The timing of bud burst and its effect on tree growth

A phenology model for estimating the timings of bud burst – one of the most influential phenological phases for the simulation of tree growth – is presented in this study. The model calculates the timings of the leafing of beech (Fagus sylvatica L.) and oak (Quercus robur L.) and the May shoot of Norway spruce (Picea abies L.) and Scots pine (Pinus sylvestris L.) on the basis of the daily maximum temperature. The data for parameterisation and validation of the model have been taken from 40 climate and 120 phenological stations in southern Germany with time series for temperature and bud burst of up to 30 years. The validation of the phenology module by means of an independent data set showed correlation coefficients for comparisons between observed and simulated values of 54% (beech), 55% (oak), 59% (spruce) and 56% (pine) with mean absolute errors varying from 4.4 days (spruce) to 5.0 days (pine). These results correspond well with the results of other – often more complex – phenology models. After the phenology module had been implemented in the tree-growth model BALANCE, the growth of a mixed forest stand with the former static and the new dynamic timings for the bud burst was simulated. The results of the two simulation runs showed that phenology has to be taken into account when simulating forest growth, particularly in mixed stands.     

Comparing phenocam color indices with phenological observations of black spruce in the boreal forest

Bud phenology identifies the growing period of trees and determines the pattern of mass and energy exchanges between forest and atmosphere over time and space. Canopy color metrics derived from phenocams have been widely used to investigate tree phenology. However, it remains unclear which color-based index better tracks the seasonal variations of tree phenology in evergreen forest ecosystems. Herein, we compared four color metrics (red chromatic coordinate (RCC), green chromatic coordinate (GCC), vegetation contrast index (VCI) and excess green index (ExG)) derived from phenocam images with bud phenological phases recorded in black spruce [Picea mariana (Mill.) B.S·P] during 2017–2020 at a boreal forest site in Quebec, Canada. Canopy redness (RCC) and greenness (GCC, ExG, and VCI) showed a bimodal and bell-shaped seasonal pattern, respectively. The phases of bud burst and bud set lasted from end-May to end-June and from mid-July to end-September, respectively. The neural network model indicated that GCC had the best predictive ability in capturing the sequential phases of bud phenology. Bud phenological phases predicted by GCC showed the highest correlation with actual bud phenological phases among four indices, with R² above 0.9 and RMSE lower than 0.5. Overall, color indices performed better when representing bud burst than bud set. Our findings improve the efficiency and confidence of the phenocam greenness index to characterize the growing season of evergreen forests.     

Multiscale modeling of spring phenology across Deciduous Forests in the Eastern United States

Phenological events, such as bud burst, are strongly linked to ecosystem processes in temperate deciduous forests. However, the exact nature and magnitude of how seasonal and interannual variation in air temperatures influence phenology is poorly understood, and model‐based phenology representations fail to capture local‐ to regional‐scale variability arising from differences in species composition. In this paper, we use a combination of surface meteorological data, species composition maps, remote sensing, and ground‐based observations to estimate models that better represent how community‐level species composition affects the phenological response of deciduous broadleaf forests to climate forcing at spatial scales that are typically used in ecosystem models. Using time series of canopy greenness from repeat digital photography, citizen science data from the USA National Phenology Network, and satellite remote sensing‐based observations of phenology, we estimated and tested models that predict the timing of spring leaf emergence across five different deciduous broadleaf forest types in the eastern United States. Specifically, we evaluated two different approaches: (i) using species‐specific models in combination with species composition information to ‘upscale’ model predictions and (ii) using repeat digital photography of forest canopies that observe and integrate the phenological behavior of multiple representative species at each camera site to calibrate a single model for all deciduous broadleaf forests. Our results demonstrate variability in cumulative forcing requirements and photoperiod cues across species and forest types, and show how community composition influences phenological dynamics over large areas. At the same time, the response of different species to spatial and interannual variation in weather is, under the current climate regime, sufficiently similar that the generic deciduous forest model based on repeat digital photography performed comparably to the upscaled species‐specific models. More generally, results from this analysis demonstrate how in situ observation networks and remote sensing data can be used to synergistically calibrate and assess regional parameterizations of phenology in models.     

