The need for a strict delimitation of early tree life stages in vegetation ecology

Abstract

AbstractAimFor practical and theoretical purposes, ecological studies commonly classify trees into five major life‐cycle stages: seed, seedling, sapling, juvenile and adult. Whereas the seed and adult stages are usually accurately delimited across studies, there are discrepancies and ambiguity in the categorization of seedlings, saplings and juveniles, which can significantly affect the conclusions of community ecology studies. Here we propose a standardized set of criteria intended for community‐level research for delimiting these three stages based on biological and ecological rationales.MethodsWe assessed the relevance of such standardization by conducting a meta‐analysis of the effects of two human‐caused disturbances (defaunation and logging) on each early tree life stage and examining differences in effect sizes and confidence intervals among: (1) studies that match our delimitation criteria, (2) studies that do not match these criteria, and (3) all studies grouped together regardless of the criteria they used.ResultsWe found stronger effects with narrower confidence intervals when considering only the studies that matched our standardized delimitation criteria. In fact, the proportion of significant effects was between 1.7 (defaunation) and 5.4 (logging) times higher in studies matching our delimitation criteria than in studies that do not match them, probably because confidence intervals were 2.3–3.1 times smaller in the former group than in the latter. For logging studies, the direction of the effects changed in 30%–50% of the cases when comparing the results from all data and studies not matching our criteria with the results of the studies matching our criteria, always from non‐significant to significant effects.ConclusionsThese findings underscore the need for an ecologically meaningful categorization of early tree life stages based on standardized measures to increase the confidence, accuracy, reproducibility and generalization in plant biology and community ecological research. Synthesis efforts will particularly benefit from this standardized protocol.