Dependence of Forest Structure and Dynamics on Substrate Age and Ecosystem Development

We quantified rates, sizes, and spatial properties of prevailing disturbance regimes in five tropical rain forest landscapes on a substrate-age gradient in Hawaii. By integrating measurements from airborne LiDAR with field studies and statistical modeling, we show that the structure and dynamics of these forests respond to processes that change during the development of ecosystems. On young substrates of 0.3 ky where forests are in primary succession and are limited by N, mean canopy height was 13 m and height decreases more than 1 m occurred in small, isolated events (power-law exponent = 1.69 ± 0.02, n = 61 gaps ha^-1). The proportion of the landscape affected by disturbance increased on high-fertility intermediate-aged substrates of 5-65 ky and canopies were heterogeneous. Frequencies of height decreases more than 1 m were n = 14, 18, and 30 gaps ha^-1 corresponding to power-law exponents of 2.188 ± 0.02, 2.220 ± 0.03, and 1.982 ± 0.02 on substrates of 5, 20, and 65 ky. There was a substantial difference between forests on a 150 ky substrate and sites of 5-65 ky; trees on the older substrate formed patchworks of stunted cloud-forest and stands of taller-stature trees. The frequency of recent disturbance events more than 1 m was n = 48 gaps ha^-1, corresponding to a power-law exponent of 1.638 ± 0.01. Across the substrate-age gradient, the proportion of each landscape that decreased in height by more than 1 m was 0.16, 0.40, 0.41, 0.36, and 0.17, respectively. These findings demonstrate that substrate age and processes associated with ecosystem development can mediate the rates, sizes, and spatial characteristics of disturbance regimes on forested landscapes, and point toward the necessity of large-area samples to obtain robust estimates of natural dynamics.