Vegetation responses in Alaskan arctic tundra after 8 years of a summer warming and winter snow manipulation experiment

We used snow fences and small (1 m²) open‐topped fiberglass chambers (OTCs) to study the effects of changes in winter snow cover and summer air temperatures on arctic tundra. In 1994, two 60 m long, 2.8 m high snow fences, one in moist and the other in dry tundra, were erected at Toolik Lake, Alaska. OTCs paired with unwarmed plots, were placed along each experimental snow gradient and in control areas adjacent to the snowdrifts. After 8 years, the vegetation of the two sites, including that in control plots, had changed significantly. At both sites, the cover of shrubs, live vegetation, and litter, together with canopy height, had all increased, while lichen cover and diversity had decreased. At the moist site, bryophytes decreased in cover, while an increase in graminoids was almost entirely because of the response of the sedge Eriophorum vaginatum. These community changes were consistent with results found in studies of responses to warming and increased nutrient availability in the Arctic. However, during the time period of the experiment, summer temperature did not increase, but summer precipitation increased by 28%. The snow addition treatment affected species abundance, canopy height, and diversity, whereas the summer warming treatment had few measurable effects on vegetation. The interannual temperature fluctuation was considerably larger than the temperature increases within OTCs (<2°C), however. Snow addition also had a greater effect on microclimate by insulating vegetation from winter wind and temperature extremes, modifying winter soil temperatures, and increasing spring run‐off. Most increases in shrub cover and canopy height occurred in the medium snow‐depth zone (0.5–2 m) of the moist site, and the medium to deep snow‐depth zone (2–3 m) of the dry site. At the moist tundra site, deciduous shrubs, particularly Betula nana, increased in cover, while evergreen shrubs decreased. These differential responses were likely because of the larger production to biomass ratio in deciduous shrubs, combined with their more flexible growth response under changing environmental conditions. At the dry site, where deciduous shrubs were a minor part of the vegetation, evergreen shrubs increased in both cover and canopy height. These changes in abundance of functional groups are expected to affect most ecological processes, particularly the rate of litter decomposition, nutrient cycling, and both soil carbon and nitrogen pools. Also, changes in canopy structure, associated with increases in shrub abundance, are expected to alter the summer energy balance by increasing net radiation and evapotranspiration, thus altering soil moisture regimes.     

雪被斑块对川西高山森林凋落叶N和P释放的影响

采用凋落物网袋法,研究冬季不同关键时期雪被斑块对川西高山森林6种代表性树种凋落物分解过程中N和P释放的影响.结果表明: 整个雪被覆盖季节凋落物N表现为富集,P表现为释放,且雪被融化期P释放速率最大.厚型和中型雪被斑块下凋落物P释放速率大于薄型和无雪被斑块,而薄型和无雪被斑块下凋落物的N释放速率明显较高.6种凋落物N释放率和释放速率与日均温呈显著负相关;除岷江冷杉外,其他树种凋落物P释放率和释放速率与日均温表现为正相关.气候变暖情景下冬季雪被覆盖的减小将促进高山森林冬季凋落物分解过程中N释放,抑制P释放.     

Leaf mineral nutrition of Arctic plants in response to warming and deeper snow in northern Alaska

