Adaptability and functional stability in forest ecosystems: a hierarchical conceptual framework

In this paper we present a hierarchical conceptual framework to study adaptability and functional stability of aggregate functions across scales in forest ecosystems in the face of environmental variability with special reference to temperate trees. Natural ecosystems display a range of adaptive responses to environmental changes, including differential gene activity and phenotypic plasticity (individual-level mechanisms), differential reproduction of genotypes (populationlevel mechanisms), and changes in species abundances (communitylevel mechanisms). We examine which of these responses are typically observed in temperate forests under changing environmental conditions. Following hierarchical adaptability theory, we regard adaptation as a never-ending multilevel hierarchical process of individual-, population- and community-level adjustments to a constantly changing environment. Adaptive adjustments at all organizational levels tend to stabilize aggregate ecosystem properties such as primary production and nutrient cycling. We propose that approximately the same plasticity limits hold for long-lived forest trees and the community of short-lived soil microorganisms. The rate of adaptive rearrangements, however, differs greatly between organisms: we suggest that a year for microorganisms is equivalent to a millennium for trees. Therefore, the forest tree communities are expected to adjust to multi-year climate oscillations as easily as microorganisms to seasonal variability. Such adjustments are made possible by increased expression or proliferation of pre-adapted genes, genotypes, and species.