RESUMEN

While there is an extensive body of research on the influence of climate warming on total soil microbial communities, our understanding of how rhizosphere and non-rhizosphere soil microorganisms respond to warming remains limited. To address this knowledge gap, we investigated the impact of 4 years of soil warming on the diversity and composition of microbial communities in the rhizosphere and non-rhizosphere soil of a temperate steppe, focusing on changes in root exudation rates and exudate compositions. We used open top chambers to simulate warming conditions, resulting in an average soil temperature increase of 1.1°C over a span of 4 years. Our results showed that, in the non-rhizosphere soil, warming had no significant impact on dissolved organic carbon concentrations, compositions, or the abundance of soil microbial functional genes related to carbon and nitrogen cycling. Moreover, soil microbial diversity and community composition remained largely unaffected, although warming resulted in increased complexity of soil bacteria and fungi in the non-rhizosphere soil. In contrast, warming resulted in a substantial decrease in root exudate carbon (by 19%) and nitrogen (by 12%) concentrations and induced changes in root exudate compositions, primarily characterized by a reduction in the abundance in alcohols, coenzymes and vitamins, and phenylpropanoids and polyketides. These changes in root exudation rates and exudate compositions resulted in significant shifts in rhizosphere soil microbial diversity and community composition, ultimately leading to a reduction in the complexity of rhizosphere bacterial and fungal community networks. Altered root exudation and rhizosphere microbial community composition therefore decreased the expression of functional genes related to soil carbon and nitrogen cycling. Interestingly, we found that changes in soil carbon-related genes were primarily driven by the fungal communities and their responses to warming, both in the rhizosphere and non-rhizosphere soil. The study of soil microbial structure and function in rhizosphere and non-rhizosphere soil provides an ideal setting for understanding mechanisms for governing rhizosphere and non-rhizosphere soil carbon and nitrogen cycles. Our results highlight the distinctly varied responses of soil microorganisms in the rhizosphere and non-rhizosphere soil to climate warming. This suggests the need for models to address these processes individually, enabling more accurate predictions of the impacts of climate change on terrestrial carbon cycling.

Asunto(s)

RESUMEN

BACKGROUND AND AIMS: Floral volatiles, visual traits, and rewards mediate attraction and defense in plant-pollinator and plant-herbivore interactions, but these floral traits may be altered by global warming through direct effects of temperature or longer term impacts on plant resources. We examined the effect of warming on floral and leaf volatile emissions, floral morphology, plant height, nectar production, and oviposition by seed predators. METHODS: We used open-top chambers that warmed plants in the field +2-3 °C on average (+6-11 °C increase in daily maxima) for 2-4 weeks across 1-3 years at 3 sites in Colorado, USA. Volatiles were sampled from two closely related species of subalpine Ipomopsis with different pollinators: I. aggregata ssp. aggregata, visited mainly by hummingbirds, and I. tenuituba ssp. tenuituba, often visited by hawkmoths. KEY RESULTS: While warming had no detected effects on leaf volatiles, the daytime floral volatiles of both I. aggregata and I. tenuituba responded in subtle ways to warming, with impacts that depended on the species, site, and year. In addition to the long-term effect of warming, temperature at the time of sampling independently affected the floral volatile emissions of I. aggregata during the day and I. tenuituba at night. Warming had little effect on floral morphology for either species, and no effect on nectar concentration, maximum inflorescence height, or flower redness in I. aggregata. However, warming increased nectar production in I. aggregata by 41%, a response that would attract more hummingbird visits, and reduced oviposition by fly seed predators by at least 72%. CONCLUSIONS: Our results suggest that floral traits can show different levels of plasticity to temperature changes in subalpine environments, with potential effects on animal behaviors that help or hinder plant reproduction. They also illustrate the need for more long-term field warming studies, as shown by responses of floral volatiles in different ways to weeks of warming versus temperature at the time of sampling.

