Canopy Structure and Environment

Precipitation

At the whole-plant level, water inputs to forests include soil moisture, ground water and below-ground aquifers in addition to precipitation. For the purpose of studying canopy, however, we can focus on precipitation.

The canopy can be thought of as having its own water cycle. Precipitation may be intercepted and returned to the atmosphere via evaporation, or channeled downwards via throughfall or stemflow (Figure 1). The relative contributions of these three processes are largely determined by local vegetation characteristics including leaf shape and texture, canopy height and closure, stem branching, and bark texture (Parker 1995). Precipitation capture and processing by forests help determine water distribution and retention at an ecosystem level, and natural or anthropogenic disturbances such as storms or deforestation therefore have major impacts on regional water budgets.

Figure 1. Routes of precipitation upon entering a canopy. (A) Interception by canopy leaves. (B) Throughfall. (C) Stemflow.

Canopy precipitation studies tend to focus on the interception term. Expressed as a formula,

I = P - TF - SF

where I = interception, P = precipitation, TF = throughfall, SF = stemflow (Crockford and Richardson 2000). Interception is influenced by climatic factors as well as canopy structure (Table 1). For example, interception is relatively high for long, low-intensity showers. In short, intense thunderstorms, a greater fraction of precipitation ends up as throughfall or stemflow.

Table 1. Effects of climate and canopy structure on the interception of precipitation by canopies. (+) indicates a positive relationship and (-) a negative relationship between each variable and the fraction of precipitation that is intercepted by canopy leaves. Adapted from Crockford and Richardson (2000).