Articulating the consequences of global climate change on terrestrial ecosystem biogeochemistry is a critical component of Arctic system studies. Leaf mineral nutrition responses of tundra plants is an important measure of changes in organismic and ecosystem attributes because leaf nitrogen and carbon contents effect photosynthesis, primary production, carbon budgets, leaf litter, and soil organic matter decomposition as well as herbivore forage quality. In this study, we used a longterm experiment where snow depth and summer temperatures were increased independently and together to articulate how a series of climate change scenarios would affect leaf N, leaf C, and leaf C:N for vegetation in dry and moist tussock tundra in northern Alaska, USA. Our findings were: 1) moist tundra vegetation is much more responsive to this suite of climate change scenarios than dry tundra with up to a 25% increase in leaf N; 2) life forms exhibit divergence in leaf C, N, and C:N with deciduous shrubs and graminoids having almost identical leaf N contents; 3) for some species, leaf mineral nutrition responses to these climate change scenarios are tundra type dependent ( Betula ), but for others ( Vaccinium vitis-idaea ), strong responses are exhibited regardless of tundra type; and 4) the seasonal patterns and magnitudes of leaf C and leaf N in deciduous and evergreen shrubs were responsive to conditions of deeper snow in winter. Leaf N is was generally higher immediately after emergence from the deep snow experimental treatments and leaf N was higher during the subsequent summer and fall, and the leaf C:N were lower, especially in deciduous shrubs. These findings indicate that coupled increases in snow depth and warmer summer temperatures will alter the magnitudes and patterns of leaf mineral nutrition and that the long term consequences of these changes may feed-forward and affect ecosystem processes.     

Shrub expansion modulates belowground impacts of changing snow conditions in alpine grasslands

Climate change is disproportionately impacting mountain ecosystems, leading to large reductions in winter snow cover, earlier spring snowmelt and widespread shrub expansion into alpine grasslands. Yet, the combined effects of shrub expansion and changing snow conditions on abiotic and biotic soil properties remains poorly understood. We used complementary field experiments to show that reduced snow cover and earlier snowmelt have effects on soil microbial communities and functioning that persist into summer. However, ericaceous shrub expansion modulates a number of these impacts and has stronger belowground effects than changing snow conditions. Ericaceous shrub expansion did not alter snow depth or snowmelt timing but did increase the abundance of ericoid mycorrhizal fungi and oligotrophic bacteria, which was linked to decreased soil respiration and nitrogen availability. Our findings suggest that changing winter snow conditions have cross-seasonal impacts on soil properties, but shifts in vegetation can modulate belowground effects of future alpine climate change.     

No increase in alpine snowbed productivity in response to experimental lengthening of the growing season

Climate change effects on snow cover and thermic regime in alpine tundra might lead to a longer growing season, but could also increase risks to plants from spring frost events. Alpine snowbeds, i.e. alpine tundra from late snowmelt sites, might be particularly susceptible to such climatic changes. Snowbed communities were grown in large monoliths for two consecutive years, under different manipulated snow cover treatments, to test for effects of early (E) and late (L) snowmelt on dominant species growth, plant functional traits, leaf area index (LAI) and aboveground productivity. Spring snow cover was reduced to assess the sensitivity of snowbed alpine species to severe early frost events, and dominant species freezing temperatures were measured. Aboveground biomass, productivity, LAI and dominant species growth did not increase significantly in E compared to L treatments, indicating inability to respond to an extended growing season. Edapho‐climatic conditions could not account for these results, suggesting that developmental constraints are important in controlling snowbed plant growth. Impaired productivity was only detected when harsher and more frequent frost events were experimentally induced by early snowmelt. These conditions exposed plants to spring frosts, reaching temperatures consistent with the estimated freezing points of the dominant species (～?10 °C). We conclude that weak plasticity in phenological response and potential detrimental effects of early frosts explain why alpine tundra from snowbeds is not expected to benefit from increased growing season length.     

