Do limiting factors at Alaskan treelines shift with climatic regimes?

Abstract

Trees at Alaskan treelines are assumed to be limited by temperature and to expand upslope and/or to higher latitudes with global warming. However, recent studies describe negative temperature responses and drought stress of Alaskan treeline trees in recent decades. In this study, we have analyzed the responses of treeline white spruce to temperature and precipitation according to different climatic regimes in Alaska, described as negative (cool) and positive (warm) phases of the Pacific Decadal Oscillation (PDO). We found that in three consecutive phases (positive from 1925-46, negative from 1947-76, and positive again from 1977-98), the growth responses to temperature and precipitation differed markedly. Before 1947, in a phase of warm winters and with summer temperatures being close to the century mean, the trees at most sites responded positively to summer temperature, as one would expect from treeline trees at northern high latitudes. Between 1947 and 1976, a phase of cold winters and average summers, the trees showed similar responses, but a new pattern of negative responses to the summer temperature of the year prior to growth coupled with positive responses to the precipitation in the same year emerged at some sites. As the precipitation was relatively low at those sites, we assume that drought stress might have played a role. However, the climate responses were not uniform but were modified by regional gradients (trees at northern sites responded more often to temperature than trees at southern sites) and local site conditions (forest trees responded more often to precipitation than treeline trees), possibly reflecting differences in energy and water balance across regions and sites, respectively. However, since the shift in the PDO in 1976 from a negative to a positive phase, the trees' climate-growth responses are much less pronounced and climate seems to have lost its importance as a limiting factor for the growth of treeline white spruce. If predictions of continued warming and precipitation increase at northern high latitudes hold true, the growth of Alaskan treeline trees will likely depend on the ratio of temperature and precipitation increase more than on their absolute values, as well as on the interaction of periodic regime shifts with the global warming trend. Once a climatic limitation is lifted, other factors, such as insect outbreaks or interspecific competition, might become limiting to tree growth.

Do Himalayan treelines respond to recent climate change? An evaluation of sensitivity indicators

“…Drought stress as a local factor impeding tree growth in recent decades has been supposed for different treeline environments (e.g. Lloyd and Fastie, 2002;Daniels and Veblen, 2004;Dulamsuren et al, 2010;Zhang et al, 2011;Fajardo and McIntire, 2012;Ohse et al, 2012).…”

Did the 1976–77 switch in the Pacific Decadal Oscillation make white spruce in the southwest Yukon more susceptible to spruce bark beetle?

“…Several studies have shown temperature increases during the latter half of the 20 th century have limited, rather than facilitated, radial growth of white spruce (Barber et al 2000;Davi et al 2003;D' Arrigo et al 2004D' Arrigo et al , 2007Wilmking et al 2004;Wilmking and Juday 2005), although this response also varies regionally and among sites (Ohse et al 2012). Dif- ferential effects seem to relate to the relative change in precipitation versus temperature with the phase change in the PDO.…”

“…These findings are consistent with other studies conducted on interior white spruce forests growing in the rain shadows of mountain ranges. At moisture-limited sites, warm temperatures limit white spruce growth (Barber et al 2000, Lloyd andFastie 2002), even at altitudinal treeline (Lloyd and Fastie 2002, Youngblut and Luckman 2008, Ohse et al 2012.…”

“…At high latitudes, cool temperatures often limit tree growth so that warmer climate is expected to benefit white spruce (Lloyd and Fastie 2002;Hogg and Wein 2005;Danby and Hik 2007a,b;Payette 2007;Boonstra et al 2008). However, white spruce (Picea glauca [Moench] Voss) often grows on relatively dry sites (Nienstaedt and Zasada 1990) where low precipitation and moisture availability (e.g., low precipitation combined with warm temperatures) can be more limiting than cool temperatures (Barber et al 2000, Lloyd and Fastie 2002, Youngblut and Luckman 2008, Ohse et al 2012. Recent research has shown non-linear or unstable climate-growth relations for white spruce in response to increased temperatures since the 1950s (Barber et al 2000;Davi et al 2003;D' Arrigo et al 2004D' Arrigo et al , 2007Wilmking et al 2004;Wilmking and Juday 2005).…”

“…Recent research has shown non-linear or unstable climate-growth relations for white spruce in response to increased temperatures since the 1950s (Barber et al 2000;Davi et al 2003;D' Arrigo et al 2004D' Arrigo et al , 2007Wilmking et al 2004;Wilmking and Juday 2005). Ohse et al (2012) have shown climate variation associated with different phases of the PDO can change the climatic factors that limit white spruce growth and directional climate warming interacts with these effects.…”