Estimation of stand crown cover using a generalized crown diameter model: application for the analysis of portuguese cork oak stands stocking evolution

摘要

A generalized non-linear tree crown diameter model was developed with theaim of allowing the computation of tree crown diameter over a large range oftree dimensions, and allowing its inclusion in forest growth and yield models.The model was formulated to provide biological meaning to the predicted values.Due to the nested structure of the data analyzed (trees within stands),both mixed- and fixed-effect models were developed. Since tree crown diameteris not frequently measured in forest inventories, the validation of the mixedmodel was carried out by considering the population specific response.The results demonstrate that when the measurements required for the mixedmodel calibration are not available, the use of the fixed effect model results inless biased and more accurate estimates. The fixed model was applied to thedata from the two last Portuguese National Forest Inventories (NFI) to analyzethe change in stand crown cover and assess the stocking evolution of cork oakstands in Portugal between 1996 and 2006. Results showed an increase in thefrequency of stands with crown cover lower than 20%, as well as a decrease inthe frequency of stands with crown cover between 20 and 40%. Average crowncover values were significantly different in the two NFI, with a decrease from28.0 to 26.5% over the considered period