雪被斑块对高山森林两种灌木凋落叶质量损失的影响

高山/亚高山森林灌木层植物凋落物的分解对于系统物质循环等过程具有重要意义, 并可能受到冬季不同厚度雪被斑块下冻融格局的影响。该文采用凋落物分解袋法, 研究了高山森林典型灌层植物华西箭竹(Fargesia nitida)和康定柳(Salix paraplesia)凋落物在沿林窗-林下形成的冬季雪被厚度梯度(厚型雪被斑块、较厚型雪被斑块、中型雪被斑块、薄型雪被斑块、无雪被斑块)上在第一年不同关键时期(冻结初期、冻结期、融化期、生长季节初期和生长季节后期)的质量损失特征。在整个冻融季节, 华西箭竹和康定柳凋落叶的平均质量损失分别占全年的(48.78 ± 2.35)%和(46.60 ± 5.02)%。冻融季节雪被覆盖斑块下凋落叶的失重率表现出厚型雪被斑块大于薄型雪被斑块的趋势,而生长季节无雪被斑块的失重率明显较高。尽管如此, 华西箭竹凋落物第一年分解表现出随冬季雪被厚度增加而增加的趋势, 但康定柳凋落物第一年失重率以薄型雪被斑块最高, 而无雪被斑块最低。同时, 相关分析表明冻融季节凋落叶的失重率与平均温度和负积温呈极显著正相关, 生长季节凋落叶的失重率与所调查的温度因子并无显著相关关系, 但全年凋落物失重率与平均温度和正/负积温均显著相关。这些结果清晰地表明, 未来冬季变暖情境下高山森林冬季雪被格局的改变将显著影响灌层植物凋落物分解, 影响趋势随着物种的差异具有明显差异。     

Role of litter decomposition for the increased primary production in areas heavily grazed by reindeer: a litterbag experiment

Heavy grazing and trampling by reindeer increase nutrient cycling and primary production in areas where grasslands have replaced shrub and moss tundra. One way in which herbivores can affect nutrient cycling is through changing the litter decomposition processes. We studied the effect of herbivory on litter decomposition rate by reciprocal transplantation of litter between lightly grazed and heavily grazed areas, using the litterbag technique. We used litter from two of the most common species on the lightly grazed side, Betula nana and Empetrum nigrum , and two of the most common species on the heavily grazed side, Carex bigelowii and Deschampsia flexuosa . We found that herbivory improved litter quality by favouring species with easily decomposed litter. However, herbivory also improved litter quality by increasing the nitrogen content and lowering the C/N ratio of each species. Decomposition rates even correlated with the abundance of the plant category in question. Shrub litter decomposed faster in the lightly grazed area where shrubs were common, and graminoid litter decomposed faster in the heavily grazed area where graminoids were common. These results indicate that the decomposer micro-organisms are adapted to the most common litter types. This study shows that detailed information about the effect of herbivory on litter quality is important to understand differences between the short-term and long-term effects of herbivory on nutrient cycling and primary production.     

川西高山林线交错带凋落叶分解初期转化酶特征

胞外酶对于有机质的降解具有重要的作用。在凋落物分解过程中,酶活性不仅受到凋落物种类或基质质量的影响,还受到环境因素的影响。转化酶催化蔗糖水解为葡萄糖和果糖,因此在凋落物分解早期,转化酶比降解难分解物质的酶具有更重要的作用。以川西高山林线交错带12种代表性凋落叶为研究对象,对林线交错带不同植被类型下的凋落叶转化酶活性以及物种和环境因子对转化酶活性的影响进行了研究。结果表明:同一植被类型下,12个物种转化酶活性具有极显著差异(P0.01)。物种、环境因子及其交互作用对转化酶活性有极显著的影响(P0.01)。初始纤维素含量与转化酶活性极显著正相关(P0.01)。初始木质素和总酚含量与转化酶活性极显著负相关(P0.01),能够共同解释转化酶活性变异的50.8%。不同植物生活型中,禾草类转化酶活性均为最高,这可能与禾草类较高的初始纤维素含量、较低的木质素和总酚含量有关。多元线性回归分析表明,凋落叶含水量能单独解释转化酶活性变量的62.1%,是环境因子中最重要的变量。从植被类型来看,大多数物种的转化酶活性在针叶林中均极显著高于高山草甸和灌丛(P0.01),这可能与针叶林中凋落叶的含水量最高且雪被最厚有关。历经一个雪被期分解后,凋落叶初始质量与环境因子的综合作用能够解释转化酶活性变异的79.1%,表明川西高山林线交错带凋落叶分解前期转化酶活性主要受初始木质素含量、总酚含量和含水量的调控。在全球气候变化情景下,凋落物水分含量的变化将会强烈的影响凋落叶分解前期的转化酶活性。     

