The Cost of Tundra Plant Structures: Evaluation of Concepts and Currencies

Carbon costs of synthesis are quite similar among parts and species of tundra plants and among chaparral shrubs, because all plant parts have substantial concentrations of certain energetically expensive constituents (either lignin or protein and tannin). I found no evidence to support the generalization that evergreen leaves are more expensive to produce than deciduous leaves. In contrast to carbon, initial nitrogen and phosphorus costs of synthesis differ substantially among plant parts and species. These costs are higher in leaves than in stems and tend to decrease from deciduous shrubs and forbs to graminoids to evergreen shrubs to mosses to lichens. Nutrient costs also vary substantially among species within a growth form. Final nitrogen and phosphorus costs are much less than initial costs because of resorption from senescing parts, whereas final carbon costs are not greatly modified by resorption. Carbon and nutrient costs are completely interconvertible, if their exchange ratios are known. However, if exchange ratios are not known, as is usually the case, carbon may in general be a more practical currency than nutrients for estimating the cost of synthesis. Nutrients may be the more useful currency in evaluating the cost of (1) tissue removal by herbivores and pathogens and (2) tissue investment in reproduction, particularly under conditions of strong nutrient limitation. Our present understanding of costs of synthesis rests on inadequate information on the exchange ratios between carbon and nutrients.