In-situ measurement of twig dieback and regrowth in mature Acer saccharum trees

Abstract

Crown shyness is thought to influence both the productivity and dynamics of forests, but few studies have examined the underlying causes of this common phenomenon. The few studies that exist suggest that crown shyness is caused by the reciprocal abrasion of neighboring tree crowns, resulting in the death of peripheral buds and/or the breakage of peripheral twigs (referred to here as twig dieback). However, twig dieback has not been directly observed due to the difficulty of accessing the crowns of mature canopy trees. In this study, we used a mobile canopy lift to obtain in-situ measurements of twig dieback in mature sugar maple (Acer saccharum Marsh.) trees. We measured: (1) percent dieback (%year−1) – the proportion of twigs that broke off or died; (2) dieback (cmyear−1) – the length of the broken or dead portion; and (3) regrowth (cmyear−1) – the length of the new twigs that sprouted from axillary buds. One third of the twigs suffered dieback over the course of 1year, resulting in 1.41cm of dieback, on average. Percent dieback was significantly higher in tree crowns located within 3m of another crown, indicating that dieback is the result of the reciprocal abrasion of neighboring crowns. Percent dieback also increased with tree size, suggesting that tree sway increases as trees grow larger, resulting in more frequent and more intense abrasion. These trends were exacerbated by similar trends in regrowth, which was slower both in large trees and in trees located with 3m of another crown. Our results suggest dieback may represent a substantial drain on both tree growth and stand productivity – a drain that increases as trees grow larger and stands mature.