Dynamics of decline and mortality of Chamaecyparis nootkatensis in southeast Alaska

Abstract

Alaska-cedar, Chamaecyparis nootkatensis (D. Don) Spach, has been dying in undisturbed forests throughout southeast Alaska for the last 100 years. To determine if decline spreads, boundaries of mortality at seven sites with decline were mapped using aerial photographs taken in 1927, 1948, 1965, and 1976. Mortality was present at all seven sites in 1927. The boundaries of mortality have since expanded, but not by more than 100 m beyond the 1927 limit. In ground surveys, dead Alaska-cedar trees, classified according to their degree of deterioration, were recorded in 427 plots along 39 transects. Fifty-five taxa of understory vegetation were also recorded from 280 plots along these 21 transects; an ordination was produced from their distribution that represented a gradient from bogs to sites with better drainage. Most mortality was associated with bog and semibog sites. Alaska-cedar has a disproportionate level of mortality (65% of basal area dead) relative to other conifers. Local spread of cedar decline occurred along this gradient, as plots with more recently killed cedar trees had high average ordination scores (better drainage) and plots with cedars killed long ago had low average scores (boggy). Snags that probably date from the onset of extensive mortality were relatively common (at least 8% of all snags) on all 23 sites where intensive mortality was surveyed on the ground. As all sites with mortality that we investigated throughout southeast Alaska have these snags, decline does not appear to have spread to new sites since its onset. Although decline is relatively species specific and has patterns of local spread, the spread of mortality is along a specific, preexisting ecological gradient. These results, and the apparent lack of any site to site spread in the last 100 years, suggest that Alaska-cedar decline is not caused by some biotic agent.