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Herbivores may increase or decrease aboveground plant productivity depending on factors such as herbivore density and habitat productivity. The grazing optimization hypothesis predicts a peak in plant production at intermediate herbivore densities, but has rarely been tested experimentally in an alpine field setting. In an experimental design with three densities of sheep (high, low, and no sheep), we harvested aboveground plant biomass in alpine grasslands prior to treatment and after five years of grazing. Biomass of vascular plants decreased at high sheep density, and marginally increased at low sheep density. The ungrazed treatment was found to be intermediate. Companion studies conducted at the same site suggest, (1) that changes in soil N-mineralization and plant community patterns are contributing to the herbivore-induced effects on plant productivity in alpine grasslands, (2) that herbivore-driven changes in plant productivity feed into the future performance for the herbivore as the marginal increase in productivity at low density corresponds with a temporal increase in lamb growth. Our study provides experimental evidence for a nonlinear effect of increased grazing on plant productivity as predicted by the grazing optimization hypothesis. This has important repercussions for ecosystem function and management, as it demonstrates how herbivore density can either increase or decrease ecosystem productivity over time.
Ecotypic specialization among populations within plant species can result in adaptational lag when the climate changes directionally. However, disturbances, whether caused by direct effects of human activities or indirect effects such as climate change, may represent zones within which natural selection is relaxed. We compared the genetically based variation in leaf morphology in Dryas octopetala within three natural populations arrayed along a snowbank gradient, to that found in a recently colonized gravel pad less than 100 m away (1600 total leaf lengths measured; 4 sites × 10 transects/site × 4 plants/transect × 10 leaves/plant). Elevated among-clone leaf length variation within the disturbed site supported the idea that disturbances may represent “hotspots” of evolutionarily significant genetic variation. In the Arctic, where colonization of disturbances is primarily by native species, adaptive evolution may be more rapid than previously thought due to relaxation of selection and subsequent mixing of previously isolated gene pools in such areas.
The seed yield of herbaceous annuals may be resource- or pollinator-limited. The extent to which either or both of these limitations negatively affect seed yield is hypothesized to vary with the timing of flowering because of seasonal changes in pollinator and resource availabilities. We tested this hypothesis by examining the relationship between seed yield and pollinator visitation rates, flowering phenology, aboveground plant biomass, and biomass allocation patterns, fruit set, and seed set, seed number per flower, and seed mass for Gentiana leucomelaena, an annual herbaceous dicot that produces four types of flowering shoots as a result of differences in flower color (blue and white) and dichogamy-type (protogyny and protandry). Bivariate plots of reproductive efficiency (total seed mass per shoot) versus vegetative mass were curvilinear and showed that early- and late-flowering shoots were more likely to be limited in seed yield than shoots flowering during the middle of the flowering season. Early-flowering shoots had greater biomass but had lower fruit set and seed set with fewer but larger seeds per fruit. These features were consistent with pollinator (visitation rate) limitations on seed yield. In contrast, late-flowering shoots had smaller biomass but had higher fruit set and seed set with many but smaller seeds per fruit. These features were consistent with resource (plant biomass) limitations on seed yield. Moreover, early-flowering shoots tended to produce white and protogynous flowers and allocated more to leaf growth, whereas late-flowering shoots tended to produce blue and protandrous flowers and allocated more to stem growth in height (perhaps to reduce shading by neighbors). In addition, the peak time of flowering shoot density coincided with the peak of reproductive efficiency. In conclusion, limitations on the seed yield of this species varied with the timing of flowering. G. leucomelaena appears to have evolved a system of changing flower color and dichogamy-type that optimizes seed yield in an alpine meadow habitat.
Knowledge on demographic responses to environmental and microhabitat factors are crucial to understand the ecological consequences of climate change predictions for species that live in isolated mountainous habitat, such as the Royle's pika. We examined the influence of snow attributes, food availability, and rock cover on adult and juvenile pika counts and juvenile emergence date based on spatiotemporal variation in these parameters at 10 permanent plots (0.25 ha squares) along an attitudinal gradient of Kedarnath Wildlife Sanctuary during the predispersal period spanning four years. Pika count and its interannual variability at plots were estimated at 4.06Mean ± 0.21SE and 19.65 ± 1.71% respectively. Regression analysis showed that spatiotemporal variation in adult counts was related to the interactive effect of snow-cover period and altitude wherein counts increased with snow-cover period only in lower altitudes (β = 0.26 ± 0.08). Juvenile count was related to spring snow-depth (β = 0.074 ± 0.037). Litter number was typically one. Our results confirm the influence of snow as thermal insulator (to pika and their food plants) on population dynamics of small mammals. We also highlight that adult and juvenile populations might be governed by different factors, invoking further studies on age-stratified assessment of population responses to climate change.
