Phenological observations of major plant growth forms and species in montane and Eriophorum vaginatum tussock tundra in central Alaska

The development stages of several tundra plant species were observed during the summers of 1977 and 1978 in different vegetation zones in a snow accumulation area and in tussock tundra. Leaf maturation and senescence tend towards synchrony regardless of the time of emergence from under the snow. Flowering stages are less synchronous and general than are the vegetation stages within a growth form.     

Differential ecophysiological response of deciduous shrubs and a graminoid to long-term experimental snow reductions and additions in moist acidic tundra, Northern Alaska

Changes in winter precipitation that include both decreases and increases in winter snow are underway across the Arctic. In this study, we used a 14-year experiment that has increased and decreased winter snow in the moist acidic tussock tundra of northern Alaska to understand impacts of variation in winter snow depth on summer leaf-level ecophysiology of two deciduous shrubs and a graminoid species, including: instantaneous rates of leaf gas exchange, and δ¹³C, δ¹⁵N, and nitrogen (N) concentrations of Betula nana, Salix pulchra, and Eriophorum vaginatum. Leaf-level measurements were complemented by measurements of canopy leaf area index (LAI) and depth of thaw. Reductions in snow lowered summer leaf photosynthesis, conductance, and transpiration rates by up to 40 % compared to ambient and deep snow conditions for Eriophorum vaginatum, and reduced Salix pulchra conductance and transpiration by up to 49 %. In contrast, Betula nana exhibited no changes in leaf gas exchange in response to lower or deeper snow. Canopy LAI increased with added snow, while reduced winter snow resulted in lower growing season soil temperatures and reduced thaw depths. Our findings indicate that the spatial and temporal variability of future snow depth will have individualistic consequences for leaf-level C fixation and water flux by tundra species, and that these responses will be manifested over the longer term by changes in canopy traits, depth of thaw, soil C and N processes, and trace gas (CO₂ and H₂O) exchanges between the tundra and the atmosphere.     

Winter precipitation and snow accumulation drive the methane sink or source strength of Arctic tussock tundra

Arctic winter precipitation is projected to increase with global warming, but some areas will experience decreases in snow accumulation. Although Arctic CH₄ emissions may represent a significant climate forcing feedback, long‐term impacts of changes in snow accumulation on CH₄ fluxes remain uncertain. We measured ecosystem CH₄ fluxes and soil CH₄ and CO₂ concentrations and ¹³C composition to investigate the metabolic pathways and transport mechanisms driving moist acidic tundra CH₄ flux over the growing season (Jun–Aug) after 18 years of experimental snow depth increases and decreases. Deeper snow increased soil wetness and warming, reducing soil %O₂ levels and increasing thaw depth. Soil moisture, through changes in soil %O₂ saturation, determined predominance of methanotrophy or methanogenesis, with soil temperature regulating the ecosystem CH₄ sink or source strength. Reduced snow (RS) increased the fraction of oxidized CH₄ (Fox) by 75–120% compared to Ambient, switching the system from a small source to a net CH₄ sink (21 ± 2 and ?31 ± 1 mg CH₄ m^?2 season^?1 at Ambient and RS). Deeper snow reduced Fox by 35–40% and 90–100% in medium‐ (MS) and high‐ (HS) snow additions relative to Ambient, contributing to increasing the CH₄ source strength of moist acidic tundra (464 ± 15 and 3561 ± 97 mg CH₄ m^?2 season^?1 at MS and HS). Decreases in Fox with deeper snow were partly due to increases in plant‐mediated CH₄ transport associated with the expansion of tall graminoids. Deeper snow enhanced CH₄ production within newly thawed soils, responding mainly to soil warming rather than to increases in acetate fermentation expected from thaw‐induced increases in SOC availability. Our results suggest that increased winter precipitation will increase the CH₄ source strength of Arctic tundra, but the resulting positive feedback on climate change will depend on the balance between areas with more or less snow accumulation than they are currently facing.     