城市化对上海市城乡梯度上植物爆芽与展叶物候的影响

城市化影响植物物候,春季物候,如爆芽和展叶是植物响应城市化的客观生物指示。然而在城乡梯度上植物物候如何变化,以及物候与城市化程度是否存在定量关系,尚不明确。利用上海市2条城乡梯度上的6种木本植物的春季物候观测数据,以及距市中心距离作为城市化程度的代理指标,研究植物的爆芽和展叶物候期在城乡梯度上的变化及其与城市化程度的关系。研究结果表明：6种城市木本植物的爆芽和展叶物候期呈现随着距市中心距离的增加而延迟的趋势;城市化程度代理指标与爆芽和展叶物候期之间存在线性关系,城市化代理指标距市中心距离可以解释物候期延迟的31.5%—96.7%;物候期延迟时间因物种和距离城市距离而异,爆芽期延迟时间为3—15 d,展叶始期延长时间为4—13 d,展叶盛期延长时间为3—9 d。上述结果对理解城市化对植物物候的影响机制,评估城市化对城市生态系统的影响具有积极意义。     

Comparison and confirmation of SNP-bud burst associations in European beech populations in Germany

European beech (Fagus sylvatica L.) is one of the most important forest tree species in Central Europe. Climate change scenarios predict an increase in annual mean temperatures that may cause earlier bud burst in spring, potentially leading to an increased late frost-risk. Despite the ecologic and economic importance of beech, knowledge about the molecular basis of bud burst is still scarce in this species. Here, an association analysis was used to detect SNPs that are significantly associated with beech bud burst. A translocation experiment was established with progenies of six different beech populations from three widely separated regions in Northeast, Central, and Southwest Germany. In total, 600 individuals of the translocation experiment were genotyped using a set of 46 SNPs located in bud burst candidate genes. The association analysis revealed seven SNPs significantly associated with bud burst, each SNP explaining only a few percent of the observed phenotypic variation. Since the same SNP set was used in a previous association analysis with European beech, we were able to compare and confirm significant associations between SNPs and bud burst in distinct beech populations growing in different environments.     

Bud-burst modelling in Siberia and its impact on quantifying the carbon budget

Vegetation phenology is affected by climate change and in turn feeds back on climate by affecting the annual carbon uptake by vegetation. To quantify the impact of phenology on terrestrial carbon fluxes, we calibrate a bud‐burst model and embed it in the Sheffield dynamic global vegetation model (SDGVM) in order to perform carbon budget calculations. Bud‐burst dates derived from the VEGETATION sensor onboard the SPOT‐4 satellite are used to calibrate a range of bud‐burst models. This dataset has been recently developed using a new methodology based on the normalized difference water index, which is able to distinguish snowmelt from the onset of vegetation activity after winter. After calibration, a simple spring warming model was found to perform as well as more complex models accounting for a chilling requirement, and hence it was used for the carbon flux calculations. The root mean square difference (RMSD) between the calibrated model and the VEGETATION dataset was 6.5 days, and was 6.9 days between the calibrated model and independent ground observations of bud‐burst available at nine locations over Siberia. The effects of bud‐burst model uncertainties on the carbon budget were evaluated using the SDGVM. The 6.5 days RMSD in the bud‐burst date (a 6% variation in the growing season length), treated as a random noise, translates into about 41 g cm^?2 yr^?1 in net primary production (NPP), which corresponds to 8% of the mean NPP. This is a moderate impact and suggests the calibrated model is accurate enough for carbon budget calculations. In addition to random differences between the calibrated model and VEGETATION data, systematic errors between the calibrated bud‐burst model and true ground behaviour may occur, because of bias in the temperature dataset or because the bud‐burst detected by VEGETATION is because of some other phenological indicator. A systematic error of 1 day in bud‐burst translates into a 10 g cm^?2 yr^?1 error in NPP (about 2%). Based on the limited available ground data, any systematic error because of the use of VEGETATION data should not lead to significant errors in the calculated carbon flux. In contrast, widely used methods based on the normalized difference vegetation index from the advanced very high resolution radiometer satellite are likely to confuse snowmelt and vegetation greening, leading to errors of up to 15 days in bud‐burst date, with consequent large errors in carbon flux calculations.     

