Russian boreal forests represent the largest forested region on Earth and comprise one-fifth of the world's forest cover. The two most common genera in Siberia are Larix and Pinus, which together cover more than 80% of the region's forested area. One observable ongoing effect of climate warming is that natural populations of Siberian larch are gradually being replaced by Scots pine. The present work focuses on comparing effects of environmental variables on sap flow density in two even-aged stands of Larix sibirica and Pinus sylvestris. While the two study stands were identical in age (49 years) with similar basal areas and leaf area index, they exhibited very different transpiration rates and response mechanisms to environmental signals. Stand water use was higher for larch than it was for pine, even though transpiration for deciduous larch trees occurred over shorter time periods. The cumulative annual transpiration of the larch stand was 284 +/- 4 mm measured over two consecutive growing seasons (2015-2016), while for pine this was 20% lower. Seasonal transpiration accounted for 50% and 40% of the reference evapotranspiration and 91% and 67% of growing season precipitation for larch and pine, respectively. Water stored in soil provided an important source of water for transpiration, observed as roughly 100 mm, which was then replenished from snowmelt the following spring. The greatest difference between two species related to how well they controlled transpiration, notably in the context of high vapor pressure deficit; under these conditions, pine maintained greater control over transpiration than larch. For all soil moisture levels measured, larch transpired more water than pine. Importantly, our results point to potential future effects of global warming, most notably an increasing decline of larch forests, changes in the ratio between latent and sensitive heat fluxes, and significant modifications in ecosystem water availability.

The effect of thawing permafrost on boreal ecosystem water cycling represents a significant knowledge gap of how climate change will affect northern landscapes. Evapotranspiration, particularly transpiration, may be changing in response to changes in permafrost conditions, vegetation, and climate. This study focuses on the effect of permafrost thaw on boreal plant transpiration over two summers with contrasting weather conditions. We quantified the response of stomatal conductance (gs), from which transpiration was calculated, of deciduous and evergreen plants to soil environmental factors that permafrost thaw affects: soil water content (S), depth of seasonal thaw (D), and soil temperature (T). We found that gs was least sensitive to T compared with S and D at both sites and across both years. At the thawing site, gs was more sensitive to S in a dry year (2009) and to D in a wet year (2010). In the wet year, S of similar to 50cm represented a threshold wherein the sensitivity of gs to T and D switched between positive (S50cm). However, the sensitivities to T and D were negative when S was consistently less than 50cm in the dry year. This is one of the first studies to explore the effect of permafrost thaw on boreal plant gs and transpiration, and our model predicted higher transpiration rates from deciduous plants located on thawing permafrost. Copyright (c) 2013 John Wiley & Sons, Ltd.

Continuous observation over the last decade has revealed evidence of abrupt land surface moistening as well as rapid soil warming within the active layer and upper part of permafrost within the central Lena River basin in eastern Siberia. The present study examined the relationship between permafrost degradation and ecohydrological change in this region. Increases in the depth of the active layer recorded since the winter of 2004 resulting from increases in moisture saturation within the soil have resulted in thawing the upper permafrost causing thermokarst subsidence, which has negatively impacted the growth of boreal (larch) forests in the region. According to multi-year sap flow measurements taken between 2006 and 2009, transpiration from larch trees (Larix cajanderi Mayr.) was significantly reduced as a result of the region's concave micro-topography, which, in conjunction with the deepening and moistening of the active layer, created perennially waterlogged conditions that left mature trees withered and dead. Several trees with reduced amounts of foliage showed a remarkable reduction in seasonal average canopy stomatal conductance during the 2009 growing season. The reduction ratio of canopy stomatal conductance within emergent trees of heights greater than 15m between 2006 and 2009 had a significant positive correlation with the increase in thickness of the active layer over that same period. These findings indicated that wetting trends in a permafrost region caused by arctic climate change may lead to unexpected ecohydrological responses with respect to permafrost degradation in eastern Siberia. Copyright (c) 2013 John Wiley & Sons, Ltd.