摘要:

ABSTRACTA personal view is presented of current approaches to scaling processes and variables in the context of better understanding ecosystem function and predicting the consequences of global environmental change. Issues considered include spatial and temporal scales of interest, the scaling process, scaling strategy, scaling problems, heterogeneity, patchiness and non‐linearity, aggregation methodology and feedbacks.Knowledge of processes in plants and vegetation is largely at small scales. The transfer of this knowledge up to larger spatial and longer temporal scales is an open‐ended process with potential errors arising from heterogeneity and patchiness in the distribution of processes and non‐linearities in the functional relationships between processes and environmental variables. Scaling now covers several orders of magnitude with respect to spatial and temporal scales with attendant risks of propagating errors.At larger scales the wide diversity of vegetation classes poses a problem, and it is suggested that this can be countered by classifying classes of vegetation (not species) into a small number of ‘functional types’ of vegetation. Scaling through summation of component processes and through derivation of appropriately averaged parameters is considered. However, the increasing role of feedbacks at larger spatial and longer temporal scales is an essential feature of the scaling process. Thus, understanding the feedbacks and including them in upscaling schemes is a major priority.A scaling strategy is outlined to minimize the propagation of errors. Because the scaling process is open‐ended it is essential that good models are used and tested at each increa

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1995.tb00620.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTPrimary producers in aquatic environments cover a range of living biomasses from 10−13to 104g dw. Benthic plants at the high end of this range contribute 5 Gt C year,1to global primary productivity. Plankton at the lower end (up to 10−4g dw) contributes about 30 Gt C year−1. While many problems of interpretation remain, in general terms the size of the organisms which dominate particular habitats can be related to the physics of water movement and its interaction with the availability of light and nutrients, the generation time of the organism, and the attentions of grazers. A second scaling problem is that of methods of studying the global energy flow and nutrient cycling roles of aquatic primary producers. Problems with scaling up from small‐scale and mesoscale to regional or global scale, and the prospects of more direct estimates of large‐scale productivity, are

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1995.tb00621.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTFew of the most common assumptions used in models of responses of plants and ecosystems to elevated CO2and climate warming have been tested under realistic life con‐ditions. It is shown that some unexpected discrepancies between predictions and experimental findings exist, suggesting that a better empirical basis is required for predictions. The following ten suggestions may improve our potential to scale up from experimental scales to the real world.(1) Experiments should be timed to account for non‐linearity in system responsiveness, asynchrony of responses and developmental differences. (2) By altering mineral nutrient supply, a wide range of CO2responses can be ‘produced’, thus requiring realistic soil conditions. (3) Distinctions should be made between ‘doubling CO2sup‐ply’ and biologically effective degrees of CO2enrichment. (4) Because of the non‐linearity of plant responses to CO2, studies of at least three instead of two CO2 concentrations are necessary to describe future trends adequately. (5) Edge effects, in particular unscreened side light, may lead to allometric anomalies, strongly constraining up‐scaling to stand‐scale CO2responses. (6) Variables such as growth, yield, net primary production and C turnover are often confused with carbon pools, carbon sequestration or net ecosystem production. (7) Mono‐ and interspecific interactions between individuals may lead to completely unpredictable CO2responses. (8) Experiments with seedlings benefit from the absence of prehistory effects but are likely to be irrelevant for the responses of larger trees which, on the other hand, may be constrained by carry‐over effects. Tree ring research indicates immediate sensitivity of large trees to environmental changes, supporting their usefulness in short‐term CO2‐enrichment experiments. (9) In predicting temperature responses, acclimation deserves more attention. (10) The significance of developmental responses is largely under‐represented in experimental research, although these responses may overrule many of the other effects of atmospheric change. Results of more realistic experiments which account for these problems will provide a better basis for modellin

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1995.tb00622.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTLeaves and herbaceous leaf canopies photosynthesize efficiently although the distribution of light, the ultimate resource of photosynthesis, is very biased in these systems. As has been suggested in theoretical studies, if a photosynthetic system is organized such that every photosynthetic apparatus photosynthesizes in concert, the system as a whole has the sharpest light response curve and is most adaptive. This condition can be approached by (i) homogenization of the light environment and (ii) acclimation of the photosynthetic properties of leaves or chloroplasts to their local light environments. This review examines these two factors in the herbaceous leaf canopy and in the leaf. Changes in the inclination of leaves in the canopy and differentiation of mesophyll into palisade and spongy tissue contribute to the moderation of the light gradient. Leaf and chloroplast movements in the upper parts of these systems under high irradiances also moderate light gradients. Moreover, acclimation of leaves and chloroplasts to the local light environment is substantial. These factors increase the efficiency of photosynthesis considerably. However, the systems appear to be less efficient than the theoretical optimum. When the systems are optically dense, the light gradients may be too great for leaves or chloroplasts to acclimate. The loss of photosynthetic production attributed to the imperfect adjustment of photosynthetic apparatus to the local light environment is most apparent when the photosynthesis of the system is in the transition between the light‐limited and light‐saturated phases. Although acclimation of the photosynthetic apparatus and moderation of light gradients are imperfect, these markedly raise the efficiency of photosynthesis. Thus more mechanistic studies on these adaptive attributes are needed. The causes and consequences of imperfect adjustment should also be investiga