Leaf phenology in relation to canopy closure in southern Appalachian trees

Leaf phenology varies markedly across tree species of temperate deciduous forests. Early leafing in spring may increase light capture and carbon gain prior to canopy closure, allowing saplings to survive in understory sites deeply shaded in midsummer. We quantified sapling leaf phenology for 18 tree species and seasonal variation in understory light availability at three sites along a ridge-slope-cove landform gradient in the Great Smoky Mountains National Park. Early leafing species (e.g., Aesculus flava, Carpinus caroliniana) broke bud an average of 24 d before late leafers (e.g., Magnolia fraseri, Nyssa sylvatica). Canopy closure occurred 14-18 d earlier and summer understory light was on average 63-74% lower on intermediate and mesic sites than on the xeric site. Early leafing species intercepted 45-80% of their growing season photon flux before canopy closure vs. 8-15% for late leafers. However, earlier leafing increased exposure to freezing temperatures by 5.5% per week near the mean time of bud break. Early leafing is strongly correlated with midsummer shade, risk of freezing temperatures, and distribution on mesic sites across a "main spectrum" of 15 deciduous species. Differences in leaf phenology and resultant impacts on spring carbon gain may help determine tree shade tolerance and distribution in southern Appalachian forests.     

Chilling and heat requirements for leaf unfolding in European beech and sessile oak populations at the southern limit of their distribution range

With global warming, an advance in spring leaf phenology has been reported worldwide. However, it is difficult to forecast phenology for a given species, due to a lack of knowledge about chilling requirements. We quantified chilling and heat requirements for leaf unfolding in two European tree species and investigated their relative contributions to phenological variations between and within populations. We used an extensive database containing information about the leaf phenology of 14 oak and 10 beech populations monitored over elevation gradients since 2005. In parallel, we studied the various bud dormancy phases, in controlled conditions, by regularly sampling low- and high-elevation populations during fall and winter. Oak was 2.3 times more sensitive to temperature for leaf unfolding over the elevation gradient and had a lower chilling requirement for dormancy release than beech. We found that chilling is currently insufficient for the full release of dormancy, for both species, at the lowest elevations in the area studied. Genetic variation in leaf unfolding timing between and within oak populations was probably due to differences in heat requirement rather than differences in chilling requirement. Our results demonstrate the importance of chilling for leaf unfolding in forest trees and indicate that the advance in leaf unfolding phenology with increasing temperature will probably be less pronounced than forecasted. This highlights the urgent need to determine experimentally the interactions between chilling and heat requirements in forest tree species, to improve our understanding and modeling of changes in phenological timing under global warming.     

Changes in the structure and function of northern Alaskan ecosystems when considering variable leaf‐out times across groupings of species in a dynamic vegetation model

The phenology of arctic ecosystems is driven primarily by abiotic forces, with temperature acting as the main determinant of growing season onset and leaf budburst in the spring. However, while the plant species in arctic ecosystems require differing amounts of accumulated heat for leaf‐out, dynamic vegetation models simulated over regional to global scales typically assume some average leaf‐out for all of the species within an ecosystem. Here, we make use of air temperature records and observations of spring leaf phenology collected across dominant groupings of species (dwarf birch shrubs, willow shrubs, other deciduous shrubs, grasses, sedges, and forbs) in arctic and boreal ecosystems in Alaska. We then parameterize a dynamic vegetation model based on these data for four types of tundra ecosystems (heath tundra, shrub tundra, wet sedge tundra, and tussock tundra), as well as ecotonal boreal white spruce forest, and perform model simulations for the years 1970–2100. Over the course of the model simulations, we found changes in ecosystem composition under this new phenology algorithm compared with simulations with the previous phenology algorithm. These changes were the result of the differential timing of leaf‐out, as well as the ability for the groupings of species to compete for nitrogen and light availability. Regionally, there were differences in the trends of the carbon pools and fluxes between the new phenology algorithm and the previous phenology algorithm, although these differences depended on the future climate scenario. These findings indicate the importance of leaf phenology data collection by species and across the various ecosystem types within the highly heterogeneous Arctic landscape, and that dynamic vegetation models should consider variation in leaf‐out by groupings of species within these ecosystems to make more accurate projections of future plant distributions and carbon cycling in Arctic regions.     