Global threats to plant-pollinator interactions are potentially serious in alpine ecosystems, which combine great diversity with particular fragility. We utilized tools from complex network theory to assess the robustness to species extinction of two Spanish alpine pollination networks. A comparison with ten additional alpine and subalpine pollination (ASP) networks allowed us to give our assessment a broader scope and provide a general view of ASP network robustness. We found a broad range of robustness among ASP networks. The two Spanish pollination networks ranked intermediate to high in robustness. This could be due to two of their structural features, connectance (proportion of potential interactions actually observed) and asymmetry (normalized difference between pollinator and plant richness), which showed a positive relationship with network robustness. A finer-scale focus on the two Spanish networks did not reveal differences between endemic and nonendemic plants in their functional role within the network but indicated that they differed in their robustness to pollinator extinction. Contrasting patterns across networks suggested that endemic robustness depends on community particularities. To improve the utility of robustness assessment as a conservation tool, we should increase our understanding on (1) the order in which network species will get extinct, (2) how species rewire once they have lost their partners, and (3) how much species depend on their mutualistic interaction.
Several factors may have interactive effects on natural 15N abundance of plant species. Some of these effects could be associated with different plant functional types, including mycorrhizal association type. Due to its high taxonomic and functional diversity, the alpine heath community in the Caucasus is a suitable object for studying 15N natural abundance of plants in relation to different functional/mycorrhizal groups, contrasting with the limited numbers of plant groups or species considered in previous studies of individual communities. The N concentration and δ15N were determined in leaves of 25 plant species from 8 functional/mycorrhizal groups from an alpine lichen heath in the Teberda Reserve, Northern Caucasus, Russia. Functional groups were represented by ericoid mycorrhizal species (ERI), ectomycorrhizal species (ECT), arbuscular mycorrhizal forbs (AM—FORB), arbuscular mycorrhizal grasses (AM—GRA), arbuscular mycorrhizal nodulated legumes (FAB—N), non-mycorrhizal graminoids (sedges and rushes) (NOM—GRA), non-mycorrhizal hemiparasites (NOM—SP), and orchids (ORC). We can summarize our results in two rankings for leaf N concentration (FAB—N > ORC > AM—FORB, ECT > NOM—SP, ERI ≥⃒ NOM—GRA, AM—GRA) and leaf δ15N signature (ORC > NOM—GRA, FAB—N > ECT ≥⃒ ERI ≥⃒ AM—FORB, NOM—SP, AM—GRA) of alpine heath species. We conclude that, within the alpine lichen heath in the Northern Caucasus, the δ15N signature of plant foliage is a relevant indicator of plant functional groups with relatively high 15N content (ORC, FAB-N, NOM-GRA), while the absence of a significant difference between relatively 15N-depleted groups (AM, ERI, and ECT species) isn't clear and may result from both processes, as the increased N isotope fractionation by arbuscular mycorrhizal fungi as the decreased role of ecto- and ericoid mycorrhizal fungi in the flux of N.
The mite fauna of patches of High Arctic moss-grass tundra of Svalbard in Petuniabukta, Billefjord (moss, grasses, Salix polaris, bare soil), and adjacent to Vestpynten, Adventfjord (moss, moss mixed with grasses, Cassiope tetragona, S. polaris), were investigated. Our aim was to describe the mite fauna of this tundra with particular focus on the stage structure that is rarely investigated. We observed that the oribatid mites were distinctly more abundant and richer in species (22) than the gamasid mites (7) and their density and diversity varied between vegetation and location. Species diversity of Oribatida and Gamasida and the Shannon H′ index of mite communities were low, and Liochthonius sellnicki or Tectocepheus velatus dominated the Oribatida and Oppiella translamellata and Diapterobates notatus were relatively abundant, while the Gamasida were dominated by Arctoseius multidentatus, with Zercon forsslundi and Z. solenites relatively abundant. The juvenile densities of the Oribatida were usually greater than adults. Eniochthonius minutissimus and Metabelba sp. are new records for Svalbard.
