ARCHITECTURE OF A COOL-TEMPERATE RAIN FOREST CANOPY

Abstract

There have been few descriptions of tree crown asymmetry and its development. Here I present the most comprehensive geometric description to date of crown asymmetry within a forest canopy and use allometric, trigonometric, and growth analyses to tentatively identify the dominant cause. A 25.6 × 25.6 m quadrat within a pure stand of Atherosperma moschatum Labillardière (Monimiaceae) in Tasmania was mapped with respect to the locations of tree trunks and crowns. Trunk girths and tree heights were also measured. Trunk girths were again measured ten years later. Standing biomass, growth rates, and structural properties of the trees were estimated from models. Crowns were more evenly spaced than trunks and showed a significant alignment by angle, both characteristics consistent with a tendency toward the optimal packing of the canopy. The displacement of crown locations from trunk locations was correlated both with the nearness of neighbors and with the east-facing aspect. For the quadrat as a whole, there was a projected tilt to the trees of 0.73 m 76.3° east of north. The displacement of crown locations from trunk locations was largely achieved early in tree development, consistent with a phototropic response to acute photosynthetically active radiation (PAR) limitation to growth in the understory, evident in the lower relative (projected canopy-area-specific) growth rates of the smaller trees, and with the propensity of A. moschatum to reproduce by vegetative sprouting. There was little evidence of size-dependent variation in the mechanical stresses within the trunks, but more detailed analysis of wood structure is needed. The overall height safety factor (a measure of buckling resistance) for the trees in the quadrat was 4.2, the highest reported factor to date for any tree species. Tasmania is an island in the “roaring forties,” and the high value of the safety factor is a likely response to strong and persistent winds. The estimates of the maximum growth rates of individual trees were 14.9 and 8.7 kg/yr for each of two models. The estimates of the growth rate for the whole stand were 12.1 and 7 m3·ha−1·yr−1 (8.5 and 4.9 metric tons per hectare per year, respectively).