Ecologists have long been intrigued by the ways co-occurring species divide limiting resources. Such resource partitioning, or niche differentiation, may promote species diversity by reducing competition. Although resource partitioning is an important determinant of species diversity and composition in animal communities, its importance in structuring plant communities has been difficult to resolve. This is due mainly to difficulties in studying how plants compete for below-ground resources. Here we provide evidence from a 15N-tracer field experiment showing that plant species in a nitrogen-limited, arctic tundra community were differentiated in timing, depth and chemical form of nitrogen uptake, and that species dominance was strongly correlated with uptake of the most available soil nitrogen forms. That is, the most productive species used the most abundant nitrogen forms, and less productive species used less abundant forms. To our knowledge, this is the first documentation that the composition of a plant community is related to partitioning of differentially available forms of a single limiting resource.

We examined the importance of temperature (7°C or 15°C) and soil moisture regime (saturated or field capacity) on the carbon (C) balance of arctic tussock tundra microcosms (intact blocks of soil and vegetation) in growth chambers over an 81-day simulated growing season. We measured gaseous CO exchanges, methane (CH) emissions, and dissolved C losses on intact blocks of tussock (Eriophorum vaginatum) and intertussock (moss-dominated). We hypothesized that under increased temperature and/or enhanced drainage, C losses from ecosystem respiration (CO respired by plants and heterotrophs) would exceed gains from gross photosynthesis causing tussock tundra to become a net source of C to the atmosphere. The field capacity moisture regime caused a decrease in net CO storage (NEP) in tussock tundra micrososms. This resulted from a stimulation of ecosystem respiration (probably mostly microbial) with enhanced drainage, rather than a decrease in gross photosynthesis. Elevated temperature alone had no effect on NEP because CO losses from increased ecosystem respiration at elevated temperature were compensated by increased CO uptake (gross photosynthesis). Although CO losses from ecosystem respiration were primarily limited by drainage, CH emissions, in contrast, were dependent on temperature. Furthermore, substantial dissolved C losses, especially organic C, and important microhabitat differences must be considered in estimating C balance for the tussock tundra system. As much as ∼ 20% of total C fixed in photosynthesis was lost as dissolved organic C. Tussocks stored ∼ 2x more C and emitted 5x more methane than intertussocks. In spite of the limitations of this microcosm experiment, this study has further elucidated the critical role of soil moisture regime and dissolved C losses in regulating net C balance of arctic tussock tundra.

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Modeled trends in phenological advancement and sensitivity for three northeastern alpine species are less pronounced compared with lower elevations in the region, and this small shift in flower timing did not increase risk of frost damage. Potential reasons for limited earlier phenological advancement at higher elevations include a slower warming trend and increased cloud exposure with elevation and/or inadequate chilling requirements.

Mount Washington, New Hampshire, has the longest northeastern U.S. mountain climatological record (1930s to present), both at the summit (1914 m) and at Pinkham Notch (612 m). Pinkham's homogenized daily temperature exhibits annual (mean 5 +0.07uC/decade, p 5 0.07; min 5 +0.11uC/decade, p 5 0.01), winter (min 5 +0.18uC/decade, p 5 0.07), spring (max 5 +0.13uC/decade, p 5 0.10), and summer (min 5 +0.11uC/decade, p 5 0.01) warming trends. Though suggesting annual, winter, and spring warming (0.05 to 0.12uC/decade), mean summit temperature trends were not significant. Pinkham shows no significant change in date of first and last snow; however, the summit does but its period of record is shorter. Onset of continuous snow cover has not changed significantly at either site. Thawing degree days trended earlier at the summit (2.8 days/decade; p 5 0.01) and Pinkham Notch (1.6 days/decade, p , 0.01), but end of continuous snow cover trended significantly earlier (1.6 days/decade; p 5 0.02) only at Pinkham. Growing degree days showed no significant trends at either location. Pinkham exhibits more climatic change than the summit but less than regional lower elevations. Thermal inversions and high incidence of cloud fog commonly at or above the regional atmospheric boundary layer may explain the summit's resistance to climate warming. Caution is needed when extrapolating climate change trends from other mountains or proximate lower elevation climate data to upper elevations.