Plant Allocation and the Multiple Limitation Hypothesis

Although optimal foraging theory applied to plant expenditure on resource uptake predicts that plant morphology and physiology should be adjusted to make all resources simultaneously limiting, two considerations make it unlikely that resource enrichment experiments will show strong responses to all nutrient additions. First, growth response to enrichment is strongly dependent on costs and demand, and detectable growth responses are only likely for "expensive" resources in high demand. Second, many resources are acquired simultaneously as a result of allocation to a single structure (e.g., various soil resources obtained by roots). This can cause the growth of an optimally foraging plant to be limited by a single resource. Altering the model to allow for more than one allocation currency (such as nitrogen and carbon) has the effect of making plant composition (and therefore demand) dependent on the supply rates of resources. This assumption can sometimes qualitatively change the above predictions, but the biological relevance of such cases is unclear. We conclude that in addition to supply and demand, the pattern of signs (positive or negative) of net correlations in uptake allocation will control the pattern of multiple limitation. Results of enrichment experiments showing limitation by only a single resource do not necessarily refute the perspective of optimal foraging.