Changes in forest productivity across Alaska consistent with biome shift

Abstract

Global vegetation models predict that boreal forests are particularly sensitive to a biome shift during the 21st century. This shift would manifest itself first at the biome's margins, with evergreen forest expanding into current tundra while being replaced by grasslands or temperate forest at the biome's southern edge. We evaluated changes in forest productivity since 1982 across boreal Alaska by linking satellite estimates of primary productivity and a large tree-ring data set. Trends in both records show consistent growth increases at the boreal-tundra ecotones that contrast with drought-induced productivity declines throughout interior Alaska. These patterns support the hypothesized effects of an initiating biome shift. Ultimately, tree dispersal rates, habitat availability and the rate of future climate change, and how it changes disturbance regimes, are expected to determine where the boreal biome will undergo a gradual geographic range shift, and where a more rapid decline.

Temperature and Snow-Mediated Moisture Controls of Summer Photosynthetic Activity in Northern Terrestrial Ecosystems between 1982 and 2011

“…A consistent long-term control of boreal forest growth by summer moisture availability has been observed in the drier continental interiors of Alaska and North-Western Canada where increased tree growth is stimulated by cooler and wetter growing seasons [23,27,28]. Hence, warming in these boreal regions has often been associated with tree growth declines rather than with increases [23], consistent in cases with co-located NDVI browning trends [18].…”

“…Using Maximum Covariance Analysis to isolate the coupled trends in winter snowpack (maximum SWE), soil moisture availability (scPDSI) and summer NDVI in North America has shown that the temporal pattern of shared trends is strongly consistent with the temporal trajectory of regional-scale declines in NDVI, tree-ring growth, forest biomass and modeled productivity previously documented in the boreal region (e.g., [15,18,35,70,91]). Furthermore, the spatial patterns of the coupled trends suggest contrasting vegetation responses to declining snowpack accumulation in the Arctic shrublands and boreal forests since the early 1990s.…”

“…Forest browning across most of Canada and Alaska since the early 1990s has led to speculation that after an initial alleviation of cold constraints during the 1980s, subsequent warming has no longer promoted northern vegetation growth because of increased temperature-induced drought stress [15,16,18,21]. This hypothesis has emphasized the role of increasing evaporative demand on surface water budget, whilst the potential influence of changes in moisture supply (snow and rainfall) and connections with concurrent changes in snow regimes have not been considered.…”

“…Furthermore, the spatial patterns of the coupled trends suggest contrasting vegetation responses to declining snowpack accumulation in the Arctic shrublands and boreal forests since the early 1990s. Thus, together with seasonal warming, declining snow cover appears to be playing an important role in driving the well-documented contrast between forest browning and tundra greening trends in North America [15,16,18]. Reduced peak snow accumulation can extend the short northern growing seasons and stimulate earlier and faster vegetation greening in some cool Arctic regions [34,90], but it may have an adverse impact in the boreal forests due to reduced water availability during summer [84] and soil insulation during the cold season [92,93].…”

“…The analysis of the global satellite record of Normalized Difference Vegetation Index (NDVI) imagery since 1981 (e.g., [14]) has shown that increases in vegetation photosynthetic activity have been stronger in Eurasia than in North America and more sustained in the colder tundra biome than over the boreal forests [15][16][17][18][19]. The most recent studies have found that after the initial increasing trend in NDVI (i.e., greening) observed in the first decade of the record [5,20], longer and hotter growing seasons appear to have resulted in reductions of summer NDVI (i.e., browning) in the drier and more continental regions of the boreal forest since the 1990s [15][16][17]21].…”