RESUMEN

How root respiration acclimates to global warming remains unclear, especially in subtropical forests that play a key role in the global carbon budget. In a large-scale in situ soil warming experiment, the occurrence of, and mechanisms controlling over, the acclimation of fine-root respiration of Cunninghamia lanceolata during the fourth year of warming were investigated. Specific respiration rates (at reference temperature of 20°C; SRR20 ) were measured with exogenous glucose addition, uncoupler addition, or no addition, and root morphological and chemical traits were also measured. Warming decreased SRR20 by 18.4% only during summer, indicating partial thermal acclimation of fine-root respiration under warming. Warming did not change fine-root N concentration, showing no possible enzyme limitation on respiration. Warming decreased root soluble sugar/starch ratio in summer, and glucose addition increased respiration only under warming, indicating a warming-induced substrate limitation on respiration. Uncoupler addition also stimulated respiration only under warming, showing a warming-induced adenylate limitation on respiration. These findings suggest that thermal acclimation of root respiration in subtropical forests, which is at least partially constrained by substrate and adenylate use, is conducive to reducing ecosystem carbon emissions and mitigating the positive feedback between atmospheric CO2 and climate warming.

Asunto(s)

RESUMEN

Due to significant differences in biotic and abiotic properties of soils compared to those of sediments, the predicted underlying microbe-mediated mechanisms of soil carbon emissions in response to warming may not be applicable for estimating similar emissions from inland water sediments. We addressed this issue by incubating different types of sediments, (including lake, small river, and pond sediments) collected from 36 sites across the Yangtze River basin, under short-term experimental warming to explore the effects of climate warming on sediment carbon emission and the underlying microbe-mediated mechanisms. Our results indicated that under climate warming CO2 emissions were affected more than CH4 emissions, and that pond sediments may yield a greater relative contribution of CO2 to total carbon emissions than lake and river sediments. Warming-induced CO2 and CH4 increases involve different microbe-mediated mechanisms; Warming-induced sediment CO2 emissions were predicted to be directly positively driven by microbial community network modularity, which was significantly negatively affected by the quality and quantity of organic carbon and warming-induced variations in dissolved oxygen, Conversely, warming-induced sediment CH4 emissions were predicted to be directly positively driven by microbial community network complexity, which was significantly negatively affected by warming-induced variations in pH. Our findings suggest that biotic and abiotic drivers for sediment CO2 and CH4 emissions in response to climate warming should be considered separately when predicting sediment organic carbon decomposition dynamics resulting from climate change.

Asunto(s)

RESUMEN

The effects of mycorrhiza and its external hyphae on the response of soil microbes to global warming remain unclear. This study investigates the role of mycorrhiza and its hyphae in regulating soil microbial community under warming by examining the microbial biomass and composition in the ingrowth cores of arbuscular mycorrhiza (AM) plant, Fargesia nitida, and ectomycorrhiza (ECM) plant, Picea asperata, with/without mycorrhiza/hyphae and experimental warming. The results showed that warming significantly increased the biomass of all soil microbes (by 19.89%-137.48%) and altered the microbial composition in both plant plots without mycorrhiza/hyphae. However, this effect was weakened in the presence of mycorrhiza or hyphae. In F. nitida plots, warming did not significantly affect biomass and composition of most soil microbial groups when mycorrhiza or hyphae were present. In P. asperata plots, warming significantly increased the total and ECM fungi (ECMF) biomass in the presence of hyphae (p < 0.05) and the total, Gn, and AM fungi (AMF) biomass in the presence of mycorrhiza (p < 0.05). Meanwhile, the response of enzyme activities to warming was also altered with mycorrhiza or hyphae. Additionally, soil microbial community composition was mainly influenced by soil available phosphorus (avaP), while enzyme activities depended on soil avaP, dissolved organic carbon (DOC), and nitrate concentrations. Our results indicate that mycorrhiza and its hyphae are essential in regulating the response of microbes to warming.

Asunto(s)

RESUMEN

Climate change is one of the primary agents of the global decline in insect abundance. Because of their narrow thermal ranges, tropical ectotherms are predicted to be most threatened by global warming, yet tests of this prediction are often confounded by other anthropogenic disturbances. We used a tropical forest soil warming experiment to directly test the effect of temperature increase on litter-dwelling ants. Two years of continuous warming led to a change in ant community between warming and control plots. Specifically, six ant genera were recorded only on warming plots, and one genus only on control plots. Wasmannia auropuctata, a species often invasive elsewhere but native to this forest, was more abundant in warmed plots. Ant recruitment at baits was best predicted by soil surface temperature and ant heat tolerance. These results suggest that heat tolerance is useful for predicting changes in daily foraging activity, which is directly tied to colony fitness. We show that a 2-year increase in temperature (of 2-4°C) can have a profound effect on the most abundant insects, potentially favouring species with invasive traits and moderate heat tolerances.

