F. I. Woodward

Ecosystem effects of biodiversity manipulations in European grasslands.

We present a multisite analysis of the relationship between plant diversity and ecosystem functioning within the European BIODEPTH network of plant-diversity manipulation experiments. We report results of the analysis of 11 variables addressing several aspects of key ecosystem processes like biomass production, resource use (space, light, and nitrogen), and decomposition, measured across three years in plots of varying plant species richness at eight different European grassland field sites. Differences among sites explained substantial and significant amounts of the variation of most of the ecosystem processes examined. However, against this background of geographic variation, all the aspects of plant diversity and composition we examined (i.e., both numbers and types of species and functional groups) produced significant, mostly positive impacts on ecosystem processes. Analyses using the additive partitioning method revealed that complementarity effects (greater net yields than predicted from monocultures due to resource partitioning, positive interactions, etc.) were stronger and more consistent than selection effects (the covariance between monoculture yield and change in yield in mixtures) caused by dominance of species with particular traits. In general, communities with a higher diversity of species and functional groups were more productive and utilized resources more completely by intercepting more light, taking up more nitrogen, and occupying more of the available space. Diversity had significant effects through both increased vegetation cover and greater nitrogen retention by plants when this resource was more abundant through N2 fixation by legumes. However, additional positive diversity effects remained even after controlling for differences in vegetation cover and for the presence of legumes in communities. Diversity effects were stronger on above- than belowground processes. In particular, clear diversity effects on decomposition were only observed at one of the eight sites. The ecosystem effects of plant diversity also varied between sites and years. In general, diversity effects were lowest in the first year and stronger later in the experiment, indicating that they were not transitional due to community establishment. These analyses of our complete ecosystem process data set largely reinforce our previous results, and those from comparable biodiversity experiments, and extend the generality of diversity–ecosystem functioning relationships to multiple sites, years, and processes.

Plant development: Signals from mature to new leaves

Stomata are microscopic pores on the surfaces of leaves, the number and density of which vary in response to changes in environmental conditions, such as carbon dioxide concentration and light. We show here that mature leaves of Arabidopsis thaliana detect and transmit this external information to new leaves of the same plant, producing an appropriate adjustment of stomatal development. As CO2 concentration controls both stomatal opening and number, and stomatal numbers also increase with higher light intensity, the large gradients of CO2 and light found within plant communities have the potential to influence stomatal development.

Photosynthetic pathway and ecological adaptation explain stomatal trait diversity amongst grasses

• The evolution of C(4) photosynthesis in plants has allowed the maintenance of high CO(2) assimilation rates despite lower stomatal conductances. This underpins the greater water-use efficiency in C(4) species and their tendency to occupy drier, more seasonal environments than their C(3) relatives. • The basis of interspecific variation in maximum stomatal conductance to water (g(max) ), as defined by stomatal density and size, was investigated in a common-environment screening experiment. Stomatal traits were measured in 28 species from seven grass lineages, and comparative methods were used to test for predicted effects of C(3) and C(4) photosynthesis, annual precipitation and habitat wetness on g(max) . • Novel results were as follows: significant phylogenetic patterns exist in g(max) and its determinants, stomatal size and stomatal density; C(4) species consistently have lower g(max) than their C(3) relatives, associated with a shift towards smaller stomata at a given density. A direct relationship between g(max) and precipitation was not supported. However, we confirmed associations between C(4) photosynthesis and lower precipitation, and showed steeper stomatal size-density relationships and higher g(max) in wetter habitats. • The observed relationships between stomatal patterning, photosynthetic pathway and habitat provide a clear example of the interplay between anatomical traits, physiological innovation and ecological adaptation in plants.

Drought limitation of photosynthesis differs between C3 and C4 grass species in a comparative experiment

Phylogenetic analyses show that C₄ grasses typically occupy drier habitats than their C₃ relatives, but recent experiments comparing the physiology of closely related C₃ and C₄ species have shown that advantages of C₄ photosynthesis can be lost under drought. We tested the generality of these paradoxical findings in grass species representing the known evolutionary diversity of C₄ NADP-me and C₃ photosynthetic types. Our experiment investigated the effects of drought on leaf photosynthesis, water potential, nitrogen, chlorophyll content and mortality. C₄ grasses in control treatments were characterized by higher CO₂ assimilation rates and water potential, but lower stomatal conductance and nitrogen content. Under drought, stomatal conductance declined more dramatically in C₃ than C₄ species, and photosynthetic water-use and nitrogen-use efficiency advantages held by C₄ species under control conditions were each diminished by 40%. Leaf mortality was slightly higher in C₄ than C₃ grasses, but leaf condition under drought otherwise showed no dependence on photosynthetic-type. This phylogenetically controlled experiment suggested that a drought-induced reduction in the photosynthetic performance advantages of C₄ NADP-me relative to C₃ grasses is a general phenomenon.