Chironomids (nonbiting midges) are used to develop centennial length temperature reconstructions for six subalpine and alpine lakes in the central Great Basin of the United States. Faunal turnover, assessed by detrended correspondence analysis (DCA), indicate that substantial compositional change in the midge communities has occurred during the past 100 years. Although the changes in composition are site-specific, increases in Dicrotendipes and decreases in Procladius characterize the late 20th century at a majority of the sites. Notable faunal turnover in midge community composition is observed at five of the six sites beginning at approximately A.D. 1970. Application of a chironomid-based mean July air temperature inference model (r2jack = 0.55, RMSEP = 0.9 °C) to the subfossil chironomid assemblages provides site-specific quantitative reconstructions of past temperature variability for the 20th and 21st centuries. Midge-inferred temperature estimates indicate that four of the six lakes were characterized by above average air temperatures during the post—A.D. 1980 interval and below average temperatures during the early 20th century. The rate of temperature change between A.D. 1920 and A.D. 2010 for these four lakes are: Smith Lake = 0.6 °C 100 yr-1; Birdeye Lake = 0.7 °C 100 yr-1; Cold Lake = 1.2 °C 100 yr-1; Stella Lake = 0.4 °C 100 yr-1. Correspondence between fluctuations in the midge-inferred temperature and instrumental measures of mean July air temperature for Nevada Climate Division #2 is also documented. This study adds to the growing body of evidence that subalpine and alpine lakes in the Intermountain West of the United States have been and are increasingly being affected by anthropogenic climate change in the early 21st century.
Woody plants are encroaching into many herbaceous-dominated communities across the globe, including arctic and alpine tundra. Quantifying the encroachment rate, testing which factors contribute to encroachment, and determining how encroachment is taking place and in which community types encroachment is occurring are essential for predicting shifts in tundra vegetation and carbon (C) storage. We examined willow cover changes from 1946 to 2008 in 18 ha of alpine tundra in Colorado using aerial photographs. We linked this pattern of change with experimental assessment of the effects of increasing summer temperatures, winter precipitation, and nitrogen (N) deposition—factors that this region has experienced over this period—on willow growth and survival. Shrub cover expanded by 441% over 62 years and is increasing at an exponential rate, corresponding to increases in C storage of 137 kg ha-1. Nitrogen and temperature facilitate willow growth and snow increases survival, although N and the combination of N plus snow decrease survival. We find clonal growth (78%) accounts for more expansion than seed dispersal (22%), and that shrubs have expanded into wet, moist, and dry meadow. In addition to a release from grazing, we suggest that global change could be driving shrub expansion.
Based on a Normalized Difference Vegetation Index (NDVI) and remote sensing data of snow cover, we analyzed the variation in NDVI in relation to trends in snow cover and vegetation of the source regions of large rivers on the Qinghai-Tibetan Plateau. We then calculated the relationship between snow cover duration, snow depth, and NDVI to reveal the effect of snow cover change on vegetation growth on a regional scale. The results show that both snow depth and duration tend to reduce gradually from northeast to southwest on the Qinghai-Tibetan Plateau. Furthermore, snow cover duration (snow depth > 0 cm) has high interannual fluctuation and generally shows an increasing trend (P < 0.01) from 1980 to 2004. The interannual fluctuations of the duration of days with snow depth ≥ 5 cm as well as the maximum and average snow depth are also quite high, but they generally show insignificant tendencies (P > 0.05) from 1980 to 2004. The snow cover characteristics (duration and depth) are insignificantly correlated to annual maximum NDVI. However, a significant positive correlation (P < 0.05) is observed between snow cover duration (snow depth > 0 cm) and the NDVI values of both April and July, and an obvious negative correlation (P < 0.05) is observed between snow depth and the NDVI value in October across all source regions from 1981 to 2004. In the study area, increasing snow depth and the prolongation of the duration of snow cover have adverse effects on vegetation growth the following year. The melting of snow brings increasing effects to the NDVI value in the spring.
While valley glaciers have received considerable attention for their contributions to summer runoff during the past decade, the contributions of rock glaciers to summer runoff patterns have largely been ignored, especially in the western United States. This article examines summer runoff from two basins in the La Sal Mountains, Utah: the non—rock glaciated Wet Fork and rock glaciated Gold Basin. Runoff events were analyzed for volume of stormflow, stormflow duration, and peak flow duration. Four events were recorded in Wet Fork (n = 4), five events were recorded in Gold Basin (n = 5), and six events at a flume immediately adjacent to the Gold Basin rock glacier (n = 6). Wet Fork hydrographs are dominated by baseflow throughout the summer and a small proportion (0.13%–0.31%) of precipitation leaves the basin as stormflow during storms. Gold Basin hydrographs are characterized by early season snowmelt with flood peaks associated with summer storms. Runoff from the gaged rock glacier represents 15%–30% of total basin runoff and is inversely related to precipitation and directly related to rainfall intensity. Removal of rock glacier hydrographs from total basin hydrographs indicates that there is increased surface runoff from alpine drainage basins that contain rock glaciers, suggesting rock glaciers act as impervious surfaces. This short-term study in Utah suggests that alpine drainage basins with rock glaciers could have greater surface runoff and higher flood peaks than drainage basins that lack rock glaciers. While the long-term effects of rock glaciers on summer water resources is still unknown, this investigation demonstrates rock glaciers may profoundly influence hydrographs in alpine drainage basins.
