为了正常的体验网站,请在浏览器设置里面开启Javascript功能!

aa

2009-09-27 19页 pdf 337KB 16阅读

用户头像

is_678658

暂无简介

举报
aa Adaptive differences in plant physiology and ecosystem paradoxes: insights from metabolic scaling theory B R I AN J . E NQU I S T *, ANDR EW J . K E RKHO F F *1 , T RAV I S E . HU XMAN * and E VAN P. E CONOMO w *Department of Ecology and Evolutionary Biology, Uni...
aa
Adaptive differences in plant physiology and ecosystem paradoxes: insights from metabolic scaling theory B R I AN J . E NQU I S T *, ANDR EW J . K E RKHO F F *1 , T RAV I S E . HU XMAN * and E VAN P. E CONOMO w *Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721, USA, wSection of Integrative Biology, University of Texas at Austin, Austin, TX 78712, USA Abstract The link between variation in species-specific plant traits, larger scale patterns of productivity, and other ecosystem processes is an important focus for global change research. Understanding such linkages requires synthesis of evolutionary, biogeogra- pahic, and biogeochemical approaches to ecological research. Recent observations reveal several apparently paradoxical patterns across ecosystems. When compared with warmer low latitudes, ecosystems from cold northerly latitudes are described by (1) a greater temperature normalized instantaneous flux of CO2 and energy; and (2) similar annual values of gross primary production (GPP), and possibly net primary production. Recently, several authors attributed constancy in GPP to historical and abiotic factors. Here, we show that metabolic scaling theory can be used to provide an alternative ‘biotically driven’ hypothesis. The model provides a baseline for understanding how potentially adaptive variation in plant size and traits associated with metabolism and biomass production in differing biomes can influence whole-ecosystem processes. The implication is that one cannot extrapolate leaf/lab/forest level functional responses to the globe without considering evolutionary and geographic variation in traits associated with metabolism. We test one key implication of this model – that directional and adaptive changes in metabolic and stoichiometric traits of autotrophs may mediate patterns of plant growth across broad temperature gradients. In support of our model, on average, mass-corrected whole-plant growth rates are not related to differences in growing season temperature or latitude. Further, we show how these changes in autotrophic physiology and nutrient content across gradients may have important implications for understanding: (i) the origin of paradoxical ecosystem behavior; (ii) the potential efficiency of whole-ecosystem carbon dynamics as measured by the quotient of system capacities for respiration, R, and assimilation, A; and (iii) the origin of several ‘ecosystem constants’ – attributes of ecological systems that apparently do not vary with temperature (and thus with latitude). Together, these results highlight the potential critical importance of community ecology and functional evolutionary/ physiological ecology for understanding the role of the biosphere within the integrated earth system. Keywords: allometry, ecosystem constants, functional traits, growth efficiency, invariants, latitudinal gradient, net primary production, scaling, stoichiometry, temperature response Received 11 July 2005; revised version received 7 January 2006 and accept 17 January 2006 Introduction Our understanding of the role that organisms play in influencing global material and energy cycles is in part constrained by a lack of knowledge of the relative influences of both abiotic and biotic features of the integrated earth environmental system (Osmond et al., 2004). It is clear, however, that the performance of autotrophs is critical in influencing ecosystem proces- sing and dynamics. For example, autotrophic respira- tion plays a substantial role in governing ecosystem carbon balance (Field et al., 1992; Ryan et al., 1995). Correspondence: Brian J. Enquist, e-mail: benquist@u.arizona.edu 1Present address: Department of Biology, Kenyon College, Gambier, OH 43022, USA. Global Change Biology (2007) 13, 591–609, doi: 10.1111/j.1365-2486.2006.01222.x r 2007 The Authors Journal compilation r 2007 Blackwell Publishing Ltd 591 Integration of basic biochemistry and biophysics of photosynthesis, whole-plant responses to regional and global climate, and ecology are essential for developing a predictive understanding of ecosystem flux (Geider et al., 2001). Therefore, accurate modeling of the re- sponse of autotrophic respiration and overall carbon balance to differing climate regimes is essential to predict the impacts of the traits that influence plant metabolism on global carbon budgets. The overall message from autecological studies is that physiological and life-history traits of plants vary in accord with changes in climate and edaphic regimes. Since the pioneering work of Clausen, Keck and Hiesey (Clausen et al., 1940) botanists have amassed a large list of examples of how differing environments select for unique life