Forest canopy change can be caused by large-scale disturbance such as insect infestation, fire and forest harvesting. These changes alter the vegetation cover and ecological integrity of watersheds, and have serious impacts on environmental values, as forested vegetation is critical for terrain stability, riparian areas, water quality and quantity. Changes in forest structure following disturbance are poorly understood and quantified. Each type of disturbance leaves different canopy complexity; for example, forest structure following infestation is markedly different than that remaining after harvesting, as the remaining canopy structure is a complex mixture of standing and fallen dead and live trees.
Many hydrologic processes are affected by these changes in forest structure. These include precipitation interception and ground shading. Interception describes the process of temporal storage of precipitation in the tree canopy, from where it either evaporates/sublimates or falls to the ground. Since interception is an important component of the water balance, deteriorating stand structure and timber harvesting are expected to increase annual and storm runoff. In watersheds with strong snowmelt regimes, the seasonal effects of large scale canopy disturbance are enhanced by the influence of canopies on wind- and radiation-related ablation losses. The loss of canopy cover and changes in structure also alter the energy balance during snow accumulation and melt, thus affecting the snow melt runoff regime. Initial results from research in this area indicates that while changes in stand structure are a major determinant of ground snow accumulation and melt rates in healthy and disturbed stands, these complex canopy responses have yet to be adequately quantified as noted in proposals by Boon and Teti.
The concept of equivalent clear-cut area (ECA) is frequently used as a measure of cumulative disturbance in a watershed. However, the ECA method has not yet been adapted for areas affected by large-scale canopy disturbance. Several stand level studies are underway (see project linkages) that compare stands with different canopy structure; however, none of these studies provides enough information to determine new guidelines for ECA calculation over larger areas. Preliminary results suggest that the percentage of disturbed trees, dead tree clustering, terrain attributes, tree size and density are the greatest factors in altering interception and snow melt, and hence the calculation of ECA in regions with large-scale forest disturbance.
A number of detailed stand-level models have been developed which attempt to quantify snow or radiation interception expected over a range of forest canopies or canopy clumping (e.g. Hedstrom and Pomeroy 1998, Chen et al, 1993, Spittlehouse et al. 2004). However, model parameterization is a major issue and is not feasible over large areas. These models also have focused on undisturbed forest stands, hence limiting our ability to predict these processes in a partly or completely disturbed forest stand. In addition, as interception and snow melt studies have historically been undertaken only in small plots, extrapolation to larger areas remains a limiting factor. As a result, the forest industry and forest hydrologists have neither the empirical data, nor the modeling output, to provide a detailed understanding of the direct linkages between canopy structure and interception/shading with respect to large-scale forest disturbance.
Snow interception strongly depends on canopy density (the ratio of canopy-covered area per unit area of ground), which is related to LAI, but also on the larger topographical settings (e.g. aspect, slope), tree species or patchiness of the forest (Pomeroy et al, 2002). Simple two-stream canopy radiation models have been successfully used to predict direct and diffuse radiation below a relative homogenous forest canopy (e.g. Chen et al. 1993), however, in sparse or dis ...
Weiler, Markus, Coops, Nicholas C.; Teti, Patrick; Boon, Sarah; Varhola, Andres; Bater, Christopher W.. 2009. Equivalent clear cut area thresholds in large-scale disturbed forests. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report. FIA2009MR437
Topic: FLNRORD Research Program
Keywords: Forest, Investment, Account, (FIA), British, Columbia
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