The Ratio of Live Crown Length to Sapwood Area as a Measure of Crown Sparseness

Leaf area density is a biologically appealing index of forest tree health because it can provide an assessment of foliage loss or retention; however, it is difficult to measure directly. In contrast, the ratio of crown length to sapwood area (CL:SA) is quite amenable to objective field measurement, and could be interpreted as an index of crown sparseness. CL:SA was computed for Douglas-fir trees from 70 plots covering a range in average size, stand density, site quality, and crown condition in the Oregon Coast Range. When expressed as cm:cm2 , the index ranged from 2 to 100 for individual trees and from 2 to 11 for plot averages based on the 200 largest trees per ha. In general, individual trees with a high CL:SA had a lower relative height within the stand. The index also increased with increasing severity of needle loss on sites where Swiss needle cast (SNC) is causing premature loss of foliage. After correcting for stand density and stand age, plot basal area growth increased significantly with a decrease in plot average CL:SA across a gradient in Swiss needle cast severity. CL:SA has strong potential for discriminating effectively among stands with varying degrees of foliage loss and growth reduction due to other disturbances as well, such as air pollution and defoliating insects. FOR. SCI. 48(1):93–100.