高山林线交错带高山杜鹃的凋落物分解

凋落物分解是维持生态系统生产力、养分循环、土壤有机质形成的关键生态过程。高山林线交错带是陆地生态系统中对气候变化响应的敏感区域。季节变化和海拔梯度上的植被类型差异可能会影响该区域凋落物的分解,进而对高山生态系统的碳氮循环产生重要影响。采用凋落物分解袋的方法,研究了川西高山林线交错带优势种高山杜鹃(Rhododendron lapponicum)凋落叶在雪被期和生长季的分解特征。结果显示:(1)季节变化和植被类型对高山杜鹃凋落物的分解均具有显著影响(P0.05),凋落叶的质量损失主要发生在生长季且在高山林线最大,暗针叶林中雪被期的质量损失略高于生长季,但差异不显著;(2)林线交错带上高山杜鹃凋落叶分解缓慢,一年干物质失重率为9.62%,拟合分解系数k为0.145;(3)高山杜鹃凋落叶的质量变化主要体现在纤维素降解显著且集中在雪被期,木质素无明显降解,在高山林线上C/N、C/P、木质素/N变化幅度较小且C、N、P的释放表现得稳定而持续。结果表明,季节性雪被对林线交错带内高山杜鹃分解的影响不仅局限在雪被期内,雪被融化期间频繁的冻融作用和雪融水淋洗作用可能会促进高山杜鹃凋落物在生长季初期的分解。总的来看,在气候变暖的情景下,雪被的缩减、生长季的延长和高山杜鹃群落的扩张可能加速高山林线交错带高山杜鹃凋落物的分解。     

Effects of fertilisation and irrigation on ‘foliar afterlife’ in alpine tundra

Question: How do increases in soil nutrient and water availability alter the nutrient fluxes through the resorption and litter decomposition pathways and how do they affect litter nutrient pools in a low‐productive alpine tundra ecosystem? Location: An alpine lichen‐rich tundra on Mt. Malaya Khati‐para in the NW Caucasus, Russia (43°27’ N, 41°42’ E; altitude 2800 m a.s.l.). Methods: We conducted a 4‐year fertilisation (N, P, N+P, lime) and irrigation experiment, and analysed the responses of nutrient resorption from senescing leaves, leaf litter quality and decomposability of six pre‐dominant vascular plant species, total plant community litter production and litter (nutrient) accumulation. Results: Vascular plant litter [N] and [P] increased 1.5 and 10 fold in response to N and P additions, due to increased concentrations of the nutrients in fresh leaves and unchanged or reduced resorption efficiency. Litter decomposability was not affected by nutrient amendments. Fertilisation enhanced litter production (180%; N+P treatment) and litter accumulation (80%; N+P), owing to tremendously increased production and low decomposability of graminoids. Together with increased litter [N] and [P] this led to great increases in total litter nutrient pools. Conclusions: Due to increased production of graminoids, nutrients added to the alpine tundra soil were mostly immobilised in recalcitrant, nutrient‐rich litter. This suggests that changing species composition in low productive ecosystems may act as an internal buffer mechanism, which under increased soil nutrient availability prevents the community from rapidly acquiring features typical of a high productive ecosystem such as high decomposability and high nutrient availability.     