A new method of survey and data analysis in phenology

Abstract. A new, computerized method is presented for the survey and analysis of phenological data on the vegetative cycle of tree species in complex forest structures. It is based on the principles of classical phenology, phytosociological sampling techniques and the main concepts of growth analysis. The method considers the development of phenological phases as a stochastic process, and allows a quantitative and mathematical-statistical comparisonbetweenphenorhythms of trees and crown sections, and correlations with environmental variables.     

Quantitative genetic architecture of adaptive phenology traits in the deciduous tree,Populus trichocarpa (Torr. and Gray)

In a warming climate, the ability to accurately predict and track shifting environmental conditions will be fundamental for plant survival. Environmental cues define the transitions between growth and dormancy as plants synchronise development with favourable environmental conditions, however these cues are predicted to change under future climate projections which may have profound impacts on tree survival and growth. Here, we use a quantitative genetic approach to estimate the genetic basis of spring and autumn phenology in Populus trichocarpa to determine this species capacity for climate adaptation. We measured bud burst, leaf coloration, and leaf senescence traits across two years (2017–2018) and combine these observations with measures of lifetime growth to determine how genetic correlations between phenology and growth may facilitate or constrain adaptation. Timing of transitions differed between years, although we found strong cross year genetic correlations in all traits, suggesting that genotypes respond in consistent ways to seasonal cues. Spring and autumn phenology were correlated with lifetime growth, where genotypes that burst leaves early and shed them late had the highest lifetime growth. We also identified substantial heritable variation in the timing of all phenological transitions (h² = 0.5–0.8) and in lifetime growth (h² = 0.8). The combination of additive variation and favourable genetic correlations in phenology traits suggests that populations of cultivated varieties of P. Trichocarpa may have the capability to adapt their phenology to climatic changes without negative impacts on growth.Subject terms: Plant breeding, Forest ecology, Evolutionary genetics     

Forecasting emergence and movement of overwintering hazelnut big bud mites from big buds

Eriophyoid big bud mites are key pests of hazelnut throughout the world, but they are difficult to control with chemicals or other methods because they are protected inside the bud. The most effective time for control is during the relatively short emergence period which is difficult for growers to predict. The key objectives of this study were to monitor mite emergence from big buds in spring, determine the phenology of mites in relation to tree phenology and weather, and identify the optimum timing for control measures. Mite emergence was found to occur between early and late spring in Canterbury, New Zealand. Mite emergence and movement occurred when daily maximum temperatures were >15 degrees C and when mean temperatures were >9 degrees C, with mite emergence increasing with temperature. The developmental status of new buds during mite emergence was a crucial factor in the infestation of new buds. An accumulated heat sum model (DD), starting at Julian date 152 and using a lower threshold temperature of 6 degrees C, predicted the onset of emergence on two cultivars and at two sites at approximately 172 DD. A regression model based on leaf number, bud length, bud width, DD and Julian date provided a more satisfactory prediction of percent accumulated mite emergence. It is recommended both peak mite emergence and the developmental status of hazelnut buds be used to optimise the time to apply control measures. The optimum time to apply a control was predicted to be before buds measure 0.5 x 0.5 mm (width x length), are enclosed within the axil, and have a rounded tip, or, when 50% accumulated mite emergence has occurred, whichever occurs first.     

Predicting spatial and temporal patterns of bud‐burst and spring frost risk in north‐west Europe: the implications of local adaptation to climate

The timing of spring bud‐burst and leaf development in temperate, boreal and Arctic trees and shrubs fluctuates from year to year, depending on meteorological conditions. Over several generations, the sensitivity of bud‐burst to meteorological conditions is subject to selection pressure. The timing of spring bud‐burst is considered to be under opposing evolutionary pressures; earlier bud‐burst increases the available growing season (capacity adaptation) but later bud‐burst decreases the risk of frost damage to actively growing parts (survival adaptation). The optimum trade‐off between these two forms of adaptation may be considered an evolutionarily stable strategy that maximizes the long‐term ecological fitness of a phenotype under a given climate. Rapid changes in climate, as predicted for this century, are likely to exceed the rate at which trees and shrubs can adapt through evolution or migration. Therefore the response of spring phenology will depend not only on future climatic conditions but also on the limits imposed by adaptation to current and historical climate. Using a dataset of bud‐burst dates from twenty‐nine sites in Finland for downy birch (Betula pubescens Ehrh.), we parameterize a simple thermal time bud‐burst model in which the critical temperature threshold for bud‐burst is a function of recent historical climatic conditions and reflects a trade‐off between capacity and survival adaptation. We validate this approach with independent data from eight independent sites outside Finland, and use the parameterized model to predict the response of bud‐burst to future climate scenarios in north‐west Europe. Current strategies for budburst are predicted to be suboptimal for future climates, with bud‐burst generally occurring earlier than the optimal strategy. Nevertheless, exposure to frost risk is predicted to decrease slightly and the growing season is predicted to increase considerably across most of the region. However, in high‐altitude maritime regions exposure to frost risk following bud‐burst is predicted to increase.     