Asunto(s)

RESUMEN

Global warming and nitrogen (N) deposition are known to unbalance the stoichiometry of carbon (C), N, and phosphorus (P) in terrestrial plants, but it is unclear how water availability regulates their effects along a natural aridity gradient. Here, we conducted manipulative experiments to determine the effects of experimental warming (WT) and N addition (NT) on plant stoichiometry in desert, typical, and meadow steppes with decreasing aridity. WT elevated air temperatures by 1.2-2.9 °C using open-top chambers. WT increased forb C:N ratio and thus its N use efficiency and competitiveness in desert steppes, whereas WT reduced forb C:N and C:P ratios in typical and meadow steppes. Plant N:P ratio, which reflects nutrient limitation, was reduced by WT in desert steppes but not for typical or meadow steppes. NT reduced plant C:N ratios and increased N:P ratios in all three steppes. NT reduced forb C:P ratios in desert and typical steppes, but it enhanced grass C:P ratio in meadow steppes, indicating an enhancement of P use efficiency and competitiveness of grasses in wet steppes. WT and NT had synergetic effects on grass C:N and C:P ratios in all three steppes, which helps to increase grasses' productivity. Under WT or NT, the changes in community C:N ratio were positively correlated with increasing aridity, indicating that aridity increases plants' N use efficiency. However, aridity negatively affected the changes in N:P ratios under NT but not WT, which suggests that aridity mitigates P limitation induced by N deposition. Our results imply that warming could shift the dominant functional group into forbs in dry steppes due to altered stoichiometry, whereas grasses become dominated plants in wet steppes under increasing N deposition. We suggest that global changes might break the stoichiometric balance of plants and water availability could strongly modify such processes in semi-arid steppes.

Asunto(s)

RESUMEN

Climate warming and invasive plant growth (plant invasion) may aggravate air pollution by affecting soil nitrogen (N) cycling and the emissions of reactive N gases, such as nitrous acid (HONO) and nitrogen oxides (NOx). However, little is known about the response of soil NOy (HONO + NOx) emissions and microbial functional genes to the interaction of climate warming and plant invasion. Here, we found that experimental warming (approximately 1.5 °C), but not Spartina alterniflora invasion, increased NOy emissions (0-140 ng N m-2 s-1) of treated wetland soils by 4-10 fold. Warming also decreased soil archaeal and fungal richness and diversity, shifted their community structure (e.g., decreased the archaeal classes Thermoplasmata and Iainarchaeia, and increased the archaeal genus Candidatus Nitrosoarchaeum, and the fungal classes Saccharomycetes and Tritirachiomycetes), and decreased the overall abundance of soil N cycling genes. Structural equation modeling revealed that warming-associated changes in edaphic factors and the microbial N cycling potential are responsible for the observed increase in soil NOy emissions. Collectively, the results showed that climate warming accelerates soil N cycling by stimulating large soil HONO and NOx emissions, and influences air quality by contributing to atmospheric reactive N and ozone cycling.

Asunto(s)

RESUMEN

Pronounced nongrowing season warming and changes in soil freeze-thaw (F-T) cycles can dramatically alter net methane (CH4 ) exchange rates between soils and the atmosphere. However, the magnitudes and drivers of warming impacts on CH4 uptake in different stages of the F-T cycle are poorly understood in cold alpine ecosystems, which have been found to be a net sink of atmospheric CH4 . Here, we reported a year-round ecosystem daily CH4 uptake in an alpine meadow on the Qinghai-Tibetan Plateau after a 5-year warming experiment that included a control, a low-level warming treatment (+2.4℃ at 5 cm soil depth), and a high-level warming treatment (+4.5℃ at 5 cm soil depth). We found that warming shortened the F-T cycle under the low-level warming and soils did not freeze under the high-level warming. Although both warming treatments increased the mean CH4 uptake rate, only the high-level warming significantly increased annual CH4 uptake compared to the control. The warming-induced stimulation of CH4 uptake mainly occurred in the cold season, which was mostly during spring thaw under low-level warming and during the frozen winter under high-level warming due to a longer period with thawed soil. We also found that warming significantly stimulated daily CH4 uptake mainly by reducing near-surface soil water content in the warm season, whereas both soil water content and temperature controlled daily CH4 uptake in different ways during the autumn freeze, frozen winter, and spring thaw periods of the control. Our study revealed a strong warming effect on CH4 uptake during the entire F-T cycle in the alpine meadow, especially the unfrozen winter. Our results also suggested the important roles of soil pH, available phosphorus, and methanotroph abundance in regulating annual CH4 uptake in response to warming, which should be incorporated into biogeochemical models for accurately forecasting CH4  fluxes under future climate scenarios.

