Explore the vast range of titles available.

Niche complementarity, in which coexisting species use different forms of a resource, has been widely invoked to explain some of the most debated patterns in ecology, including maintenance of diversity and relationships between diversity and ecosystem function. However, classical models assume resource specialization in the form of distinct niches, which does not obviously apply to the broadly overlapping resource use in plant communities. Here we utilize an experimental framework based on competition theory to test whether plants partition resources via classical niche differentiation or via plasticity in resource use. We explore two alternatives: niche preemption, in which individuals respond to a superior competitor by switching to an alternative, less-used resource, and dominant plasticity, in which superior competitors exhibit high resource use plasticity and shift resource use depending on the competitive environment. We determined competitive ability by measuring growth responses with and without neighbors over a growing season and then used 15 N tracer techniques to measure uptake of different nitrogen (N) forms in a field setting. We show that four alpine plant species of differing competitive abilities have statistically indistinguishable uptake patterns (nitrate > ammonium > glycine) in their fundamental niche (without competitors) but differ in whether they shift these uptake patterns in their realized niche (with competitors). Competitively superior species increased their uptake of the most available N form, ammonium, when in competition with the rarer, competitively inferior species. In contrast, the competitively inferior species did not alter its N uptake pattern in competition. The existence of plasticity in resource use among the dominant species provides a mechanism that helps to explain the manner by which plant species with broadly overlapping resource use might coexist.

To evaluate nitrogen (N) saturation in xeric environments, we measured hydrologic N losses, soil N pools, and microbial processes, and developed an N-budget for a chaparral catchment (Sierra Nevada, California) exposed to atmospheric N inputs of approximately 8.5 kg N ha⁻¹ y⁻¹. Dual-isotopic techniques were used to trace the sources and processes controlling nitrate (NO₃ ⁻) losses. The majority of N inputs occurred as ammonium. At the onset of the wet season (November to April), we observed elevated streamwater NO₃ ⁻ concentrations (up to 520 µmol l⁻¹), concomitant with the period of highest gaseous N-loss (up to 500 ng N m⁻² s⁻¹) and suggesting N-saturation. Stream NO₃ ⁻ δ¹⁵N and δ¹⁸O and soil N measurements indicate that nitrification controlled NO₃ ⁻ losses and that less than 1% of the loss was of atmospheric origin. During the late wet season, stream NO₃ ⁻ concentrations decreased (to <2 µmol l⁻¹) as did gaseous N emissions, together suggesting conditions no longer indicative of N-saturation. We propose that chaparral catchments are temporarily N-saturated at ≤8.5 kg N ha⁻¹ y⁻¹, but that N-saturation may be difficult to reach in ecosystems that inherently leak N, thereby confounding the application of N-saturation indicators and annual N-budgets. We propose that activation of N sinks during the typically rainy winter growing season should be incorporated into the assessment of ecosystem response to N deposition. Specifically, the N-saturation status of chaparral may be better assessed by how rapidly catchments transition from N-loss to N-retention.

We used tree ring data (AD 1601-–2007) to examine the occurrence of and climatic influences on spruce beetle ( Dendroctonus rufipennis ) outbreaks in south-central and southwest Alaska and found evidence of regional-scale outbreaks dating from the mid-1700s, related to climate variability at multiple temporal scales. Over interannual time scales (∼∼1-–3 years), El Niño years, combined with severe late-summer drought, appeared to contribute significantly to spruce beetle outbreaks in the study area. Over multidecadal time scales (up to ∼∼40 years), cool-phase Pacific Decadal Oscillation (PDO) conditions tended to precede beetle outbreaks, regardless of the phase of El Niño-Southern Oscillation (ENSO). All sites showed low-severity disturbances attributed to spruce beetle damage, most notably during the 1810s. During other major periods of disturbance (i.e., 1870s, 1910s, 1970s), the effects of spruce beetle outbreaks were of moderate or higher severity. The highly synchronized timing of spruce beetle outbreaks at interannual to multidecadal scales, and particularly the association between cool-phase PDO conditions and beetle disturbance, suggests that climate (i.e., temperature, precipitation) is a primary driver of outbreaks in the study area. Our disturbance chronologies (mid-1700s to present) suggest that recent irruptions (1990s to present) in south-central and southwest Alaska are within the historical geographic range, but that outbreaks since the 1990s show greater spatiotemporal synchrony (i.e., more sites record high-severity infestations) than at any other time in the past ∼∼250 years.