浙江天童常绿阔叶林栲树爆芽物候格局研究

芽的活动和芽库动态反映了植物对环境的适应方式以及生活史对策。为了研究栲树（Castanopsis farge-sii）爆芽物候特征及其生活史对策,在2008年3～6月间对栲树自然种群的爆芽物候及芽变化进行了观察,定期记录了栲树的爆芽情况,包括被标记标准枝的芽数、芽所处阶段等参数,分析研究了栲树的爆芽物候特征。结果表明：栲树芽的发育集中在3～4月下旬,芽发育过程依其形态可分为：休眠期（第一阶段B0）、萌动期（第二阶段B1）、爆芽期（第三～第五阶段B2～B4）3个时期。爆芽各阶段种群、个体、小枝的物候参数基本相同。爆芽物候进程在第三阶段为双峰曲线,在第四阶段和第五阶段呈单峰曲线。在林下和林窗两种生境下第三阶段爆芽物候在时间上基本相同,而第四、第五阶段在时间上林下比林窗滞后。栲树在爆芽阶段并不存在早开始早结束的关系,虽然芽进入各阶段的时间不同,但最终出叶时间相对集中。作为亚热带常绿阔叶树种栲树,爆芽物候受气候等多方面因素的影响,其中春季温度为最重要的影响因素,长期适应的结果使得栲树爆芽物候与环境条件的变化相协调。     

The growth and decline of a population of the spruce aphid Elatobium abietinum during a three year study,and the changing pattern of fecundity,recruitment and alary polymorphism in a Northern Ireland Forest

The population size and structure of the green spruce aphid was followed throughout the spring — summer cycle on the same group of trees in a low-elevation coastal Sitka spruce forest for three consecutive years. The relationship between the pattern of change and the phenology of bud burst, which heralds a marked change in needle sap quality, suggests that yearly differences in the winter temperature regime may affect the duration of the population growth phase and hence the peak numbers attained in late spring.An index of population growth rate was sufficiently sensitive to aphid fecundity during the population cycles of two years to suggest that the changing rate of fertility was the decisive process governing changes in population size. The commitment of aphids to alate development was greater than that recorded elsewhere in Britain but did little to effect population decline since the contribution of alatae to larviposition was substantial while seasonally pulsed.     

Predicting adaptation of phenology in response to climate change, an insect herbivore example

Climate change has led to an advance in phenology in many species. Synchrony in phenology between different species within a food chain may be disrupted if an increase in temperature affects the phenology of the different species differently, as is the case in the winter moth egg hatch–oak bud burst system. Operophtera brumata (winter moth) egg hatch date has advanced more than Quercus robur (pedunculate oak) bud burst date over the past two decades. Disrupted synchrony will lead to selection, and a response in phenology to this selection may lead to species genetically adapting to their changing environment. However, a prerequisite for such genetic change is that there is sufficient genetic variation and severe enough fitness consequences. So far, examples of observed genetic change have been few. Using a half-sib design, we demonstrate here that O. brumata egg-hatching reaction norm is heritable, and that genetic variation exists. Fitness consequences of even a few days difference between egg hatch and tree bud opening are severe, as we experimentally determined. Estimates of genetic variation and of fitness were then combined with a climate scenario to predict the rate and the amount of change in the eggs' response to temperature. We predict a rapid response to selection, leading to a restoration of synchrony of egg hatch with Q. robur bud opening. This study shows that in this case there is a clear potential to adapt – rapidly – to environmental change. The current observed asynchrony is therefore not due to a lack of genetic variation and at present it is unclear what is constraining O. brumata to adapt. This kind of model may be particularly useful in gaining insight in the predicted amount and rate of change due to environmental changes, given a certain genetic variation and selection pressure.     

