Spring Leaf Phenology Research Articles

Uniforming spring phenology under non-uniform climate warming across latitude in China

Temperature is the primary factor controlling plant phenology. As temperature changes with latitude, leaf phenology in spring always shows a significant latitudinal pattern. However, under asymmetric warming at the low and high latitudes, the variability of the spring leaf phenology with latitude is becoming unclear. Based on the 23,094 observations of the leaf unfolding date (LUD) for woody species located in eastern China within latitudes 23–49°N, we analyzed the variability of LUD and its latitudinal sensitivity (Slat, days °N−1, expressed in delayed days per degree in latitude) during 1963–2008. The results showed an earlier LUD at the mid- (−2.2 days decade−1) and high (−2.5 days decade−1) latitude regions, while a stable LUD at the low-latitude regions during 1963–2008. However, the temperature sensitivity of LUD (ST, days °C−1, expressed in advanced days per degree in temperature) remained stable across the latitudes although a slight decreasing trend from 1963 to 2008. As a result, the non-uniform optimal preseason warming with latitude (Tlat, °C °N−1, expressed in the increase of temperature per degree in latitude) decreased Slat from 2.38 (days °N−1) in 1963 to 1.55 (days °N−1) in 2008. Further analyses indicated that the Growing Degree Hours (GDH) played a critical role in these processes, although the Chilling Hours (CH) showed significant variability after 1991. Our results provide evidence that the change in the balance of CH and GDH across latitude induced declining Slat over the last 40 years in eastern China. Furthermore, it may continue under the future climate warming scenarios and ultimately has important consequences on the structure and function of ecosystems.

Intraspecific variation in spring leaf phenology and duration of leaf expansion in relation to leaf habit and leaf size of temperate tree species

Spring leaf phenology has been intensively studied in temperate deciduous broad-leaved tree species, but the phenology of evergreen broad-leaved tree species has seldom been focused on. Evaluation of the difference in spring leaf phenology between coexisting deciduous and evergreen species is essential to predict their responses to climate change. In this study, spring leaf phenology was investigated for the 12 deciduous and 12 evergreen broad-leaved species coexisting in a warm-temperate forest in Japan, based on the predictions that selection pressure for earlier leaf production in spring results in less intraspecific variation in phenology at a given timing (SDmax) and a shorter duration of leaf expansion in deciduous than in evergreen species. In contrast to this prediction, SDmax did not differ between deciduous and evergreen species and was not related to leaf area or LMA. On the other hand, the duration of leaf expansion was longer in evergreen than in deciduous species, and was positively correlated with LMA. The leaves of greater LMA required longer periods of leaf expansion, probably due to a higher cost of and/or more conservative leaf development in the face of herbivory. As a consequence, the timing of full leaf expansion was delayed not only by later budburst (approximately two weeks) but also by a longer duration of leaf expansion (approximately two weeks) in evergreen than in deciduous broad-leaved species, which would probably influence the productivity of newly emerging leaves in spring.

A comparison of ground-based methods for obtaining large-scale, high-resolution data on the spring leaf phenology of temperate tree species.

Phenological variation in spring leafing between and within species can determine plant responses to warmer winter and spring temperatures in the short term. Methods are needed for monitoring canopy development that can be replicated on a large-scale, while retaining fine-scale resolution at the level of individual trees. Citizen science has the potential to provide this, but a range of approaches exist in terms of the phenophase recorded (e.g. budburst or leaf expansion), how the phenophase is characterised (first events or intensity monitoring) and the portion of tree crown assessed and observation frequency. A comparison of spring budburst and leaf expansion of four tree species (Fraxinus excelsior, Fagus sylvatica, Quercus robur and Acer pseudoplatanus) was monitored in one woodland using (1) counts of expanded leaves on three crown sections, (2) percentage estimates of expanded leaves across the whole crown and (3) a greenness index from photography. Logistic growth models were applied to make comparisons. First-event dates were found to be misleading due to high variation in leaf development rates within and between species. Percentage estimates and counts produced similar estimates of leaf expansion timing and rate. The greenness index produced similar estimates of timing, but not rate, and was compromised by practicalities of photographing individual crowns in closed-canopy woodland. Citizen scientists could collect data across the period of spring leafing, with visual counts and/or estimates made every 3-4days, subject to tests of reliability in pilot citizen science studies.

