Moving beyond the economics spectrum?
The study of plant ecological strategy axes has emerged as an important area of research in the past decade. A strategy axis is defined by suites of covarying traits that are studied across species. One of the axes that has received considerable attention is the suite of traits related to photosynthesis and leaf life span, recently dubbed the ‘leaf economics spectrum’ (Wright et al., 2004), with a continuum of variation from species with short-lived, high specific leaf area (SLA) leaves of high nutrient concentrations and photosynthetic rates, to those with the opposite set of trait values at the other end. The basic contrast of these photosynthetic strategies was initially recognized in comparisons of evergreen vs deciduous species (e.g. Mooney & Dunn, 1970). Kikuzawa (1991) and others then introduced the critical distinction between leaf life span (the length of time that an individual leaf persists on a tree) and leaf habit (whether all leaves fall off the canopy at any one time). Reich et al. (1997) were the first to recognize clearly the convergence in leaf function across diverse ecosystems, setting the stage for the global synthesis described by Wright et al. Traits comprising the woody tissue of plants, while long appreciated for their structural and hydraulic importance (Tyree & Ewers, 1991; Niklas, 1992), have not been as successfully placed within a general predictive framework.
The first set of papers in the Virtual Special Issue of New Phytologist on plant ecological strategy axes continue to probe the generality of the physiological mechanisms that generate trade-offs in leaf form and function, and their broader ecological and evolutionary implications. He et al. (2006) addressed a gap in global knowledge, studying several dozen species of the Tibetan plateau. They found that the overall pattern of trait correlations was similar at this climatic extreme, further supporting the generality of the relationships, although photosynthetic rates were slightly lower relative to leaf nitrogen. Wright et al. (2006) assessed patterns in leaf dark respiration, and reported shifts in respiration relative to other traits as a function of irradiance and site temperature. The temperature effects, however, appear to be direct effects on respiration, as there are no detectable differences when respiration is measured at a common temperature. Leishman et al. (2007) examined data on native vs invasive species, and found that invasives are shifted towards the ‘fast’ end of the spectrum, along the shared axis of trait covariation. In an interesting twist, Lusk & Warton (2007) have discovered that the relationship between leaf traits and regeneration strategies in forest plants depend on leaf habit and ontogeny. In deciduous plants, the correlations between leaf mass per area (the inverse of SLA) and shade tolerance shift between seedlings and saplings, and in different light environments, suggesting a more complex set of growth strategies in strongly seasonal environments.
In comparative studies, leaf life span is viewed as a species trait, and metabolic traits are generally reported for newly matured leaves. Within plants, leaf life span varies among leaves and reflects the dynamics of self-shading, nitrogen resorption and leaf carbon balance. Oikawa et al. (2006) examine the role of soil fertility on internal canopy dynamics, and Pornon & Lamaze (2007) consider variation in leaf life span within the canopy, and the relative efficiency of nutrient use among different sets of leaves. These papers suggest an important avenue for future research to integrate more thoroughly our understanding of within-canopy nitrogen allocation and leaf function with the broad patterns of interspecific variation and ecological strategies.
Studies of leaf function in an evolutionary and comparative framework provide an important perspective on our knowledge of ecological strategies. Karst & Lechowicz (2007) examined leaf traits of temperate, understory ferns, and found that they fall within the envelopes reported for seed plants at a global scale, again supporting the generality of the overall relationships. Watanabe et al. (2007) examined leaf nutrient concentrations in a taxonomic framework, and found that about 25% of the variation in various nutrients is associated with plant family. The significance of such patterns in relation to the evolution of leaf function through time remains virtually unexplored.
The structural and functional properties of wood have also received considerable recent attention from plant biologists exploring the possibilities of adaptive trade-offs or patterns of coordinated evolution among the anatomical and physiological traits making up this complex tissue. Are there broadly generalizable strategy axes among woody tissue traits analogous to those of the leaf economics spectrum? Woody growth serves three primary roles in plants: conducting water and nutrients from root to shoot; providing structural support for leaves; and serving as a storage reservoir. The cellular building blocks all plants must work with in growing wood are the hollow conducting elements (xylem vessels and tracheids), thick-walled fibers and parenchyma cells. If the numbers and dimensions of these cells can vary independently, a great variety in wood anatomy is possible, with a correspondingly wide range in stem structural and functional performance. For example, wood composed primarily of large-diameter, thin-walled vessels bears a low carbon construction cost and has high water-conducting capacity, but provides relatively weak structural support and low resistance to drought-induced rupture of the water column (embolism). Reducing vessel diameter and adding fibers and parenchyma all increase wood density and both structural and functional integrity but at the cost of reduced hydraulic conductivity, higher construction costs and lower whole-plant growth rate.
The second group of papers in this Virtual Special Issue all consider aspects of the structural and water-conducting properties of wood and their variation within and across plant communities. Working in the wet tropical forest of Bolivia, van Gelder et al. (2006) examined the mechanical property of wood from 30 species with contrasting juvenile-stage light requirements, finding consistent differences between shade-tolerant species (dense and tough wood) and pioneer species (little structural margin of safety from breakage). In an accompanying commentary, Falster (2006) presents an overview of the engineering principles involved in wood biomechanics and puts variation in wood density in a broader context of variation in other traits of importance to plant performance in the tropical forest understory, such as seed size and leaf economics. Moving to the Mediterranean environment of coastal California and a different set of environmental demands on woody plant vasculature, Preston et al. (2006) analyzed patterns of variation in xylem vessel density and lumen area in 51 species, ranging from vines to trees. Their results, including evidence from phylogenetic independent contrasts, suggested opposing evolutionary trends in these two key determinants of wood density. Within a much narrower phylogenetic range (nine species of Rhamnaceae from California), Pratt et al. (2007) found a positive relationship between the evolution of high stem wood density and resistance to embolism under water stress. Interestingly, there was no apparent trade-off between wood density and hydraulic conductance in these species. Anfodillo et al. (2006), in a survey of 30 species, provide broad support for the model of West et al. (1999), predicting changes in vessel lumen area with tree height that minimize conduit length effects on hydraulic conductance.
How have wood vascular traits evolved in response to changes in water availability? Three final papers presented here address this question, taking advantage of known phylogenies of species growing across distinct precipitation gradients. Among nine Cordia species from the neotropics, Choat et al. (2007) reported greater resistance to embolism in drier site species, but interspecific variation in other wood traits that was independent of mean annual precipitation. A similar result was reported by Bhaskar et al. (2007) among six species pairs of chaparral shrubs from California and Mexico. In Pereskia, a basal group of Cactacea lacking stem succulence, Edwards (2006) found strong support for correlated evolution among several stem and leaf traits of importance in plant water relations but found no evidence for any trait correlations with habitat water availability. These results highlight, yet again, the importance of taking phylogeny into account when making inferences from comparative studies about adaptive trait evolution.
Collectively, these papers affirm the predictive value of the underlying relationships among plant functional traits, and the utility of the leaf economic spectrum as a framework to understand variation among sites and among different groups of species. The substantive progress relating wood anatomical traits to plant function and ecological adaptation suggests that additional strategy axes soon may be uncovered to complement this framework.