histories and physiological traits (Mooney & Billings, 1961; Klikoff, 1966; Strain, 1966; McNaughton, 1967). Differences in local climate and abiotic regimes can also act as a filter ‘selecting’ those combinations of organismal traits that ensure that the organism main- tains a positive carbon and energy balance (Criddle et al., 1994; Weiher & Keddy, 1995; McGill et al., 2006). Thus, as a result of acclimatization and environmental selection of traits, differing ecosystems tend to be characterized by plants with unique physiological and life-history adaptations for that specific environment (Schimper, 1903; Shields, 1950; Mooney, 1977; Reich et al., 1999; Fonseca et al., 2000; Kleidon & Mooney, 2000; Schippers et al., 2001; Wright et al., 2001; Nicotra et al., 2002). Although evidence for physiological and life-history adaptation is abundant, relatively little is known about the importance of local adaptations in influencing eco- system processes across broad scale gradients. Can adaptive and directional shifts in functional traits influence ecosystem processes? Patterns of plant trait variation due to both species replacements and within-species variation across re- source and environmental gradients (light, water, nu- trients, and temperature) are thought to reflect local adaptation (Reich et al., 2003). For example, within species, there is a rich literature on acclimation, the adaptive adjustments of physiology to temperature and other environmental factors (see Criddle et al., 1994; Atkin & Tjoelker, 2003; Talts et al., 2004; Atkin et al., 2005; Galme´s et al., 2005). Between species inhabiting differing environments, the optimal temperatures for photosynthesis and overall optimal growth are gener- ally correlated with the temperature range experienced by plants during the growing season (Amthor, 1989; Larcher, 1995; Cunningham et al., 1999; Saxe et al., 2001). Cold adapted plants tend to have physiological adapta- tions associated with the rate of metabolism. For exam- ple, they tend to have higher rates of cellular respiration and carbon assimilation at a given temperature than plants grown in warm environments (Will, 2000; Galme´s et al., 2005; but see Wright et al., 2006). Changes in respiration often reflects (i) an increase in the poten- tial rates of respiratory activity per unit mitochondrial volume (Klikoff, 1966; Miroslavov & Kravkina, 1991) in addition to; (ii) a change in the proteins and efficiency of terminal oxidase (Ribas Carbo et al., 2000; Kurimoto et al., 2004); and (iii) an overall altering of the tempera- ture dependance of metabolism by changes in biochem- ical activation energies as measured by the Arrhenius temperature coefficient (Criddle et al., 1994). In common garden experiments, plant respiration rates are gener- ally higher for plants originating colder sites (Mooney, 1963; Criddle et al., 1994; Oleksyn et al., 1998). Acclima- tion of respiration and photosynthesis strongly suggests that factors other than reaction kinetics regulate plant flux. In addition to changes in rates of carbon fluxes associated with respiration and photosynthesis the effi- ciency of carbon use (the ratio of organismal net pri- mary production divided by gross primary production (NPP/GPP), a measure of what fraction of total carbon assimilated becomes incorporated into biomass) may also vary across plants across temperature gradients (Chambers et al., 2004). Despite the many examples of physiological adapta- tion and geographic variation in functional groups, it is still not clear if such evolutionary and ecological changes in organismal traits systematically alter large- scale ecosystem processes (Ackerly & Monson, 2003). Further, what specific adaptive differences in plant traits could modify ecosystem processes? Recent ana- lyses suggest that adaptive variation in traits that influence plant metabolism can have substantial impact on the carbon balance of ecosystems (Luo et al., 2001; Kerkhoff et al., 2005; Wythers et al., 2005). Here, we ask whether plant physiological adaptation can mediate the influence of abiotic drivers on ecosystem processes such as primary production or nutrient cycling across the globe. We build upon a growing awareness of the importance of functional traits (see McGill et al., 2006) by mechanistically emphasizing the fundamental role of potential variation in organismal physiology, instead of climate alone, in influencing variability in ecosystem fluxes (Kerkhoff et al., 2005). This paper has three objectives: 1. We first highlight a prominent yet paradoxical cross- ecosystem finding that relates environmental tem- perature and ecosystem energetics. We show how this pattern has important implications for under- standing the response of the biosphere to aspects of global change. To account for this pattern we review 592 B . J . E NQU I S T et al. r 2007 The Authors Journal compilation r 2007 Blackwell Publishing Ltd, Global Change Biology, 13, 591–609 a novel model for scaling organismal metabolism from cells to ecosystems that builds upon metabolic scaling theory (West et al., 1997; Enquist et al., 1998, 2003; Brown et al., 2004; Kerkhoff et al., 2005). 