Shrubline but not treeline advance matches climate velocity in montane ecosystems of south‐central Alaska

Tall shrubs and trees are advancing into many tundra and wetland ecosystems but at a rate that often falls short of that predicted due to climate change. For forest, tall shrub, and tundra ecosystems in two pristine mountain ranges of Alaska, we apply a Bayesian, error‐propagated calculation of expected elevational rise (climate velocity), observed rise (biotic velocity), and their difference (biotic inertia). We show a sensitive dependence of climate velocity on lapse rate and derive biotic velocity as a rigid elevational shift. Ecosystem presence identified from recent and historic orthophotos ~50 years apart was regressed on elevation. Biotic velocity was estimated as the difference between critical point elevations of recent and historic logistic fits divided by time between imagery. For both mountain ranges, the 95% highest posterior density of climate velocity enclosed the posterior distributions of all biotic velocities. In the Kenai Mountains, mean tall shrub and climate velocities were both 2.8 m y^?1. In the better sampled Chugach Mountains, mean tundra retreat was 1.2 m y^?1 and climate velocity 1.3 m y^?1. In each mountain range, the posterior mode of tall woody vegetation velocity (the complement of tundra) matched climate velocity better than either forest or tall shrub alone, suggesting competitive compensation can be important. Forest velocity was consistently low at 0.1–1.1 m y^?1, indicating treeline is advancing slowly. We hypothesize that the high biotic inertia of forest ecosystems in south‐central Alaska may be due to competition with tall shrubs and/or more complex climate controls on the elevational limits of trees than tall shrubs. Among tall shrubs, those that disperse farthest had lowest inertia. Finally, the rapid upward advance of woody vegetation may be contributing to regional declines in Dall's sheep (Ovis dalli), a poorly dispersing alpine specialist herbivore with substantial biotic inertia due to dispersal reluctance.     

Shifts of tundra bacterial and archaeal communities along a permafrost thaw gradient in Alaska

Understanding the response of permafrost microbial communities to climate warming is crucial for evaluating ecosystem feedbacks to global change. This study investigated soil bacterial and archaeal communities by Illumina MiSeq sequencing of 16S rRNA gene amplicons across a permafrost thaw gradient at different depths in Alaska with thaw progression for over three decades. Over 4.6 million passing 16S rRNA gene sequences were obtained from a total of 97 samples, corresponding to 61 known classes and 470 genera. Soil depth and the associated soil physical–chemical properties had predominant impacts on the diversity and composition of the microbial communities. Both richness and evenness of the microbial communities decreased with soil depth. Acidobacteria, Verrucomicrobia, Alpha‐ and Gamma‐Proteobacteria dominated the microbial communities in the upper horizon, whereas abundances of Bacteroidetes, Delta‐Proteobacteria and Firmicutes increased towards deeper soils. Effects of thaw progression were absent in microbial communities in the near‐surface organic soil, probably due to greater temperature variation. Thaw progression decreased the abundances of the majority of the associated taxa in the lower organic soil, but increased the abundances of those in the mineral soil, including groups potentially involved in recalcitrant C degradation (Actinomycetales, Chitinophaga, etc.). The changes in microbial communities may be related to altered soil C sources by thaw progression. Collectively, this study revealed different impacts of thaw in the organic and mineral horizons and suggests the importance of studying both the upper and deeper soils while evaluating microbial responses to permafrost thaw.     

Plant production and nitrogen accumulation above- and belowground in low and tall birch tundra communities: the influence of snow and litter

Background and Aim

A vegetation transition to taller and denser deciduous shrub tundra is currently occurring in many locations across the low Arctic, and is associated with climate change. Here, we investigated if deeper snow is a mechanism for enhanced shrub growth.

Methods

To determine if a moderate and climatically realistic increase in snow depth can enhance shrub productivity, we compared growth responses between ambient and experimentally deepened snow plots in low birch hummock tundra. To determine the potential influence of factors other than deepened snow that are associated with taller, denser shrubs, we also compared shrub growth between low birch hummock and tall birch-dominated tundra.

Results

Neither deciduous shrub above- nor belowground production nor nitrogen accumulation was enhanced by deepened snow. However, deciduous birch shrub new shoot production was 23× larger and total vascular shoot to belowground biomass ratios were higher in the tall birch tundra than the birch hummock (~0.7 and ~0.4, respectively), indicating that the combination of deeper snow together with other internal feedbacks greatly enhanced birch growth.

Conclusions

Together, our results strongly suggest that the much larger litter production in tall birch ecosystems is an important internal feedback that may or may not interact with deeper snow to promote birch growth in tall shrub tundra.

     

Pollen and plant diversity relationships in a Mediterranean montane area

Vegetation History and Archaeobotany - Understanding the significance of pollen diversity is key to reconstructing plant diversity over long timescales. Here we present quantitative pollen-plant...     

