The Impact of Windstorm Disturbance on the Forest Structural Attributes in Oriental Beech-Hornbeam Mixed Stands of Hyrcanian Region

Aims: In this study, the response of individual trees and the stand structure to windstorm damage were investigated in a mixed broad-leaf forest stands located in Darabkola Forest, Sari, northern Iran. A field survey was conducted over an area of ۲۶۱۲ ha of the given stands. Material & Methods: A full inventory was performed for all trees with diameter at breast height≥ ۱۰cm and all damaged (i.e., snapped, snags, branch loss, broken trunk or uprooted) trees were identified within the study area. Tree height was measured using Laser Distance Meter for trees more than ۱۰ m high and a Clinometer for all trees <۱۰m. The percentage of trees damaged was the dependent variable; but, independent variables included number, mean height (m), total basal area (m۲) and volume trees (m۳). Independent t-test was performed to compare the mean of trees based on diameter classes. Also, the affection of height on susceptibility to windstorm effects was tested by comparing mean trees height within each class. Findings: The number of the damaged trees varied with species, ranging from ۰.۰۴ % (۲ individuals) in Tilia begonifolia Stev to ۵۳.۷ % (۲۷۰۹ individuals) in Fagus orientalis L. Among the damaged trees ۵۰۵۴ recorded, ۲۲۳۱ (۴۴.۲۰ %) had been snapped and ۷۸۷ (۱۵.۶ %) uprooted, and ۲۰۲۸ (۴۰.۲۰ %) were snags. Among the species, Fagus orientalis L. (۴۷.۷ %), Alnus subcordata C. A. Mey. (۱۹.۷ %) and Carpinus betulus L. (۱۸.۸ %) were most susceptible to uprooting, whereas Populus caspica Bornm, Quercus castaneifolia C. A. Mey. and Ulmus glabra Huds. were the least susceptible to uprooting. The species differed significantly in their susceptibility to snapping. Trees ≥۶۰cm dbh were more likely to be snapped (۷۸ %) or snags (۶۹ %), whereas trees <۶۰ cm dbh were more presumably to be uprooted (۵۰ %). Also, about one third (۳۳ %) of total trees larger than ۶۰ cm (D.B.H) have showed distinct effects and individual trees >۲۰ m were more commonly uprooted. The number of snapped trees, snags and uprooted trees had a sharp increase in trees taller than ۲۰m. Conclusion: Our finding showed that Oriental beech and Common hornbeam to be species that were most susceptible to both being snapped and snagged, whereas Caucasian alder had the most uprooting damage among the other species. Overall, features like species, D.B.H, height and height to diameter ratio (H/D.B.H), were intensively related with the type and severity of windstorm damage. The findings showed that windstorms cause substantial structural effects in mixed stands of beech and hornbeam.

کلیدواژه ها:

Stand structure ، Natural disturbance ، Mixed forest