Influence of canopy structure on the understory environment in tall, old-growth, conifer forests

The effect of the spatial distribution of trees and foliage on understory c onditions was examined in six tall old-growth forests along the Pacific Coa st: two sites each in Washington, Oregon, and California. Detailed field me asurements of crown parameters were collected on over 9000 trees encompassi ng over 14.5 ha in the stands. Crown parameters were used to construct a sp atially explicit model useful in analyzing the variability of crown distrib utions in both vertical and horizontal dimensions. Sapwood measurements of over 400 trees in combination with published equations and 240 hemispherica l photos were used to assess leaf area and understory light levels, respect ively. Shrub and herb cover was used as a biological indicator of growing c onditions in the understory. Although leaf area is often assumed to be corr elated with the amount of light penetrating the canopy, this is not the cas e in tall, old-growth forests. The semivariance of the horizontal distribut ion of canopy volume was strongly correlated with shrub cover and understor y light levels and was an overall predictor of canopy structure. This varia bility gives rise to potentially higher understory light levels and shrub c over values when compared with a forest lacking this vertical heterogeneity and may allow the stand to support a higher volume of foliage.