The alpine treeline in northern Fennoscandia is composed primarily of mountain birch (Betula pubescens ssp. czerepanovii), a deciduous tree that experiences episodic defoliation due to outbreaks of the autumnal moth (Epirrita autumnata) and winter moth (Operophtera brumata). Here, we use an extensive dendroecological data set to reconstruct historic defoliating outbreaks and relate them to climatic conditions. Our data are from 25 sites in eight valleys in northern Sweden. We used the computer program OUTBREAK to reconstruct moth outbreaks. The reconstructed outbreak record matches the historical record well. There is a significant, but weak relationship between the outbreak severity and temperatures in February, April, July, and August of the outbreak year. Temperatures in the previous May and November were also positively correlated with outbreak severity. For seasonally aggregated temperatures, only autumn temperatures are correlated with outbreak severity. There was no significant correlation between NAO index and outbreak severity. A spatiotemporal semivariogram analysis showed that sites within approximately 100 km of each other show similar patterns in outbreak severity. Our analyses suggest that moths are affected by climatic variations. The influence of climate on outbreaks is weak because background climatic conditions alone cannot induce an outbreak. Outbreaks also depend on nonclimatic factors, such as tree age, and the outbreak status of neighboring areas.
The accuracy of daily mean surface pressure from five meteorological reanalyses is assessed against in situ observations from automatic weather stations in East Antarctica for 2005 to 2008. The in situ observations are from Zhongshan, LGB69, EAGLE, and Dome A. The five reanalyses all explain more than 87% of the average variance and have annual root mean square errors between 15 hPa and 45 hPa. The ERA Interim reanalysis performs best against both criteria. The NCEP-1, NCEP-2, and 20CRv2 reanalyses have negative biases of 29.7 hPa, 25.9 hPa, and 11.1 hPa, respectively, while ERA Interim and JCDAS have positive biases of 4.9 hPa and 14.9 hPa. The reanalyses do not show obvious seasonal differences. The errors generally tend to decrease from the coast to the interior of the East Antarctic ice sheet, although there are regional differences between the performance of the different reanalyses. ERA Interim is superior to other reanalyses, probably because of its 4D assimilation scheme, which is strongly guided by satellite observations. The three NCEP reanalyses perform worst; their assimilation scheme is more constrained by limited observations and 20CRv2 has less input data, assimilating only surface pressure observations. Despite deficiencies and limitations, the reanalyses are still powerful tools for climate studies in the Antarctic region. However, more in situ observations are required, especially from the vast interior of Antarctica.
Increasing temperatures in arctic regions are causing earlier spring snowmelt, leading to earlier plant emergence, which could lengthen the period of carbon uptake. Warming is also leading to a shift from graminoid to deciduous shrub-dominated tundra, and in many areas deciduous shrubs are becoming taller. As taller shrubs become increasingly dominant, arctic landscapes may retain more snow, which could lengthen spring snow cover duration and offset advances in the start of the growing season that are expected as a result of earlier spring snowmelt. As a consequence, deeper snow and later snowmelt in taller shrub tundra could delay plant emergence and shorten the period of carbon uptake. This study tracked leaf development of two abundant deciduous shrubs, Betula nana and Salix pulchra, and compared individuals along a natural shrub height gradient on the North Slope of Alaska. We measured spring snow depth and snow cover duration, bud and developing leaf nitrogen content, as well as the timing of budburst and leaf expansion. Taller deciduous shrubs in shrub-dominated communities had deeper snow surrounding them, and became snow-free 1 to 6 days later, delaying budburst by 2 to 12 days relative to shorter deciduous shrubs in graminoid-dominated communities. However, leaf development of tall shrubs caught up to that of short shrubs; occasionally, tall shrubs reached full leaf expansion 1 to 4 days before short shrubs, indicating more rapid leaf development. This convergence in the timing of later leaf development stages is potentially enabled by approximately 16% to 25% greater nitrogen in buds and developing leaves of taller shrubs compared with shorter shrubs. Our findings suggest that delayed snowmelt in areas dominated by taller shrubs may have a short-lived impact on the timing of leaf development, likely resulting in no difference in duration of peak photosynthetic period between tall and short stature shrubs.