Direct and indirect control by snow cover over decomposition in alpine tundra along a snowmelt gradient

We assessed direct and indirect effects of snow cover on litter decomposition and litter nitrogen release in alpine tundra. Direct effects are driven by the direct influence of snow cover on edaphoclimatic conditions, whereas indirect effects result from the filtering effect of snow cover on species’ abundance and traits. We compared the in situ decomposition of leaf litter from four dominant plant species (two graminoids, two shrubs) at early and late snowmelt locations using a two-year litter-bag experiment. A seasonal experiment was also performed to estimate the relative importance of winter and summer decomposition. We found that growth form (graminoids vs. shrubs) are the main determinants of decomposition rate. Direct effect of snow cover exerted only a secondary influence. Whatever the species, early snowmelt locations showed consistently reduced decomposition rates and delayed final stages of N mineralization. This lower decomposition rate was associated with freezing soil temperatures during winter. The results suggest that a reduced snow cover may have a weak and immediate direct effect on litter decomposition rates and N availability in alpine tundra. A much larger impact on nutrient cycling is likely to be mediated by longer term changes in the relative abundance of lignin-rich dwarf shrubs.     

Local snow melt and temperature—but not regional sea ice—explain variation in spring phenology in coastal Arctic tundra

The Arctic is undergoing dramatic environmental change with rapidly rising surface temperatures, accelerating sea ice decline and changing snow regimes, all of which influence tundra plant phenology. Despite these changes, no globally consistent direction of trends in spring phenology has been reported across the Arctic. While spring has advanced at some sites, spring has delayed or not changed at other sites, highlighting substantial unexplained variation. Here, we test the relative importance of local temperatures, local snow melt date and regional spring drop in sea ice extent as controls of variation in spring phenology across different sites and species. Trends in long‐term time series of spring leaf‐out and flowering (average span: 18 years) were highly variable for the 14 tundra species monitored at our four study sites on the Arctic coasts of Alaska, Canada and Greenland, ranging from advances of 10.06 days per decade to delays of 1.67 days per decade. Spring temperatures and the day of spring drop in sea ice extent advanced at all sites (average 1°C per decade and 21 days per decade, respectively), but only those sites with advances in snow melt (average 5 days advance per decade) also had advancing phenology. Variation in spring plant phenology was best explained by snow melt date (mean effect: 0.45 days advance in phenology per day advance snow melt) and, to a lesser extent, by mean spring temperature (mean effect: 2.39 days advance in phenology per °C). In contrast to previous studies examining sea ice and phenology at different spatial scales, regional spring drop in sea ice extent did not predict spring phenology for any species or site in our analysis. Our findings highlight that tundra vegetation responses to global change are more complex than a direct response to warming and emphasize the importance of snow melt as a local driver of tundra spring phenology.     

Environmental effects on CO2 efflux from riparian tundra in the northern foothills of the Brooks Range,Alaska, USA

Summary Carbon dioxide efflux and soil microenvironmental factors were measured diurnally in Carex aquatilus-and Eriophorum angustifolium-dominated riparian tundra communities to determine the relative importance of soil environmental factors controlling ecosystem carbon dioxide exchange with the atmosphere. Measurements were made weekly between 18 June and 24 July 1990. Diurnal patterns in carbon dioxide efflux were best explained by changes in soil temperature, while seasonal changes in efflux were correlated with changes in depth to water table, depth to frozen soil and soil moisture. Carbon dioxide efflux rates were lowest early in the growing season when high water tables and low soil temperatures limited microbial and root activity. Individual rainfall events that raised the water table were found to strongly reduce carbon dioxide efflux. As the growing season progressed, rainfall was low and depth to water table and soil temperatures increased. In response, carbon dioxide efflux increased strongly, attaining rates late in the season of approximately 10 g CO₂ m^–2 day^–1. These rates are as high as maxima recorded for other arctic sites. A mathematical model is developed which demonstrates that soil temperature and depth to water table may be used as efficient predictors of ecosystem CO₂ efflux in this habitat. In parallel with the field measurements of CO₂ efflux, microbial respiration was studied in the laboratory as a function of temperature and water content. Estimates of microbial respiration per square meter under field conditions were made by adjusting for potential respiring soil volume as water table changed and using measured soil temperatures. The results indicate that the effect of these factors on microbial respiration may explain a large part of the diurnal and seasonal variation observed in CO₂ efflux. As in coastal tundra sites, environmental changes that alter water table depth in riparian tundra communities will have large effects on ecosystem CO₂ efflux and carbon balance.     

