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Keywords:

  • biodiversity;
  • ecosystem function;
  • gradient;
  • natural experiment;
  • photosynthesis;
  • stomatal optimization

There is often tension in the ecological sciences between the search for general patterns and the pursuit of the unique and the interesting exception. Both are valuable motivations – and often the unique provides the exception that proves the rule or sparks the search for a more fundamental generality. The field of plant ecophysiology has made some important contributions through searching for, and testing, the general implications of phenomena that are observed at the level of the plant organ, cell and even smaller scales. Identifying the implications of the evolution of distinct photosynthetic pathways has been one such area – and in general we now understand the ecological value and meaning of the derived C4 and Crassulacean acid metabolism (CAM) photosynthesis pathways in comparison with the ancestral C3 pathway in relation to environmental conditions (although there are still several unanswered questions which indicate that we lack a complete general understanding). Robust general ecophysiological patterns are particularly useful at present, in providing material for the development of predictive models of vegetation function at large spatial scales that are needed in the area of global-change research (e.g. Woodward & Lomas, 2004). The development of models of vegetation structure and function, with varying degrees of abstraction, was a major achievement of the global-change ecological community in the latter decade or two of the last century, and this development is ongoing (e.g. Scheiter & Higgins, 2009). It is notable that such models have been able to reproduce credibly the geographical distribution of major vegetation types of the world, and provide valuable estimates of the fluxes of carbon (C) and water vapour from the terrestrial surface, necessary for an ever-improving understanding of the human impact on the global C and hydrological cycles.

‘… the trick in identifying a general pattern is to find a signal of this ‘time-integrated’ stomatal behavior.’

The abstractions and assumptions involved in simulating the function of different major plant functional types are a key aspect of building such models, and thus the identification of general patterns in the way that plant species respond functionally to environmental drivers greatly reduces the complexity necessary for inclusion in these simulations. This is one of the main underlying motivations for the work by Prentice et al. in this issue of New Phytologist (pp. 169–180). Their paper takes a very useful look at an unresolved issue of universal environmental moisture drivers of the control of C uptake across different plant life-forms. Because C and water are two resources that are vital for plant survival and growth, it has been accepted for some time that stomatal regulation optimizes C uptake relative to water loss, and this is reflected in the ‘time-averaged drawdown of CO2 concentration’ during photosynthesis, as the authors explain. Because stomatal behavior is responsive to both environmental and physiological cues over short periods of time, even minutes in some species and under some conditions (e.g. the passage of light flecks across subcanopy leaves; Mitchell & Pearcy, 2000), the trick in identifying a general pattern is to find a signal of this ‘time-integrated’ stomatal behavior. Prentice et al. do this by using the δ13C stable C-isotope ratio, which, as the authors explain effectively, averages the long-term C drawdown of CO2 by plant leaves.

As Prentice et al. point out, there are two competing views on how stomatal function might be controlled along environmental moisture gradients. The idea that seems more appealing from a ‘generalizable’ perspective is that all species will approximate a similar relationship between CO2 drawdown and a moisture index continuum – that is, plants in general will show increasingly more conservative water use with decreasing environmental moisture index. If such a relationship holds, it greatly simplifies the modeling of vegetation photosynthetic and water-use characteristics at large spatial scales. The alternative view is that individual species will show distinct patterns, possibly reflecting specific adaptations to particular sites and to particular conditions. This could take the form of what Prentice et al. term ‘offsets’ in the drawdown–moisture relationship, representing C-uptake and water-use patterns that are more finely tuned across particular sections of the moisture-availability continuum (see Fig. 1 for three possible examples). This would be much harder to cope with in a general model, and would require a consideration of how ‘species’ composition’ might turnover along moisture continua if credible simulations of the finer patterns of C and water relations along these continua are to be made. Dynamic global vegetation models are ill-equipped to deal with this complexity because they almost ignore biodiversity. Prentice et al. discuss evidence for both hypotheses, and also how the latter hypothesis would provide a role for biodiversity turnover (beta-diversity) for achieving functional ‘homeostasis’ along a moisture continuum; in other words suggesting a role for biodiversity in potentially altering vegetation function at large scales. All in all, this is an interesting and important issue to resolve both from a theoretical and a practical application perspective.

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Figure 1.  Possible species-specific deviations from a universal trend relating CO2 drawdown (inferred from δ13C) to moisture index (MI), represented by the dashed line. Species geographic range limits mean that they occupy different parts of the moisture gradient. The lines labeled Sp1, Sp2 and Sp3 represent the δ13C : MI relationship for three species that replace one another along the moisture continuum. In Sp1, which shows a steeper response to MI than the universal trend, the species shows more conservative water use than the general pattern at a lower MI (‘a’ in the diagram), and less conservative water use than the general pattern at drier sites (‘b’ in the diagram). The pattern for Sp2 shows more conservative water use than the universal trend for this species, regardless of MI, and Sp3 shows less conservative water use, regardless of MI.

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Prentice et al. use a ‘natural experiment’ approach, in the form of a long rainfall gradient, to provide the raw material to test the hypothesis. Such an approach is challenging logistically, but has the tremendous advantage of avoiding artifacts associated with more controlled experiments, especially in relation to long-lived plants with extensive rooting systems. This ambitious design is matched by a sampling regime that covers an impressive number of species and sites, and is certainly sufficient to allow a solid analysis of the question. The gradient they chose to work on is in China, thus also exploring a flora that has not traditionally been investigated for this type of work.

Using this impressive data set, the authors show quite conclusively that the ‘generalizable’ hypothesis for CO2 drawdown (‘universal scaling’) is better supported by the data they obtain than the alternative ‘homeostatic role for beta-diversity’. A clear gradient-wide relationship emerges from the data that supports expectations of how moisture index controls CO2 drawdown, but most importantly, there is relatively little species-specific and growth-form-specific deviation from this form, even in species that replace one another along the gradient. While this may be rather disappointing to those who seek a wide-ranging role for biodiversity in underpinning all aspects of ecosystem function, it will be encouraging for the dynamic vegetation modelers, in the sense that there is a justifiable reason for not worrying too much about species turnover in simulating this aspect, at least. It will be interesting to see how this conclusion holds up as the results from other transect studies emerge, and also how some of the ancillary issues discussed by Prentice et al. are further investigated.

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