Asunto(s)

RESUMEN

The variation in fine root respiration with root age provides insight into root adaptation to climate warming, but the mechanism is poorly understood. In this study, we investigated the respiratory response of fine roots (<1 mm and 1-2 mm) of different ages (2-, 4- and 6-month old) of Chinese fir (Cunninghamia lanceolata (Lamb.)) seedlings to soil warming (4 °C above the control using cable heating). Fine roots were excised to measure the specific respiration rate at a reference temperature of 20 °C (SRR20), and root morphological and chemical traits were measured. Soil warming significantly increased SRR20 by 40% compared with the control, potentially indicating limited acclimation on a short time scale (6 months). However, soil warming increased SRR20 significantly in 2-month-old roots (by 72%) compared with 4- and 6-month-old roots, leading to a steeper decline in SRR20 with root age. This result suggests possible increased nutrient uptake efficiency in young fine roots under warmer temperatures. Soil warming significantly increased specific root length (SRL) but not root tissue nitrogen concentration (RTN). The variation in SRR20 between warming treatments, but not across root ages, was predicted by SRL and RTN individually or together. Our findings conclusively indicate that soil warming increased the respiration cost of young fine roots, which was predicted by adjusting for SRL and RTN, indicating that Chinese fir may adopt a faster fine root turnover strategy to enhance nutrient uptake and soil exploitation under warmer temperatures. Future studies should simultaneously investigate age-related root respiration and nutrient uptake in warming experiments to better understand the effects of warming on root metabolic activity.

Asunto(s)

RESUMEN

Net ecosystem CO2 exchange is the result of net carbon uptake by plant photosynthesis and carbon loss by soil and plant respiration. Temperature increases due to climate change can modify the equilibrium between these fluxes and trigger ecosystem-climate feedbacks that can accelerate climate warming. As these dynamics have not been well studied in dry shrublands, we subjected a Mediterranean shrubland to a 10-year night-time temperature manipulation experiment that analyzed ecosystem carbon fluxes associated with dominant shrub species, together with several plant parameters related to leaf photosynthesis, leaf morphology, and canopy structure. Under moderate night-time warming (+0.9°C minimum daily temperature, no significant reduction in soil moisture), Cistus monspeliensis formed shoots with more leaves that were relatively larger and denser canopies that supported higher plant-level photosynthesis rates. Given that ecosystem respiration was not affected, this change in canopy morphology led to a significant enhancement in net ecosystem exchange (+47% at midday). The observed changes in shoot and canopy morphology were attributed to the improved nutritional state of the warmed plants, primarily due to changes in nitrogen cycling and higher nitrogen resorption efficiency in senescent leaves. Our results show that modifications in plant morphology triggered by moderate warming affected ecosystem CO2  fluxes, providing the first evidence for enhanced daytime carbon uptake in a dry shrubland ecosystem under experimental warming.

Asunto(s)