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.     

Local adaptations to frost in marginal and central populations of the dominant forest tree Fagus sylvatica L. as affected by temperature and extreme drought in common garden experiments

Local adaptations to environmental conditions are of high ecological importance as they determine distribution ranges and likely affect species responses to climate change. Increased environmental stress (warming, extreme drought) due to climate change in combination with decreased genetic mixing due to isolation may lead to stronger local adaptations of geographically marginal than central populations. We experimentally observed local adaptations of three marginal and four central populations of Fagus sylvatica L., the dominant native forest tree, to frost over winter and in spring (late frost). We determined frost hardiness of buds and roots by the relative electrolyte leakage in two common garden experiments. The experiment at the cold site included a continuous warming treatment; the experiment at the warm site included a preceding summer drought manipulation. In both experiments, we found evidence for local adaptation to frost, with stronger signs of local adaptation in marginal populations. Winter frost killed many of the potted individuals at the cold site, with higher survival in the warming treatment and in those populations originating from colder environments. However, we found no difference in winter frost tolerance of buds among populations, implying that bud survival was not the main cue for mortality. Bud late frost tolerance in April differed between populations at the warm site, mainly because of phenological differences in bud break. Increased spring frost tolerance of plants which had experienced drought stress in the preceding summer could also be explained by shifts in phenology. Stronger local adaptations to climate in geographically marginal than central populations imply the potential for adaptation to climate at range edges. In times of climate change, however, it needs to be tested whether locally adapted populations at range margins can successfully adapt further to changing conditions.     

Simulating stand climate, phenology, and photosynthesis of a forest stand with a process-based growth model

In the face of climate change and accompanying risks, forest management in Europe is becoming increasingly important. Model simulations can help to understand the reactions and feedbacks of a changing environment on tree growth. In order to simulate forest growth based on future climate change scenarios, we tested the basic processes underlying the growth model BALANCE, simulating stand climate (air temperature, photosynthetically active radiation (PAR) and precipitation), tree phenology, and photosynthesis. A mixed stand of 53- to 60-year-old Norway spruce (Picea abies) and European beech (Fagus sylvatica) in Southern Germany was used as a reference. The results show that BALANCE is able to realistically simulate air temperature gradients in a forest stand using air temperature measurements above the canopy and PAR regimes at different heights for single trees inside the canopy. Interception as a central variable for water balance of a forest stand was also estimated. Tree phenology, i.e. bud burst and leaf coloring, could be reproduced convincingly. Simulated photosynthesis rates were in accordance with measured values for beech both in the sun and the shade crown. For spruce, however, some discrepancies in the rates were obvious, probably due to changed environmental conditions after bud break. Overall, BALANCE has shown to respond to scenario simulations of a changing environment (e.g., climate change, change of forest stand structure).     

胡杨枝芽生长特征及其展叶物候特征

以5个不同发育阶段的胡杨(Populus euphratica Oliv.)个体为研究对象,观测记录了枝芽展叶物候、枝芽生长特征和叶形变化的空间分布规律。结果表明:不同发育阶段的胡杨个体以及同一个体树冠的不同层次,其枝芽生长及其展叶物候期表现出不同的时空特征。随着树龄的增加和树冠层次的增高(由基向顶),当年新生枝条长度、枝条叶片数和叶形指数逐渐减小,但叶面积和叶片干重逐渐增大。5个不同发育阶段胡杨个体均表现出展叶物候始于树冠顶层,依次向下结束于树冠基部;展叶物候期共性表现在枝芽萌动期均在4月上旬,起始展叶期集中在4月中旬,展叶终期则在5月上旬到下旬;树龄较大的个体其枝芽萌动期、起始展叶期、展叶终期较树龄较小的个体早;其枝芽萌动期到展叶终期的时间进程较树龄较小的个体短;不同发育阶段的个体枝芽萌动期出现的时间较为离散,起始展叶期和展叶终期出现的时间较为集中。相关分析表明,出叶周期与枝条长度、枝条叶片数量和叶形指数呈极显著正相关,与叶面积和叶片干重呈显著负相关。