Plastic Response of Crown Architecture to Crowding in Understorey Trees of Two Co-dominating Conifers

Abstract

Crown architecture and growth rate of trunk height, trunk diameter and lateral branches of understorey trees (5–10m tall) were compared between two co-dominating conifers, Abies sachalinensisand Picea glehnii, in relation to the index of local crowding intensity, W, represented as a function of density, distance and basal area of taller neighbours. For the two conifers, the growth of trunk height and diameter was decreased and crowns became flat with increasing W, keeping crown projection area. Self-pruning of lower branches was more intense in Abiesthan in Piceain crowded conditions, while both conifers showed similar crown forms in less crowded conditions. These results suggest that the growth in lateral branches exceeded that in height in crowded conditions, especially in Abies. Tree age of both conifers increased with increasing W, resulting from the low growth rate in crowded conditions. The age of the longest and lowest branch of Picea, up to 150 years, was positively correlated with the tree age ranging from 70 to 250 years, whereas that of Abieswas constant at around 30 years irrespective of tree age varying from 40 to 140 years. This result agrees with the observation that aged Abieshad more flat-shaped crowns than in aged Piceain crowded conditions. These results suggest that each conifer adapted to crowding in different ways: high elongation of branches with high turnover rate for Abiesand vice versafor Picea.