RESUMEN

We investigated the effects of warming on soil nitrogen cycling process in alpine scrub ecosystem, with an in-situ simulated warming experiment at Sibiraea angustata alpine scrubland on the eastern Qinghai-Tibet Plateau, China. We examined the responses of soil nitrogen transformation rate to warming in three critical periods (the early, late, and non-growing seasons). The results showed that warming increased soil temperature by 1.2 ℃, but decreased soil moisture by 2.5%. The soil net nitrogen mineralization rates (i.e., ammonification and nitrification) in the growing season were significantly higher than those in the non-growing season. The rates of soil net nitrogen fixation in the non-growing season were significantly higher than that in the growing season. Soil nitrification was the major process of soil nitrogen transformation in the early growing season, while soil ammonification was the major one in the late growing season and non-growing season. The effects of experimental warming on soil nitrogen transformation differed among those three periods. Experimental warming significantly increased soil net ammonification, nitrification, nitrogen mine-ralization and fixation in the early growing season, and enhanced soil net nitrification and nitrogen mineralization in the non-growing season. However, warming significantly decreased soil net nitrification, nitrogen mineralization and fixation in the late growing season and soil net ammonification in the non-growing season. Moreover, warming did not affect soil net nitrogen fixation rates in the non-growing season and soil net nitrification rates in the late growing season. Future climate warming would significantly change soil nitrogen transformation by accelerating soil nitrogen cycling in the alpine scrub ecosystem on the eastern Qinghai-Tibet Plateau.

Asunto(s)

RESUMEN

Although there is abundant evidence that plant phenology is shifting with climatic warming, the magnitude and direction of these shifts can depend on the environmental context, plant species, and even the specific phenophase of study. These disparities have resulted in difficulties predicting future phenological shifts, detecting phenological mismatches and identifying other ecological consequences. Experimental warming studies are uniquely poised to help us understand how climate warming will impact plant phenology, and meta-analyses allow us to expose broader trends from individual studies. Here, we review 70 studies comprised 1226 observations of plant phenology under experimental warming. We find that plants are advancing their early-season phenophases (bud break, leaf-out, and flowering) in response to warming while marginally delaying their late-season phenophases (leaf coloration, leaf fall, and senescence). We find consistency in the magnitude of phenological shifts across latitude, elevation, and habitat types, whereas the effect of warming on nonnative annual plants is two times larger than the effect of warming on native perennial plants. Encouragingly for researchers, plant phenological responses were generally consistent across a variety of experimental warming methods. However, we found numerous gaps in the experimental warming literature, limiting our ability to predict the effects of warming on phenological shifts. In particular, studies outside of temperate ecosystems in the Northern Hemisphere, or those that focused on late-season phenophases, annual plants, nonnative plants, or woody plants and grasses, were underrepresented in our data set. Future experimental warming studies could further refine our understanding of phenological responses to warming by setting up experiments outside of traditionally studied biogeographic zones and measuring multiple plant phenophases (especially late-season phenophases) across species of varying origin, growth form, and life cycle.

Asunto(s)

RESUMEN

PREMISE: Climate change is having major impacts on alpine and arctic regions, and inter-annual variations in temperature are likely to increase. How increased climate variability will impact plant reproduction is unclear. METHODS: In a 4-year study on fruit production by an alpine plant community in northern Sweden, we applied three warming regimes: (1) a static level of warming with open-top chambers (OTC), (2) press warming, a yearly stepwise increase in warming, and (3) pulse warming, a single-year pulse event of higher warming. We analyzed the relationship between fruit production and monthly temperatures during the budding period, fruiting period, and whole fruit production period and the effect of winter and summer precipitation on fruit production. RESULTS: Year and treatment had a significant effect on total fruit production by evergreen shrubs, Cassiope tetragona, and Dryas octopetala, with large variations between treatments and years. Year, but not treatment, had a significant effect on deciduous shrubs and graminoids, both of which increased fruit production over the 4 years, while forbs were negatively affected by the press warming, but not by year. Fruit production was influenced by ambient temperature during the previous-year budding period, current-year fruiting period, and whole fruit production period. Minimum and average temperatures were more important than maximum temperature. In general, fruit production was negatively correlated with increased precipitation. CONCLUSIONS: These results indicate that predicted increased climate variability and increased precipitation due to climate change may affect plant reproductive output and long-term community dynamics in alpine meadow communities.

