Contrasting long-term alpine and subalpine precipitation trends in a mid-latitude North American mountain system,Colorado Front Range,USA

Background: Long-term climate trends in mountain systems often vary strongly with elevation.

Aims: To evaluate elevation dependence in long-term precipitation trends in subalpine forest and alpine tundra zones of a mid-continental, mid-latitude North American mountain system and to relate such dependence to atmospheric circulation patterns.

Methods: We contrasted 59-year (1952–2010) precipitation records of two high-elevation climate stations on Niwot Ridge, Colorado Front Range, Rocky Mountains, USA. The sites, one in forest (3022 m a.s.l.) and the other in alpine tundra (3739 m), are closely located (within 7 km horizontally, ca. 700 m vertically), but differ with respect to proximity to the mountain-system crest (the Continental Divide).

Results: The sites exhibited significant differences in annual and seasonal precipitation trends, which depended strongly on their elevation and distance from the Continental Divide. Annual precipitation increased by 60 mm (+6%) per decade at the alpine site, with no significant change at the subalpine site. Seasonally, trends at the alpine site were dominated by increases in winter, which we suggest resulted from an increase in orographically generated precipitation over the Divide, driven by upper-air (700 hPa) north-westerly flow. Such a change was not evident at the subalpine site, which is less affected by orographic precipitation on north-westerly flow.

Conclusions: Elevation dependence in precipitation trends appears to have arisen from a change in upper-air flow from predominantly south-westerly to north-westerly. Dependence of precipitation trends on topographic position and season has complex implications for the ecology and hydrology of Niwot Ridge and adjacent watersheds, involving interactions among physical processes (e.g. snowpack dynamics) and biotic responses (e.g. in phenologies and ecosystem productivity).     

Interpretation of the last‐glacial vegetation of eastern‐central Europe using modern analogues from southern Siberia

Aim Interpretation of fossil pollen assemblages may benefit greatly from comparisons with modern palynological and vegetation analogues. To interpret the full‐ and late‐glacial vegetation in eastern‐central Europe we compared fossil pollen assemblages from this region with modern pollen assemblages from various vegetation types in southern Siberia, which presumably include the closest modern analogues of the last‐glacial vegetation of central Europe. Location Czech and Slovak Republics (fossil pollen assemblages); Western Sayan Mountains, southern Siberia (modern pollen assemblages). Methods Eighty‐eight modern pollen spectra were sampled in 14 vegetation types of Siberian forest, tundra and steppe, and compared with the last‐glacial pollen spectra from seven central European localities using principal components analysis. Results Both full‐ and late‐glacial pollen spectra from the valleys of the Western Carpathians (altitudes 350–610 m) are similar to modern pollen spectra from southern Siberian taiga, hemiboreal forest and dwarf‐birch tundra. The full‐glacial and early late‐glacial pollen spectra from lowland river valleys in the Bohemian Massif (altitudes 185–190 m) also indicate the presence of patches of hemiboreal forest or taiga. Other late‐glacial pollen spectra from the Bohemian Massif suggest an open landscape with steppe or tundra or a mosaic of both, possibly with small patches of hemiboreal forest. Main conclusions Our results are consistent with the hypothesis that during the full glacial and late glacial, the mountain valleys of the north‐western Carpathians supported taiga or hemiboreal forest dominated by Larix, Pinus cembra, Pinus sylvestris and Picea, along with some steppic or tundra formations. Forests tended to be increasingly open or patchy towards the west (Moravian lowlands), gradually passing into the generally treeless landscape of Bohemia, with possible woodland patches in locally favourable sites.     

Climate and species affect fine root production with long-term fertilization in acidic tussock tundra near Toolik Lake,Alaska

Long-term fertilization of acidic tussock tundra has led to changes in plant species composition, increases in aboveground production and biomass and substantial losses of soil organic carbon (SOC). Root litter is an important input to SOC pools, although little is known about fine root demography in tussock tundra. In this study, we examined the response of fine root production and live standing fine root biomass to short- and long-term fertilization, as changes in fine root demography may contribute to observed declines in SOC. Live standing fine root biomass increased with long-term fertilization, while fine root production declined, reflecting replacement of the annual fine root system of Eriophorum vaginatum, with the long-lived fine roots of Betula nana. Fine root production increased in fertilized plots during an unusually warm growing season, but remained unchanged in control plots, consistent with observations that B. nana shows a positive response to climate warming. Calculations based on a few simple assumptions suggest changes in fine root demography with long-term fertilization and species replacement could account for between 20 and 39% of the observed declines in SOC stocks.     

