Warmer Sites Research Articles

The temperature optima for pollen germination and pollen tube growth of coconut (Cocos nucifera L.) strongly depend on the growth temperature

Abstract Understanding trait variation in response to temperature is important to predict how crops respond to rising temperature. Although we have a sound understanding of the effects of increasing temperature on growth and development of crops, a robust assessment of how crop reproductive processes are affected by climate warming is still lacking. In this study, we experimentally investigate how the growth temperature affects the cardinal temperatures of in vitro pollen germination of widely distributed tree crop species Cocos nucifera L. (cultivar Sri Lankan Tall). We hypothesize that temperature optima for pollen germination and pollen tube growth would be determined by the growth temperature. Our results showed that the temperature optima of pollen germination and pollen tube growth were higher at relatively warmer sites (sites where the mean annual temperature ∼ 28°C) compared to the cooler sites (sites where the mean annual temperature ∼ 22°C). The two processes were better coordinated at warmer sites. We speculate that tropical tree species that are currently growing in relatively cooler environments may have the capacity to perform their reproductive physiological functions in future warmer climates without any substantial negative impacts. Findings of this study should prove useful in quantifying the potential impacts of climate warming on tropical agro-ecosystems, improving the representation of plant reproduction in crop models.

Intensified Positive Arctic–Methane Feedback under IPCC Climate Scenarios in the 21st Century

The positive Arctic–methane (CH 4 ) feedback forms when more CH 4 is released from the Arctic tundra to warm the climate, further stimulating the Arctic to emit CH 4 . This study utilized the CLM-Microbe model to project CH 4 emissions across five distinct Arctic tundra ecosystems on the Alaska North Slope, considering three Shared Socioeconomic Pathway (SSP) scenarios using climate data from three climate models from 2016 to 2100. Employing a hyper-resolution of 5 m × 5 m within 40,000 m 2 domains accounted for the Arctic tundra’s high spatial heterogeneity; three sites were near Utqiaġvik (US-Beo, US-Bes, and US-Brw), with one each in Atqasuk (US-Atq) and Ivotuk (US-Ivo). Simulated CH 4 emissions substantially increased by a factor of 5.3 to 7.5 under the SSP5–8.5 scenario compared to the SSP1–2.6 and SSP2–4.5 scenarios. The projected CH 4 emissions exhibited a stronger response to rising temperature under the SSP5–8.5 scenario than under the SSP1–2.6 and SSP2–4.5 scenarios, primarily due to strong temperature dependence and the enhanced precipitation-induced expansion of anoxic conditions that promoted methanogenesis. The CH 4 transport via ebullition and plant-mediated transport is projected to increase under all three SSP scenarios, and ebullition dominated CH 4 transport by 2100 across five sites. Projected CH 4 emissions varied in temperature sensitivity, with a Q 10 range of 2.7 to 60.9 under SSP1–2.6, 3.8 to 17.6 under SSP2–4.5, and 5.7 to 17.2 under SSP5–8.5. Compared with the other three sites, US-Atq and US-Ivo were estimated to have greater increases in CH 4 emissions due to warmer temperatures and higher precipitation. The fact that warmer sites and warmer climate scenarios had higher CH 4 emissions suggests an intensified positive Arctic–CH 4 feedback in the 21st century. Microbial physiology and substrate availability dominated the enhanced CH 4 production. The simulated intensified positive feedback underscores the urgent need for a more mechanistic understanding of CH 4 dynamics and the development of strategies to mitigate CH 4 across the Arctic.

Leaf antioxidant activity in Colombian elite Hevea brasiliensis genotypes as a breeding strategy for water deficit tolerance under Amazonia conditions.