Initial Stages of Tundra Shrub Litter Decomposition May Be Accelerated by Deeper Winter Snow But Slowed Down by Spring Warming

The Arctic climate is projected to change during the coming century, with expected higher air temperatures and increased winter snowfall. These climatic changes might alter litter decomposition rates, which in turn could affect carbon (C) and nitrogen (N) cycling rates in tundra ecosystems. However, little is known of seasonal climate change effects on plant litter decomposition rates and N dynamics, hampering predictions of future arctic vegetation composition and the tundra C balance. We tested the effects of snow addition (snow fences), warming (open top chambers), and shrub removal (clipping), using a full-factorial experiment, on mass loss and N dynamics of two shrub tissue types with contrasting quality: deciduous shrub leaf litter (Salix glauca) and evergreen shrub shoots (Cassiope tetragona). We performed a 10.5-month decomposition experiment in a low-arctic shrub tundra heath in West-Greenland. Field incubations started in late fall, with harvests made after 249, 273, and 319 days of field incubation during early spring, summer and fall of the next year, respectively. We observed a positive effect of deeper snow on winter mass loss which is considered a result of observed higher soil winter temperatures and corresponding increased winter microbial litter decomposition in deep-snow plots. In contrast, warming reduced litter mass loss during spring, possibly because the dry spring conditions might have dried out the litter layer and thereby limited microbial litter decomposition. Shrub removal had a small positive effect on litter mass loss for C. tetragona during summer, but not for S. glauca. Nitrogen dynamics in decomposing leaves and shoots were not affected by the treatments but did show differences in temporal patterns between tissue types: there was a net immobilization of N by C. tetragona shoots after the winter incubation, while S. glauca leaf N-pools were unaltered over time. Our results support the widely hypothesized positive linkage between winter snow depth and litter decomposition rates in tundra ecosystems, but our results do not reveal changes in N dynamics during initial decomposition stages. Our study also shows contrasting impacts of spring warming and snow addition on shrub decomposition rates that might have important consequences for plant community composition and vegetation-climate feedbacks in rapidly changing tundra ecosystems.     

雪被覆盖对高山森林凋落物分解过程中水溶性和有机溶性组分含量的影响

季节性雪被可能通过冻结、淋溶以及冻融循环等对高山森林凋落物水溶性和有机溶性组分含量产生影响.本文采用凋落物分解袋法,以川西高山森林典型乔木（四川红杉、岷江冷杉、红桦、方枝柏）和灌木（高山杜鹃、康定柳）凋落物为研究对象,研究了雪被覆盖不同时期（雪被形成期、雪被覆盖期和雪被融化期）和雪被厚度（厚型雪被、中型雪被、薄型雪被和无雪被）下凋落物水溶性和有机溶性组分含量的动态变化特征.结果表明： 在一个冬季的分解过程中,6种凋落物水溶性组分含量在雪被形成期和融化期降低而雪被覆盖期增加,但除高山杜鹃凋落物有机溶性组分含量在雪被覆盖期增加外,其他5种凋落物有机溶性组分含量在整个冬季呈降低趋势.相对于凋落物有机溶性组分含量,不同厚度雪被斑块对凋落物水溶性组分含量变化的影响更大,且主要表现在雪被形成期和雪被覆盖期.相对于其他雪被斑块,薄型雪被斑块更加显著地促进了高山柳和高山杜鹃凋落物水溶性组分含量降低,但显著抑制了方枝柏凋落物水溶性组分含量降低,而其他凋落物水溶性组分含量变化在不同斑块间无显著差异.冬季高山森林雪被对凋落物水溶性和有机溶性组分含量的影响主要受控于凋落物质量.     