Decline in the strength of genetic controls on aspen environmental responses from seasonal to century‐long phenomena

AbstractUnderstanding intra‐specific variation in climate sensitivity could improve the prediction of tree responses to climate change. We attempted to identify the degree of genetic control of tree phenology and growth of trembling aspen (Populus tremuloides Mchx.) in a natural stand of this species in northwestern Quebec. We mapped and genotyped 556 aspen trees growing within the plot, using seven nuclear microsatellite loci for clone identification. We selected 13 clones (n of trees per clone >5, in total 350 trees) and evaluated the explanatory power of clone identity in (a) variability of spring leaf phenology and (b) short‐ and long‐term growth responses. The clone's identity explained 43% of the variability in spring leaf phenology, between 18% and 20% of variability in response to monthly climate variables significantly affecting growth, between 8% and 26% of growth response to insect outbreaks, and 12% in the long‐term growth rates. Strong clonal control of aspen phenology and moderate control of growth responses to monthly weather do not result in an equally large impact on long‐term growth rates. The result suggests an important role of environmental extremes and within community interactions as factors averaging aspen growth performance at the stand level.

Canopy damage by spring frost in European beech along the Apennines: effect of latitude, altitude and aspect

Late spring frost plays a major role in the structure and function of forest ecosystems with potential consequences on species distribution at both local and regional scales. Paradoxically, in a warmer world the incidence and impact of frost is increasing because of earlier leaf unfolding and flowering as a response to warmer temperatures. In this regard, European Beech (Fagus sylvatica L.), a native tree species widely distributed in European forests, is considered particularly sensitive to changes in spring temperature regimes associated with climate change and thus especially subject to the risk of frost damage. Although several studies concerning F. sylvatica frost damage have been conducted in northern and central Europe, no extensive studies are available for the southern part of its range, i.e. central and southern Italy as well as Greece.In this paper the effect of a late spring frost occurring at the end of April 2016 is extensively documented with high spatial detail all along the Apennine Chain through satellite image data. Three different remote-sensing greenness indexes, namely the normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), and the greenness index (GI) derived from Landsat-8 satellite images acquired from May to July in the years 2014, 2015, and 2016 at a spatial resolution of 30 m, were used to gauge the spatial response of common beech forests to this late frost event with relation to latitude, altitude and slope exposure. Frost damage was evaluated as a difference (Δ) of NDVI, GI and EVI between the mean of years 2014 and 2015 (i.e. MRY, mean of reference years), and 2016 (i.e. FEY, frost reference year). The three satellite remote-sensing indexes were efficient at detecting leaf damage with detailed spatial resolution and proved consistent with one another.The greatest damage occurred in the middle altitudinal range between 1500 and 1700 m a.s.l. with a decreasing trend toward both lower and higher elevations due to warmer temperatures below, and delayed phenology above. Exposure also influenced frost injury, with south-facing slopes of the mountain more damaged than the north. This difference was due to earlier spring leaf phenology of southern beech trees in response to a greater heat sum in the warm weeks preceding. Less damage in the northern Apennines is consistent with the spatial extent of minimum freezing temperatures. To sum up, frost damage is strongly related to site-specific conditions, i.e. on the one hand to minimum temperatures, and on the other to the phenological stage of the trees involving both altitude and exposure. Hence focusing on detailed sub-regional studies can be helpful for predicting future consequences of climate change on forests.

Intraspecific differences in spring leaf phenology in relation to tree size in temperate deciduous trees.

