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Phylogenetic analyses show that C grasses typically occupy drier habitats than their C relatives, but recent experiments comparing the physiology of closely related C and C species have shown that advantages of C photosynthesis can be lost under drought. We tested the generality of these paradoxical findings in grass species representing the known evolutionary diversity of C NADP-me and C photosynthetic types. Our experiment investigated the effects of drought on leaf photosynthesis, water potential, nitrogen, chlorophyll content and mortality. C grasses in control treatments were characterized by higher CO assimilation rates and water potential, but lower stomatal conductance and nitrogen content. Under drought, stomatal conductance declined more dramatically in C than C species, and photosynthetic water-use and nitrogen-use efficiency advantages held by C species under control conditions were each diminished by 40%. Leaf mortality was slightly higher in C than C 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 C NADP-me relative to C grasses is a general phenomenon. [ABSTRACT FROM AUTHOR]

1. Plant–soil interactions play a central role in the biogeochemical carbon (C), nitrogen (N) and hydrological cycles. In the context of global environmental change, they are important both in modulating the impact of climate change and in regulating the feedback of greenhouse gas emissions (CO2, CH4 and N2O) to the climate system. 2. Dynamic global vegetation models (DGVMs) represent the most advanced tools available to predict the impacts of global change on terrestrial ecosystem functions and to examine their feedbacks to climate change. The accurate representation of plant–soil interactions in these models is crucial to improving predictions of the effects of climate change on a global scale. 3. In this paper, we describe the general structure of DGVMs that use plant functional types (PFTs) classifications as a means to integrate plant–soil interactions and illustrate how models have been developed to improve the simulation of: (a) soil carbon dynamics, (b) nitrogen cycling, (c) drought impacts and (d) vegetation dynamics. For each of these, we discuss some recent advances and identify knowledge gaps. 4. We identify three ongoing challenges, requiring collaboration between the global modelling community and process ecologists. First, the need for a critical evaluation of the representation of plant–soil processes in global models; second, the need to supply and integrate knowledge into global models; third, the testing of global model simulations against large-scale multifactor experiments and data from observatory gradients. 5. Synthesis. This paper reviews how plant–soil interactions are represented in DGVMs that use PFTs and illustrates some model developments. We also identify areas of ecological understanding and experimentation needed to reduce uncertainty in future carbon coupled climate change predictions. [ABSTRACT FROM AUTHOR]

The response of plant respiration ( R) to temperature is an important component of the biosphere's response to climate change. At present, most global models assume that R increases exponentially with temperature and does not thermally acclimate. Although we now know that acclimation does occur, quantitative incorporation of acclimation into models has been lacking. Using a dataset for 19 species grown at four temperatures (7, 14, 21, and 28 °C), we have assessed whether sustained differences in growth temperature systematically alter the slope and/or intercepts of the generalized log–log plots of leaf R vs. leaf mass per unit leaf area (LMA) and vs. leaf nitrogen (N) concentration. The extent to which variations in growth temperature account for the scatter observed in log–log R–LMA–N scaling relationships was also assessed. We show that thermal history accounts for up to 20% of the scatter in scaling relationships used to predict R, with the impact of thermal history on R–LMA–N generalized scaling relationships being highly predictable. This finding enabled us to quantitatively incorporate acclimation of R into a coupled global climate–vegetation model. We show that accounting for acclimation of R has negligible impact on predicted annual rates of global R, net primary productivity (NPP) or future atmospheric CO2 concentrations. However, our analysis suggests that accounting for acclimation is important when considering carbon fluxes among thermally contrasting biomes (e.g. accounting for acclimation decreases predicted rates of R by up to 20% in high-temperature biomes). We conclude that acclimation of R needs to be accounted for when predicting potential responses of terrestrial carbon exchange to climatic change at a regional level. [ABSTRACT FROM AUTHOR]

This paper describes experiments carried out to investigate the causes of high leaf nitrogen concentrations and high δ13C values in Alchemilla alpina L. growing at high altitudes. We investigated whether genetic adaptation, high levels of nitrogen input or low temperatures could account for these trends. In a field experiment, plants from two altitudes in the Scottish Highlands were transplanted to Great Dun Fell, a site in the Pennines of northern England. The experimental design was fully factorial : two altitudinal origins × two altitudes of growth × two nitrogen levels. A second experiment used a controlled environment to test the effects of temperature alone. The effects of altitude in the field transplant experiment were very similar to those in naturally growing plants. Leaf nitrogen concentration and δ13C were both higher at the high altitude, whilst growth declined and nitrogen per leaf was unaffected. An increase in potassium concentration with altitude was also found, Nitrogen addition caused increased leaf nitrogen concentrations but also increased nitrogen per leaf; δ13C was not affected and potassium and phosphorus concentrations decreased. The addition of nitrogen also increased mortality. Altitude of origin had relatively few effects but the population from the higher altitude did have a higher specific leaf area. Low temperature in the controlled environment caused increased δ13C, decreased leaf size and increased nitrogen and carbon contents, although the effect was less clear than the effects of altitude in the field. Gas exchange measurements suggested that the δ13C effect was caused by a reduction in stomatal conductance. We conclude that the effects of altitude on this species are principally the result of direct environmental modifications to growth rather than genetic adaptation. Of the various factors that change with altitude, temperature and a short growing season are particularly important; enhanced nitrogen supply through atmospheric deposition promotes increasing leaf nitrogen concentrations but must be considered in conjunction with other variables. [ABSTRACT FROM AUTHOR]

Elevated levels of Co2 in the atmosphere are expected to affect plant performance and may alter global temperature patterns. Changes in mean air temperatures that might be induced by rising levels of CO2 and other greenhouse gases could also be accompanied by increased variability in daily temperatures such that acute increases in air temperature may be more likely than at present. Consequently, we investigated whether plants grown in a CO2 enriched atmosphere would be differently affected by a heat shock than plants grown at ambient CO2 levels. Plants of a C3 annual (Abutilon theophrasti), a C3 annual crop (Sinapis alba) and a C4 annual (Amaranthus retroflexus) were grown from seed in growth chambers under either 400 or 700cm ³ m-3 CO2, and were fertilized with either a high or low nutrient regime. Young seedlings of S. alba, as well as plants of all species in either the vegetative or reproductive phase of growth were exposed to a 4-h heat chock in which the temperature was raised an additional 14-23°C (depending on plant age). Total biomass and reproductive biomass were examined to determine the effect of CO2, nutrient and heat shock treatments on plant performance heat shock, CO2, and nutrient treatments, all had sonic significant effects on plant performance, but plants from both CO2 treatments responded similarly to heat shocks. We also found, as expected, that plants grown under high CO2 had dramatically decreased tissue N concentrations relative to plants grown under ambient conditions. We predicted that high-CO2-grown plants would be more susceptible to a heat shock than ambient-CO2-grown plants, because the reduced N concentrations of high-CO2 grown plants could result in the reduced synthesis of heat shock proteins and reduced thermotolerance. Although we did not examine heat shock proteins, our results showed little relationship between plant nitrogen status and the ability of a plant to tolerate an acute increase in temperature. [ABSTRACT FROM AUTHOR]