Architecture of Cottonwood as an Index of Browsing History in Yellowstone

Abstract

I determined the history of browsing by elk (Cervus elaphus) on narrowleaf cottonwood (Populus angustifolia) at a site in northern Yellowstone National Park (YNP). I aged cottonwoods in three stands by dendrochronology and classified them into four architectural categories that I postulated were produced by four browsing regimes: 1) uninterrupted-growth type (light-to-moderate browsing), 2) arrested-type (intense browsing), 3) retrogressed-type (a change from light-to-moderate browsing to intense browsing), and 4) released-type (a change from intense browsing to light-to-moderate browsing). Cottonwood trees established prior to 1947 were of uninterrupted-growth type architecture. Trees established between 1947 and 1968 were of uninterrupted- and released-type architectures. With the exception of individuals short enough to be protected from winter browsing by snowpack, all individuals established after 1968 were of arrested- and retrogressed-type architectures. Cottonwoods in the study area experienced the following browsing history: (1) light-to-moderate until 1951, (2) intense from 1952 to 1962, (3) light-to-moderate from 1963 to 1974, and (4) intense since 1975. Architecture-based methods can be used to determine the rank order sequence in which elk currently prefer different species of browse. Once determined, that rank order sequence can be used to test competing hypotheses about the declines in woody plants in YNP.