Asunto(s)

RESUMEN

Global warming and nitrogen (N) deposition are known to affect root dynamics in grasslands. However, previous studies were based only on a single ecosystem type, so it is unclear how warming and N addition affect root traits (root biomass, root-shoot ratio, root production and turnover) along the aridity gradient. In this study, we conducted an experiment to determine the effects of warming and N addition on root traits in desert, typical, and meadow grasslands in northern China, where the aridity gradually decreases from west to east across the region. Warming increased root-shoot ratio in dry year due to decline in soil water, but had a downward trend in root production and turnover in all three grasslands. N addition decreased root-shoot ratio in humid year due to increase in soil N, whereas did not significantly affect root production in any grasslands and increased root turnover in desert and meadow grasslands rather than typical grassland. Warming combined with N addition had negatively additive effects on root turnover in typical and meadow grasslands rather than desert grassland. N addition-induced changes in root biomass and root-shoot ratio were negatively affected by aridity in dry year. Aridity positively affected responses of root production and turnover to warming but negatively affected those responses to N addition. However, root-shoot ratio, root production and turnover under warming combined with N addition were not affected by aridity. Our results suggest that warming suppresses root carbon (C) input but N addition may exacerbate it in temperate grasslands, and warming combined with N addition suppresses it only in wet grasslands. Aridity promotes root C input under warming but suppresses it under N addition. However, aridity may little affect soil C and nutrient dynamics under global warming combined with N deposition in temperate grasslands in the future.

Asunto(s)

RESUMEN

Soil temperature is an important determinant of carbon (C) and nitrogen (N) cycling in terrestrial ecosystems, but its effects on soil organic carbon (SOC) and total nitrogen (TN) dynamics as well as rice biomass in rice paddy ecosystems are not fully understood. We conducted a five-year soil warming experiment in a single-cropping paddy field in Japan. Soil temperatures were elevated by approximate 2 °C with heating wires during the rice growing season and by approximate 1 °C with nighttime thermal blankets during the fallow season. Soil samples were collected in autumn after rice harvest and in spring after fallow each year, and anaerobically incubated at 30 °C for four weeks to determine soil C decomposition and N mineralization potentials. The SOC and TN contents, rice biomass, dissolved organic carbon (DOC) and microbial biomass carbon (MBC) concentrations were measured in the study. Soil warming did not significantly enhance rice aboveground and root biomasses, but it significantly decreased SOC and TN contents and thus decreased soil C decomposition and N mineralization potentials due to depletion of available C and N. Moreover, soil warming significantly decreased DOC concentration but significantly increased MBC concentration. The ratios of C decomposition potential to N mineralization potential, decomposition potential to SOC, and N mineralization to TN were not affected by soil warming. There were significant seasonal and annual variations in SOC, C decomposition and N mineralization potentials, soil DOC and MBC under each temperature treatments. Our study implied that soil warming can decrease soil C and N stocks in paddy ecosystem probably via stimulating microbial activities and accelerating the depletion of DOC. This study further highlights the importance of long-term in situ observation of C and N dynamics and their availabilities in rice paddy ecosystems under increasing global warming scenarios.

Asunto(s)

RESUMEN

Climate warming is often seasonally asymmetric with a higher temperature increase toward winters than summers. However, the effect of winter-biased warming on plant reproductive phenology has been seldom investigated under natural field conditions. The goal of this study was to determine the effects of winter-biased warming on plant reproductive phenologies. In an alpine meadow of Tibetan Plateau, we deployed six large (15 m × 15 m × 2.5 m height) open top chambers (three warmed chambers and three non-warmed chambers) to achieve winter-biased warming (i.e., a small increase in annual mean temperature with a greater increase towards winter than summer). We investigated three phenophases (onset and offset times and duration) for both the flowering and fruiting phenologies of 11 common species in 2017 and 8 species in 2018. According to the vernalization theory, we hypothesized that mild winter-biased warming would delay flowering and fruiting phenologies. The data indicated that the phenological responses to warming were species-specific (including positive, neutral, and negative responses), and the number of plant species advancing flowering (by averagely 4.5 days) and fruiting onset times (by averagely 3.6 days) was higher than those delaying the times. These changes were inconsistent with the vernalization hypothesis (i.e. plants need to achieve a threshold of chilling before flowering) alone, but can be partly explained by the accumulated temperature hypothesis (i.e. plants need to achieve a threshold of accumulative temperature before flowering) and/or the overtopping hypothesis (i.e. plants need to reach community canopy layer before flowering). The interspecific difference in the response of reproductive phenology could be attributed to the variation in plant traits including plant height growth, the biomass ratio of root to shoot, and seed mass. These results indicate that a mild winter-biased warming may trigger significant change in plant reproductive phenology in an alpine meadow.