This study evaluated the foliar antioxidant activity in nine Hevea brasiliensis genotypes from the ECC-1 (Élite Caquetá Colombia) selection and IAN 873 cultivar (control) in trees in the growth stage in two large-scale clonal trials in response to different climatic (semi-humid warm and humid warm sites) and seasonal (dry and rainy periods) conditions in the Colombian Amazon. The results indicated that Reactive Oxygen Species (ROS) production increased under conditions of lower water availability (dry period), leading to lipid peroxidation, high defense of photosynthetic pigments, and development of better osmotic adjustment capacity in the ECC 64, IAN 873, ECC 90, and ECC 35 genotypes due to high concentrations of carotenoids (0.40 mg g-1), reducing sugars (65.83 μg mg-1), and malondialdehyde (MDA) (2.44 nmol ml-1). In contrast, during the rainy period, a post-stress action was observed due to high contents of proline and total sugars (39.43 μg g-1 and 173.03 μg g-1, respectively). At the site level, with high Photosynthetically Active Radiation (PAR) values (1143 moles photons m-2 s-1), temperature (32.11°C), and lower precipitation (135 mm), higher antioxidant activity (chlorophylls a, b and total, carotenoids, and proline) was recorded at the humid warm site, demonstrating that the ECC 90, ECC 64, and ECC 66 genotypes are tolerant to water deficit compared to IAN 873. The ECC 64 genotype, independent of seasonal changes and site conditions, presented the highest contents in Chl a, total Chl, reducing sugars, total sugars, and MDA, showing a tendency to adapt to fluctuating conditions. This study showed that water fluctuations do not cause the same metabolic responses, these vary within the same species, depending on their developmental stage and the climatic and seasonal variations characteristic of the Colombian Amazon.

Cold Climate Factors in Nitrogen Management for Maize

Among essential crop nutrients, nitrogen is the greatest management challenge in maize (Zea mays L.) production due to high requisite rates as well as dynamic transformations and losses. Climate plays a role in N management through changes in crop calendars, soil properties, agronomic practices, and yield effects. This study focuses on climate influences on maize N management and the objectives are to (i) review cold climate factors impacting economic optimum N rates (EONR), (ii) discuss approaches and climate considerations in estimating optimum N rates, and (iii) illustrate unexplored climate aspects related to optimum N rate assessment. Cold climate effects are expressed through inherent soil properties, agronomic management, and N fertilizer management. Most current N rate calculators do not explicitly account for climate factors, but implicitly integrate them through regional calibrations. Yield and EONR data from the US Corn Belt region indicate a positive correlation where lower means are associated with colder climates. High variability within climate regions is explained by differences in annual production environments, notably seasonal weather. Soil health models show that colder climates in the US are associated with higher stocks of soil organic matter, especially labile fractions. Adapt-N model simulations of a colder (North Central Wisconsin; 45.50, −89.70) and warmer (South Central Illinois; 38.50, −89.70) Corn Belt location show that higher soil organic N stocks do not increase crop N availability, presumably due to temperature-constrained N mineralization rates. The EONR for the colder site is 58 kg N ha−1 lower than the warmer site, which is well explained by differences in yield potential. Overall, abductive inferences suggest that colder climates are generally associated with higher levels of organic N stocks, but lower yields and crop N demands lessen EONRs. Seasonal weather and interactions with soil and agronomic factors also critically impact EONR, which can be assessed with model-based decision tools.

Thermophilisation of Afromontane forest stands demonstrated in an elevation gradient experiment

Abstract. The response of tropical trees and tree communities to climate change is crucial for the carbon storage and biodiversity of the terrestrial biosphere. Trees in tropical montane rain forests (TMFs) are considered particularly vulnerable to climate change, but this hypothesis remains poorly evaluated due to data scarcity. To reduce the knowledge gap in the response of TMF trees to warming, we established a field experiment along a 1300–2400 m elevation gradient as a proxy for warming in Rwanda. Seedling-size trees of 20 species native to montane forests in eastern and central Africa were planted in multi-species plots at three sites along the gradient. They have overlapping distributions but primarily occur in either transitional rain forests (∼ 1600–2000 ma.s.l.) or mid-elevation TMFs (∼ 2000–3000 ma.s.l.), with both early- (ES) and late-successional (LS) species represented in each elevation origin group. Tree growth (diameter and height) and survival were monitored regularly over 2 years. We found that ES species, especially from lower elevations, grew faster at warmer sites, while several of the LS species, especially from higher elevations, did not respond or grew slower. Moreover, a warmer climate increased tree mortality in LS species, but not much in ES species. ES species with transitional rain forest origin strongly increased proportional to stand basal area at warmer sites, while TMF species declined, suggesting that lower-elevation ES species will have an advantage over higher-elevation species in a warming climate. The risk of higher-elevation and LS species of becoming outcompeted by lower-elevation and ES species due to a thermophilisation response in a warmer climate has important implications for biodiversity and carbon storage of Afromontane forests.

Assisted Migration of Sequoiadendron Genotypes for Conservation and Timber: Performance and Morphology in a Warmer Climate Outside of Their Range

Sequoiadendron giganteum (giant sequoia) has a fragmented distribution of 75 groves found along the western slope of the Sierra Nevada Mountains, California, USA. Outplanting and range expansion or assisted migration of this iconic species for the objectives of genetic conservation and timber production would be supported by information on growth and morphology to guide seed-collection decisions. We measured and assessed giant sequoia planted as seedlings and clonal stock originating from 22 groves in two common-garden experiments at Foresthill, California, north of the current species range, after 29 growing seasons. Traits examined were tree-size parameters, fluting and asymmetry of the lower stem, basal swelling, fullness of the live crown, epicormic sprouting, and heartwood decay resistance in cut stumps. Performance in terms of tree size after 29 years varied widely among genotypes with different grove origins. Morphology and decay resistance also exhibited some variation according to grove origins. The seedling stock outperformed the clonal stock of the same grove origins in terms of size and is therefore recommended when faster early growth is desired to outcompete other trees or for other management objectives. However, more fluting was exhibited by the larger fast-growing giant sequoia, while fewer seedlings had epicormic sprouts than the clonal stock of the same grove origins. At our warm low-elevation study site, giant sequoia from Mountain Home, Giant Forest, and Converse Basin consistently exhibited above-average growth among other giant sequoia in a pure planting and in an intimate mixture with five common conifer associates. Therefore, seed collected from these three groves should perform relatively well at other locations with a similar climate. When conservation of the species and its genetic diversity is the primary objective, we recommend collecting from a wide range of groves and undertaking assisted migration by planting at multiple locations inside and outside giant sequoia’s range as a hedge against the loss of native groves.

Earlier and more uniform spring green-up linked to lower insect richness and biomass in temperate forests

Urbanization and agricultural intensification are considered the main causes of recent insect decline in temperate Europe, while direct climate warming effects are still ambiguous. Nonetheless, higher temperatures advance spring leaf emergence, which in turn may directly or indirectly affect insects. We therefore investigated how Sentinel-2-derived start of season (SOS) and its spatial variability (SV-SOS) are affected by spring temperature and whether these green-up variables can explain insect biomass and richness across a climate and land-use gradient in southern Germany. We found that the effects of both spring green-up variables on insect biomass and richness differed between land-use types, but were strongest in forests. Here, insect richness and biomass were higher with later green-up (SOS) and higher SV-SOS. In turn, higher spring temperatures advanced SOS, while SV-SOS was lower at warmer sites. We conclude that with a warming climate, insect biomass and richness in forests may be affected negatively due to earlier and more uniform green-up. Promising adaptation strategies should therefore focus on spatial variability in green-up in forests, thus plant species and structural diversity.

Drivers and spatiotemporal patterns of post-drought growth resilience of four temperate broad-leaved trees

With the intensification of climate warming, more frequent and hotter droughts are a significant risk to forest ecosystems around the globe, including temperate biomes. To accurately predict the response of forests to drought, it is necessary to provide comprehensive information on the resilience of tree growth, its spatiotemporal changes and drivers. This information is often lacking for broadleaf species in temperate regions. To fulfill this aim, we analyzed 3177 ring-width series from 1981 trees covering 40 sites for four dominant broad-leaf tree species (Fraxinus mandshurica, Phellodendron amurense, Juglans mandshurica, and Quercus mongolica) in temperate forests in northeastern China. We quantified the resilience (resistance and recovery) of tree growth to extreme droughts. Minimum temperatures in most seasons and warm conditions and drought stress in early summer were the main factors limiting growth. Among the four tree species, Q. mongolica had the weakest growth resistance to drought and the fastest growth recovery from drought. Juglans mandshurica had a significantly (p < 0.05) higher drought resilience than F. mandshurica and P. amurense. Tree resilience presented a clear spatial pattern driven by differences in site condition and local climate. The resistance (recovery) to drought decreased (increased) with increasing latitude and longitude, and increased (decreased) with elevation. Q. mongolica exhibited a decrease in resistance and increase in recovery through time, whereas the other three tree species showed the opposite pattern. Fraxinus mandshurica, J. mandshurica, and Q. mongolica had high resistance (recovery) in warmer-wetter (colder-drier) regions. In contrast, P. amurense had reduced recovery in cold sites and low resistance in warm sites. The resistance and recovery indices of these sympatric tree species confirmed that different tree species have distinct strategies to cope with drought that is influenced by tree condition and local environment. The influence of biogeographical factors is often more important than other factors and should be considered when studying forest resilience to drought.

Long-term field translocation differentially affects arbuscular mycorrhizal and ectomycorrhizal trees in a sub-tropical forest

Arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) trees are commonly represented in tropical forests, but their response to warming is generally not known. We conducted a long-term (9-year) field experiment in subtropical China by translocating AM tree species (Machilus breviflora and Schima superba) and EcM tree species (Syzygium rehderianum and Castanopsis hystrix) from a cooler high-elevation site (300 m) to a 1 °C warmer low-elevation site (30 m). Here, we report leaf chemical and physiological data from the last three years (Years 7–9) of the translocation experiment. Translocation to warmer site induced higher air (Tair) and soil (Tsoil) temperatures, but lower soil volumetric water content (SVWC). Translocation treatment significantly increased the light-saturated photosynthetic rate (Asat) for AM trees due to higher stomatal conductance (gs) and Rubisco carboxylation capacity (Vcmax), generating increased growth in AM trees. Translocation treatment reduced the maximum efficiency of photosynthetic quantum yield (Fv/Fm) and growth in EcM trees. Translocation treatment increased leaf turgor loss point (TLP) in AM trees, but reduced TLP in EcM trees. Translocation treatment increased foliar nitrogen (N) concentrations and phosphorus (P) concentrations for AM trees. In EcM trees, stable foliar N and P concentrations were related to higher N and P resorption efficiencies. Translocation treatment was more beneficial for the growth of AM trees than EcM trees, while EcM trees were more tolerant of soil water deficit. Differential responses to these co-occurring climate drivers may alter the composition of tree functional groups and ecosystem function in tropical forests in response to future warmer and drier climate conditions.

Temperature variability during the growing season affects the quality attributes of table grapes in Pothwar-insight from a new emerging viticulture region in South Asia.

Rising air temperature due to climate change has posed a mammoth challenge to global viticulture and key berry quality traits are compromised. Exploring the effects of seasonal temperature variability on berry ripening and quality attributes in different viticulture regions may help in sustainable viticulture industry. The present research was designed to explore the effect of temperature variables on key quality attributes of table grape cultivars in Pothwar region of Pakistan. Key berry quality traits such as total soluble solids (TSS), titratable acidity (TA), maturity indices (MI), ascorbic acid, sugars, total polyphenol contents (TPC) and total anthocyanin contents (TAC) were unlocked for four important table grape cultivars under varying environmental conditions at Chakwal and Islamabad districts for two consecutive vintages of 2019 and 2020. The district Chakwal has up to 0.92 °C, 1.35 °C, 1.12°C and 0.81°C higher Tmin, Tmax, Tmean anddiurnal temperature variation (DTV)respectively, compared to Islamabad particularly for the 2019 vintage. The results of the present study revealed that the warmer site (Chakwal) has significantly (P ≤0.05) higher juice pH, TSS (°brix) and maturity indices (MI) particularly for the relatively hotter vintage of 2019. Interestingly, MI was 33% higher for the relatively warmer vintage of 2019 compared to 2020 with relatively lower acidity (up to 38%). Moreover, higher titratable acidity (11.2%), ascorbic acid (28.5%), polyphenols (20.3%) and anthocyanins (10.6%) were noticed for the colder Islamabad compared to Chakwal. Although elevated temperature for warmer location and vintage favoured berry ripening, however key biochemical attributes such as titratable acidity, ascorbic acid, polyphenols and anthocyanins were negatively affected. The findings of the present research provide useful insight into the impact of growing season temperature on key berry attributes and may help devise adaptation strategies to improve berry quality.

Teosinte populations exhibit weak local adaptation to their rhizosphere biota despite strong effects of biota source on teosinte fitness and traits.

While biotic interactions often impose selection, species and popula- tions vary in whether they are locally adapted to biotic interactions. Evo- lutionary theory predicts that environmental conditions drive this variable local adaptation by altering the fitness impacts of species interactions. To investigate the influence of an environmental gradient on adaptation be- tween a plant and its associated rhizosphere biota, we cross-combined teosinte (Zea mays ssp. mexicana) and rhizosphere biota collected across a gradient of decreasing temperature, precipitation, and nutrients in a greenhouse common garden experiment. We measured both fitness and phenotypes expected to be influenced by biota, including concentrations of nutrients in leaves. Independent, main effects of teosinte and biota source explained most variation in teosinte fitness and traits. For example, biota from warmer sites provided population-independent fitness benefits across teosinte hosts. Effects of biota that depended on teosinte genotype were often not specific to their local hosts, and most traits had similar relation- ships to fitness across biota treatments. However, we found weak patterns of local adaptation between teosinte and biota from colder sites, suggest- ing environmental gradients may alter the importance of local adaptation in teosinte-biota interactions, as evolutionary theory predicts.

The role of surface energy fluxes in determining mixing layer heights

The atmospheric mixing layer height (MLH) is a critical variable for understanding and constraining ecosystem and climate dynamics. Past MLH estimation efforts have largely relied on data with low temporal (radiosondes) or spatial (reanalysis) resolutions. This study is unique in that it utilized continuous point-based ceilometer- and radiosonde-derived measurements of MLH at surface flux tower sites to identify the surface influence on MLH dynamics. We found a strong correlation (R2 = 0.73-0.91) between radiosonde MLH and ceilometer MLH at two sites with co-located observations. Seasonally, mean MLH was the highest at all sites during the summer, while the highest annual mean MLH was found at the warm and dry sites, dominated by high sensible heat fluxes. At daily time scales, surface fluxes of sensible heat, latent heat, and vapor pressure deficit had the largest influence on afternoon MLH. However, at best, the identified forcing variables and surface fluxes only accounted for ∼38-65% of the variability in MLH under all sky conditions, and ∼53-76% of the variability under clear skies. These results highlight the difficulty in using single-point observations to explain MLH dynamics but should encourage the use of ceilometers or similar atmospheric measurements at surface flux sites in future studies.

Genome-wide DNA methylation patterns in bumble bee (Bombus vosnesenskii) populations from spatial-environmental range extremes

Unraveling molecular mechanisms of adaptation to complex environments is crucial to understanding tolerance of abiotic pressures and responses to climatic change. Epigenetic variation is increasingly recognized as a mechanism that can facilitate rapid responses to changing environmental cues. To investigate variation in genetic and epigenetic diversity at spatial and thermal extremes, we use whole genome and methylome sequencing to generate a high-resolution map of DNA methylation in the bumble bee Bombus vosnesenskii. We sample two populations representing spatial and environmental range extremes (a warm southern low-elevation site and a cold northern high-elevation site) previously shown to exhibit differences in thermal tolerance and determine positions in the genome that are consistently and variably methylated across samples. Bisulfite sequencing reveals methylation characteristics similar to other arthropods, with low global CpG methylation but high methylation concentrated in gene bodies and in genome regions with low nucleotide diversity. Differentially methylated sites (n = 2066) were largely hypomethylated in the northern high-elevation population but not related to local sequence differentiation. The concentration of methylated and differentially methylated sites in exons and putative promoter regions suggests a possible role in gene regulation, and this high-resolution analysis of intraspecific epigenetic variation in wild Bombus suggests that the function of methylation in niche adaptation would be worth further investigation.

The functional mechanism behind the latitudinal pattern of liana diversity: Freeze-thaw embolism reduces the ecological performance of liana species.

There is a strong decrease in liana diversity along latitudinal and altitudinal gradients at a global scale, and there is a marked difference in liana diversity between tropical and temperate ecosystems. From these observations, it has been proposed that cold temperatures would restrict the ecological patterns of liana because of their vascular system's vulnerability to freeze-thaw embolism. Our objective was to establish the functional mechanism that drives the loss of liana diversity along a latitudinal temperature gradient. We evaluate the ecological performance of liana in 10 different species based on the apical growth rate, as well as functional traits associated with efficiency (maximum hydraulic conductivity and percentage conductivity lost) and safety of water transport (vessel diameter, vessel density, wood density, and root pressure). We found that at the colder (more southern) site within the latitudinal gradient, liana species showed lower performance, with a fivefold decrease in their apical growth rate as compared to the warmer (more northern) sites. We postulate that this lower performance results from a much lower water transport efficiency (26.1-fold decrease as compared to liana species that inhabit warmer sites) that results from higher freeze-thaw (37.5% of PLC) and reduction of vessel diameter (3 times narrower). These results are unmistakable evidence that cold temperature restricts liana performance: in a cold environment, liana species exhibit a strong decrease in performance, low efficiency, and higher safety of water transport. Conversely, at warmer sites, we found that liana species exhibit functional strategies associated with higher performance, higher efficiency, and lower safety of water transport capacity. This trade-off between efficiency and safety of water transport and their effects on performance could explain the latitudinal pattern of liana diversity.

Diagnosing evapotranspiration responses to water deficit across biomes using deep learning.

Accounting for water limitation is key to determining vegetation sensitivity to drought. Quantifying water limitation effects on evapotranspiration (ET) is challenged by the heterogeneity of vegetation types, climate zones and vertically along the rooting zone. Here, we train deep neural networks using flux measurements to study ET responses to progressing drought conditions. We determine a water stress factor (fET) that isolates ET reductions from effects of atmospheric aridity and other covarying drivers. We regress fET against the cumulative water deficit, which reveals the control of whole-column moisture availability. We find a variety of ET responses to water stress. Responses range from rapid declines of fET to 10% of its water-unlimited rate at several savannah and grassland sites, to mild fET reductions in most forests, despite substantial water deficits. Most sensitive responses are found at the most arid and warm sites. A combination of regulation of stomatal and hydraulic conductance and access to belowground water reservoirs, whether in groundwater or deep soil moisture, could explain the different behaviors observed across sites. This variety of responses is not captured by a standard land surface model, likely reflecting simplifications in its representation of belowground water storage.

Investigating the thermal state of permafrost with Bayesian inverse modeling of heat transfer

Abstract. Long-term measurements of permafrost temperatures do not provide a complete picture of the Arctic subsurface thermal regime. Regions with warmer permafrost often show little to no long-term change in ground temperature due to the uptake and release of latent heat during freezing and thawing. Thus, regions where the least warming is observed may also be the most vulnerable to permafrost degradation. Since direct measurements of ice and liquid water contents in the permafrost layer are not widely available, thermal modeling of the subsurface plays a crucial role in understanding how permafrost responds to changes in the local energy balance. In this work, we first analyze trends in observed air and permafrost temperatures at four sites within the continuous permafrost zone, where we find substantial variation in the apparent relationship between long-term changes in permafrost temperatures (0.02–0.16 K yr−1) and air temperature (0.09–0.11 K yr−1). We then apply recently developed Bayesian inversion methods to link observed changes in borehole temperatures to unobserved changes in latent heat and active layer thickness using a transient model of heat conduction with phase change. Our results suggest that the degree to which recent warming trends correlate with permafrost thaw depends strongly on both soil freezing characteristics and historical climatology. At the warmest site, a 9 m borehole near Ny-Ålesund, Svalbard, modeled active layer thickness increases by an average of 13 ± 1 cm K−1 rise in mean annual ground temperature. In stark contrast, modeled rates of thaw at one of the colder sites, a borehole on Samoylov Island in the Lena River delta, appear far less sensitive to temperature change, with a negligible effect of 1 ± 1 cm K−1. Although our study is limited to just four sites, the results urge caution in the interpretation and comparison of warming trends in Arctic boreholes, indicating significant uncertainty in their implications for the current and future thermal state of permafrost.

Older forests function as energetic and demographic refugia for a climate-sensitive species.

More frequent and extreme heat waves threaten climate-sensitive species. Structurally complex, older forests can buffer these effects by creating cool microclimates, although the mechanisms by which forest refugia mitigate physiological responses to heat exposure and subsequent population-level consequences remain relatively unexplored. We leveraged fine-scale movement data, doubly labeled water, and two decades of demographic data for the California spotted owl (Strix occidentalis occidentalis) to (1) assess the role of older forest characteristics as potential energetic buffers for individuals and (2) examine the subsequent value of older forests as refugia for a core population in the Sierra Nevada and a periphery population in the San Bernardino Mountains. Individuals spent less energy moving during warmer sampling periods and the presence of tall canopies facilitated energetic conservation during daytime roosting activities. In the core population, where tall-canopied forest was prevalent, temperature anomalies did not affect territory occupancy dynamics as warmer sites were both less likely to go extinct and less likely to become colonized, suggesting a trade-off between foraging opportunities and temperature exposure. In the peripheral population, sites were more likely to become unoccupied following warm summers, presumably because of less prevalent older forest conditions. While individuals avoided elevated energetic expenditure associated with temperature exposure, behavioral strategies to conserve energy may have diverted time and energy from reproduction or territory defense. Conserving older forests, which are threatened due to fire and drought, may benefit individuals from energetic consequences of exposure to stressful thermal conditions.

Integration of a Frost Mortality Scheme Into the Demographic Vegetation Model FATES

AbstractFrost is damaging to plants when air temperature drops below their tolerance threshold. The set of mechanisms used by cold‐tolerant plants to withstand freezing is called “hardening” and typically take place in autumn to protect against winter damage. The recent incorporation of a hardening scheme in the demographic vegetation model FATES opens up the possibility to investigate frost mortality to vegetation. Previously, the hardening scheme was used to improve hydraulic processes in cold‐tolerant plants. In this study, we expand upon the existing hardening scheme by implementing hardiness‐dependent frost mortality into CLM5.0‐FATES to study the impacts of frost on vegetation in temperate and boreal sites from 1950 to 2015. Our results show that the original freezing mortality approach of FATES, where each plant type had a fixed freezing tolerance threshold—an approach common to many other dynamic vegetation models, was restricted to predicting plant type distribution. The main results emerging from the new scheme are a high autumn and spring frost mortality, especially at colder sites, and increasing mid‐winter frost mortality due to global warming, especially at warmer sites. We demonstrate that the new frost scheme is a major step forward in dynamically representing vegetation in ESMs by for the first time including a level of frost tolerance that is responding to the environment and includes some level of cost (implicitly) and benefit. By linking hardening and frost mortality in a land surface model, we open new ways to explore the impact of frost events in the context of global warming.

Interspecific differences in thermal tolerance landscape explain aphid community abundance under climate change

A single critical thermal limit is often used to explain and infer the impact of climate change on geographic range and population abundance. However, it has limited application in describing the temporal dynamic and cumulative impacts of extreme temperatures. Here, we used a thermal tolerance landscape approach to address the impacts of extreme thermal events on the survival of co-existing aphid species (Metopolophium dirhodum, Sitobion avenae and Rhopalosiphum padi). Specifically, we built the thermal death time (TDT) models based on detailed survival datasets of three aphid species with three ages across a broad range of stressful high (34–40 °C) and low (−3∼−11 °C) temperatures to compare the interspecific and developmental stage variations in thermal tolerance. Using these TDT parameters, we performed a thermal risk assessment by calculating the potential daily thermal injury accumulation associated with the regional temperature variations in three wheat-growing sites along a latitude gradient. Results showed that M. dirhodum was the most vulnerable to heat but more tolerant to low temperatures than R. padi and S. avenae. R. padi survived better at high temperatures than Sitobion avenae and M. dirhodum but was sensitive to cold. R. padi was estimated to accumulate higher cold injury than the other two species during winter, while M. dirhodum accrued more heat injury during summer. The warmer site had higher risks of heat injury and the cooler site had higher risks of cold injury along a latitude gradient. These results support recent field observations that the proportion of R. padi increases with the increased frequency of heat waves. We also found that young nymphs generally had a lower thermal tolerance than old nymphs or adults. Our results provide a useful dataset and method for modelling and predicting the consequence of climate change on the population dynamics and community structure of small insects.

Canopy composition drives variability in urban growing season length more than the heat island effect

The elevated heat of urban areas compared to their surroundings makes humid temperate cities a useful preview of future climate effects on natural forest phenology. The utility of this proxy rests on the expectation that trees in urban areas alter their phenology in response to warmer site conditions in spring and fall. However, it is possible that apparent lengthening of the growing season is instead governed by human-driven tree species selection and plant functional type (PFT; trees, shrubs, turfgrass) heterogeneity typical of managed landscapes. Without the use of highly spatially and temporally resolved remote sensing data, the roles of tree taxonomy and local site characteristics (e.g., impervious cover) in controlling phenology remain confounded. To understand the drivers of earlier start of season (SOS) and later end of season (EOS) among urban trees, we estimated individual tree phenology using >130 high-resolution satellite images per year (2018–2020) for ~10,000 species-labeled trees in Washington, DC. We found that species identity alone accounted for 4× more variability in the timing of SOS and EOS compared with a tree's planting location characteristics. Additionally, the urban mix of PFTs may be more responsible for apparent advances in SOS (by between 1.8 ± 1.3 and 3.5 ± 1.3 days) than heat per se. The results of this study caution against associating longer growing seasons in cities—observed in moderate to coarse resolution remote sensing imagery—to within-species phenological plasticity and demonstrate the power of high-resolution satellite data for tracking tree phenology in biodiverse environments.