Spring leaf phenology strongly influences plant productivity in temperate deciduous forests. Many studies have detected earlier budburst and leaf maturation in smaller trees within species, and have discussed the adaptive significance of increasing carbon gain before canopy closure in small trees. However, some previous studies have found the opposite pattern, and the physiological and environmental bases for this discrepancy are incompletely understood. We investigated the spring leaf phenology of 11 deciduous species in a cool-temperate forest in Japan for 2 years with different amounts of snowfall, and also gathered data on the day of budburst from multiple studies to assess whether and how the timing of budburst is related to tree size, phylogeny, temperature and annual snowfall of study sites. We found that differences in the timing of budburst and leaf maturation between saplings (<1 m height) and short trees (1-10 m height) are larger than those between short trees and tall trees (>10 m height), resulting in non-linear relationships between timing and height in most species. Cumulative degree-days to the day of budburst were smaller in saplings than in tall trees, probably because saplings are selected to outcompete the other individuals/species to become established. Moreover, phylogenetic relatedness did not explain the difference in spring leaf phenology between saplings and tall trees in the literature survey. In addition, our literature survey showed that budburst occurred earlier in saplings of most species in sites with less snowfall, whereas budburst occurred earlier in tall trees at sites with heavy snowfall. These results suggest that the opposite patterns found in some studies may be due to (i) differences in the target size, as saplings show larger phenological discrepancies than short and tall trees, and (ii) the microclimate experienced by the tree, as sites with heavy snow show delayed sapling phenology.

A new algorithm for the estimation of leaf unfolding date using MODIS data over China’s terrestrial ecosystems

Using solely vegetation indices (VIs) from remote sensing is not always sufficient to accurately detect spring leaf phenology, i.e., the leaf unfolding date (LUD). Several current phenology products failed to provide reliable LUD estimates for specific regions and plant functional types, e.g., evergreen species at mid-low latitudes. Therefore, increasing efforts have been made to improve LUD modeling by combining VIs and meteorological variables. Temperature before the growing season (‘preseason’ henceforth) plays an important role in regulating spring phenology. With ground observations of LUD (LUDOBS) across different plant functional types (PFTs) in China during 2001–2014, we analyzed the response of LUDOBS to preseason temperature temporally and spatially, and proposed an improved LUD modeling algorithm by developing a temperature-based scale factor to adjust the traditional VI-based (i.e., two band enhanced vegetation index (EVI2)) LUD estimates. We found that the new algorithm can better characterize the spatial and temporal patterns of LUD variability for different PFTs, especially for evergreen species where MODIS phenology product failed to provide reliable LUD estimates. Furthermore, we investigated the spatio-temporal patterns of LUD over China with respect to both different vegetation types and climate systems. We showed that for ∼70% pixels, our new model predicted an overall later LUDs than MODIS phenology product, possibly suggesting an overestimated greening potential of China’s terrestrial ecosystems. Our study suggests that preseason temperature plays a previously neglected role in modeling spring LUD and instead of using VIs or temperature alone, a combination of temperature and VIs can improve the prediction of spring phenology.

Later springs green-up faster: the relation between onset and completion of green-up in deciduous forests of North America.

In deciduous forests, spring leaf phenology controls the onset of numerous ecosystem functions. While most studies have focused on a single annual spring event, such as budburst, ecosystem functions like photosynthesis and transpiration increase gradually after budburst, as leaves grow to their mature size. Here, we examine the "velocity of green-up," or duration between budburst and leaf maturity, in deciduous forest ecosystems of eastern North America. We use a diverse data set that includes 301 site-years of phenocam data across a range of sites, as well as 22years of direct ground observations of individual trees and 3years of fine-scale high-frequency aerial photography, both from Harvard Forest. We find a significant association between later start of spring and faster green-up: - 0.47 ± 0.04 (slope ± 1 SE) days change in length of green-up for every day later start of spring within phenocam sites, - 0.31 ± 0.06days/day for trees under direct observation, and - 1.61 ± 0.08days/day spatially across fine-scale landscape units. To explore the climatic drivers of spring leaf development, we fit degree-day models to the observational data from Harvard Forest. We find that the default phenology parameters of the ecosystem model PnET make biased predictions of leaf initiation (39days early) and maturity (13days late) for red oak, while the optimized model has biases of 1day or less. Springtime productivity predictions using optimized parameters are closer to results driven by observational data (within 1%) than those of the default parameterization (17% difference). Our study advances empirical understanding of the link between early and late spring phenophases and demonstrates that accurately modeling these transitions is important for simulating seasonal variation in ecosystem productivity.

