Response of Leaf Area Index to Density for Two Contrasting Tree Species

It is commonly assumed that mature forest stands with closed canopies support constant amounts (weight or area) of foliage, independent of stand density. For stand leaf area to be constant, mean leaf area must be plastic with respect to density. We examined the relationship between density and both leaf area index and mean leaf area for two contrasting tree species, lodgepole pine (Pinuscontorta var. latifolia Engelm.) and subalpine fir (Abieslasiocarpa (Hook.) Nutt.). In lodgepole pine, leaf area index tended to be constant over a wide range of absolute and relative densities, but in subalpine fir, leaf area index increased with density. Consistent with these results, mean leaf area of lodgepole pine was more plastic with respect to density than mean leaf area of subalpine fir. The presumption of stable leaf area index independent of stand density, therefore, may not be as general as usually assumed owing to differential responses of mean leaf area to density. Differences in plasticity between the two species were attributed to differences in relative shade tolerance and the effect of shade on competitive interactions at high densities