of metsulfuron pesticide were degraded in microcosms containing C. nutans plants than in those without plants. By day 131, 27.4% of 14C added from metsulfuron was respired in the planted treatment and only 22.8% from the non-planted treatment. This would suggest that the presence of decomposing weed litter (dying weed plants) has the potential to enhance the decomposition of added xenobiotics such as herbicides.

Linking litter decomposition of above‐ and below‐ground organs to plant–soil feedbacks worldwide

Summary Conceptual frameworks relating plant traits to ecosystem processes such as organic matter dynamics are progressively moving from a leaf‐centred to a whole‐plant perspective. Through the use of meta‐analysis and global literature data, we quantified the relative roles of litters from above‐ and below‐ground plant organs in ecosystem labile organic matter dynamics. We found that decomposition rates of leaves, fine roots and fine stems were coordinated across species worldwide although less strongly within ecosystems. We also show that fine roots and stems had lower decomposition rates relative to leaves, with large differences between woody and herbaceous species. Further, we estimated that on average below‐ground litter represents approximately 33 and 48% of annual litter inputs in grasslands and forests, respectively. These results suggest a major role for below‐ground litter as a driver of ecosystem organic matter dynamics. We also suggest that, given that fine stem and fine root litters decompose approximately 1.5 and 2.8 times slower, respectively, than leaf litter derived from the same species, cycling of labile organic matter is likely to be much slower than predicted by data from leaf litter decomposition only. Synthesis . Our results provide evidence that within ecosystems, the relative inputs of above‐ versus below‐ground litter strongly control the overall quality of the litter entering the decomposition system. This in turn determines soil labile organic matter dynamics and associated nutrient release in the ecosystem, which potentially feeds back to the mineral nutrition of plants and therefore plant trait values and plant community composition.

John Edward Atkinson : an appreciation

Immediately upon graduating from the University of Durham in 1961 with a BA (Hons) in Classical and General Literature, John Atkinson set off for the former Rhodesia, where he served for two years as assistant lecturer in the then University College of Rhodesia and Nyasaland. There T.F. Carney's ideas and interests exercised a powerful, long-lasting influence on him. After a short stay of 18 months as a lecturer at the University of South Africa (Unisa), the honorand took up another lectureship at the University of Cape Town in January 1965. In the following forty years he progressed through the ranks to full professor and was the last Dean of the Faculty of Arts, before its amalgamation into the Faculty of Humanities.

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