Variations in the temperature sensitivity of spring leaf phenology from 1978 to 2014 in Mudanjiang, China.

Continuous long-term temperature sensitivity (ST) of leaf unfolding date (LUD) and main impacting factors in spring in the period 1978-2014 for 40 plant species in Mudanjiang, Heilongjiang Province, Northeast China, were analyzed by using observation data from the China Phenological Observation Network (CPON), together with the corresponding meteorological data from the China Meteorological Data Service Center. Temperature sensitivities, slopes of the regression between LUD and mean temperature during the optimum preseason (OP), were analyzed using 15-year moving window to determine their temporal trends. Major factors impacting ST were then chosen and evaluated by applying a random sampling method. The results showed that LUD was sensitive to mean temperature in a defined period before phenophase onset for all plant species analyzed. Over the period 1978-2014, the mean ST of LUD for all plant species was - 3.2 ± 0.49days°C-1. The moving window analysis revealed that 75% of species displayed increasing ST of LUD, with 55% showing significant increases (P < 0.05). ST for the other 25% exhibited a decreasing trend, with 17% showing significant decreases (P < 0.05). On average, ST increased by 16%, from - 2.8 ± 0.83days°C-1 during 1980-1994 to - 3.30 ± 0.65days°C-1 during 2000-2014. For species with later LUD and longer OP, ST tended to increase more, while species with earlier LUD and shorter OP tended to display a decreasing ST. The standard deviation of preseason temperature impacted the temporal variation in ST. Chilling conditions influenced ST for some species, but photoperiod limitation did not have significant or coherent effects on changes in ST.

Interspecific and interannual variation in the duration of spring phenophases in a northern mixed forest

A wide range of intra- and interspecific variation occurs in spring leaf phenology as a result of biotic factors such as, life strategy, ecological niche and genetic adaptation, and abiotic factors such as environmental condition. Whereas knowing when the start of bud-burst occurs is necessary for determining the beginning of the growing season, and the subsequent start of carbon uptake, the duration of phenophases is equally important (to estimate the rate of carbon uptake, for example), but rarely reported. Here, we investigate variation in the timing and duration of 3 key phenophase categories (bud-open, leaf-out, full-leaf unfolded) from a range of 8 broadleaf and 2 conifer species in a mixed forest in northern Wisconsin, USA over a 5-year period. As expected, the start of each phenophase category varied across species and years and an earlier start to one phenophase did not necessarily result in an earlier start to subsequent phenophases nor did it mean a faster or slower progression. Ecological niche was not always a useful predictor of the timing or duration of the spring phenology season. The spring phenology season from bud-burst to full leaf open for the entire forest community took an average of 13days ranging from 12 to 18days across species. Bud-open and leaf-out lasted an average of 4days whereas, full-leaf unfolding lasted 5 and again there were variations among species. Full leaf unfolded for A. incana lasted significantly (p<0.001) longer than other species. Variation in the duration of the spring phenology season among years closely tracked local seasonal air-temperature based on growing degree hours (GDH). These results could be used to help determine the relationship between phenology and the potential for carbon storage in early spring in a mixed forest and highlight the value of direct field observation data at species level, the detail of which cannot, at present, be captured by satellite remote sensing.

Contrasting growth phenology of native and invasive forest shrubs mediated by genome size.

Examination of the significance of genome size to plant invasions has been largely restricted to its association with growth rate. We investigated the novel hypothesis that genome size is related to forest invasions through its association with growth phenology, as a result of the ability of large-genome species to grow more effectively through cell expansion at cool temperatures. We monitored the spring leaf phenology of 54 species of eastern USA deciduous forests, including native and invasive shrubs of six common genera. We used new measurements of genome size to evaluate its association with spring budbreak, cell size, summer leaf production rate, and photosynthetic capacity. In a phylogenetic hierarchical model that differentiated native and invasive species as a function of summer growth rate and spring budbreak timing, species with smaller genomes exhibited both faster growth and delayed budbreak compared with those with larger nuclear DNA content. Growth rate, but not budbreak timing, was associated with whether a species was native or invasive. Our results support genome size as a broad indicator of the growth behavior of woody species. Surprisingly, invaders of deciduous forests show the same small-genome tendencies of invaders of more open habitats, supporting genome size as a robust indicator of invasiveness.

