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• Transmittance and phenology data were collected in oak stands during leaf unfolding. • Modelling transmittance provided parameters characterizing tree foliage development. • Growing season onset can continuously be monitored with light sensors. • The relationship between observed bud burst date and transmittance is complex. • Indices based on transmittance could be used to give an estimate of bud burst date. Better knowledge and a more complete long-term monitoring of tree species phenology and tree foliage development are crucial to accurately estimating the impact of climate change on forest ecosystems functioning and on forest species distribution. We set up global solar radiation sensors under the forest canopy and continuously recorded solar radiation in order to follow the development of sessile oak (Quercus petraea L.) foliage in pure stands over four consecutive years and for two levels of stand density. At the same time, we made phenological observations to determine bud burst date. One of our goals was to link observed bud burst dates and transmittance measurements. Our results show that solar radiation transmittance during the leaf unfolding period followed a sigmoid-shaped curve; and that it was possible to fit a generalized logistic model to determine a set of parameters characterizing tree foliage and its development during the unfolding period. Among these parameters, we suggest that the date at which solar radiation interception by foliage reaches 50% could be used to monitor the beginning of the growing season over the long term. The relationship between observed bud burst date and the transmittance model parameters was more complex than we expected. We tested several indices to deduce the date of bud burst from transmittance. We showed that when radiation interception due to foliage reached 10%, this indicated the observed bud burst date. Finally, a linear model including parameters from the generalized logistic models of transmittance as predictors explained 57% of the variability in bud burst date. Complementary research combining light measurements and phenological observations over a longer period of time is needed to know whether these relationships could help estimate bud burst date regardless of the year or the stand density. [ABSTRACT FROM AUTHOR]

• We examined timing of spring phenology in subtropical trees under climatic warming. • We applied process-based tree phenology modelling. • We identified tree traits that restrict the acceleration of spring phenology. • The results explain the declined temperature sensitivity of phenology often found. • Understanding of tree ecophysiology is needed in climatic change impact studies. Global warming has generally advanced the spring phenology of extratropical trees. In several cases, however, the advancing has levelled off, indicating a declining temperature sensitivity of phenological timing. The potential reasons for the decline have been actively debated, but no direct experimental evidence has been produced to support any of the theories put forward. With the aid of scenario simulations, we examined which ecophysiological tree traits restrict the advancing of the onset of spring phenology in four subtropical tree species under global warming. In the simulations, we applied process-based tree spring phenology models formulated on the basis of results of experiments specifically designed for examining the ecophysiological responses addressed. We identified three restricting ecophysiological traits: 1) the chilling effect operates at relatively low temperatures only, 2) the temperature sensitivity of spring phenology is low in the temperature range of +10 to +20 °C which is critical under climatic warming in subtropical conditions; and 3) the winter rest is deep. Unexpectedly, a high chilling requirement was not included amongst the restricting ecophysiological traits. Our experimentally-based results show that the spring phenology of the trees under climatic warming is significantly affected by seemingly small and usually neglected details of the ecophysiological responses to chilling and forcing temperatures. [ABSTRACT FROM AUTHOR]

• How environmental cues interactively regulate spring phenology remains largely unstudied. • A novel method to test vegis theory, interaction of chilling and forcing, was introduced. • Vegis effect was found in two species, while effects of photoperiod were generally minor. • In the other species, the timing of leaf-out was mainly regulated by forcing. • This implies that species-specific models are needed in global change research. Climatic warming is currently changing the spring phenology of extratropical trees, and this has several important effects on the trees and ecosystems. The major climatic cues regulating the spring phenology are winter chilling, spring forcing, and photoperiod. The interactions between these three remain largely unstudied because most studies concentrate on the effects of one cue, or maximally two, at a time. We studied the effects and interactions of chilling duration, forcing temperature, and forcing photoperiod simultaneously in four subtropical tree species. The main emphasis in our experiments was on the interaction of chilling duration and forcing temperature. The existence of this interaction was suggested in the 'Vegis theory', put forward decades ago but largely forgotten since. We also introduced a novel method for testing the theory experimentally. We found support for the Vegis theory in two of the four species examined. In the other two species the leaf-out timing was largely controlled by spring forcing. The effects of photoperiod were generally minor. Our results show that there are major differences between sympatric subtropical tree species in their phenological responses to environmental cues. These differences need to be addressed in the development of process-based tree phenology models. Our results further suggest that different subtropical trees respond differently to climatic warming because of differences related to the Vegis theory. This hypothesis remains to be tested in further studies. [ABSTRACT FROM AUTHOR]

Inter-annual variation in climate is reflected by changes in the timing of phenology. Over the last decades a considerable number of models have been developed in order to explain the inter-annual variation of spring phenology in trees. Contrary to empirical models, “process-based” models aim at simulating physiological processes in order to yield more realistic predictions of growing season onset dates. Despite the increasing knowledge on the environmental controls of seasonal dormancy in trees, the detailed action and interaction of the involved environmental drivers (chilling, photoperiod and warm temperature) remains to be elucidated. This study aims at a uniform comparison of a wide range of existing models (and new recombinations), on a multitude of long-term observation series in six tree species across central Europe, using extensive cross-validation. Even though the assessed models differ in the phases of dormancy and environmental drivers accounted for, they yielded a surprisingly similar quality of prediction of leaf unfolding dates. Depending on the species, the lowest average prediction errors for leaf unfolding ( RMSE ) ranged from 7 to 9 days for the dataset pooled across sites and years and from 4 to 6 days for site-specific predictions, in absence of any obvious geographical pattern. Simple models, that feature ecodormancy release only, performed similar or better than more complex models, which additionally include endodormancy release through chilling temperatures. Model parameterisation tended to converge towards similar behaviour and models with many parameters tended to overfit on the 40 year time-series of leaf unfolding. Additionally, all models tended to underestimate the inter-annual variation of leaf unfolding and failed to predict very early or late dates of leaf unfolding in certain years. The transfer of site-specific parameters to other sites was associated with an almost doubling of the average prediction error, independent of distance and climatic similarity between the calibration and validation sites. The findings challenge the accurate implementation of the physiological processes controlling spring phenology in the models and highlight shortcomings associated with model parameterisation on observational time-series only. [ABSTRACT FROM AUTHOR]