Large and small herbivores have strong effects on tundra vegetation in Scandinavia and Alaska

1. Large and small mammalian herbivores are present in most vegetated areas in the Arctic and often have large impacts on plant community composition and ecosystem functioning. The relative importance of different herbivores and especially how their specific impact on the vegetation varies across the Arctic is however poorly understood.
2. Here, we investigate how large and small herbivores influence vegetation density and plant community composition in four arctic vegetation types in Scandinavia and Alaska. We used a unique set of exclosures, excluding only large (reindeer and muskoxen) or all mammalian herbivores (also voles and lemmings) for at least 20 years.
3. We found that mammalian herbivores in general decreased leaf area index, NDVI, and abundance of vascular plants in all four locations, even though the strength of the effect and which herbivore type caused these effects differed across locations. In three locations, herbivore presence caused contrasting plant communities, but not in the location with lowest productivity. Large herbivores had a negative effect on plant height, whereas small mammalian herbivores increased species diversity by decreasing dominance of the initially dominating plant species. Above‐ or belowground disturbances caused by herbivores were found to play an important role in shaping the vegetation in all locations.
4. Synthesis: Based on these results, we conclude that both small and large mammalian herbivores influence vegetation in Scandinavia and Alaska in a similar way, some of which can mitigate effects of climate change. We also see important differences across locations, but these depend rather on local herbivore and plant community composition than large biogeographical differences among continents.

     

Spatial variation in anuran richness,diversity, and abundance across montane wetland habitat in Volcanoes National Park,Rwanda

The spatial distribution of species has long sparked interest among ecologists and biogeographers, increasingly so in studies of species responses to climate change. However, field studies on spatial patterns of distribution, useful to inform conservation actions at local scales, are still lacking for many regions, especially the tropics. We studied elevational trends and species‐area relationships among anurans in wetland habitats within Volcanoes National Park (VNP) in Rwanda, part of the biodiverse Albertine Rift region. In VNP, wetlands are key sites for anuran reproduction, and anurans are likely threatened by wetland desiccation which has occurred for the last few decades. Between 2012 and 2017, we sampled anuran communities in ten VNP wetlands located along an elevational gradient of c. 600 m (from 2,546 to 3,188 m a.s.l.) and found at least eight species, including at least two Albertine Rift Endemics. We show that species richness, diversity, and abundance likely decline with a decrease in wetland size and with an increase in elevation, though additional sampling (e.g., at night) might be needed to derive definite conclusions. Larger wetlands at lower elevations contained most species and individuals, which indicates the potential threat of wetland size reduction (through desiccation) for anuran conservation. However, we also found that wetlands differed in species composition and that some species (e.g., Sclerophrys kisoloensis) were likely restricted in distribution to only a few of the smaller wetlands—suggesting that the conservation of each individual wetland should be prioritized, regardless of size. We propose that all wetlands in VNP require additional conservation measures, which should be based on knowledge gathered through long‐term monitoring of anuran communities and research on drivers of wetland decline. Only such extended research will allow us to understand the response of anurans in VNP to threats such as climate change and wetland desiccation.     

雪灾后粤北山地常绿阔叶林优势树种幼苗更新动态

对遭受2008年雪灾破坏的车八岭山地常绿阔叶林进行连续3 a(2008-2010年)的监测,研究在冠层恢复过程中优势树种的林下幼苗动态,旨在了解幼苗灾后的更新规律及其对冠层结构变化(以叶面积指数LAI的变化来表示)的响应。结果表明,12个优势种的林下幼苗对冠层恢复有明显的响应。非参数的多元方差分析(perMANOVA)显示,优势种幼苗的组成和分布存在着极显著的年际差异(P<0.001);其中2008年与2009年及2010年的差异均极显著(P<0.001);2009年与2010年的差异不显著。这与冠层LAI的变化情况相应:2008年LAI最低,2009年LAI值迅速增加;2010与2009年相比LAI增长缓慢,并逐渐趋于稳定。不同的优势树种幼苗对样方中LAI变化的响应不同,阳性树种幼苗的相对多度和频度一般会随林分郁闭度的增加而锐减,阴性及耐阴树种幼苗数量则随冠层恢复而增加。随着冠层恢复,林下指示种截然变化。2008年的林下幼苗指示种为8个喜光的种类,而2010年则仅见较耐阴的香楠Randia canthioides 为指示种。指示种分析从另一个角度反映了幼苗对冠层恢复的响应。     

