Production Efficiency in Subalpine Conifers: The Influence of Crown Size and Canopy Structure
Leaf area efficiency (E) was examined in 49 destructively sampled Abies lasiocarpa (Hook.) Nutt from several stands. Efficiency was found to decline with increasing crown leaf area (LA). This decrease was associated with a decrease in foliage ratio, that is, the proportion of assimilating tissue in the crown. This ratio has been suggested as an indirect measure of a tree's respiratory demand per unit of LA. A lower ratio implies that a smaller percentage of carbon fixed by the crown is available for stemwood growth, thus reducing E.
Efficiency declined more rapidly when only LA on branches assumed to be exporting carbohydrates to the stem (LA,) was considered; even though the percent LA. of crowns increased as total LA increased. The greater proportion of LA. on trees with high LA was attributed to greater crown length, which on large trees results in a greater proportion of LA exposed to a favorable light environment. However, the continued decrease in foliage ratio with increasing crown size appears to more than offset the increase in percent LAX, causing the decrease in E with increasing LA.
Efficiency of a tree is also affected by its position within the canopy. In A. lasiocarpa, greater LA is strongly associated with increased height in relatively small trees. I examined efficiency of trees relative to other trees in the same stand and found E to increase with greater LA in small trees. Greater E with increasing LA was attributed to the improved canopy position associated with increased height. In larger trees, greater LA was no longer associated with substantial increases in height and E declined as LA increased, presumably because of the decreasing foliage ratio.
The pattern of change in E with increasing LA of A. lasiocarpa was compared with that of Pinus contorta var. latifolia Engelm. Differences between the two species were attributed to differences in canopy structure. Greater canopy stratification in A. lasiocarpa, combined with greater canopy coverage and leaf area index, suggests that the light environment of relatively small trees is strongly affected by canopy position and, therefore, height. In A. lasiocarpa, then, E increases with LA as long as there are associated increases in height. In P. contorta, limited canopy stratification results in few trees being located in a poor light environment. Increased LA is not associated with improved canopy position and, therefore, E decreases with increasing LA, even in relatively small trees, due to the decreasing foliage ratio. A general conceptual model is proposed to explain production-leaf area relations in forest-grown trees, based on changes in relative tree size and differences in canopy structure.
Roberts, S.D. (1991). Production efficiency in subalpine conifers: the influence of crown size and canopy structure. Ph.D. 76p.