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Browsing by herbivores profoundly affects the nature and extent of woody plant communities through a variety of mechanisms, including alterations to disturbance regimes (such as fire frequency), indirect effects on nutrient cycling and soil fertility, and changes to net primary production (Hobbs 1996). Plants have evolved myriad ways to avoid being eaten, ranging from the development of spiny, unpalatable foliage to the production of secondary metabolites as defense compounds (Bennett and Wallsgrove 1994). In analyzing the responses of plants to herbivory and the consequences of these responses for plant–plant competition, there has often been an implicit assumption that such defense strategies come at a cost to the individual (more defense means less growth or reproduction), thereby providing the basis for fundamental ecological and evolutionary hypotheses about allocation of limited resources (Strauss et al. 2002). Several hypotheses have been used as a framework for investigating the patterns and trade-offs of plant defense against herbivores (Stamp 2003), especially in the ecological literature. These hypotheses have been used singularly, or even in combination, to try and produce a unifying theory of plant– herbivore ecology and evolution. All of these hypotheses have been criticized in the literature. Although there are many instances when hypotheses have predicted empirical observations correctly, there are also many instances when either they have not, or they have been misinterpreted (Stamp 2003). One of these hypotheses, the carbon/nutrient balance (CNB) hypothesis, was originally developed to explain the influence of soil nutrients and shade on plant defense chemistry (Bryant et al. 1983). The CNB hypothesis suggests that species adapted to fertile sites (such as early-successional species) will respond to herbivory by utilizing stored resources for compensatory growth, while slower-growing species adapted to relatively infertile soils will instead protect their leaves by investing more carbon in anti-herbivory defense compounds. Such a hypothesis is appealing, as it makes specific predictions about patterns of allocation to plant secondary metabolite production, which have often been confirmed by empirical observation (reviewed by Stamp 2003). However, the CNB hypothesis has also been criticized, as there are many instances when predictions do not match observations (e.g., Hamilton et al. 2001, Koricheva 2002). In this issue, Calder et al. (2011) report that conifer expansion reduces the competitive ability of Populus tremuloides (trembling aspen). The transition from aspen to conifer forests in western North America often coincides with a transition to soils with lower pH and lower nitrogen concentrations (Jerabkova et al. 2006), at least partly mediated by the chemical composition of conifer needles being added to the litter. The loss of early-successional species, such as aspen, in these systems over time has been attributed to their physiological preference for nitrogen uptake as NO3, which may become less available in soils as conifer dominance increases (Kronzucker et al. 1997, Min et al. 1998, Kronzucker et al. 2003). While changes in nutrient availability may partly explain this successional trajectory, the canopies of evergreen conifers and deciduous broad-leaf trees also provide very different light environments for seedlings. Calder et al. (2011) show that by reducing light availability and altering soil characteristics, a conifer species (Abies lasiocarpa, subalpine fir) inhibited the growth of competing aspen trees. However, these abiotic changes induced by conifer dominance had another effect: they led to lower concentrations of phenolic glycosides (PGs) and condensed tannins (CTs) in aspen leaves, two groups of Commentary

Tree competition and defense against herbivores: currency matters when counting the cost