• The biological realism of process-based tree phenology models needs to be improved. • A framework for spring phenology modelling on an experimental basis was developed. • The experiments explicitly addressed the physiological responses being modelled. • The models developed for four subtropical tree species showed reasonable accuracy. • The framework can be applied to boreal and temperate trees as well. Process-based phenological models are currently used for assessing the effects of climatic warming on the timing of spring phenological events, such as leafout and flowering, in trees. However, the biological realism of the models may be undermined by the practices of often formulating the models solely on the basis of observational records of the phenological event rather than addressing the physiological processes actually modelled. Here we introduce a framework for developing process-based phenological models on the basis of experiments explicitly designed for this purpose and apply the framework to developing process-based models for leafout in four subtropical tree species. Our method is based on a hypothetico-deductive approach, where the air temperature responses of the simulated processes are inferred from their implications for the occurrence and timing of leafout in the experimental conditions. That approach has only rarely been taken with boreal or temperate trees, and to the best of our knowledge, never before with subtropical trees. Big differences were found between the four species in the air temperature responses modelled, and these differences implied major differences in the dormancy dynamics predicted for the four species. Together with the results of a recent modelling study based on observational data, our results highlight the importance of experimental studies for the development of biologically realistic process-based models of spring phenology in trees. The framework developed in the present study can be applied to developing such models for all tree species that show the phenomena of rest (endodormancy) and chilling requirement, no matter whether the trees are boreal, temperate, or subtropical. [ABSTRACT FROM AUTHOR]

Highlights: [•] Beech phenology is assessed against growth and survival at its southern range limit. [•] Phenology segregated according to location of, and elevation at, provenance origin. [•] Southern, and some central, European provenances performed best. [•] Early-flushing is positively correlated with the height of 10-year old beech trees. [•] Surprisingly, local adaptation promotes frost avoidance above drought preemption. [Copyright &y& Elsevier]

Abstract: Climate change has already affected the phenology of several species. To be able to assess the impacts of climate change under various climate scenarios, we need superior models of the phenology of different species. Linear regression methods alone are of limited value for the analyses of natural indicators or phenological data because most time series of naturally occurring events in ecosystems do change in a nonlinear way. In this paper, we applied a Bayesian probability approach to investigate time series of the phenological phase of bud burst in Norway spruce (Picea abies (L.) Karst.) and mean monthly/weekly temperatures of corresponding climate stations in Germany. In these temperature and Norway spruce bud burst time series we detected years with the highest probability for discontinuities. We analysed rates of change and the relationship between temperature changes and bud burst of Norway spruce in the 51-year period 1953–2003. We used a Bayesian method for a coherence analysis between phenological onset dates and an effective temperature generated as a weighted average of monthly and weekly means from January to May. Weight coefficients were obtained from an optimization of the coherence factor by simulated annealing. In all investigated cases we found coherence factors that suggested a relationship between temperature and phenological time series. Norway spruce bud burst and mean temperature times series of April and May exhibited abrupt changes, particularly at the beginning of the 1980s. April and May temperature time series revealed an increased warming until 2003, and bud burst events advanced. Norway spruce bud burst, in particular, exhibited responses to temperatures of the previous (April) and current month (May). We suggest that besides commonly used sums of daily mean temperatures, forcing temperatures in phenology models should also include solutions where weighted effective temperatures in a sensitive time span are considered. [Copyright &y& Elsevier]

• Chilling effects are caused in subtropical trees by unexpectedly high temperatures. • To explain this novel finding, a hypothesis was put forward. • For testing the hypothesis, a method using climatological analyses was introduced. • The hypothesis was supported by preliminary results. • The chilling temperature range varied among the tree species and their life stages. Spring phenology is a key phenomenon mediating the effects of climate change on terrestrial plants and ecosystems, but in regard to subtropical trees, the dormancy mechanisms that regulate spring phenology are still poorly understood. It has been suggested recently that similarly to temperate and boreal trees, subtropical tree species also show endodormancy and a chilling requirement of endodormancy release. However, there are no previous experimental results on the chilling temperature range that is effective for endodormancy release in subtropical trees. We studied endodormancy and the chilling requirement in four subtropical tree species experimentally. In addition to chilling in natural conditions, we applied controlled chilling at several constant temperatures ranging from -2 to +15 °C. Our results show endodormancy and a chilling requirement in the tree species studied and reveal several differences among the four species in the manifestation and depth of endodormancy. Most importantly, our findings indicate that contrary to the prevailing mainline conception that chilling temperatures are generally restricted to those below +10 °C, higher temperatures of up to +15 °C are also effective for endodormancy release in the subtropical tree species examined. An exact upper threshold of +10.4 °C has been experimentally established for boreal Betula pubescens. We hypothesized that this difference would be explained by differences in the occurrence of low autumn temperatures between the two respective climates. We developed a method for testing the hypothesis by analysing long-term climatic records in relation to the experimental findings. Tentative results supported our hypothesis. On the basis of this novel result, we put forward the testable generalized hypothesis that in any climatic conditions where trees show endodormancy, the range of temperatures physiologically effective in endodormancy release represents the range of typical autumn air temperatures occurring in those particular climatic conditions. Image, graphical abstract [ABSTRACT FROM AUTHOR]