Lindsey J. Thompson

Biodiversity and plant productivity in a model assemblage of plant species

We examined productivity as a function of biotic diversity. We manipulated plant species richness as an experimental factor to determine if productivity (net above ground primary productivity or NPP) is affected by changes in plant diversity (species richness). We constructed 164 assemblages that varied in species richness and measured their biomass at the end of one growing season. The plants were drawn from a pool of 16 species of self-pollinating annual herbs common to English weedy fields. On average, species-poor assemblages were less productive. Results also showed, however, that species-poor assemblages had wider ranges of possible productivities than more diverse assemblages.

The effect of grasses on the quality of transmitted radiation and its influence on the growth of white clover Trifolium repens

SummaryPlants of white clover Trifolium repens were grown under the canopies of three grass species, Lolium perenne, Agrostis tenuis and Holcus lanatus, and under simulated canopies of black polythene and controls were exposed to unfiltered natural radiation. The canopies were adjusted so that they transmitted equal intensities of Photosynthetically Active Radiation (P.A.R.). The ratio of red to far red radiation (ζ) was unchanged under the black polythene canopies but was reduced under canopies of Lolium and Agrostis and even more so under Holcus. The effect of canopy filtered radiation on the growth of clover was greatly to reduced internode length, mean number of nodes, the number of branched nodes and the number of rooted nodes and greatly to increase petiole length. The effect of canopies of Holcus was greater than that of the other grass species both in its effect on ζ and on the responses of the clover plant to its shade.

The effect of earthworms and snails in a simple plant community

Snails and earthworms affected the dynamics of a simple, three-species plant community, in the Ecotron controlled environment facility. Earthworms enhanced the establishment, growth and cover of the legume Trifolium dubium, both via the soil and interactions with other plant species. Worms increased soil phosphates, increased root nodulation in T. dubium, and enabled T. dubium seedlings to establish in the presence of grass (Poa annua) litter, by increasing soil heterogeneity. Worms also buried the seeds of Poa annua and Senecio vulgaris, reducing the germination of new seedlings. Snails reduced nitrogen-fixing Trifolium dubium and increased cover of plant litter, thereby reducing ammonia-nitrogen concentrations in the soil. These effects and their interactions demonstrate that the detritivore food chain, and earthworms in particular, cannot be ignored if we are to understand the spatial and temporal dynamics of plant communities.

Below-ground microbial community development in a high temperature world

The response of above-ground plant and ecosystem processes to climate change are likely to be influenced by both direct and indirect effects of elevated temperature on soil biota and their activities. This study examined the effects of elevated atmospheric temperature on the development of the soil microbial community in a model terrestrial ecosystem facility. The model system was characterized by a soil of low nutrient availability, a condition that simulates most native terrestrial plant communities. The experiment was run over three plant generations, broadly mimicking the early stages of a plant succession, and showed that microbial biomass, measured using phospholipid fatty acid (PLFA) analysis, increased significantly in response to elevated temperature during the first generation only. This increase was unrelated to changes in plant productivity or soil C-availability, and was largely due to a direct effect of elevated temperature on fast-growing Gram-positive bacteria. Slow growing soil micoorganisms such as fungi and actinomycetes were unaffected by elevated temperature throughout the experimental period. Measures of microbial biomass, microbial respiration and N-mineralization were also unaffected by elevated atmospheric temperature over the three generations. The lack of effects on the soil microbial community is thought to be due to the fact that elevated temperature did not influence root biomass or soil C-availability. We suggest that the observed reductions in above-ground plant productivity, in response to elevated temperature, will become apparent in the longer term when litter decomposition pathways are more established. The temporal measures of PLFA and microbial biomass indicated that over the experimental period rapid initial changes occurred in most soil biological characteristics, followed by periods of stabilization during later plant succession. These changes were associated with increases in above-ground plant productivity and amounts of available C in the soil. In contrast, total microbial biomass declined during the last plant generation. Reductions in the diversity of PLFAs in later plant generations appeared to be associated with an increase in the proportion of fatty acids associated with fungi, relative to those from bacteria. These changes are likely to be related to increased competition for resources within the soil, and an associated reduction in N- and C-availability. These changes appear to be broadly consistent with those reported for other studies on the successional development of soil microbial and plant communities. Overall, our data suggest that elevated atmospheric temperature has little effect on the development of below-ground microbial communities and their activities in soils of low nutrient status.

The influence of natural canopy density on the growth of white clover, Trifolium repens

Two genotypes of white clover, Trifolium repens, were propagated clonally and grown under grass or woodland canopies. Canopy densities varied such that they transmitted a complete range (0 to 100%) of the available photosynthetically active radiation (PAR). As % PAR decreased (i.e. with more shading) the development of branches was suppressed, and internode and petiole elongation increased. In general, however, the responses were non-linear, and elongation of internodes and petioles decreased again at the lowest values of % PAR. The genotype from the open habitat was, in general, more sensitive to variations in % PAR under lower light conditions. It reached maximum internode and petiole elongation under higher light conditions than did the genotype from the more shaded habitat (...)

The effects of soil warming on plant recruitment

We demonstrate, using a simple field experiment, that soil warming is sufficient, independent of other cues, to potentially alter the recruitment of Betula pendula seedlings. This study suggests that changing temperature, and correlated changes in edaphic factors, can alter patterns in plant recruitment via differential effects on seed dormancy. Increased soil temperatures (of less than 5 °C) throughout the year led to a significant increase in the number of birch seedlings that emerged from soil samples collected in the spring. No concomitant changes in the emergence of other species was observed. Such temperature induced changes in seedbank dynamics and their subsequent effects on the competitive interactions within a plant community suggest far reaching ecological consequences of current increases in global temperature.

Biotic diversity and ecosystem processes: using the Ecotron to study a complex relationship

Abstract Global change research examines a variety of effects that changing environmental conditions may have on ecosystems. One aspect, a decline in species richness (biodiversity) observable in most ecosystems, has been less well studied, partly due to the complexity and long time necessary for experimentation. Model ecosystems (micro- or mesocosms), however, provide a feasible means for testing the many hypothetical relationships proposed to exist between diversity and ecosystem behaviour that observation and theory support. This article examines these hypotheses and shows how mesocosm studies conducted in the Ecotron, a controlled environmental facility, provide insights into this complex issue.