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1995.tb00623.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTA simple ‘big leaf’ ecosystem gas exchange model was developed, using eddy covariance data collected at an undisturbed tropical rainforest in south‐western Amazonia (Brazil). The model used mechanistic equations of canopy biochemistry combined with an empirical stomatal model describing responses to light, temperature and humidity. After calibration, the model was driven using hourly data from a weather station at the top of the tower at the measurement site, yielding an estimate of gross primary productivity (annual photosynthesis) in 1992/1993 of about 200 mol C m−2year−. Although incoming photon flux density emerged as the major control on photosynthesis in this forest, at a given PAR CO2assimilation rates were higher in the mornings than in the afternoons. This was attributable to stomatal closure in the afternoon in response to increasing canopy‐to‐air vapour pressure differences. Although most morning gas exchange was clearly limited by the rate of electron transport, afternoon gas exchange was generally observed to be very nearly co‐limited by both Rubisco activity (Vmax) and electron transport rate. The sensitivity of the model to changes in nitrogen allocation showed that the modelled ratio of Vmaxto electron transport (Jmax) served nearly to maximize the annual carbon gain, and indeed, would have resulted in almost maximum annual carbon gain at the pre‐industrial revolution atmospheric CO2concentration of 27 Pa. Modelled gross primary productivity (GPP) was somewhat lower at 27 Pa, being about 160 mol C m−2year−1. The model suggests that, in the absence of any negative feedbacks on GPP, future higher concentrations of atmospheric CO2will continue to increase the GPP of this rainforest, up to about 230 mo

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1995.tb00624.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTIn order to parametrize a leaf submodel of a canopy level gas‐exchange model, a series of photosynthesis and stomatal conductance measurements were made on leaves of white oak (Quercus albaL.) and red maple (Acer rubrumL.) in a mature deciduous forest near Oak Ridge, TN. Gas‐exchange characteristics of sun leaves growing at the top of a 30 m canopy and of shade leaves growing at a depth of 3–4 m from the top of the canopy were determined. Measured rates of net photosynthesis at a leaf temperature of 30°C and saturating photosynthetic photon flux density, expressed on a leaf area basis, were significantly lower (P = 0.01; n = 8) in shade leaves (7.9μmol m−2s−1) than in sun leaves (11–5μmol m−2s−1). Specific leaf area increased significantly with depth in the canopy, and when photosynthesis rates were expressed on a dry mass basis, they were not significantly different for shade and sun leaves. The percentage leaf nitrogen did not vary significantly with height in the canopy; thus, rates expressed on a per unit nitrogen basis were also not significantly different in shade and sun leaves. A widely used model integrating photosynthesis and stomatal conductance was parametrized independently for sun and shade leaves, enabling us to model successfully diurnal variations in photosynthesis and evapotranspiration of both classes of leaves. Key photosynthesis model parameters were found to scale with leaf nitrogen levels. The leaf model parametrizations were then incorporated into a canopy‐scale gas‐exchange model that is discussed and tested in a companion paper (Baldocchi&Harley 1995,Plant, Cell and Env

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1995.tb00625.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTThe scaling of CO2and water vapour transfer from leaf to canopy dimensions was achieved by integrating mechanistic models for physiological (photosynthesis, stomatal conductance and soil/root and bole respiration) and micrometeorological (radiative transfer, turbulent transfer and surface energy exchanges) processes. The main objectives of this paper are to describe a canopy photosynthesis and evaporation model for a temperate broadleaf forest and to test it against field measurements. The other goal of this paper is to use the validated model to address some contemporary ecological and physiological questions concerning the transfer of carbon and water between forest canopies and the atmosphere. In particular, we examine the role of simple versus complex radiative transfer models and the effect of environmental (solar radiation and CO2) and ecophysiological (photo‐synthetic capacity) variables on canopy‐scale carbon and water vapour flu

