1515Plant, Cell and Environment (1999) 22, 1515-1526 g sm , maximum stomatal conductance for water vapour; J S , sap flux density; k/A L , leaf-specific hydraulic conductance; LAI, leaf area index; DY S-L , water potential difference between soil and leaf. INTRODUCTIONAs the vapour pressure deficit between leaf and air (D) increases, stomata generally respond by partial closure (Lange et al. 1971). In most cases, stomatal conductance (g s ) decreases exponentially with increasing D (Massman & Kaufmann 1991; McCaughey & Iacobelli 1994; Monteith 1995). The stomatal closure response to increasing D generally results in a non-linear increase in transpiration rate (per unit leaf area, E) to a plateau and in some cases a decrease at high D (Jarvis 1980; Monteith 1995;Pataki, Oren & Smith 1999). By avoiding high E that would otherwise be caused by increasing D, stomatal closure avoids the corresponding decline in plant water potential (Saliendra, Sperry & Comstock 1995). It is a reasonable premise that the closure response evolved to prevent excessive dehydration and physiological damage.It is established that the cue for the closure response is linked to E rather than D (Mott & Parkhurst 1991) and is therefore fundamentally a feedback response to water loss from the leaf tissue. The only known mechanism by which the plant can sense E is a change in the water potential (or its proxy, relative water content) of cells in the leaf. However, the identity of these cells, and the details of the signal transduction are unknown. Nevertheless, these results argue for an analysis of stomatal responses to D from the standpoint of the regulation of E (Monteith 1995) and water potential (Saliendra et al. 1995).In this paper, we focus on the sensitivity of the stomatal response to D, where sensitivity refers to the magnitude of the reduction in g s with increasing D. While most plants exhibit a decline in g s with D, there is considerable variation at the intra-and interspecific levels in the sensitivity of the response (e.g. Whitehead, Okali & Fasehun 1981;Aphalo & Jarvis 1991; McNaughton & Jarvis 1991). It is commonly observed that greater sensitivity is associated with a higher g s at low D (Kaufmann 1982; McNaughton & Jarvis 1991;Yong, Wong & Farquhar 1997). Here we test the generality of this relationship for data obtained by both porometric and sap flux methods across a variety of species ABSTRACTResponses of stomatal conductance (g s ) to increasing vapour pressure deficit (D) generally follow an exponential decrease described equally well by several empirical functions. However, the magnitude of the decrease -the stomatal sensitivity -varies considerably both within and between species. Here we analysed data from a variety of sources employing both porometric and sap flux estimates of g s to evaluate the hypothesis that stomatal sensitivity is proportional to the magnitude of g s at low D (£ 1 kPa). To test this relationship we used the function g s = g sref -m · lnD where m is the stomatal sensitivity and g sref = g s a...

A highly controversial issue in global biogeochemistry is the regulation of terrestrial carbon (C) sequestration by soil nitrogen (N) availability. This controversy translates into great uncertainty in predicting future global terrestrial C sequestration. We propose a new framework that centers on the concept of progressive N limitation (PNL) for studying the interactions between C and N in terrestrial ecosystems. In PNL, available soil N becomes increasingly limiting as C and N are sequestered in long-lived plant biomass and soil organic matter. Our analysis focuses on the role of PNL in regulating ecosystem responses to rising atmospheric carbon dioxide concentration, but the concept applies to any perturbation that initially causes C and N to accumulate in organic forms. This article examines conditions under which PNL may or may not constrain net primary production and C sequestration in terrestrial ecosystems. While the PNL-centered framework has the potential to explain diverse experimental results and to help researchers integrate models and data, direct tests of the PNL hypothesis remain a great challenge to the research community.