Long‐term experimental warming alters community composition of ascomycetes in Alaskan moist and dry arctic tundra

Arctic tundra regions have been responding to global warming with visible changes in plant community composition, including expansion of shrubs and declines in lichens and bryophytes. Even though it is well known that the majority of arctic plants are associated with their symbiotic fungi, how fungal community composition will be different with climate warming remains largely unknown. In this study, we addressed the effects of long‐term (18 years) experimental warming on the community composition and taxonomic richness of soil ascomycetes in dry and moist tundra types. Using deep Ion Torrent sequencing, we quantified how OTU assemblage and richness of different orders of Ascomycota changed in response to summer warming. Experimental warming significantly altered ascomycete communities with stronger responses observed in the moist tundra compared with dry tundra. The proportion of several lichenized and moss‐associated fungi decreased with warming, while the proportion of several plant and insect pathogens and saprotrophic species was higher in the warming treatment. The observed alterations in both taxonomic and ecological groups of ascomycetes are discussed in relation to previously reported warming‐induced shifts in arctic plant communities, including decline in lichens and bryophytes and increase in coverage and biomass of shrubs.     

Arctic warming: nonlinear impacts of sea‐ice and glacier melt on seabird foraging

Arctic climate change has profound impacts on the cryosphere, notably via shrinking sea‐ice cover and retreating glaciers, and it is essential to evaluate and forecast the ecological consequences of such changes. We studied zooplankton‐feeding little auks (Alle alle), a key sentinel species of the Arctic, at their northernmost breeding site in Franz‐Josef Land (80°N), Russian Arctic. We tested the hypothesis that little auks still benefit from pristine arctic environmental conditions in this remote area. To this end, we analysed remote sensing data on sea‐ice and coastal glacier dynamics collected in our study area across 1979–2013. Further, we recorded little auk foraging behaviour using miniature electronic tags attached to the birds in the summer of 2013, and compared it with similar data collected at three localities across the Atlantic Arctic. We also compared current and historical data on Franz‐Josef Land little auk diet, morphometrics and chick growth curves. Our analyses reveal that summer sea‐ice retreated markedly during the last decade, leaving the Franz‐Josef Land archipelago virtually sea‐ice free each summer since 2005. This had a profound impact on little auk foraging, which lost their sea‐ice‐associated prey. Concomitantly, large coastal glaciers retreated rapidly, releasing large volumes of melt water. Zooplankton is stunned by cold and osmotic shock at the boundary between glacier melt and coastal waters, creating new foraging hotspots for little auks. Birds therefore switched from foraging at distant ice‐edge localities, to highly profitable feeding at glacier melt‐water fronts within <5 km of their breeding site. Through this behavioural plasticity, little auks maintained their chick growth rates, but showed a 4% decrease in adult body mass. Our study demonstrates that arctic cryosphere changes may have antagonistic ecological consequences on coastal trophic flow. Such nonlinear responses complicate modelling exercises of current and future polar ecosystem dynamics.     

