Responses of the circumpolar boreal forest to 20th century climate variability

Abstract

We examined relationships between tree ring-width and climate at 232 sites around the circumpolar boreal forest to explore variability in two types of response to temperature: a browning response characterized by inverse correlations between growth and temperature, and a greening response characterized by positive correlations between growth and temperature. We used moving-window correlation analysis for eight 30-year time windows, lagged by 10 years, to characterize the climate response at each site from 1902 to 2002. Inverse growth responses to temperature were widespread, occurring in all species, all time periods, and in nearly all geographic areas. The frequency of the browning response increased after 1942, while the frequency of the greening response declined. Browning was concentrated in five species (Picea abies, Picea glauca, Picea mariana, Picea obovata and Pinus banksiana), and occurred more frequently in the warmer parts of species' ranges, suggesting that direct temperature stress might be a factor. In some species, dry sites were also more likely to experience browning; moisture stress might thus be an additional explanation in some cases. As inverse responses to temperature are widespread, and occur in a broad array of species, there is unlikely to be any single explanation for their occurrence.

Forests synchronize their growth in contrasting Eurasian regions in response to climate warming

“…The growth reduction in Mediterranean forests may be caused by increasing water limitations (6), but the lack of a positive growth response to warming in boreal forests is puzzling. Various hypotheses have been proposed to explain this phenomenon (27), but temperature-induced drought or direct heat stress are plausible constraints of forest productivity in high latitudes (16).…”

“…Boreal forests in Siberia, limited mainly by low temperatures (15,16), show high synchrony irrespective of species. In contrast, the primarily water-limited forests in Spain (14,19) have lower synchrony.…”

“…Indeed, climate has changed markedly over the last decades, prompting an array of physiological reactions in trees that could strengthen growth-climate relationships, thereby enhancing spatial synchrony. Such tree responses may be linked to global shifts in the timing of plant activity (2), drought stress in mid-latitudes (6,14), or an uncoupling of air and soil thermal regimes in the early growing season (15) and direct heat stress (16) in high latitudes, among other factors. Changing tree growth patterns associated with enhanced synchrony in response to warming have been reported at small geographical scales (<150 km) (14-18, but see ref.…”

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

“…Local and regional responses of boreal vegetation to moisture variability have typically been inferred indirectly from correlations with soil moisture forcings, such as precipitation and temperature (e.g., [23,39,40]). In other cases, comparisons have been made with simple meteorological drought indices that often neglect soil properties, snow dynamics and vegetation (e.g., [21,35,41]).…”

A sensitive slope: estimating landscape patterns of forest resilience in a changing climate

“…In the circumboreal forests of the northern hemisphere, growth records recorded in tree rings indicate that boreal conifers are already responding to regional changes in climate (Lloyd and Bunn 2007). Upland populations of white spruce (Picea glauca) in Alaska and Yukon have shown a consistent signal of reduced radial growth during recent warm decades (Barber et al 2000, Hogg andWein 2005), and many treeline forests in northwestern North America show an increasing frequency of negative growth responses to temperature (Wilmking et al 2004, Pisaric et al 2007).…”