Palaeo-ecophysiological perspectives on plant responses to global change

Taxonomic classifications of plant species, based on morphological characteristics, provide a stable and robust approach for inferring taxonomic and phylogenetic relationships between extant and extinct species. This implies that, although evolution is a continuous process for a species, there is no whole-scale change in those suites of morphological characteristics that define higher order (genus and greater) relationships. Recent research suggests that a higher order characteristic - stomatal density - may reflect not only the atmospheric C0(2) concentration during initial evolution, but may also strongly constrain the responses of higher order plant groups to future C0(2)-enrichment.

Climate change and the British scene

The concentration of CO2 in the atmosphere has been steadily increasing since the onset of the industrial revolution (Houghton et al. 1990; Keeling et al. 1989). General circulation models (GCMs) of the earths climatic system predict that the continued increase in the atmospheric concentrations of CO2 and other greenhouse gases will cause climatic warming (Houghton et al. 1990, 1992; Wigley & Raper 1992). This is a clear and neat solution. However, the global climate system is neither as neat nor as simple. The changes in global temperatures over the last 40 years do not closely mirror the changes in C02, so clearly the greenhouse effect of increased CO2 is not a simple relationship. The Intergovernmental Panel on Climate Change (IPCC) has investigated some of the atmospheric and terrestrial processes which influence the climate system (Houghton et al. 1990, 1992). Wigley & Raper (1992) describe how future climate can be influenced by a range of factors including changes in policies on greenhouse gas emissions, the influences of SO2 and sulphate aerosol production, cloud formation, uncertainties in the global carbon cycle and the negative feedback on halocarbon effects through the depletion of stratospheric ozone. New information on all of these influences leads to current model projections of global warming for the year 2050, which are about 1 ?C less than the 1990 report of the IPCC (Houghton et al. 1990). The model projections are still likely to be in error because the degree of the feedback between terrestrial and oceanic ecosystems and climate, through changes in the fluxes of CO2 and water vapour, is still unknown (Wigley & Raper 1992). The nature and degree of feedbacks between climate and parts of the climate system are major limitations to adequate predictions of future climate. Taking account of the new projections of climatic change (Wigley & Raper 1992) and the modelling of Climate Change Impact Review Group (CCIRG) (1991), then it is possible to suggest the following changes in UK temperatures by the year 2050: Summer Temperature + 1.1 ?C, uniformly over UK Winter Temperature + 1.7?C, northern limit of UK + 1.2?C, southern limit of UK (linear south/north gradient). GCM predictions of future precipitation are notoriously unreliable (Schneider 1992). However, CCIRG (1991) suggests that the summer precipitation is most likely to remain unchanged, while the winter totals may increase by about 4%. A future problem will be that the basically physical portrayal of the climate system will, increasingly, incorporate feedbacks which are under human control, e.g. C02, CH4, NO, and CFC emissions. Therefore the climate model will be more like an economic model (The Economist 1992) in which predictions from the model can lead to changes in climate, through changes in policy on emissions. If the economic analogy is carried on then it is likely that the fundamental uncertainties which characterise economic models may then spread to climate models, suggesting a decreased potential for adequate future predictions of climate. In spite of this consideration, it is important to consider the prospects for climatic change and for the probable influence of these changes on the biosphere. Just as there is a human effect on climate there is also a human effect on conservation of natural, living resources. If we are convinced of likely changes in climate then there must be argument and discussion about management responses to these changes. Although it may prove impossible to prevent changes in vegetation, for example, it is possible that greater weight should be given to the conservation of species and this must be accommodated somewhere in the models. This argument strongly favours the conservation and maintenance of diversity, in this case as an end in itself, and international bodies such as IGBP (1992), WCMC (1992) and SCOPE (1990) are now actively pursuing the importance of diversity to the continued functioning of ecosystems and their responses to environmental change. There are many aspects of the responses of British vegetation to global change which might be used to forecast changes in their ecology. In this paper we illustrate some effects of future climatic warming on the British landscape by reference to native and introduced species. Special emphasis is placed on the native pine forests of Scotland and two introduced and aggressively invasive species Fallopia japonica and Impatiens glandulifera. In the next 100 years the responses shown by pine forests to the climatic amelioration of the late-glacial (Dubois & Ferguson 1985) may be repeated as global change takes place, but this time the response may be to conditions which have not been previously experienced for at least the last 200 000 years (Houghton et al. 1990). For native pine 391

Ecology: Forest air conditioning

During the growing season, with photosynthesis at its peak, leaf temperatures remain constant over a wide latitudinal range. This is a finding that overturns a common assumption and has various ramifications.