季节性雪被对高山森林凋落物分解的影响

季节性雪被可能对高山森林凋落物分解产生重要影响, 但一直没有深入的研究。该文采用凋落物分解袋法, 于2010-2012年雪被覆盖下几个关键时期(冻结初期、深冻期和融化期)以及生长季节, 研究了川西高山森林代表性树种岷江冷杉(Abies faxoniana)、红桦(Betula albosinensis)、四川红杉(Larix mastersiana)和方枝柏(Sabina saltuaria)凋落叶在不同厚度冬季雪被下的分解动态。经过两年的分解, 不同雪被覆盖下岷江冷杉凋落物分解率为33.98%-39.55%, 红桦为46.49%-48.22%, 四川红杉为42.30%-44.93%, 方枝柏为40.34%-43.84%。相对于无雪被覆盖环境, 厚型雪被覆盖均小幅提高了4种凋落物两年的失重率(1.57%-5.57%)。3个针叶树种(岷江冷杉、四川红杉和方枝柏) Olson凋落物分解系数k均以厚型雪被覆盖最大, 薄型雪被覆盖最小, 而阔叶树种红桦分解系数k则表现为无雪被>薄型雪被>较厚型雪被>厚型雪被>中型雪被。尽管在第二年生长季中雪被对红桦凋落物分解的促进作用不明显, 但雪被覆盖明显促进了两年各个关键时期岷江冷杉、四川红杉和方枝柏凋落物的分解。第一年雪被期凋落物分解对当年分解总量的贡献达42.5%-65.5%, 季节性雪被变化明显改变了凋落物冬季分解格局, 对深冻期凋落物分解过程影响尤为显著。综上所述, 当前气候变化情景下冬季雪被的减少可能减缓该区森林凋落物分解过程, 但相对于易分解的阔叶凋落物, 针叶凋落物的响应特征可能更为强烈。     

不同厚度雪被对高山森林6种凋落物分解过程中酸溶性和酸不溶性组分的影响

高山森林冬季不同厚度雪被格局可能通过影响凋落物的分解过程中酸溶性和酸不溶性组分特征,改变凋落物分解过程,但缺乏必要关注。采用凋落物分解袋法,研究了高山森林林窗中央至林下形成的天然雪被厚度梯度(厚型雪被、中型雪被、薄型雪被和无雪被)覆盖下,6种典型物种岷江冷杉(Abies faxoniana)、红桦(Betula albo-sinensis)、四川红杉(Larix mastersiana)、方枝柏(Sabina saltuaria)、康定柳(Salix paraplesia)和高山杜鹃(Rhododendron lapponicum)凋落物在不同关键时期(雪被形成期、雪被覆盖期和雪被融化期)的酸溶性组分和酸不溶性组分变化特征。经历一个冬季的分解后,6种凋落物酸溶性组分绝对含量呈降低趋势,除红桦外5种凋落物酸不溶性组分绝对含量呈增加趋势。不同厚度雪被显著影响雪被覆盖期和融化期凋落物酸不溶性和酸溶性组分绝对变化量;其中方枝柏、红桦和康定柳凋落物酸不溶性组分增加量在厚型雪被下显著高于其它雪被覆盖;而相对于阔叶凋落物酸溶性组分变化量在薄型雪被和无雪被梯度达到最大值,针叶凋落物酸溶性组分在厚型雪被下具有最大的变化量。一个冬季分解结束后,表征6种凋落物酸溶性和酸不溶性组分含量相对比例的LCI指数(Lignocellulose index)总体升高,雪被对LCI指数的影响主要表现在雪被覆盖期和融化期,且方枝柏、岷江冷杉和康定柳凋落物LCI在冬季分解后均在厚型雪被达到最高值。同时统计分析结果表明,物种极显著影响冬季不同阶段凋落物酸溶性和酸不溶性组分的变化。这些结果意味着气候变暖情景下,高山森林冬季雪被和冻融格局的改变将显著影响凋落物分解过程中酸溶性、酸不溶性组分以及LCI指数代表的抵抗性组分结构的变化,且影响趋势受到凋落物质量的调控。     

Effects of forest gap on hemicellulose dynamics during foliar litter decomposition in an subalpine forest

