Divergence, inertia, and change: what variability in the response of boreal forest trees to recent warming tells us about ecological sensitivity to abrupt climate change

Background/Question/Methods   Between 1900 and 2003, the Arctic warmed faster than the rest of the Northern Hemisphere; the rate of warming in the Arctic accelerated to ~0.4°C/decade in the last few decades of the 20th century (ACIA, 2005). Although the growth of individual organisms may respond nearly instantaneously to changing climate, populations of long-lived species are likely to exhibit substantial inertia and may thus display lagged responses to warming. The degree of inertia in a population may vary depending on a species' regeneration niche: disturbance-dependent species (e.g., black spruce) would be expected to respond less rapidly to changing climate than species (e.g., white spruce) whose regeneration is less dependent on disturbance. In this talk, I will synthesize previously published tree-ring records from the Alaskan boreal forest to answer two questions. (1) How rapidly have tree populations responded to warming, and how does the rate of response vary among species and regions? (2)Are the patterns of response observed at the population level consistent with those observed at the level of individual organisms?

Results/Conclusions   The rate of population response to warming varied substantially between species. The majority of white spruce populations examined showed an increase in density and/or a change in distribution during the last several decades. In contrast, the northern limit of black spruce in Alaska remained stable, despite evidence that temperature limits the growth of black spruce trees at its distributional limit. This difference may reflect differences in the species' regeneration niches. Black spruce is dependent upon fire for successful sexual reproduction, even at its northern limit in Alaska, and this imposes a significant constraint on the rate at which black spruce populations respond to warming, particularly in areas (like the Brooks Range) where fires occur infrequently. A second important finding to emerge from this synthesis is that the response of individual trees to warming differs, in some locations, from the response observed at the population level. In a number of sites where populations respond positively to warming, individual tree growth shows a predominantly negative response: increased warming, at least after 1950, was associated at these sites with reduced growth. This apparent mismatch between the response of populations and that of the individuals making up those populations likely reflects the different time scales at which individuals and populations respond to change, in which case the response of individual trees to warming may provide a ‘leading indicator' of future population-level responses.