RESUMEN

The effects of climate change on tropical forests may have global consequences due to the forests' high biodiversity and major role in the global carbon cycle. In this study, we document the effects of experimental warming on the abundance and composition of a tropical forest floor herbaceous plant community in the Luquillo Experimental Forest, Puerto Rico. This study was conducted within Tropical Responses to Altered Climate Experiment (TRACE) plots, which use infrared heaters under free-air, open-field conditions, to warm understory vegetation and soils + 4°C above nearby control plots. Hurricanes Irma and María damaged the heating infrastructure in the second year of warming, therefore, the study included one pretreatment year, one year of warming, and one year of hurricane response with no warming. We measured percent leaf cover of individual herbaceous species, fern population dynamics, and species richness and diversity within three warmed and three control plots. Results showed that one year of experimental warming did not significantly affect the cover of individual herbaceous species, fern population dynamics, species richness, or species diversity. In contrast, herbaceous cover increased from 20% to 70%, bare ground decreased from 70% to 6%, and species composition shifted pre to posthurricane. The negligible effects of warming may have been due to the short duration of the warming treatment or an understory that is somewhat resistant to higher temperatures. Our results suggest that climate extremes that are predicted to increase with climate change, such as hurricanes and droughts, may cause more abrupt changes in tropical forest understories than longer-term sustained warming.

RESUMEN

A warming Arctic has been associated with increases in aboveground plant biomass, specifically shrubs, and changes in vegetation cover. However, the magnitude and direction of changes in NDVI have not been consistent across different tundra types. Here we examine the responsiveness of fine-scale NDVI values to experimental warming at eight sites in northern Alaska, United States. Warming in our eight sites ranged in duration from 2­23 seasons. Dry, wet and moist tundra communities were monitored for canopy surface temperatures and NDVI in ambient and experimentally-warmed plots at near-daily frequencies during the summer of 2017 to assess the impact of the warming treatment on the magnitude and timing of greening. Experimental warming increased canopy-level surface temperatures across all sites (+0.47 to +3.14˚C), with the strongest warming effect occurring during June and July and for the southernmost sites. Green-up was accelerated by warming at six sites, and autumn senescence was delayed at five sites. Warming increased the magnitude of peak NDVI values at five sites, decreased it at one site, and at two sites it did not change. Warming resulted in earlier peak NDVI at three sites and no significant change in the other sites. Shrub and graminoid cover was positively correlated with the magnitude of peak NDVI (r=0.37 to 0.60) while cryptogam influence was mixed. The magnitude and timing of peak NDVI showed considerable variability across sites. Warming extended the duration of the summer green season at most sites due to accelerated greening in the spring and delayed senescence in the autumn. We show that in a warmer Arctic (as simulated by our experiment) the timing and total period of carbon gain may change. Our results suggest these changes are dependent on community composition and abundance of specific growth forms and therefore will likely impact net primary productivity and trophic interactions.

RESUMEN

Belowground climate change responses remain a key unknown in the Earth system. Plant fine-root response is especially important to understand because fine roots respond quickly to environmental change, are responsible for nutrient and water uptake, and influence carbon cycling. However, fine-root responses to climate change are poorly constrained, especially in northern peatlands, which contain up to two-thirds of the world's soil carbon. We present fine-root responses to warming between +2 °C and 9 °C above ambient conditions in a whole-ecosystem peatland experiment. Warming strongly increased fine-root growth by over an order of magnitude in the warmest treatment, with stronger responses in shrubs than in trees or graminoids. In the first year of treatment, the control (+0 °C) shrub fine-root growth of 0.9 km m-2 y-1 increased linearly by 1.2 km m-2 y-1 (130%) for every degree increase in soil temperature. An extended belowground growing season accounted for 20% of this dramatic increase. In the second growing season of treatment, the shrub warming response rate increased to 2.54 km m-2 °C-1 Soil moisture was negatively correlated with fine-root growth, highlighting that drying of these typically water-saturated ecosystems can fuel a surprising burst in shrub belowground productivity, one possible mechanism explaining the "shrubification" of northern peatlands in response to global change. This previously unrecognized mechanism sheds light on how peatland fine-root response to warming and drying could be strong and rapid, with consequences for the belowground growing season duration, microtopography, vegetation composition, and ultimately, carbon function of these globally relevant carbon sinks.