Twentieth-century warming and the dendroclimatology of declining yellow-cedar forests in southeastern Alaska

Abstract

Decline of yellow-cedar ( Chamaecyparis nootkatensis ((D. Don) Spach) has occurred on 200 000 ha of temperate rainforests across southeastern Alaska. Because declining forests appeared soon after the Little Ice Age and are limited mostly to low elevations (whereas higher elevation forests remain healthy), recent studies have hypothesized a climatic mechanism involving early dehardening, reduced snowpack, and freezing injury. This hypothesis assumes that a specific suite of microclimatic conditions occurs during late winter and declining cedar populations across the region have responded similarly to these conditions. Based on the first geographically extensive tree ring chronologies constructed for southeastern Alaska, we tested these assumptions by investigating regional climatic trends and the growth responses of declining cedar populations to this climatic variation. Warming winter trends were observed for southeastern Alaska, resulting in potentially injurious conditions for yellow-cedar due to reduced snowfall and frequent occurrence of severe thaw–freeze events. Declining cedar forests shared a common regional chronology for which late-winter weather was the best predictor of annual growth of surviving trees. Overall, our findings verify the influence of elevational gradients of temperature and snow cover on exposure to climatic stressors, support the climatic hypothesis across large spatial and temporal scales, and suggest cedar decline may expand with continued warming.

The altitude-for-latitude disparity in the range retractions of woody species

“…Recent studies have documented large-scale dieback events affecting hundreds or thousands of square kilometres of forest within the lowland distribution of various North American trees, including yellow cedar (Chamaecyparis nootkatentis) [46], yellow birch (Betula alleghaniensis) [47], the pinyon pine (Pinus edulis) [5] and trembling aspen [48,49] (Figure 2). Comparable dieback events are reported in Chilean cedar (Austrocedrus chilensis) at the forest-steppe ecotone in Northern Patagonia [50] and from European beech in Northern Eurasia [51], eucalypts in northeast Australia [52] and various trees and shrubs in the Mediterranean [53].…”

“…A common factor inducing these dieback events is elevated temperature, which acts during the summer to exacerbate drought [5,24,33,[48][49][50][51]53,55] or during winter to cause a premature start to growth and subsequent winter injury [24,46,47]. Elevated temperature and drought can directly elevate mortality [4,27,30,31,48,53,55,56], increase the susceptibility of species to attack by pests and pathogens [5,28,49,57], or favour outcompetition owing to reduced reproduction and relative competitive ability [12,26,29,55].…”

“…The studies cited above [5,[46][47][48][49][50][51][52][53]58] provide compelling evidence of increased mortality within the lowland distribution of various woody species, in some cases at subcontinental scales [5,47,48]. These mortality events are linked directly or indirectly to increasing temperatures, yet observed regional mass mortality remains poorly linked with the range retractions predicted to accompany current and future global temperature changes.…”

“…Consequently, where woody species die over large areas of their latitudinal range, rapid replacement by more drought tolerant species is unlikely without human intervention owing to the dispersal lag of the incoming species [8,24,[80][81][82][83][84]. In such a scenario, recent studies demonstrate that, rather than observing ecotone shifts, ecotone expansions and rapid alterations to plant communities are likely to occur [4,5,8,41,46], including the formation of novel species assemblages driven by expansion of non-native species [8,85] or the absence of competitors [3]. Community shifts could be persistent and accompanied by changes in the properties and services of ecosystem [82], such as decreased soil stability and elevated erosion [4,5,23].…”

“…Consequently, rapid lowland range retractions might be imminent, with large-scale dieback poised to occur on a sub-continental scale in some species, as recent reports from North America demonstrate [5,7,30,37,46,47,55,72]. Such impacts will result in significant changes to plant community composition and associated biodiversity, ecosystem function and carbon dynamics.…”