2. Next, we show that a trait-based elaboration of metabolic scaling theory specifies how directional shifts in plant traits across latitudinal/temperature gradients can influence ecosystem behavior. In parti- cular, recent work by Kerkhoff et al. (2005) highlights the importance of plant tissue nutrient stoichiometry and growth efficiency. We provide empirical evi- dence showing that the growth rates of trees (ad- justed for average mass) does not appear to vary significantly and systematically in response to a broad temperature gradient. This result is consistent with the Kerkhoff et al. (2005) model indicating that variation in traits associated with organismal growth and metabolism, due to selection for increased growth rates in cold environments, can in turn yield the ‘paradoxical responses’ of whole-ecosystems mentioned above. 3. Finally, we explore the implications of an approxi- mate invariance in growth rate with latitude/ temperature for autotrophic respiration and net eco- system primary production. Specifically, we show that the ratio between ecosystem capacities for re- spiration and net assimilation is invariant with respect to a temperature gradient, providing one of several ‘ecosystem invariants.’ Latitude, temperature, and paradoxical patterns of ecosystem flux and production constants A physical explanation for large-scale variability in ecosystem flux along temperature and latitudinal gradients Temperature is fundamental in influencing the kinetics of biochemical reactions. In general, rates of biologically mediated conversions are tightly linked to changes in temperature (Johnson et al., 1974; Lloyd & Taylor, 1994). It is widely thought that increases in global temperature will bring about increases in the metabolic activity of organisms within terrestrial ecosystems. Recently, how- ever, utilizing a network of CO2 and H2O flux monitor- ing stations across Europe (EUROFLUX), Valentini et al. (2000) found no trend in annual ecosystem GPP across European latitudes north of the Mediterranean. Further, a recent analysis by Kerkhoff et al. (2005) showed that variation in instantaneous rates of net primary produc- tivity, showed little to no variation with latitude and growing season temperature. These results are surpris- ing as they run counter to the prominent paradigm that cold, high latitude ecosystems are less productive than warmer, lower latitude ecosystems (Lieth, 1975). Valentini et al. (2000) hypothesized that the apparent constancy in GPP was not due to functional trait or diversity differences between sites but instead due to the relatively high abundance of soil carbon and recent warming of northerly latitudes. However, others have suggested that there are no clear trends of decreasing soil carbon with increasing mean annual temperature (Thornley & Cannell, 2001). In contrast, Kerkhoff et al. (2005) hypothesized that the relative constancy of in- stantaneous rates of NPP with temperature was due to possibly adaptive differences in growth rates across temperature gradients. An alternative hypothesis: the three A’s – Acclimation Adaptation, Assembly – can negate physical drivers of ecosystem flux and production The findings of Giardina & Ryan (2000) (Liski et al., 1999) and Baldocchi et al. (2001) may offer another insight into the relative insensitivity of GPP and annual NPP (ANPP) with latitude noted by Valentini et al. (2000) and Kerkhoff et al. (2005). Giardina & Ryan (2000) found that soil decomposition rates across a global-scale gra- dient in mean annual temperature were remarkably constant. Baldocchi et al. (2001) noted that the tempera- ture optimum for ecosystem photosynthesis appeared to change with mean growing season temperature. Similar to findings from comparative ecophysiology of leaves (Niinemets et al., 1999), ‘cold’ ecosystems seemed to have lower temperature optima for photosynthesis and ‘warm’ ecosystems had higher temperature optima (Fig. 1). The findings from Baldocchi et al. suggest that photoautotrophic processes may systematically vary across broad gradients. However, the specific mechan- isms behind such shifts in the optimum temperature for ecosystem photosynthesis are not clear. If, as proposed for autotrophs, whole-ecosystem respiration acclimates to ecosystem photosynthate supply (i.e. primary pro- duction; Dewar et al., 1999), then the temperature re- sponse of whole-ecosystem carbon flux and biomass production will likely also be altered. Building upon the findings of Valentini et al. (2000) and Giardina & Ryan (2000), Enquist et al. (2003) used data from FLUXNET (http://daac.ornl.gov/FLUXNET/) to document a related pattern of ecosystem invariance. Across a variety of arid and mesic sites in both Europe and North America, CO2 and energy flux was charac- terized by a similar exponential functional response with temperature the Boltzmann or Van’t Hoff reac- tion rate rule (Gillooly et al., 2001). However, when