Field evidence for earlier leaf-out dates in alpine grassland on the eastern Tibetan Plateau from 1990 to 2006.

Worldwide, many plant species are experiencing an earlier onset of spring phenophases due to climate warming. Rapid recent temperature increases on the Tibetan Plateau (TP) have triggered changes in the spring phenology of the local vegetation. However, remote sensing studies of the land surface phenology have reached conflicting interpretations about green-up patterns observed on the TP since the mid-1990s. We investigated this issue using field phenological observations from 1990 to 2006, for 11 dominant plants on the TP at the levels of species, families (Gramineae-grasses and Cyperaceae-sedges) and vegetation communities (alpine meadow and alpine steppe). We found a significant trend of earlier leaf-out dates for one species (Koeleria cristata). The leaf-out dates of both Gramineae and Cyperaceae had advanced (the latter significantly, starting an average of 9 days later per year than the former), but the correlation between them was significant. The leaf-out dates of both vegetation communities also advanced, but the pattern was only significant in the alpine meadow. This study provides the first field evidence of advancement in spring leaf phenology on the TP and suggests that the phenology of the alpine steppe can differ from that of the alpine meadow. These findings will be useful for understanding ecosystem responses to climate change and for grassland management on the TP.

Effect of open-field experimental warming on the leaf phenology of oriental oak (Quercus variabilis) seedlings

Aims An open-field warming experiment enables us to test the effects of projected temperature increase on change in plant phenology with fewer confounding factors and to study phenological response to temperature ranges beyond natural variability. This study aims to (i) examine the effect of temperature increase on leaf unfolding and senescence of oriental oak (Quercus variabilis Blume) under experimental warming and (ii) measure temperature-related parameters used in estimating phenological response to temperature elevation. Methods Using an open-field warming system with infrared heaters, we increased the air temperature by ~3°C in the warmed plots compared with that of the control plots consistently for 2 years. Leaf unfolding and senescence dates of Q. variabilis seedlings were recorded and temperature-related phenological parameters were analysed. Important Findings The timing of leaf unfolding was advanced by 3–8 days (1.1– 3.0 days/°C) and the date of leaf senescence was delayed by 14–19 days (5.0–7.3 days/°C) under elevated air temperatures. However, the cumulative degree days (CDD) of leaf unfolding were not significantly changed by experimental warming, which suggest the applicability of a constant CDD value to estimate the change in spring leaf phenology under 3°C warming. Consistent ranges of advancement and temperature sensitivity in spring phenology and delayed autumn phenology and proposed temperature parameters from this study might be applied to predict future phenological change.

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.

Sensitivity of leaf unfolding to experimental warming in three temperate tree species

Phenological events are sensitive to climate change, and a clear advancement of spring leaf phenology has been well documented. However, the observed phenological trends cannot be directly used to forecast future changes because the underlying mechanisms are still unclear. We conducted a two-year experimental warming experiment using one-meter-high saplings of birch (Betula pendula L.), oak (Quercus robur L.) and beech (Fagus sylvatica L.). The main objective of this study was to quantify the temperature sensitivity of leaf unfolding to six different warming levels. The results suggested that leaf unfolding dates were significantly advanced with warming for all species in both experimental years. However, different warming treatments elicited different leaf unfolding responses. At low and moderate warming (<+4°C), the sensitivity of leaf unfolding to temperature (changes in days per °C) increased with warming in all three studied species. For birch, the temperature sensitivity continued to increase with additional warming. For oak and beech, in contrast, the temperature sensitivity dramatically decreased when warming exceeded +4°C. Our findings confirm that continued and intense warming may not result in further advances of leaf unfolding in the late successional species oak and beech, if the warming exceeds a certain threshold leading to insufficient chilling temperatures for dormancy release.