Identifying Differences in Carbon Exchange among Arctic Ecosystem Types

Our objective was to determine how varied is the response of C cycling to temperature and irradiance in tundra vegetation. We used a large chamber to measure C exchange at 23 locations within a small arctic catchment in Alaska during summer 2003 and 2004. At each location, we determined light response curves of C exchange using shade cloths, twice during a growing season. We used data to fit a simple photosynthesis-irradiance, respiration-temperature model, with four parameters. We used a maximum likelihood technique to determine the acceptable parameter space for each light curve, given measurement uncertainty. We then explored which sites and time periods had parameter sets in common—an indication of functional similarity. We found that seven distinct parameter sets were required to explain observed C flux responses to temperature and light variation at all sites and time periods. The variation in estimated maximum photosynthetic rate (P_max) was strongly correlated with measurements of site leaf area index (LAI). The behavior of tussock tundra sites, the dominant vegetation of arctic tundra, could largely be described with a single parameter set, with a P_max of 9.7 μmol m⁻² s⁻¹. Tussock tundra sites had, correspondingly, similar LAI (mean = 0.66). Non-tussock sites (for example, sedge and shrub tundras) had larger spatial and temporal variations in both C dynamic parameters (P_max varying from 9.7–25.7 μmol m⁻² s⁻¹) and LAI (0.6–2.0). There were no clear relationships between dominant non-tussock vegetation types and a particular parameter set. Our results suggest that C dynamics of the acidic tussock tundra slopes and hilltops in northern Alaska are relatively simply described during the peak growing season. However, the foot-slopes and water tracks have more variable patterns of LAI and C exchange, not simply related to the dominant vegetation type.     

Trophic transfer and trophic modification of fatty acids in high Arctic lakes

1. A comparative study of fatty acid (FA) profiles in particulate matter (seston) and the key grazer Daphnia was performed in six high Arctic ponds (79°N, Svalbard). The ponds were all small and shallow, but followed a strong gradient with respect to nutrient content and optical properties. 2. A distinct locality‐specific pattern was detected by principal component analysis of FA profiles, where samples from each locality clustered both with regard to seston and Daphnia. Linear discriminant analysis using nine sestonic fatty acids as discriminant functions gave on average a correct prediction of the Daphnia lake membership in 47% of cases, with very high predictability in some lakes but poor predictability in others. 3. No significant correlation was detected between FA and nutrient concentration or levels of dissolved organic carbon. The major determinant of FA profiles as judged from a redundancy analysis was the taxonomic composition of phytoplankton communities, notably the biomass of Chlorophyceae. 4. The FA profiles of Daphnia were for some FAs strongly enriched relative to the seston, while diluted for others. Among the polyunsaturated fatty acids (PUFAs), a pronounced magnification of eicosapentaenoic acid (EPA, 20 : 5 n‐3), and to some extent 18 : 3 n‐3 and 20 : 4 n‐6 was found, while docosahexaenoic acid (DHA, 22 : 6 n‐3) contributed in general less to FAs in Daphnia than in seston and was hardly detectable in Daphnia from most localities. 5. The overall content of PUFAs in Daphnia was consistently high, close to 40% of total FA in all investigated localities, despite major differences in seston PUFA content. Daphnia thus acts as a regulator with regard to overall PUFAs, reflecting its physiological constraints and relatively fixed demands for PUFAs in general. The distinct locality‐specific profiles in Daphnia strongly suggest a kind of FA‐fingerprint, but our data do not allow strict statements on the use of specific FAs as trophic markers.     

Long-term functional plasticity in plant hydraulic architecture in response to supplemental moisture

Background and Aims

Plasticity in structural and functional traits related to water balance may determine plant performance and survival in ecosystems characterized by water limitation or high levels of rainfall variability, particularly in perennial herbaceous species with long generation cycles. This paper addresses whether and the extent to which several such seasonal to long-term traits respond to changes in moisture availability.

Methods

Using a novel approach that integrates ecology, physiology and anatomy, a comparison was made of lifetime functional traits in the root xylem of a long-lived perennial herb (Potentilla diversifolia, Rosaceae) growing in dry habitats with those of nearby individuals growing where soil moisture had been supplemented for 14 years. Traditional parameters such as specific leaf area (SLA) and above-ground growth were also assessed.

Key Results

Individuals from the site receiving supplemental moisture consistently showed significant responses in all considered traits related to water balance: SLA was greater by 24 %; roots developed 19 % less starch storing tissue, an indicator for drought-stress tolerance; and vessel size distributions shifted towards wider elements that collectively conducted water 54 % more efficiently – but only during the years for which moisture was supplemented. In contrast, above-ground growth parameters showed insignificant or inconsistent responses.

Conclusions

The phenotypic changes documented represent consistent, dynamic responses to increased moisture availability that should increase plant competitive ability. The functional plasticity of xylem anatomy quantified in this study constitutes a mechanistic basis for anticipating the differential success of plant species in response to climate variability and change, particularly where water limitation occurs.