The importance of interspecific competition as a cause of resource partitioning among species has been widely assumed but rarely tested. Using neighbor removals in combination with ⁱ⁵N tracer additions in the field, we examined variation among three alpine species in the uptake of ⁱ⁵N-NH₄⁺, ⁱ⁵N-NO₃⁻, and ⁱ⁵N-13C-[2]-glycine in intact neighborhoods, when paired with a specific neighbor, and when all neighbors were removed. Species varied in the capacity to take up ⁱ⁵N-labeled NH₄⁺, NO₃⁻, and glycine in intact neighborhoods and in interspecific pairs. When interspecific neighbor pairs were compared with no neighbor controls, neighbors reduced ⁱ⁵N uptake in target species by as much as 50%, indicating competition for N. Furthermore, neighbor identity influenced the capacity of species to take up different forms of N. Thus, competition within interspecific neighbor pairs often caused reduced uptake of a particular form of N, as well as shifts to uptake of an alternative form of N. Such shifts in resource use as a result of competition are an implicit assumption in studies of resource partitioning but have rarely been documented. Our study suggests that plasticity in the uptake of different forms of N may be a mechanism by which co-occurring plants reduce competition for N.

Primary succession is a fundamental process in macroecosystems; however, if and how soil development influences microbial community structure is poorly understood. Thus, we investigated changes in the bacterial community along a chronosequence of three unvegetated, early successional soils (~20-year age gradient) from a receding glacier in southeastern Peru using molecular phylogenetic techniques. We found that evenness, phylogenetic diversity, and the number of phylotypes were lowest in the youngest soils, increased in the intermediate aged soils, and plateaued in the oldest soils. This increase in diversity was commensurate with an increase in the number of sequences related to common soil bacteria in the older soils, including members of the divisions Acidobacteria, Bacteroidetes, and Verrucomicrobia. Sequences related to the Comamonadaceae clade of the Betaproteobacteria were dominant in the youngest soil, decreased in abundance in the intermediate age soil, and were not detected in the oldest soil. These sequences are closely related to culturable heterotrophs from rock and ice environments, suggesting that they originated from organisms living within or below the glacier. Sequences related to a variety of nitrogen (N)-fixing clades within the Cyanobacteria were abundant along the chronosequence, comprising 6-40% of phylotypes along the age gradient. Although there was no obvious change in the overall abundance of cyanobacterial sequences along the chronosequence, there was a dramatic shift in the abundance of specific cyanobacterial phylotypes, with the intermediate aged soils containing the greatest diversity of these sequences. Most soil biogeochemical characteristics showed little change along this ~20-year soil age gradient; however, soil N pools significantly increased with soil age, perhaps as a result of the activity of the N-fixing Cyanobacteria. Our results suggest that, like macrobial communities, soil microbial communities are structured by substrate age, and that they, too, undergo predictable changes through time.

Moderate Resolution Imaging Spectroradiometer (MODIS) daily snow cover products provide an opportunity for determining snow onset and melt dates across broad geographic regions; however, cloud cover and polar darkness are limiting factors at higher latitudes. This study presents snow onset and melt dates for Alaska, portions of western Canada and the Russian Far East derived from Terra MODIS snow cover daily 500 m grid data (MOD10A1) and evaluates our method for filling data gaps caused by clouds or polar darkness. Pixels classified as cloud or no data were reclassified by: spatial filtering using neighboring pixel values; temporal filtering using pixel values for days before/after cloud cover; and snow-cycle filtering based on a time series assessment of a pixel’s position within snow accumulation, cover or melt periods. During the 2012 snow year, these gap-filling methods reduced cloud pixels from 27.7% to 3.1%. A total of 12 metrics (e.g., date of first and last snow, date of persistent snow cover and periods of intermittence) for each pixel were calculated by snow year. A comparison of MODIS-derived snow onset and melt dates with in situ observations from 244 weather stations generally showed an early bias in MODIS-derived dates and an effect of increasing cloudiness exacerbating bias. Our results show that mean regional duration of seasonal snow cover is 179–311 days/year and that snow cover is often intermittent, with 41% of the area experiencing ≥2 snow-covered periods during a snow season. Other regional-scale patterns in the timing of snow onset and melt are evident in the yearly 500 m gridded products publically available at http://static.gina.alaska.edu/NPS_products/MODIS_snow/.

