To better understand the factors controlling the growth of larch trees in Arctic taiga-tundra boundary ecosystem, we conducted field measurements of photosynthesis, tree size, nitrogen (N) content, and isotopic ratios in larch needles and soil. In addition, we observed various environmental parameters, including topography and soil moisture at four sites in the Indigirka River Basin, near Chokurdakh, northeastern Siberia. Most living larch trees grow on mounds with relatively high elevations and dry soils, indicating intolerance of high soil moisture. We found that needle delta(13)c was positively correlated with needle N content and needle mass, and these parameters showed spatial patterns similar to that of tree size. These results indicate that trees with high needle N content achieved higher rates of photosynthesis, which resulted in larger amounts of C assimilation and larger C allocation to needles and led to larger tree size than trees with lower needle N content. A positive correlation was also found between needle N content and soil NK4+ pool. Thus, soil inorganic N pool may indicate N availability, which is reflected in the needle N content of the larch trees. Microtopography plays a principal role in N availability, through a change in soil moisture. Relatively dryer soil of mounds with higher elevation and larger extent causes higher rates of soil N production, leading to increased N availability for plants, in addition to larger rooting space for trees to uptake more N. (C) 2014 Elsevier B.V. and NIPR. All rights reserved.

The Arctic has experienced rapid warming and, although there are uncertainties, increases in precipitation are projected to accompany future warming. Climate changes are expected to affect magnitudes of gross ecosystem photosynthesis (GEP), ecosystem respiration (ER) and the net ecosystem exchange of CO2 (NEE). Furthermore, ecosystem responses to climate change are likely to be characterized by nonlinearities, thresholds and interactions among system components and the driving variables. These complex interactions increase the difficulty of predicting responses to climate change and necessitate the use of manipulative experiments. In 2003, we established a long-term, multi-level and multi-factor climate change experiment in a polar semidesert in northwest Greenland. Two levels of heating (30 and 60Wm2) were applied and the higher level was combined with supplemental summer rain. We made plot-level measurements of CO2 exchange, plant community composition, foliar nitrogen concentrations, leaf 13C and NDVI to examine responses to our treatments at ecosystem- and leaf-levels. We confronted simple models of GEP and ER with our data to test hypotheses regarding key drivers of CO2 exchange and to estimate growing season CO2-C budgets. Low-level warming increased the magnitude of the ecosystem C sink. Meanwhile, high-level warming made the ecosystem a source of C to the atmosphere. When high-level warming was combined with increased summer rain, the ecosystem became a C sink of magnitude similar to that observed under low-level warming. Competition among our ER models revealed the importance of soil moisture as a driving variable, likely through its effects on microbial activity and nutrient cycling. Measurements of community composition and proxies for leaf-level physiology suggest GEP responses largely reflect changes in leaf area of Salix arctica, rather than changes in leaf-level physiology. Our findings indicate that the sign and magnitude of the future High Arctic C budget may depend upon changes in summer rain.

Because of their vast size, grazing lands have the potential to sequester significant quantities of carbon, slowing the increase in atmospheric CO, and reducing the risk of global warming. Although CO2 uptake during the growing season can be substantial, losses during winter months reduce annual sequestration, potentially turning grazing lands into net carbon sources. The goal of this research was to quantify the magnitude of winter fluxes for humid-temperate pastures in the northeastern USA. The study was conducted from 2003 to 2005 on two pastures in the ridge and valley region of central Pennsylvania, one dominated by a mix of cool-season grasses and the other transitioning from an alfalfa to mixed-grass pasture. Pasture-scale CO2 fluxes were quantified using eddy covariance techniques. The alfalfa pasture was less of a CO2 source to the atmosphere during winter months (1 December to 31 March) than the grass pasture, averaging 2.68 g CO2 m(-2) day(-1) compared with 3.09 g CO2 m(-2) day(-1) for the grass pasture. Cumulative efflux for the winter months averaged 326 g CO2 m(-2) (88 g C m(-2)) for the alfalfa and 375 g CO2 m(-2) (101 g C m(-2)) for the grass pasture. In the absence of snow cover, eddy covariance measurements estimated that photosynthetic CO2, uptake occurred at temperatures below 0 degrees C. Canopy and leaf chamber measurements in the field and in controlled environments suggested minimum temperatures for photosynthetic CO2 uptake of about -4 degrees C. Even when daytime uptake occurred, nighttime efflux from the system was greater so that the pastures remained CO2 sources throughout the winter. Published by Elsevier B.V.