BIOMASS ALLOCATION IN PLANTS: ONTOGENY OR OPTIMALITY? A TEST ALONG THREE RESOURCE GRADIENTS

We examined biomass allocation patterns throughout the entire vegetative growth phase for three species of annual plants along three separate gradients of resource availability to determine whether observed patterns of allocational plasticity are consistent with optimal partitioning theory. Individuals of the annual plant species Abutilon theophrasti, Chenopodium album, and Polygonum pensylvanicum were grown from locally field-gathered seed in controlled greenhouse conditions across gradients of light, nutrients, and water. Frequent harvests were used to determine the growth and allocation (root vs. shoot, and leaf area vs. biomass) responses of these plants over a 57-d period. Growth analysis revealed that each species displayed significant plasticity in growth rates and substantial amounts of ontogenetic drift in root:shoot biomass ratios and ratios of leaf area to biomass across each of the three resource gradients. Ontogenetically controlled comparisons of root:shoot and leaf area ratios across light and nutrient gradients were generally consistent with predictions based on optimal partitioning theory; allocation to roots decreased and leaf area increased under low light and high nutrient conditions. These trends were confirmed, though were less dramatic, in allometric plots of biomass allocation throughout ontogeny. These species did not alter biomass allocation (beyond ontogenetic drift) in response to the broadly varying water regimes. Furthermore, many of the observed differences in biomass allocation were limited to a given time during growth and development. We conclude that, for these rapidly growing annual species, plasticity in biomass allocation patterns is only partially consistent with optimal partitioning theory, and that these plastic responses are ontogenetically constrained. Further, while these species did adjust biomass allocation patterns in response to light and nutrient availability, they did not adjust biomass allocation in response to water availability, despite dramatic plasticity in growth rates along all three resource gradients. Our results support a developmentally explicit model of plasticity in biomass allocation in response to limiting resources.