Climate change predictions derived from coupled carbon-climate models are highly dependent on assumptions about feedbacks between the biosphere and atmosphere. One critical feedback occurs if C uptake by the biosphere increases in response to the fossil-fuel driven increase in atmospheric [CO 2] (''CO2 fertilization''), thereby slowing the rate of increase in atmospheric [CO 2]. Carbon exchanges between the terrestrial biosphere and atmosphere are often first represented in models as net primary productivity (NPP). However, the contribution of CO 2 fertilization to the future global C cycle has been uncertain, especially in forest ecosystems that dominate global NPP, and models that include a feedback between terrestrial biosphere metabolism and atmospheric [CO 2] are poorly constrained by experimental evidence. We analyzed the response of NPP to elevated CO 2 (Ϸ550 ppm) in four free-air CO 2 enrichment experiments in forest stands. We show that the response of forest NPP to elevated [CO 2] is highly conserved across a broad range of productivity, with a stimulation at the median of 23 ؎ 2%. At low leaf area indices, a large portion of the response was attributable to increased light absorption, but as leaf area indices increased, the response to elevated [CO 2] was wholly caused by increased light-use efficiency. The surprising consistency of response across diverse sites provides a benchmark to evaluate predictions of ecosystem and global models and allows us now to focus on unresolved questions about carbon partitioning and retention, and spatial variation in NPP response caused by availability of other growth limiting resources.CO2 fertilization ͉ global change ͉ leaf area index ͉ net primary productivity A nalysis and prediction of the effects of human activities, particularly the combustion of fossil fuels, on climate and the biological, physical, and social responses to changing climate require an integrated view of the complex interactions between the biosphere and atmosphere. Carbon cycle models are now being coupled to atmosphere-ocean general circulation climate models to achieve a dynamic analysis of the relationships between C emissions, atmospheric chemistry, biosphere activity, and climatic change (1-3).Exchanges between the terrestrial biosphere and atmosphere are represented in models using empirical and theoretical expressions of net primary productivity (NPP), the net fixation of C by green plants into organic matter, or the difference between photosynthesis and plant respiration. Because the photosynthetic uptake of carbon that drives NPP is not saturated at current atmospheric concentrations (4), NPP should increase as fossilfuel combustion adds to the atmospheric [CO 2 ]. Increased C uptake into the biosphere in response to rising [CO 2 ] (''CO 2 fertilization'') can create a negative feedback that slows the rate of increase in atmospheric [CO 2 ] (3, 5). Hence, assumptions regarding CO 2 fertilization of the terrestrial biosphere greatly affect predictions of future atmospheric [CO 2 ] (3)...

Deforestation in mid-to high latitudes is hypothesized to have the potential to cool the Earth's surface by altering biophysical processes [1][2][3] . In climate models of continental-scale land clearing, the cooling is triggered by increases in surface albedo and is reinforced by a land albedo-sea ice feedback 4,5 . This feedback is crucial in the model predictions; without it other biophysical processes may overwhelm the albedo effect to generate warming instead 5 . Ongoing land-use activities, such as land management for climate mitigation, are occurring at local scales (hectares) presumably too small to generate the feedback, and it is not known whether the intrinsic biophysical mechanism on its own can change the surface temperature in a consistent manner 6,7 . Nor has the effect of deforestation on climate been demonstrated over large areas from direct observations. Here we show that surface air temperature is lower in open land than in nearby forested land. The effect is 0.85 6 0.44 K (mean 6 one standard deviation) northwards of 456 N and 0.21 6 0.53 K southwards. Below 356 N there is weak evidence that deforestation leads to warming. Results are based on comparisons of temperature at forested eddy covariance towers in the USA and Canada and, as a proxy for small areas of cleared land, nearby surface weather stations. Night-time temperature changes unrelated to changes in surface albedo are an important contributor to the overall cooling effect. The observed latitudinal dependence is consistent with theoretical expectation of changes in energy loss from convection and radiation across latitudes in both the daytime and night-time phase of the diurnal cycle, the latter of which remains uncertain in climate models 8 .The latitudinal gradient of land-use impact is evident in the comparison of the surface air temperature recorded at FLUXNET (www.fluxnet.ornl.gov) forest towers 9 (Supplementary Table 1 and Supplementary Fig. 1) and surface weather stations in North America (Fig. 1a). Here we use the surface stations as proxies for cleared land. In accordance with the requirement of the World Meteorological Organization, these stations are located in open grassy fields that have biophysical characteristics similar to those of open land, such as being covered by snow in northern latitudes in the winter 10 . Latitude accounts for 31% of the variations in the temperature difference DT between the forest sites and the adjacent open lands (number of site pairs n 5 37). The rate of change in DT with latitude is 20.070 6 0.010 K per degree (mean 6 one standard error, s.e., P , 0.005). At these sites, the annual net all-wave radiation R n