Environmental effects on molecular and phenotypic variation in populations of Eruca sativa across a steep climatic gradient

In Israel Eruca sativa has a geographically narrow distribution across a steep climatic gradient that ranges from mesic Mediterranean to hot desert environments. These conditions offer an opportunity to study the influence of the environment on intraspecific genetic variation. For this, we combined an analysis of neutral genetic markers with a phenotypic evaluation in common‐garden experiments, and environmental characterization of populations that included climatic and edaphic parameters, as well as geographic distribution. A Bayesian clustering of individuals from nine representative populations based on amplified fragment length polymorphism (AFLP) divided the populations into a southern and a northern geographic cluster, with one admixed population at the geographic border between them. Linear mixed models, with cluster added as a grouping factor, revealed no clear effects of environment or geography on genetic distances, but this may be due to a strong association of geography and environment with genetic clusters. However, environmental factors accounted for part of the phenotypic variation observed in the common‐garden experiments. In addition, candidate loci for selection were identified by association with environmental parameters and by two outlier methods. One locus, identified by all three methods, also showed an association with trichome density and herbivore damage, in net‐house and field experiments, respectively. Accordingly, we propose that because trichomes are directly linked to defense against both herbivores and excess radiation, they could potentially be related to adaptive variation in these populations. These results demonstrate the value of combining environmental and phenotypic data with a detailed genetic survey when studying adaptation in plant populations.     

Adaptive plasticity in wing melanisation of a montane butterfly across a Himalayan elevational gradient

1. Traits that are significant to the thermal ecology of temperate or montane species are expected to prominently co-vary with the thermal environment experienced by an organism. The Himalayan Pieris canidia butterfly exhibits considerable variation in wing melanisation. We investigated: (i) whether variation in wing melanisation and (ii) activity period of this montane butterfly was influenced by the seasonally and elevationally changing thermal landscape.
2. We discovered that wing melanisation varied across elevation, seasons, sex, and wing surfaces, with the variation strongly structured in space and time: colder seasons and higher elevations produced more melanic individuals. Notably, melanisation did not vary uniformly across all wing surfaces: (i) melanisation of the ventral hindwing co-varied much more prominently with elevation, but (ii) melanisation on all other surfaces varied with seasonal changes in the thermal environment.
3. Observed wing surface-specific patterns indicated thermoregulatory function for this variation in melanisation. Such wing surface-specific responses to seasonal and elevational variation in temperature have rarely been reported in montane insects.
4. Moreover, daily and seasonal thermal cycles were found to strongly influence activity periods of this species, suggesting the potential limits to wing melanisation plasticity.
5. Overall, these results showed that the seasonal and elevational gradients in temperature influence the thermal phenotype as well as activity periods of this Himalayan butterfly. It will be critical to study the phenotypic evolution of such montane insects in response to the ongoing climate change, which is already showing significant signs in this iconic mountain range.

     

Ectomycorrhizal fungal community structure across a bog-forest ecotone in southeastern Alaska

We examined the ectomycorrhizal (ECM) fungal community across a bog-forest ecotone in southeastern Alaska. The bog and edge were both characterized by poorly drained Histosols and a continuous layer of Sphagnum species, ericaceous shrubs, Carex species, and shore pine [Pinus contorta Dougl. ex Loud. var. contorta]. The forest had better-drained Inceptisols and Spodosols, a tree community comprised of western hemlock [Tsuga heterophylla (Raf.) Sarg.], yellow cedar (Thuja plicata Donn ex D. Don.), Sitka spruce [Picea sitchensis (Bong.) Carr.] and shore pine, and an understorey of ericaceous shrubs and herbs. ECM root tip density (tips cm^–3 soil) was significantly greater in the forest than the edge or bog and ECM colonization was significantly different in all three plant communities. The below ground ECM fungal taxa were analyzed using molecular techniques (PCR-RFLP and DNA sequencing). Three ECM fungal taxa, Suillus tomentosus (Kauffman) Singer, Cenococcum geophilum Fr.:Fr, and a Russula species, differed in relative frequency, yet were among the four most frequent in all three plant communities. Although differences in ECM fungal richness were observed across plant communities, unequal sampling of ECM roots due to root density and colonization differences confounded richness comparisons. Using resampling procedures for creating taxon-accumulation curves as a function of sampled ECM roots revealed similarities in cumulative ECM fungal taxa richness across the ecotone.