Scaling of a tree: How Picea sitchensis development, neighborhood forest density, and forest structure influence canopy complexity in Olympic rainforests

Abstract

Picea sitchensis is dominant tree in coastal forest ecosystems whose large appendages provide canopy habitat for epiphytes, mammals, and birds. Growth and development of P. sitchensis is described across three scales—tree, neighborhood, and landscape—to illustrate ways this species can accelerate old-growth structure in young forests. We climbed, cored, and three-dimensionally mapped 60 P. sitchensis from 14 to 495-cm diameter, 10 to 94-m tall, and 18 to 390 yr age and developed predictive equations of aboveground biomass and leaf area with > 90 % accuracy. Both Pseudotsuga menziesii and Sequoia sempervirens are radically out-grown by P. sitchensis, which becomes heavier (155 ± 9 Mg) than any living P. menziesii and almost half as heavy as any living S. sempervirens in < 500 yr. Measurements of 30-m plots around 36 P. sitchensis trees were used to quantify neighborhood-scale competition on habitat-related appendage development. Appendage diameters and crown volume decrease ~20 % per 25 % increase in neighborhood density, and dense neighborhoods retard development > 15-cm-daimeter appendages by > 100 yr relative to unencumbered trees. Compared to P. menziesii, P. sitchensis accumulates crown complexity twice as fast, yet lives only half as long. At landscape-scale, P. sitchensis-dominated valleys were contrasted with upland P. menziesii-dominated forests based on LiDAR of the Hoh River watershed on the Olympic Peninsula. Valley bottom forests differ from upland forests by consisting of small patches of tall trees (11 %), dense shorter trees (19 %), and gaps (7.4 %) dispersed in a matrix of open forest with scattered large trees (63 %). In contrast, upland forests consist of larger patches of tall (13 %), dense (58 %), and open forest (27 %) with fewer gaps (2.1 %). Valley-associated disturbances increase the amount of open-canopy forest, while upland processes increase closed-canopy tall forests, dense shorter forests, and aggregation of forest patches. The largest trees (top 10 %) are twice as densely distributed in valley-bottom (0.05 tree ha-1) compared to upland (0.02 tree ha-1) forests. Arrangement of tall P. sitchensis within a matrix of more open conditions in valley-bottoms versus large blocks of tall P. menziesii in upland forests accounts for this major landscape-scale difference in canopy structure. The integrated management implications relevant to P. sitchensis are: (1) P. sitchensis is valuable for management to accelerate old-growth structure because of its rapid growth but should be included with longer lived species, (2) competition within 30-m strongly depresses large appendage growth so some trees should be left with ≥ 30-m growing space, and (3) forest treatments emulate a matrix of open-canopy forest with of scattered tall trees interspersed with pockets of dense tall trees to create optimal conditions for growing the largest and most complex individuals.