Plant energetics and population density”

Abstract

Enquist et al.¹ present an interesting analysis of the link between plant size, allometry and mortality. But I believe that their claim that it has a functional basis is misleading when it is scaled up to whole populations.

Main

Transpiration in plants is strongly driven by environmental conditions². It is therefore difficult to compare the transpiration rate per plant among different species unless these plants are exposed to the same environment. This does not apply here, as Enquist et al. derived their transpiration rates from a re-analysis of data from the literature. The confounding effect is negligible at the scale of individual plants, given the wide range of rates and dimensions reviewed. But when scaling up to the whole canopy, environmental driving variables such as radiation, water availability and site fertility predominate and tend to obscure any effects of plant size on the function of stands of species. This is clear from a comparison of maximum conductances and assimilation rates among biomes across the world³.

The transpiration rates that have been scaled up to the level of populations by Enquist et al.¹ (see their Fig. 4) seem surprisingly high: rates of about 100 l m^-2 per day (that is, millimetres per day) far exceed the maximum values of 3-12 mm per day derived from a global comparison of plant canopies all over the world⁴. Another meta-analysis of evapotranspiration in coniferous forests and grasslands gave maximum values of 6-7 mm per day (ref. 5).

What is most important, however, is that the conclusions drawn by Enquist et al. from their upscaling to whole populations are misleading. It is debatable whether “total energy use or productivity of plants in ecosystems is⃛ invariant with respect to body size”: stand chronosequences in forest tree species⁶ indicate that, after canopy closure at the polestage, the leaf-area index tends to decline (as a result of self-thinning, among other processes). In mature canopies, this has a marginal effect on the interception of radiant energy and gross primary production⁷. Net assimilation and above-ground allocation, however, are further reduced by increasing respiratory costs, nutrient immobilization in soil litters and hydraulic constraints^7,10, which are all a direct result of increasing body size, contributing to the well-known decline in forest growth with tree dimensions and age^6,8.

Even the conservative nature of forest evapotranspiration on the regional scale¹¹ seems to be true more at the community than at the population level, resulting from interaction among overstorey and understorey processes³. On the contrary, there are considerable changes in transpiration with stand development¹². The results of Enquist et al. do not seem to alter this picture.

Reply See Also - Dewar