Applications of structural equation modeling (SEM) in forestry research

Forest ecosystem dynamics are driven by a complex array of simultaneous cause-and-efect relationships. In recent years, the functional traits of plants their morphological, anatomical, and physiological characteristics have been broadly used in research on forest ecology to understand how vegetation properties and function change along key ecological gradients. Presently, several comparative studies have dealt with the relationships among traits by using simple bivariate approaches, while few of them scaled up to multivariate ones, which are capable of accounting for the complexity of the relationships among variables. Indeed, many changes in tree structure and function occurring throughout ontogeny likely involve trade-ofs which are dificult to discern. Therefore, although multiple regression is sometimes used to examine interacting traits, understanding complex multivariate data requires specialized powerful techniques such as Structural Equation Modeling (SEM). The SEM approach may be particularly useful at modeling complex and more realistic functional relationships in trees. Its strength consists in the ability to resolve the uncertainties in regression models by taking into account the nature and magnitude of direct and indirect efects of multiple interacting variables. Here, we discuss the SEM model applications on forest ecological studies, and we report a case study of SEM application to investiga - te the mechanisms influencing the apical dominance ratio (ADR), recently reported as suitable indicator of the growth conditions favoring silver fir (Abies alba Mill.) natural regeneration in Mediterranean areas. The core objective of the application study was to provide a conceptual framework to understand the mechanisms linking a set of morphological and physiological traits to ADR, and to quantify the system of interrelationships explaining this linkage. Our SEM model demonstrated that ADR is influenced by the synergistic efects of several traits linked to whole-tree architecture and physiology not apparent in univariate relationships. We argue that ADR can express the short-term physiological adaptive responses of silver fir saplings under Mediterranean climatic constraints.