DEVELOPMENTAL PLASTICITY ALLOWSBETULA NANATO DOMINATE TUNDRA SUBJECTED TO AN ALTERED ENVIRONMENT

Abstract

We investigated how three co-dominant arctic shrubs (Betula nana, Salix pulchra, and Ledum palustre ssp. decumbens) responded to long-term treatment with N+P fertilizers and greenhouses in a factorial field experiment at Toolik Lake, Alaska. Our goal was to understand the relationship between growth of individuals and species abundance in the community, and the mechanism by which one species achieves dominance under changed environmental conditions. We compared aboveground growth and allocation patterns in individual ramets 15 yr of age with community abundance measured by quadrat harvests. Ramets of all three species substantially increased their stem biomass with fertilization, but the increase was much larger for Betula than for the other two species. In quadrat sampling, only Betula appreciably increased its biomass per unit area with fertilization or greenhouse treatment. For Salix in all treatments, and Ledum in the two fertilizer treatments, ramet density per unit area decreased more than growth of surviving 15-yr-old ramets was promoted, so community biomass of these species declined. In contrast, Betula increased its ramet density in all treatments by producing new shoots from stems older than 15 yr, even though stem mortality was also increased in the two fertilizer treatments. Betula increased its growth in part by a major change in allocation, from producing mostly short shoots to producing many more long shoots. As a result, the number of branches and the rate of production of new meristems greatly increased. This developmental plasticity allowed extensive growth that led to development of a dense canopy and imposed light limitation on the other species. The flexible growth strategy of Betula points to the importance of meristem availability and developmental constraints in determining plant response to environmental change. Developmental controls over meristem availability are not usually considered in ecological paradigms for allocation, but they may be useful for predicting plant response to changes in nutrient availability in other ecosystems.