In the N-limited alpine tundra, plants may utilize a diversity of N sources (organic and inorganic N) in order to meet their nutritional requirements. To characterize species-level differences in traits related to N acquisition, we analyzed foliar δ15N, nitrate reductase activity (NRA) and mycorrhizal infection in co-occurring alpine species during the first half of the growing season and compared these traits to patterns of N uptake using a15N (15N-NH₄⁺,15N-NO₃⁻) or13C,15N ([1]-13C-15N-glycine) tracer addition in the greenhouse.13C enrichment in belowground tissue indicated that all species were capable of taking up labeled glycine, although only one species showed uptake of glycine potentially exceeding that of inorganic N. Species showing the most depleted foliar δ15N and elevated NRA in the field also tended to show relatively high rates of NO₃⁻ uptake in the greenhouse. Likewise, species showing the most enriched foliar δ15N also showed high rates of NH₄⁺ uptake. The ratio of NO₃⁻:NH₄⁺ uptake rates and growth rate explained 64% and 72% of the variance in foliar δ15N, respectively, suggesting that species differ in the ability to take up NO₃⁻ and NH₄⁺ in the field and that such differences may enable species to partition soil N on the basis of N form.

As an estimate of species-level differences in the capacity to take up different forms of N, we measured plant uptake of ^{15}\\mathrm{N}-\\mathrm{N}{\\mathrm{H}}_{4}^{+}$, ^{15}\\mathrm{N}-\\mathrm{N}{\\mathrm{O}}_{3}^{-}$ and 15N, [1]-13C glycine within a set of herbaceous species collected from three alpine community types. Plants grown from cuttings in the greenhouse showed similar growth responses to the three forms of N but varied in the capacity to take up $\\mathrm{N}{\\mathrm{H}}_{4}^{+}$, $\\mathrm{N}{\\mathrm{O}}_{3}^{-}$ and glycine. Glycine uptake ranged from approximately 42% to greater than 100% of $\\mathrm{N}{\\mathrm{H}}_{4}^{+}$ uptake; however, four out of nine species showed significantly greater uptake of either $\\mathrm{N}{\\mathrm{H}}_{4}^{+}$ or $\\mathrm{N}{\\mathrm{O}}_{3}^{-}$ than of glycine. Relative concentrations of exchangeable N at the sites of plant collection did not correspond with patterns of N uptake among species; instead, species from the same community varied widely in the capacity to take up $\\mathrm{N}{\\mathrm{H}}_{4}^{+}$, $\\mathrm{N}{\\mathrm{O}}_{3}^{-}$, and glycine, suggesting the potential for differentiation among species in resource (N) use.

Aim: A critical concern for boreal ecosystems centres on broad-scale responses to warming, i.e. declining growth and mortality, or enhanced growth and greater productivity. However, few studies have synthesized tree growth along biogeographic gradients to address this issue. This study develops a broader understanding of how growth of a dominant conifer has responded to recent warming near the western forest margin of Alaska. Location: Alaska, United States. Methods: Thirty Picea glauca sites in south-west Alaska (1216 trees ≥ 4 cm dbh) were evaluated for growth differences from the southern boreal forest to the western forest-tundra margin across low-elevation forests and woodlands, and altitudinal tree line. Regional climate records were used to evaluate (1) whether tree growth near western tree line showed greater sensitivity to temperature and/or precipitation than southern boreal sites, (2) if the climategrowth response varied through time, across ecoregions and ecosystems, with spruce beetle disturbance, and by tree age, and (3) if there was a temperature threshold that limited growth. Results: Positive growth trends since the 1980s in many open stands were consistent with the predicted expansion of western and altitudinal tree line. However, growth levelled off with temperatures ≥ 13°C at all but altitudinal tree line. An increasingly positive effect of precipitation on growth occurred after 1985, particularly at boreal tree line and closed forest sites. Closed-canopy forests showed lower rates of growth, greater spruce beetle activity and less potential for resiliency to warming than other ecosystem types. Main Conclusions: Warming has led to markedly different growth responses according to ecosystem type and ecoregion near the forest-tundra margin. Altitudinal tree line sites showed consistently positive growth with warmer temperatures in recent decades, whereas low-elevation forests had reduced growth and greater beetle mortality. Strong positive correlations between growth and summer-fall precipitation since the mid-1980s suggest that precipitation will become an increasingly important factor with further warming.