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1995.tb00626.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTA simple analytical scheme, involving the distribution of nitrogen, to scale up photosynthesis from leaf to canopy is proposed. The scheme is based on the assumption that there are two pools of nitrogen in leaves: nitrogen in photosynthetic, degradable structures (Np) and nitrogen in non‐photosynthetic and non‐degradable structures (Ns). The rate of photon‐saturated photosynthesis, Fm, is assumed to be proportional to Npand is distributed inside the canopy similarly to photon flux density (PFD). Prior assumptions of an optimum distribution of nitrogen are not a prerequisite. Calculations made with the scheme lead to development of the hypothesis that the canopy can be treated as a ‘big leaf’ on the time scales involved in acclimation of photosynthesis to PFD. Simulations using parameters for tree species with different requirements for PFD show that shade‐tolerant species may have denser canopies than sun‐demanding species because of smaller amounts of non‐photosynthetic structural nitrogen and/or supporting tissue

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1995.tb00627.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTA model is presented which solves simultaneously for leaf‐scale stomatal conductance, CO2assimilation and the energy balance as a function of leaf position within canopies of well‐watered vegetation. Fluxes and conductances were calculated separately for sunlit and shaded leaves. A linear dependence of photosynthetic capacity on leaf nitrogen content was assumed, while leaf nitrogen content and light intensity were assumed to decrease exponentially within canopies. Separate extinction coefficients were used for diffuse and direct beam radiation. An efficient Gaussian integration technique was used to compute fluxes and mean conductances for the canopy. The multilayer model synthesizes current knowledge of radiation penetration, leaf physiology and the physics of evaporation and provides insights into the response of whole canopies to multiple, interacting factors. The model was also used to explore sources of variation in the slopes of two simple parametric models (nitrogen‐ and light‐use efficiency), and to set bounds on the magnitudes of the parameters.For canopies low in total N, daily assimilation rates are ∼10% lower when leaf N is distributed uniformly than when the same total N is distributed according to the exponentially decreasing profile of absorbed radiation. However, gains are negligible for plants with high N concentrations. Canopy conductance, Gcshould be calculated as Gc=Aσ(fslgsl+fshgsh), where Δ is leaf area index, fsiand fshare the fractions of sunlit and shaded leaves at each level, and gsiand gshare the corresponding stomatal conductances. Simple addition of conductances without this weighting causes errors in transpiration calculated using the ‘big‐leaf’ version of the Penman‐Monteith equation. Partitioning of available energy between sensible and latent heat is very responsive to the parameter describing the sensitivity of stomata to the atmospheric humidity deficit. This parameter also affects canopy conductance, but has a relatively small impact on canopy assimilation.Simple parametric models are useful for extrapolating understanding from small to large scales, but the complexity of real ecosystems is thus subsumed in unexplained variations in parameter values. Simulations with the multilayer model show that both nitrogen‐ and radiation‐use efficiencies depend on plant nutritional status and the diffuse component of incident radiation, causing a 2‐ to 3‐fold vari

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1995.tb00628.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTTwo independent methods of estimating gross ecosystem production (GEP) were compared over a period of 2 years at monthly integrals for a mixed forest of conifers and deciduous hardwoods at Harvard Forest in central Massachusetts. Continuous eddy flux measurements of net ecosystem exchange (NEE) provided one estimate of GEP by taking day to night temperature differences into account to estimate autotrophic and heterotrophic respiration. GEP was also estimated with a quantum efficiency model based on measurements of maximum quantum efficiency (Qmax), seasonal variation in canopy phenology and chlorophyll content, incident PAR, and the constraints of freezing temperatures and vapour pressure deficits on stomatal conductance. Quantum efficiency model estimates of GEP and those derived from eddy flux measurements compared well at monthly integrals over two consecutive years (R2= 0–98).Remotely sensed data were acquired seasonally with an ultralight aircraft to provide a means of scaling the leaf area and leaf pigmentation changes that affected the light absorption of photosynthetically active radiation to larger areas. A linear correlation between chlorophyll concentrations in the upper canopy leaves of four hardwood species and their quantum efficiencies (R2= 0–99) suggested that seasonal changes in quantum efficiency for the entire canopy can be quantified with remotely sensed indices of chlorophyll. Analysis of video data collected from the ultralight aircraft indicated that the fraction of conifer cover varied from<7% near the instrument tower to about 25% for a larger sized area. At 25% conifer cover, the quantum efficiency model predicted an increase in the estimate of annual GEP of<5% because unfavourable environmental conditions limited conifer photosynthesis in much of the non‐growing season when hardwoods lacked l

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1995.tb00629.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY