Codominant water control on global interannual variability and trends in land surface phenology and greenness

Identifying the relative importance of climatic and other environmental controls on the interannual variability and trends in global land surface phenology and greenness is challenging. Firstly, quantifications of land surface phenology and greenness dynamics are impaired by differences between satellite data sets and phenology detection methods. Secondly, dynamic global vegetation models (DGVMs) that can be used to diagnose controls still reveal structural limitations and contrasting sensitivities to environmental drivers. Thus, we assessed the performance of a new developed phenology module within the LPJmL (Lund–Potsdam–Jena managed Lands) DGVM with a comprehensive ensemble of three satellite data sets of vegetation greenness and ten phenology detection methods, thereby thoroughly accounting for observational uncertainties. The improved and tested model allows us quantifying the relative importance of environmental controls on interannual variability and trends of land surface phenology and greenness at regional and global scales. We found that start of growing season interannual variability and trends are in addition to cold temperature mainly controlled by incoming radiation and water availability in temperate and boreal forests. Warming‐induced prolongations of the growing season in high latitudes are dampened by a limited availability of light. For peak greenness, interannual variability and trends are dominantly controlled by water availability and land‐use and land‐cover change (LULCC) in all regions. Stronger greening trends in boreal forests of Siberia than in North America are associated with a stronger increase in water availability from melting permafrost soils. Our findings emphasize that in addition to cold temperatures, water availability is a codominant control for start of growing season and peak greenness trends at the global scale.     

Divergent carbon cycle response of forest and grass‐dominated northern temperate ecosystems to record winter warming

Northern temperate ecosystems are experiencing warmer and more variable winters, trends that are expected to continue into the foreseeable future. Despite this, most studies have focused on climate change impacts during the growing season, particularly when comparing responses across different vegetation cover types. Here we examined how a perennial grassland and adjacent mixed forest ecosystem in New Hampshire, United States, responded to a period of highly variable winters from 2014 through 2017 that included the warmest winter on record to date. In the grassland, record‐breaking temperatures in the winter of 2015/2016 led to a February onset of plant growth and the ecosystem became a sustained carbon sink well before winter ended, taking up roughly 90 g/m² more carbon during the winter to spring transition than in other recorded years. The forest was an unusually large carbon source during the same period. While forest photosynthesis was restricted by leaf‐out phenology, warm winter temperatures caused large pulses of ecosystem respiration that released nearly 230 g C/m² from February through April, more than double the carbon losses during that period in cooler years. These findings suggest that, as winters continue to warm, increases in ecosystem respiration outside the growing season could outpace increases in carbon uptake during a longer growing season, particularly in forests that depend on leaf‐out timing to initiate carbon uptake. In ecosystems with a perennial leaf habit, warming winter temperatures are more likely to increase ecosystem carbon uptake through extension of the active growing season. Our results highlight the importance of understanding relationships among antecedent winter conditions and carbon exchange across land‐cover types to understand how landscape carbon exchange will change under projected climate warming.     

Onset of spring starting earlier across the Northern Hemisphere

Recent warming of Northern Hemisphere (NH) land is well documented and typically greater in winter/spring than other seasons. Physical environment responses to warming have been reported, but not details of large‐area temperate growing season impacts, or consequences for ecosystems and agriculture. To date, hemispheric‐scale measurements of biospheric changes have been confined to remote sensing. However, these studies did not provide detailed data needed for many investigations. Here, we show that a suite of modeled and derived measures (produced from daily maximum–minimum temperatures) linking plant development (phenology) with its basic climatic drivers provide a reliable and spatially extensive method for monitoring general impacts of global warming on the start of the growing season. Results are consistent with prior smaller area studies, confirming a nearly universal quicker onset of early spring warmth (spring indices (SI) first leaf date, ?1.2 days decade^?1), late spring warmth (SI first bloom date, ?1.0 days decade^?1; last spring day below 5°C, ?1.4 days decade^?1), and last spring freeze date (?1.5 days decade^?1) across most temperate NH land regions over the 1955–2002 period. However, dynamics differ among major continental areas with North American first leaf and last freeze date changes displaying a complex spatial relationship. Europe presents a spatial pattern of change, with western continental areas showing last freeze dates getting earlier faster, some central areas having last freeze and first leaf dates progressing at about the same pace, while in portions of Northern and Eastern Europe first leaf dates are getting earlier faster than last freeze dates. Across East Asia last freeze dates are getting earlier faster than first leaf dates.     

The timing of autumn senescence is affected by the timing of spring phenology: implications for predictive models

Autumn senescence regulates multiple aspects of ecosystem function, along with associated feedbacks to the climate system. Despite its importance, current understanding of the drivers of senescence is limited, leading to a large spread in predictions of how the timing of senescence, and thus the length of the growing season, will change under future climate conditions. The most commonly held paradigm is that temperature and photoperiod are the primary controls, which suggests a future extension of the autumnal growing season as global temperatures rise. Here, using two decades of ground‐ and satellite‐based observations of temperate deciduous forest phenology, we show that the timing of autumn senescence is correlated with the timing of spring budburst across the entire eastern United States. On a year‐to‐year basis, an earlier/later spring was associated with an earlier/later autumn senescence, both for individual species and at a regional scale. We use the observed relationship to develop a novel model of autumn phenology. In contrast to current phenology models, this model predicts that the potential response of autumn phenology to future climate change is strongly limited by the impact of climate change on spring phenology. Current models of autumn phenology therefore may overpredict future increases in the length of the growing season, with subsequent impacts for modeling future CO₂ uptake and evapotranspiration.     

Four ways to define the growing season

What is addressed as growing season in terrestrial ecosystems is one of the main determinants of annual plant biomass production globally. However, there is no well-defined concept behind. Here, we show different facets of what might be termed growing season, each with a distinct meaning: (1) the time period during which a plant or a part of it actually grows and produces new tissue, irrespective of net carbon gain (growing season sensu stricto). (2) The period defined by developmental, that is, phenological markers (phenological season). (3) The period during which vegetation as a whole achieves its annual net primary production (NPP) or a net ecosystem production (NEP), expressed as net carbon gain (productive season) and (4) the period during which plants could potentially grow based on meteorological criteria (meteorological season). We hypothesize that the duration of such a ‘window of opportunity’ is a strong predictor for NPP at a global scale, especially for forests. These different definitions have implications for the understanding and modelling of plant growth and biomass production. The common view that variation in phenology is a proxy for variation in productivity is misleading, often resulting in unfounded statements on potential consequences of climatic warming such as carbon sequestration.     

Phenology shifts at start vs. end of growing season in temperate vegetation over the Northern Hemisphere for the period 1982–2008

Changes in vegetative growing seasons are dominant indicators of the dynamic response of ecosystems to climate change. Therefore, knowledge of growing seasons over the past decades is essential to predict ecosystem changes. In this study, the long‐term changes in the growing seasons of temperate vegetation over the Northern Hemisphere were examined by analyzing satellite‐measured normalized difference vegetation index and reanalysis temperature during 1982–2008. Results showed that the length of the growing season (LOS) increased over the analysis period; however, the role of changes at the start of the growing season (SOS) and at the end of the growing season (EOS) differed depending on the time period. On a hemispheric scale, SOS advanced by 5.2 days in the early period (1982–1999) but advanced by only 0.2 days in the later period (2000–2008). EOS was delayed by 4.3 days in the early period, and it was further delayed by another 2.3 days in the later period. The difference between SOS and EOS in the later period was due to less warming during the preseason (January–April) before SOS compared with the magnitude of warming in the preseason (June–September) before EOS. At a regional scale, delayed EOS in later periods was shown. In North America, EOS was delayed by 8.1 days in the early period and delayed by another 1.3 days in the later period. In Europe, the delayed EOS by 8.2 days was more significant than the advanced SOS by 3.2 days in the later period. However, in East Asia, the overall increase in LOS during the early period was weakened in the later period. Admitting regional heterogeneity, changes in hemispheric features suggest that the longer‐lasting vegetation growth in recent decades can be attributed to extended leaf senescence in autumn rather than earlier spring leaf‐out.     

Daylength helps temperate deciduous trees to leaf‐out at the optimal time

Global warming has led to substantially earlier spring leaf‐out in temperate‐zone deciduous trees. The interactive effects of temperature and daylength underlying this warming response remain unclear. However, they need to be accurately represented by earth system models to improve projections of the carbon and energy balances of temperate forests and the associated feedbacks to the Earth's climate system. We studied the control of leaf‐out by daylength and temperature using data from six tree species across 2,377 European phenological network ( www.pep725.eu ), each with at least 30 years of observations. We found that, in addition to and independent of the known effect of chilling, daylength correlates negatively with the heat requirement for leaf‐out in all studied species. In warm springs when leaf‐out is early, days are short and the heat requirement is higher than in an average spring, which mitigates the warming‐induced advancement of leaf‐out and protects the tree against precocious leaf‐out and the associated risks of late frosts. In contrast, longer‐than‐average daylength (in cold springs when leaf‐out is late) reduces the heat requirement for leaf‐out, ensuring that trees do not leaf‐out too late and miss out on large amounts of solar energy. These results provide the first large‐scale empirical evidence of a widespread daylength effect on the temperature sensitivity of leaf‐out phenology in temperate deciduous trees.     

Phenology model from surface meteorology does not capture satellite-based greenup estimations

Seasonal temperature change in temperate forests is known to trigger the start of spring growth, and both interannual and spatial variations in spring onset have been tied to climatic variability. Satellite dates are increasingly being used in phenology studies, but to date that has been little effort to link remotely sensed phenology to surface climate records. In this research, we use a two‐parameter spring warming phenology model to explore the relationship between climate and satellite‐based phenology. We employ daily air temperature records between 2000 and 2005 for 171 National Oceanographic and Atmospheric Administration weather stations located throughout New England to construct spring warming models predicting the onset of spring, as defined by the date of half‐maximum greenness (D₅₀) in deciduous forests as detected from Moderate Resolution Imaging Spectrometer. The best spring warming model starts accumulating temperatures after March 20th and when average daily temperatures exceed 5°C. The accumulated heat sums [heating degree day (HDD)] required to reach D₅₀ range from 150 to 300 degree days over New England, with the highest requirements to the south and in coastal regions. We test the ability of the spring warming model to predict phenology against a null photoperiod model (average date of onset). The spring warming model offers little improvement on the null model when predicting D₅₀. Differences between the efficacies of the two models are expressed as the ‘climate sensitivity ratio’ (CSR), which displays coherent spatial patterns. Our results suggest that northern (beech‐maple‐birch) and central (oak‐hickory) hardwood forests respond to climate differently, particularly with disparate requirements for the minimum temperature necessary to begin spring growth (3 and 6°C, respectively). We conclude that spatial location and species composition are critical factors for predicting the phenological response to climate change: satellite observations cannot be linked directly to temperature variability if species or community compositions are unknown.     

Leaf and root phenology and biomass of Eriophorum vaginatum in response to warming in the Arctic

北极地区气候变暖对莎草地上和地下部分物候和生物量的影响 北极地区的土壤和植被中存储了大量的碳,在气候变化的大背景下,北极升温速度几乎是地球其他地区的2倍。由于缺乏同步测量,尚不清楚北极地区植被地上部分和地下部分的物候和生物 量对气候变化的反应。在2016和2017年的生长季节,我们在北极的Toolik Lake站点上建立了一个移植花园,并沿纬度变化从高到低依次从 Sagwon、Toolik Lake和Coolfoot这3个站点移植了3种不同生态类型的莎草(Eriophorum vaginatum)。一半莎草用作增温处理,另一半为控制处理。我们用物候相机、手持式光谱仪和微根窗分别观测3种生态型莎草的绿色度、归一化植被指数(NDVI)和根长动态,根据NDVI和根长计算了叶片和细根的生物量。研究发现,莎草的叶和根生长是异步的,叶片开始生长的时间比根系生长的时间早约28天。气温升高1°C会延迟叶片衰老的时间,从而延长生长期,但温度升高对根系物候没有显着影响,变暖在统计学上没有增加叶片和根的生物量。此外,叶片生长的季节动态受气温的影响,但根的生长与土壤融化深度有关。因此,我们建议在使用碳和养分循环模型时,应将叶和根成分分开考虑,因为地上和地下的以及功能属性可能对气候变暖有不同的反应。     

Phenological responses of <Emphasis Type="Italic">Ulmus pumila</Emphasis> (Siberian Elm) to climate change in the temperate zone of China

Using Ulmus pumila (Siberian Elm) leaf unfolding and leaf fall phenological data from 46 stations in the temperate zone of China for the period 1986–2005, we detected linear trends in both start and end dates and length of the growing season. Moreover, we defined the optimum length period during which daily mean temperature affects the growing season start and end dates most markedly at each station in order to more precisely and rationally identify responses of the growing season to temperature. On average, the growing season start date advanced significantly at a rate of −4.0 days per decade, whereas the growing season end date was delayed significantly at a rate of 2.2 days per decade and the growing season length was prolonged significantly at a rate of 6.5 days per decade across the temperate zone of China. Thus, the growing season extension was induced mainly by the advancement of the start date. At individual stations, linear trends of the start date correlate negatively with linear trends of spring temperature during the optimum length period, namely, the quicker the spring temperature increased at a station, the quicker the start date advanced. With respect to growing season response to interannual temperature variation, a 1°C increase in spring temperature during the optimum length period may induce an advancement of 2.8 days in the start date of the growing season, whereas a 1°C increase in autumn temperature during the optimum length period may cause a delay of 2.1 days in the end date of the growing season, and a 1°C increase in annual mean temperature may result in a lengthening of the growing season of 9 days across the temperate zone of China. Therefore, the response of the start date to temperature is more sensitive than the response of the end date. At individual stations, the sensitivity of growing season response to temperature depends obviously on local thermal conditions, namely, either the negative response of the start date or the positive response of the end date and growing season length to temperature was stronger at warmer locations than at colder locations. Thus, future regional climate warming may enhance the sensitivity of plant phenological response to temperature, especially in colder regions.     

Satellite Data-Based Phenological Evaluation of the Nationwide Reforestation of South Korea

Through the past 60 years, forests, now of various age classes, have been established in the southern part of the Korean Peninsula through nationwide efforts to reestablish forests since the Korean War (1950–53), during which more than 65% of the nation''s forest was destroyed. Careful evaluation of long-term changes in vegetation growth after reforestation is one of the essential steps to ensuring sustainable forest management. This study investigated nationwide variations in vegetation phenology using satellite-based growing season estimates for 1982–2008. The start of the growing season calculated from the normalized difference vegetation index (NDVI) agrees reasonably with the ground-observed first flowering date both temporally (correlation coefficient, r = 0.54) and spatially (r = 0.64) at the 95% confidence level. Over the entire 27-year period, South Korea, on average, experienced a lengthening of the growing season of 4.5 days decade⁻¹, perhaps due to recent global warming. The lengthening of the growing season is attributed mostly to delays in the end of the growing season. The retrieved nationwide growing season data were used to compare the spatial variations in forest biomass carbon density with the time-averaged growing season length for 61 forests. Relatively higher forest biomass carbon density was observed over the regions having a longer growing season, especially for the regions dominated by young (<30 year) forests. These results imply that a lengthening of the growing season related to the ongoing global warming may have positive impacts on carbon sequestration, an important aspect of large-scale forest management for sustainable development.     

Satellite Evidence of Phenological Differences Between Urbanized and Rural Areas of the Eastern United States Deciduous Broadleaf Forest

We used a 10-year record (1990–99) of composited and cloud-screened reflectances from the Advanced Very High Resolution Radiometer (AVHRR) to test for phenological differences between urban and rural areas in the eastern United States deciduous broadleaf forest (DBF). We hypothesized that well-documented urban heat island effects would be associated with alterations in temperature-sensitive vegetation phenology. Our objectives were thus (a) to investigate possible differences in the start of the growing season (SOS) and end of the growing season (EOS) between the urban and DBF land covers, (b) to investigate related differences in greenness amplitude and fractional cover, and (c) to develop a generalized additive model (GAM) to predict the spatial variation of observed differences. By analyzing individual 1° latitude by 1° longitude blocks, we found that, on average, urbanization is associated with a growing season expansion of 7.6 days. Most of this effect is caused by an earlier SOS in urban areas. In all cases, urban regions had lower fractional cover and greenness amplitude. The GAM model failed to produce a viable model for differences in EOS, probably because it is dominated by photoperiod controls with only a minor temperature impact. SOS differences were predicted with an accuracy of about 2.4 days, with a GAM consisting of smoothed functions of mean annual average temperature, urban fractional cover, and the urban vs DBF greenness amplitude difference. We speculate that evidence of a phenological response to warming indicates that global warming, without reduction in DBF vegetation cover and greenness amplitude, may increase carbon sequestration in mesic deciduous forests. Received 6 June 2001; accepted 23 October 2001.     

Phenology of temperate trees in tropical climates

Several North American broad-leaved tree species range from the northern United States at 47°N to moist tropical montane forests in Mexico and Central America at 15–20°N. Along this gradient the average minimum temperatures of the coldest month (T _Jan), which characterize annual variation in temperature, increase from –10 to 12°C and tree phenology changes from deciduous to leaf-exchanging or evergreen in the southern range with a year-long growing season. Between 30 and 45°N, the time of bud break is highly correlated with T _Jan and bud break can be reliably predicted for the week in which mean minimum temperature rises to 7°C. Temperature-dependent deciduous phenology—and hence the validity of temperature-driven phenology models—terminates in southern North America near 30°N, where T _Jan>7°C enables growth of tropical trees and cultivation of frost-sensitive citrus fruits. In tropical climates most temperate broad-leaved species exchange old for new leaves within a few weeks in January-February, i.e., their phenology becomes similar to that of tropical leaf-exchanging species. Leaf buds of the southern ecotypes of these temperate species are therefore not winter-dormant and have no chilling requirement. As in many tropical trees, bud break of Celtis, Quercus and Fagus growing in warm climates is induced in early spring by increasing daylength. In tropical climates vegetative phenology is determined mainly by leaf longevity, seasonal variation in water stress and day length. As water stress during the dry season varies widely with soil water storage, climate-driven models cannot predict tree phenology in the tropics and tropical tree phenology does not constitute a useful indicator of global warming.     

日光诱导叶绿素荧光对亚热带常绿针叶林物候的追踪

植被物候期(春季返青和秋季衰老)是表征生物响应和陆地碳循环的基础信息。由于常绿针叶林冠层绿度的季节变动较弱,遥感提取常绿针叶林的物候信息存在着较大的不确定性,是目前区域物候监测中的难点。利用MODIS植被指数(归一化植被指数NDVI和增强型植被指数EVI)、GOME-2日光诱导叶绿素荧光(SIF)和通量数据(总初级生产力GPP)估算2007—2011年亚热带常绿针叶林物候期,用来比较三类遥感指数估算常绿针叶林物候的差异。结果表明:基于表征光合作用物候的通量GPP数据估算得到5年内亚热带常绿针叶林生长季开始时间(SOS_(GPP))为第63天,生长季结束时间(EOS_(GPP))为第324天,生长季长度为272天;基于反映植被光合作用特征的SIF曲线获得物候信息要滞后GPP物候期,其中生长季开始时间滞后19天,生长季结束时间滞后2天;基于传统植被指数NDVI和EVI的物候期滞后GPP物候期的时间要大于SIF滞后期,其中植被指数SOS滞后SOS_(GPP)31天,植被指数EOS滞后EOS_(GPP)10—17天。虽然基于3种遥感指数估算的春季和秋季物候都滞后于通量GPP的物候期,但是卫星SIF的物候信息能够更好地捕捉常绿针叶林的生长阶段。同时,春季温度是影响森林生长季开始时间的最重要因素;秋季水分和辐射是影响生长季结束时间的关键因素。由此可见,SIF估算的亚热带常绿针叶林的春季和秋季物候的滞后时间要短于传统植被指数,能更好地追踪常绿林光合作用的季节性,为深入研究陆地生态系统碳循环及其对气候变化的响应提供重要的基础。     

From observations to experiments in phenology research: investigating climate change impacts on trees and shrubs using dormant twigs

Background and Aims Climate change is advancing the leaf-out times of many plant species and mostly extending the growing season in temperate ecosystems. Laboratory experiments using twig cuttings from woody plant species present an affordable, easily replicated approach to investigate the relative importance of factors such as winter chilling, photoperiod, spring warming and frost tolerance on the leafing-out times of plant communities. This Viewpoint article demonstrates how the results of these experiments deepen our understanding beyond what is possible via analyses of remote sensing and field observation data, and can be used to improve climate change forecasts of shifts in phenology, ecosystem processes and ecological interactions.Scope The twig method involves cutting dormant twigs from trees, shrubs and vines on a single date or at intervals over the course of the winter and early spring, placing them in containers of water in controlled environments, and regularly recording leaf-out, flowering or other phenomena. Prior to or following leaf-out or flowering, twigs may be assigned to treatment groups for experiments involving temperature, photoperiod, frost, humidity and more. Recent studies using these methods have shown that winter chilling requirements and spring warming strongly affect leaf-out and flowering times of temperate trees and shrubs, whereas photoperiod requirements are less important than previously thought for most species. Invasive plant species have weaker winter chilling requirements than native species in temperate ecosystems, and species that leaf-out early in the season have greater frost tolerance than later leafing species.Conclusions This methodology could be extended to investigate additional drivers of leaf-out phenology, leaf senescence in the autumn, and other phenomena, and could be a useful tool for education and outreach. Additional ecosystems, such as boreal, southern hemisphere and sub-tropical forests, could also be investigated using dormant twigs to determine the drivers of leaf-out times and how these ecosystems will be affected by climate change.     

Key canopy traits drive forest productivity

Quantifying the mechanistic links between carbon fluxes and forest canopy attributes will advance understanding of leaf-to-ecosystem scaling and its potential application to assessing terrestrial ecosystem metabolism. Important advances have been made, but prior studies that related carbon fluxes to multiple canopy traits are scarce. Herein, presenting data for 128 cold temperate and boreal forests across a regional gradient of 600 km and 5.4°C (from 2.4°C to 7.8°C) in mean annual temperature, I show that stand-scale productivity is a function of the capacity to harvest light (represented by leaf area index, LAI), and to biochemically fix carbon (represented by canopy nitrogen concentration, %N). In combination, LAI and canopy %N explain greater than 75 per cent of variation in above-ground net primary productivity among forests, expressed per year or per day of growing season. After accounting for growing season length and climate effects, less than 10 per cent of the variance remained unexplained. These results mirror similar relations of leaf-scale and canopy-scale (eddy covariance) maximum photosynthetic rates to LAI and %N. Collectively, these findings indicate that canopy structure and chemistry translate from instantaneous physiology to annual carbon fluxes. Given the increasing capacity to remotely sense canopy LAI, %N and phenology, these results support the idea that physiologically based scaling relations can be useful tools for global modelling.     

Antagonistic effects of growing season and autumn temperatures on the timing of leaf coloration in winter deciduous trees

Autumn phenology remains a relatively neglected aspect in climate change research, which hinders an accurate assessment of the global carbon cycle and its sensitivity to climate change. Leaf coloration, a key indicator of the growing season end, is thought to be triggered mainly by high or low temperature and drought. However, how the control of leaf coloration is split between temperature and drought is not known for many species. Moreover, whether growing season and autumn temperatures interact in influencing the timing of leaf coloration is not clear. Here, we revealed major climate drivers of leaf coloration dates and their interactions using 154 phenological datasets for four winter deciduous tree species at 89 stations, and the corresponding daily mean/minimum air temperature and precipitation data across China's temperate zone from 1981 to 2012. Results show that temperature is more decisive than drought in causing leaf coloration, and the growing season mean temperature plays a more important role than the autumn mean minimum temperature. Higher growing season temperature and lower autumn minimum temperature would induce earlier leaf coloration date. Moreover, the mean temperature over the growing season correlates positively with the autumn minimum temperature. This implies that growing season mean temperature may offset the requirement of autumn minimum temperature in triggering leaf coloration. Our findings deepen the understanding of leaf coloration mechanisms in winter deciduous trees and suggest that leaf life‐span control depended on growing season mean temperature and autumn low temperature control and their interaction are major environmental cues. In the context of climate change, whether leaf coloration date advances or is delayed may depend on intensity of the offset effect of growing season temperature on autumn low temperature.     

Divergent phenological response to hydroclimate variability in forested mountain watersheds

Mountain watersheds are primary sources of freshwater, carbon sequestration, and other ecosystem services. There is significant interest in the effects of climate change and variability on these processes over short to long time scales. Much of the impact of hydroclimate variability in forest ecosystems is manifested in vegetation dynamics in space and time. In steep terrain, leaf phenology responds to topoclimate in complex ways, and can produce specific and measurable shifts in landscape forest patterns. The onset of spring is usually delayed at a specific rate with increasing elevation (often called Hopkins' Law; Hopkins, 1918), reflecting the dominant controls of temperature on greenup timing. Contrary with greenup, leaf senescence shows inconsistent trends along elevation gradients. Here, we present mechanisms and an explanation for this variability and its significance for ecosystem patterns and services in response to climate. We use moderate‐resolution imaging spectro‐radiometer (MODIS) Normalized Difference Vegetation Index (NDVI) data to derive landscape‐induced phenological patterns over topoclimate gradients in a humid temperate broadleaf forest in southern Appalachians. These phenological patterns are validated with different sets of field observations. Our data demonstrate that divergent behavior of leaf senescence with elevation is closely related to late growing season hydroclimate variability in temperature and water balance patterns. Specifically, a drier late growing season is associated with earlier leaf senescence at low elevation than at middle elevation. The effect of drought stress on vegetation senescence timing also leads to tighter coupling between growing season length and ecosystem water use estimated from observed precipitation and runoff generation. This study indicates increased late growing season drought may be leading to divergent ecosystem response between high and low elevation forests. Landscape‐induced phenological patterns are easily observed over wide areas and may be used as a unique diagnostic for sources of ecosystem vulnerability and sensitivity to hydroclimate change.     

A natural heating experiment: Phenotypic and genotypic responses of plant phenology to geothermal soil warming

Under global warming, the survival of many populations of sedentary organisms in seasonal environments will largely depend on their ability to cope with warming in situ by means of phenotypic plasticity or adaptive evolution. This is particularly true in high‐latitude environments, where current growing seasons are short, and expected temperature increases large. In such short‐growing season environments, the timing of growth and reproduction is critical to survival. Here, we use the unique setting provided by a natural geothermal soil warming gradient (Hengill geothermal area, Iceland) to study the response of Cerastium fontanum flowering phenology to temperature. We hypothesized that trait expression and phenotypic selection on flowering phenology are related to soil temperature, and tested the hypothesis that temperature‐driven differences in selection on phenology have resulted in genetic differentiation using a common garden experiment. In the field, phenology was related to soil temperature, with plants in warmer microsites flowering earlier than plants at colder microsites. In the common garden, plants responded to spring warming in a counter‐gradient fashion; plants originating from warmer microsites flowered relatively later than those originating from colder microsites. A likely explanation for this pattern is that plants from colder microsites have been selected to compensate for the shorter growing season by starting development at lower temperatures. However, in our study we did not find evidence of variation in phenotypic selection on phenology in relation to temperature, but selection consistently favoured early flowering. Our results show that soil temperature influences trait expression and suggest the existence of genetically based variation in flowering phenology leading to counter‐gradient local adaptation along a gradient of soil temperatures. An important implication of our results is that observed phenotypic responses of phenology to global warming might often be a combination of short‐term plastic responses and long‐term evolutionary responses, acting in different directions.     

Consistent shifts in spring vegetation green‐up date across temperate biomes in China, 1982–2006

Understanding spring phenology changes in response to the rapid climate change at biome‐level is crucial for projecting regional ecosystem carbon exchange and climate–biosphere interactions. In this study, we assessed the long‐term changes and responses to changing climate of the spring phenology in six temperate biomes of China by analyzing the global inventory monitoring and modeling studies (GIMMS) NOAA/AVHRR Normalized Difference Vegetation Index (NDVI) and concurrent mean temperature and precipitation data for 1982–2006. Results show that the spring phenology trends in the six temperate biomes are not continuous throughout the 25 year period. The spring phenology in most areas of the six biomes showed obvious advancing trends (ranging from ?0.09 to ?0.65 day/yr) during the 1980s and early 1990s, but has subsequently suffered consistently delaying trends (ranging from 0.22 to 1.22 day/yr). Changes in spring (February–April) temperature are the dominating factor governing the pattern of spring vegetation phenology in the temperate biomes of China. The recently delayed spring phenology in these temperate biomes has been mainly triggered by the stalling or reversal of the warming trend in spring temperatures. Results in this study also reveal that precipitation during November–January can explain 16.1% (P < 0.05), 20.9% (P < 0.05) and 14.2% (P < 0.05) of the variations in temperate deciduous forest (TDF), temperate steppe (TS), temperate desert (TD) respectively, highlighting the important role of winter precipitation in regulating changes in the spring vegetation phenology of water–limited biomes.