 亚高山森林林窗可能通过改变冬季雪被格局和生长季水热环境影响林窗内凋落物中半纤维素的分解动态, 但目前对此还缺乏研究。采用凋落物分解袋法, 以亚高山森林5种典型物种岷江冷杉(Abies faxoniana)、红桦(Betula albosinensis)、四川红杉(Larix mastersiana)、方枝柏(Sabina saltuaria)和高山杜鹃(Rhododendron lapponicum)凋落物为研究对象, 研究雪被形成期、雪被覆盖期、雪被融化期和生长季节从林窗中心、林冠林窗、扩展林窗到郁闭林下物种凋落物的半纤维素变化特征。经历一年分解后, 5种凋落物的半纤维素均呈现净累积现象。针、阔叶凋落物半纤维素分别在雪被覆盖期和融化期表现出相对较高的损失率。在雪被覆盖期和融化期, 凋落物半纤维素在林窗中心和林冠林窗具有相对较高的损失率; 而在生长季节, 林窗中心呈现相对较低的凋落物半纤维素累积率。统计分析结果表明凋落物分解过程中半纤维素损失率与环境因子和凋落物质量因子均显著相关。这些结果表明亚高山森林林窗对凋落物分解过程中半纤维素损失率具有显著影响, 分别促进了半纤维素在冬季的损失以及抑制了半纤维素在生长季节的累积, 意味着亚高山森林林窗的形成有利于凋落物半纤维素的降解。     

Contrasting effects of hemiparasites on ecosystem processes: can positive litter effects offset the negative effects of parasitism?

Hemiparasites are known to influence community structure and ecosystem functioning, but the underlying mechanisms are not well studied. Variation in the impacts of hemiparasites on diversity and production could be due to the difference in the relative strength of two interacting pathways: direct negative effects of parasitism and positive effects on N availability via litter. Strong effects of parasitism should result in substantial changes in diversity and declines in productivity. Conversely, strong litter effects should result in minor changes in diversity and increased productivity. We conducted field-based surveys to determine the association of Castilleja occidentalis with diversity and productivity in the alpine tundra. To examine litter effects, we compared the decomposition of Castilleja litter with litter of four other abundant plant species, and examined the decomposition of those four species when mixed with Castilleja. Castilleja was associated with minor changes in diversity but almost a twofold increase in productivity and greater foliar N in co-occurring species. Our decomposition trials suggest litter effects are due to both the rapid N loss of Castilleja litter and the effects of mixing Castilleja litter with co-occurring species. Castilleja produces litter that accelerates decomposition in the alpine tundra, which could accelerate the slow N cycle and boost productivity. We speculate that these positive effects of litter outweigh the effects of parasitism in nutrient-poor systems with long-lived hemiparasites. Determining the relative importance of parasitism and litter effects of this functional group is crucial to understand the strong but variable roles hemiparasites play in affecting community structure and ecosystem processes.     

Changes in alpine snowbed‐wetland vegetation over three decades in northern Norway

We have quantified floristic changes in alpine snowbeds and wetland vegetation during three decades and analyzed to what extent these changes are related to initial variations in snow cover duration and distance to groundwater level. Vascular plant species richness and total plant cover were estimated along three transects in northern Norway. Three different vegetation zones were identified along the original transects: relatively dry snowbeds, wet snowbeds and wetlands. The resampling shows major changes in species richness and plant cover. In general, there was a net immigration of species and 13 new species were found. Five rare species with initial low cover were lost. In the dry and wet snowbeds, species richness and total plant cover increased, mostly because of invasion by shrubs, graminoids and herbs. A general trend was that species indicating high soil moisture were strongly reduced. In the wetland zones there were no significant floristic changes but hygrophilous species had decreased and were replaced by graminoids and shrub species with lower water requirements. These floristic changes were significantly related to snow and soil moisture conditions, important factors for rate and direction of change. Contrasting vegetation responses within very short distances demonstrate the importance of detailed knowledge of the actual microhabitats when effects of climate change in alpine habitats are considered.     