the total annual ecosystem respiration was plotted as a function of annual temperature no significant relation- E COLOGY, O RGAN I SMAL ENERG E T I C S AND ECO S Y S T EM PARADOX E S 593 r 2007 The Authors Journal compilation r 2007 Blackwell Publishing Ltd, Global Change Biology, 13, 591–609 ship was found (Fig. 2). More importantly, they found that when the instantaneous rates of ecosystem respira- tion were standardized for a given temperature, colder and higher latitude ecosystems actually exchanged CO2 and energy at three- to sixfold greater rates than war- mer low latitude ecosystems. Figure 3 shows a positive correlation between temperature-standardized flux (at 20 1C) and latitude for all sites used in the analysis (r25 0.569, n5 46, F5 58.06, Po0.0001 data from En- quist et al., 2003). Enquist et al. concluded that taking the annualized and instantaneous findings together illus- trates a paradox – why should it be that (i) there are no significant differences in annual fluxes across diverse ecosystems yet (ii) instantaneous fluxes of colder sites are much greater than warmer sites? Enquist et al. (2003) provided a mechanistic model to account for the observed temperature response function of ecosystem respiration. Based on this model, they outlined several hypotheses to explain the increase in instantaneous rates of respiration at a standardized temperature and the approximate constancy in total annual respiration across latitude. One possibility proposed by Enquist et al. is the organismal-centered hypothesis that focuses on the importance of local adaptations or acclimation of cellular metabolism (b0 in the model of Enquist et al., 2003) and turnover in the presence and relative abundance of species. Below, we revisit the Enquist et al. (2003) model to assess the hypothesis that the approximate invariance of annual ecosystem flux and the increase in the rates of instantaneous flux across sites (with temperature or latitude) results from adaptive changes in organismal metabolism across broad gradients. Focusing more spe- cifically on the autotrophic community alone, we utilize Fig. 1 Data from Baldocchi et al. (2001) (Fig. 9) showing a change in the temperature optimum for CO2 uptake and the mean summer temperature for several sites in the FLUXNET dataset. The positive correlation indicates that the photosyn- thetic temperature response curves of entire ecosystems varies in direct proportion to the mean growing season temperature experienced by that ecosystem. ‘Cold ecosystems’ have lower optimal temperatures for optimum photosynthesis than ‘warm ecosystems.’ The slope of the line is close to 1.0 indicating that the ecosystem response in optimal photosynthesis temperature is closely matches a change in growing season temperature. Fig. 2 Relationship between the annual night-time CO2 flux (average rate per second) and the average annual night-time temperature for several FLUXNET sites. Data from Enquist et al. (2003). Temperature, T, is plotted as inverse temperature as measures in kelvins (K). The differing symbol numbers refer to different sites as originally listed in Enquist et al. (2003). The solid symbols are for European sites and the open symbols are for North American sites Numbers on the upper x-axis are tempera- ture in degrees C. Fig. 3 Increase in the temperature normalized (at 20 1C) instan- taneous nightly ecosystem energy flux (Be) with latitude. Note, temperature normalized data are natural log transformed. The positive correlation indicates that at a given temperature, high latitude sites flux energy and carbon at greater rates than low latitude sites. Sites include all of the site years listed in Enquist et al. (2003). The dataset is dominated by forest ecosystems although there are a few grassland and arid sites included. 594 B . J . E NQU I S T et al. r 2007 The Authors Journal compilation r 2007 Blackwell Publishing Ltd, Global Change Biology, 13, 591–609 a recently modified version of this model (Kerkhoff et al., 2005) to argue that the above paradoxical variation in ecosystem fluxes primarily reflects local adaptations to cooler temperatures and shorter growing seasons that involve how stoichiometric changes influences plant metabolism and the efficiency of biomass production. A general model for scaling organismal metabolisms from cells to ecosystems Enquist et al. (2003) utilized metabolic scaling theory (West et al., 1997; Enquist et al., 1998; Brown et al., 2004) to derive a general equation for how temperature and plant size will influence ecosystem flux. Metabolic scaling theory builds upon the approach advocated by Harte (2002). Specifically, we take ‘a Fermi approach’ (or a zeroth-order model a la West et al., 1997), in that our goal is to construct the
/
本文档为【aa】,请使用软件OFFICE或WPS软件打开。作品中的文字与图均可以修改和编辑, 图片更改请在作品中右键图片并更换,文字修改请直接点击文字进行修改,也可以新增和删除文档中的内容。
[版权声明] 本站所有资料为用户分享产生,若发现您的权利被侵害,请联系客服邮件isharekefu@iask.cn,我们尽快处理。 本作品所展示的图片、画像、字体、音乐的版权可能需版权方额外授权,请谨慎使用。 网站提供的党政主题相关内容(国旗、国徽、党徽..)目的在于配合国家政策宣传,仅限个人学习分享使用,禁止用于任何广告和商用目的。
热门搜索

历史搜索

    清空历史搜索