Is escaping deer browse just a matter of time in Picea sitchensis? A chemical and dendroecological approach

Abstract

We combined chemical and dendroecological analyses to understand the mechanisms that are involved in escaping deer browse by young Sitka spruce (Picea sitchensis) exposed to browsing by Sitka black-tailed deer (Odocoileus hemionus sitchensis) on Haida Gwaii (British Columbia, Canada). We compared chemical defences (terpenes), nutritive compounds (nitrogen, non-structural constituents, cellulose, and lignin), as well as age and radial growth of two young spruce categories growing side by side: (1) stunted spruces that were heavily browsed, shorter than the browse line, and (2) escaped spruces that were taller than the browse line but still browsed below the browse line. Escaped and stunted spruces did not differ in terpene concentrations, or in nutritive compound contents, suggesting that they had similar palatability. Escaped spruces were older that stunted spruces. Stunted and escaped trees had similar slow growth when young, suggesting no difference in initial browsing between the two spruce categories. For escaped spruce, there was a dramatic increase in radial growth at about 12-13 years old, suggesting that the apex of the trees had escaped deer browse. Because the two categories of spruces were equally accessible and did not differ in chemical defences or in nutritive compounds, and because escaped spruces were older than stunted trees and had a similar slow radial growth in their first 12-13 years, we conclude that morphological differences between stunted and escaped browsed trees are due to age and that it is only a matter of time before spruce escape deer on Haida Gwaii.

Growth change of young Picea sitchensis in response to deer browsing

“…Thus although their growth is severely reduced by browsing, spruce tree will eventually reach the browsing limit and escape deer. These results are consistent with those of Duncan et al (1994) and of Vila et al (2002) but are species dependant. Indeed, under high browsing pressure, the escaping of western redcedar on the Queen Charlotte Islands, is essentially a function of a better potential in some individuals to produce chemical defenses and of ecological context (e.g.…”

“…Age at release varied between 8 and 25 years. Vila et al (2002) by sampling equally accessible spruce in the sites 1 and 2 showed that accessibility, nutritive values and chemical compounds were not correlated to escaping browsing. This study shows that tree age is.…”

“…Variations in tree palatability, visibility or shape have been shown to influence sensitivity to browsing (Chouinard and Filion, 2001;Vourc'h et al, 2001Vourc'h et al, , 2002. However, in the case of Sitka spruce Vila et al (2002) showed that there was no evidence for chemical nor nutritive variation coupled with variation in browsing impact. Although height and radial growth were essentially a function of tree height with respect to the browsing limit they were nevertheless influenced by site conditions.…”

Effects of Twenty Years of Deer Exclusion on Woody Vegetation at Three Life-History Stages in a Mid-Atlantic Temperate Deciduous Forest

“…5B). Once the leader stem is out of the browse zone (approximately 2 m), deer-caused mortality is less likely (Vila et al 2002). These larger saplings might have reached this height prior to the installation of the deer exclosure, and so did not benefit from the reduced browsing pressure.…”

Response of young Tsuga heterophylla to deer browsing: developing tools to assess deer impact on forest dynamics

“…While dendrochronologists have used the influence of disturbances recorded in tree ring series to reconstruct spatial and temporal fluctuations of several insect species (Swetnam and Lynch 1989;Morin and Laprise 1990;Veblen et al 1991;Gross 1992;Jardon et al 1994;Krause and Morin 1995;Krause and Raffa 1996;Weber 1997) few studies have used ring-width analysis to assess the influence of past damage by ungulates on woody plant growth and dynamics (Motta 1996;Payette 1998, 2000;Chouinard and Filion 2001;Vila et al 2001Vila et al , 2002.…”