Canopy Gap Dynamics

Introduction

Although canopy structure is often thought of in terms of a positive, space filling model, it can be useful to use a common artist's approach of modelling the negatives, the "empty" areas not occupied by plant matter. The term gap or canopy gap is generally used to refer to such empty areas within forest canopies. Spatial heterogeneity of canopy structure means that gaps of widely varying shape and size exist throughout a forest stand.

Gaps range from subcanopy spaces between individual trees or branches (Connell et al. 1997) to the more classical complete gaps, consisting of holes in the canopy that extend all the way to ground level (Brokaw 1982). The latter are relatively scarce (Table 1), but are probably the most studied. The distribution of gap types appears to be similar in temperate and tropical forests (Bongers 2001).

Table 1. Patterns of vertical vegetation structure. Four different gap types are recognized and the total percentage of each type is given for forests in different areas. Note that one vegetation pattern (empty-closed-empty) is present in both overstory and understory gap types. From Bongers (2001).

In the 1950s researchers first began describing canopy gaps as incidental side effects of treefalls, viewing them as areas of local disturbance with little relevance to overall forest function. Gap formation is now generally treated as an integral part of forest climax communities, and an important source of environmental heterogeneity.

The ecological characteristics of a gap are very different from those of the surrounding forest. Gaps are brighter and warmer due to increased irradiance; their surface soils contain more water, presumably due to the reduction in transpiration by plants (Denslow 1987). Treefall gaps may exhibit substantial soil composition changes, as deep soil attached to tree roots is brought to the surface (Denslow 1987).

Gap Dynamics and Canopy Structure

One of the most dramatic and obvious differences between closed forests and gaps, particularly ground-level treefall gaps, is in their vegetation. The treefall itself is a source of direct mortality to understory plants. In addition, the new higher-light environment creates a habitat which favors fast-growing, high-light species over the slow-growing shade tolerant plants typical of the forest understory. Habitat change is more extreme in large openings, such as those caused by the blowdown of several trees in a windstorm; the toppling of a single small tree causes relatively little change to the original understory environment.

Many tree species depend on gaps for seed germination and seedling growth, and gap dynamics are therefore closely linked to the "next generation" of trees. This creates a potentially complex interdependence between two major ecological features of forests: Canopy composition and structure affects gap size, shape, and frequency, while gap characteristics affect seedling establishment and thus the future canopy composition.