A crown model for simulating long-term stand and gap dynamics in northern hardwood forests

Abstract

Evaluation of ecosystem management alternatives in forests will require new or revised models capable of simulating the development of multi-aged stands managed on long rotations and with unconventional stand structures. In this paper we describe the development and testing of an individual tree, crown-based model (CANOPY) designed to simulate canopy gap dynamics and stand structural changes in mature and old-growth northern hardwood forests dominated by Acer saccharum. The model was calibrated with data from 63 plots in managed and unmanaged stands, with sample trees ranging in age from 17–311 years. For A. saccharum, R 2 values ranged from 0.76 to 0.81 for prediction of basal area increment, 0.45–0.50 for crown radial increment, and 0.63–0.79 for height increment. Equations using field-measured crown size and crown competition variables provided only slightly better predictions than conventional mensurational variables such as DBH and plot stocking level. Use of the model for long-term (250 year) projections of cumulative DBH and crown radius provided a good match to largely independent size-age trends in the data base, but total height appeared to be moderately overestimated for trees >100 years old. Both the field data and simulations suggest that basal area increment of individual trees does not decline with age in these species but actually increases up to the observed maximum of 300 years, which is consistent with nearly linear diameter-age trends often reported for shade-tolerant species. Lateral crown growth of trees with exposed crown margins, however, showed curvilinear decreasing trends with age. These findings suggest that the common assumption of constant radial closure of canopy gaps by border trees could potentially overestimate rates of gap closure and underestimate the probability of successful gap capture by saplings in multi-aged stands.