Influence of leaf phenology and site nitrogen on invasive species establishment in temperate deciduous forest understories

Invasion by nonnative species is considered one of the greatest threats to ecosystem structure and function worldwide. Human disturbance and landscape fragmentation create edge habitats that are often higher in resource (i.e., light and nutrient) availability and are noted for having a high degree of invasive species establishment and colonization. In contrast, interior forest understories, which are often light- and nutrient-limited, are typically less susceptible to invasion. We assessed invasive species presence in interior, intact, temperate forest understories in relation to canopy-induced environmental conditions. We hypothesized that nonnative invasive species presence would be greatest under those canopy tree species that allow higher understory light and/or soil nitrate levels as a result of inherent differences in canopy transmission, spring leaf phenology, and nitrogen fixation. Five native forest canopy types were used, Populus tremuloides (quaking aspen), Acer saccharum (sugar maple), Quercus spp./Carya spp. (oak/hickory), Fraxinus americana (white ash), evergreen species mix Pinus spp./Tsuga canadensis (pine/hemlock), along with one nonnative canopy species, Robinia pseudoacacia (black locust). Measures of understory light availability post leaf-expansion were significantly higher under quaking aspen than other canopies with the exception of black locust and white ash. Canopy bud break and leaf flush occurred first in quaking aspen stands followed by sugar maple, oak/hickory, white ash and black locust stands, respectively. As expected, soil analyses showed higher nitrate levels in the nitrogen-fixing black locust stands, but unexpectedly, also in the white ash stands. Under these two canopy types, invasive species cover and relative abundance were significantly higher suggesting the importance of greater resource availability in invasive plant success. As such, intact forest stands with greater light transmission, later phenology, and nitrate fixation act as islands of invasion in resource-limited forested landscapes.

Spring leaf phenology and the diurnal temperature range in a temperate maple forest

Spring leaf phenology in temperate climates is intricately related to numerous aspects of the lower atmosphere [e.g., surface energy balance, carbon flux, humidity, the diurnal temperature range (DTR)]. To further develop and improve the accuracy of ecosystem and climate models, additional investigations of the specific nature of the relationships between spring leaf phenology and various ecosystem and climate processes are required in different environments. This study used visual observations of maple leaf phenology, below-canopy light intensities, and micrometeorological data collected during the spring seasons of 2008, 2009, and 2010 to examine the potential influence of leaf phenology on a seasonal transition in the trend of the DTR. The timing of a reversal in the DTR trend occurred near the time when the leaves were unfolding and expanding. The results suggest that the spring decline in the DTR can be attributed primarily to the effect of canopy closure on daily maximum temperature. These findings improve our understanding of the relationship between leaf phenology and the diurnal temperature range in temperate maple forests during the spring. They also demonstrate the necessity of incorporating accurate phenological data into ecosystem and climate models and warrant a careful examination of the extent to which canopy phenology is currently incorporated into existing models.

Modeling the date of leaf appearance in low‐arctic tundra

AbstractOne of the reported changes of arctic ecosystems in response to warming climate is the advance of the leaf appearance in spring. Such phenological changes play a role in the structural changes within tundra ecosystem communities. Recently, we developed a model that estimates the leaf appearance date for deciduous trees in taiga. We apply this model to the whole low‐arctic tundra, and we compare the simulated green‐up dates with the green‐up dates obtained from satellite observations and to in situ measurements of deciduous shrub leaf appearance. The model, although calibrated for taiga, performs remarkably well in tundra, with root mean square error ranging between 4 and 8 days for most of the tundra region, the same order as in taiga regions. The results seem to indicate that air temperature is the main factor controlling spring leaf phenology in tundra, just as in taiga, although these results do not permit us to reject soil temperature as the main trigger for leaf appearance in tundra. Because our model performs in tundra as well as in taiga, it can be used across the ecotone, and during a northward migration of the species from the taiga to the low‐arctic region. The leaf appearance model and the satellite observations reveal that leaf appearance has tended to occur earlier by approximately 10 days both in Alaska since 1975, and in west Siberian tundra since 1965.