Potential and limitations of current concepts regarding the response of clonal plants to environmental heterogeneity

Plant ecologists have spent considerable effort investigating the physiological mechanisms and ecological consequences of clonal growth in plants. One line of research is concerned with the response of clonal plants to environmental heterogeneity. Several concepts and hypotheses have been formulated so far, suggesting that intra-clonal resource translocation, morphological plasticity on different organizational levels (e.g. leaves, ramets, fragments), and other features of clonal plants may represent potentially adaptive traits enabling stoloniferous and rhizomatous species to cope better with habitat patchiness. Although each of these concepts contributes substantially to our understanding of the ecology of clonal species, it is difficult to combine them into a consistent theoretical framework. This apparent lack of conceptual coherence seems partly be caused by an uncritical use of the term habitat heterogeneity. Researchers have not always acknowledged the fact that heterogeneity may refer to a number of fundamentally different aspects of environmental variability (i.e. scale, contrast, predictability, temporal vs. spatial heterogeneity), and that each of these aspects may, on one hand, allow for the evolution of specific plant responses to heterogeneity and, on the other, severely constrain the viability of potentially adaptive traits. Since adaptive responses are operational only in a narrow range of conditions (delimited by external environmental conditions and constraints internal to plants) it seems imperative to clearly define the context and the limits within which concepts regarding clonal plants' responses to heterogeneity are valid. In this paper an attempt is made to review a number of these concepts and to try and identify the necessary conditions for them to be operational. Special attention is paid (1) to different aspects of environmental heterogeneity and how they may affect clonal plants, and (2) to possible constraints (e.g. sectoriality, perception of environmental signals, morphological plasticity) on plant responses to patchiness.