Crown openness as influenced by tree and site characteristics for yellow birch, sugar maple, and eastern hemlock

Crown openness (CO) of mature trees influences light transmission within the forest canopy. However, in modeling, this variable is often considered constant within species, and its potential regional variability is ignored. The objective of this study was to evaluate if CO values of yellow birch (Betula alleghaniensis Britt.), sugar maple (Acer saccharum Marsh.), and eastern hemlock (Tsuga canadensis (L.) Carriere) vary according to the following factors: (i) species, (ii) regional actual evapotranspiration (AET), (iii) tree size (i.e., diameter at breast height, DBH), and (iv) angle of transmission from zenith. To achieve this, CO was evaluated for 136 yellow birches, 109 sugar maples, and 68 hemlocks from different regions of western Quebec, southern Ontario, and northern Michigan. Results showed that all of the studied factors affected CO. While dominant trees can intercept light laterally as well as vertically, smaller trees are more efficient at intercepting light vertically. Increasing AET is associated with more open crowns. Given its importance in light transmission in the understory, a better understanding of how CO varies between individuals, species, and regions is needed.