Direct and indirect effects of ski run management on alpine Orthoptera

Alpine landscapes are heavily influenced by ski run management, which can have severe impacts on alpine biodiversity. To assess these impacts on alpine Orthoptera, we compared species richness and species abundance in 41 plot pairs on ski runs and adjacent off-slope control plots in three ski resorts in Austria and Germany. A mixed modelling approach was used to assess the impacts of ski run preparation, artificial snow-making and environmental variables such as altitude, cover of dwarf shrubs and the application of fertilizer. Ski run plots showed a significantly lower species richness and number of individuals than control plots. Moreover, artificial snow led to a further decrease in species number. Hierarchical variance partitioning revealed that Orthoptera community composition is best predicted by environmental variables indirectly related to ski run management (fertilization, cover of dwarf shrubs) and to altitude. Only one out of five species significantly decreased in abundance after artificial snow-making. Other species were more sensitive to fertilizing and altitude. Dwarf shrubs were negatively associated with ski run management but positively associated with abundance of three species and species richness. Our data provide evidence for both direct and indirect consequences of ski runs and artificial snow-making on alpine Orthoptera. Overall, Orthoptera communities are suitable indicators for human-induced changes in alpine environments. In particular, a shift towards generalist species such as Chorthippus parallelus along with a decrease in typical alpine species gives cause for concern as this implies a homogenisation of biodiversity owing to ski run management.     

N2-Fixing Red Alder Indirectly Accelerates Ecosystem Nitrogen Cycling

Symbiotic N₂-fixing tree species can accelerate ecosystem N dynamics through decomposition feedbacks via both direct and indirect pathways. Direct pathways include the production of readily decomposed leaf litter and increased N supply to decomposers, whereas indirect pathways include increased tissue N and altered detrital dynamics of non-fixing vegetation. To evaluate the relative importance of direct and indirect pathways, we compared 3-year decomposition and N dynamics of N₂-fixing red alder leaf litter (2.34% N) to both low-N (0.68% N) and high-N (1.21% N) litter of non-fixing Douglas-fir, and decomposed each litter source in four forests dominated by either red alder or Douglas-fir. We also used experimental N fertilization of decomposition plots to assess elevated N availability as a potential mechanism of N₂-fixer effects on litter mass loss and N dynamics. Direct effects of N₂-fixing red alder on decomposition occurred primarily as faster N release from red alder than Douglas-fir litter. Direct increases in N supply to decomposers via experimental N fertilization did not stimulate decomposition of either species litter. Fixed N indirectly influenced detrital dynamics by increasing Douglas-fir tissue and litter N concentrations, which accelerated litter N release without accelerating mass loss. By increasing soil N, tissue N, and the rate of N release from litter of non-fixers, we conclude that N₂-fixing vegetation can indirectly foster plant–soil feedbacks that contribute to the persistence of elevated N availability in terrestrial ecosystems.     

Shrub canopies influence soil temperatures but not nutrient dynamics: An experimental test of tundra snow–shrub interactions

Shrubs are the largest plant life form in tundra ecosystems; therefore, any changes in the abundance of shrubs will feedback to influence biodiversity, ecosystem function, and climate. The snow–shrub hypothesis asserts that shrub canopies trap snow and insulate soils in winter, increasing the rates of nutrient cycling to create a positive feedback to shrub expansion. However, previous work has not been able to separate the abiotic from the biotic influences of shrub canopies. We conducted a 3‐year factorial experiment to determine the influences of canopies on soil temperatures and nutrient cycling parameters by removing ~0.5 m high willow (Salix spp.) and birch (Betula glandulosa) shrubs, creating artificial shrub canopies and comparing these manipulations to nearby open tundra and shrub patches. Soil temperatures were 4–5°C warmer in January, and 2°C cooler in July under shrub cover. Natural shrub plots had 14–33 cm more snow in January than adjacent open tundra plots. Snow cover and soil temperatures were similar in the manipulated plots when compared with the respective unmanipulated treatments, indicating that shrub canopy cover was a dominant factor influencing the soil thermal regime. Conversely, we found no strong evidence of increased soil decomposition, CO₂ fluxes, or nitrate or ammonia adsorbtion under artificial shrub canopy treatments when compared with unmanipulated open tundra. Our results suggest that the abiotic influences of shrub canopy cover alone on nutrient dynamics are weaker than previously asserted.