Forest Canopy Stratification-Is It Useful?

Abstract

It has long been recognized that the forest canopy has a complex structure that is significant for environmental interactions, regeneration, growth, and biotic habitat. Not only is the structure variously complex, but also there are many ways to conceptualize that complexity. Yet the persistent theme when considering the structure of canopies continues to be that of stratification: whether structural units are arranged in layers above the ground. We examined the use of the terms "stratification," "layering," and others in connection with canopy structure and found they had various meanings (often only implied) that were difficult to reconcile and to measure. We applied the definitions to the structure of a single, well-studied canopy located in Virginia, U.S.A., and found they failed to define consistently and clearly the presence, number, or location of strata. Additionally, we found the concept had limitations related to scale dependence, point of reference, and spatial averaging. Thus, asserting that a forest is stratified or naming the number of layers generally provided no guide to its structure. We propose alternative ways of conceptualizing and studying the forest canopy that avoid most of the problems associated with stratification. Among these are direct measurement and mapping of structural and environmental variables that have clear potential connections with canopy functions and viewing the distribution of structures or environmental conditions within the canopy as ecological gradients.

Forest and woodland stand structural complexity: Its definition and measurement

“…These definitions failed to consistently and clearly identify the presence, number or location of strata. Parker and Brown (2000) attributed this to the presence of trees of different shade tolerance, which formed a continuous canopy from ground layer to canopy top. They suggested that it would be more meaningful to define strata in terms of the ecological gradients created by foliage (e.g.…”

“…When calculating FHD, different studies have assessed vegetation density in different ways (Parker and Brown, 2000;Erdelen, 1984), and unlike MacArthur and MacArthur's original study in which class intervals related to distinct life forms, the delineation of height classes is often arbitrary (e.g. Sullivan et al, 2001;Tanabe et al, 2001).…”

“…Parker and Brown (2000) reviewed the concept of ''canopy stratification'' in the ecological literature, and demonstrated that the term had a variety of meanings that were both difficult to measure and to reconcile. As a practical demonstration of this ambiguity, they applied a number of definitions of stratification to a mixed-oak forest canopy in south-western Virginia, USA.…”

Forest canopy studies as an emerging field of science

“…A growing literature and textbooks now document our expanding knowledge of the composition, structure, and function of canopy biota, and their responses to changes in environmental conditions (Lowman and Rinker 2004;Lowman and Moffett 1993;Lowman and Nadkarni 1995;Mulkey et al 1996), including general review papers (Lowman 2009;Parker and Brown 2000) and reviews of specific topics, e.g., epiphytes (Benzing 1990), canopy insects (Basset et al 2003), and tree physiology (Ryan 2002). Canopy symposia are convened at scientific meetings, and international canopy conferences are held every 4 years.…”

Vertical stratification and effects of crown damage on maximum tree height in mixed conifer–broadleaf forests of Yakushima Island, southern Japan

“…Several factors are involved in the development of vertical stratification in forests, such as differences among species in the timing of establishment after initial disturbance, height-growth rate, inherent maximum height, and vulnerability to crown damage, which may offset height growth (Palik and Pregitzer 1991;Ishii et al 2000b;Parker and Brown 2000). Yakushima Island is in an area often hit by typhoons, and many canopy trees show evidence of crown damage (Bellingham et al 1995).…”