Effectiveness of freezing temperatures on dormancy release of temperate woody species.

BACKGROUND AND AIMS: Spring phenological change of plants in response to global warming may affect many ecological processes and functions. Chilling temperature regulates budburst date by releasing dormancy. However, whether freezing temperature (<0°C) contributes to dormancy release is still debated. Our poor understanding of the role of chilling makes estimating shifts in budburst date difficult. METHODS: A two-year chilling-forcing experiment was explicitly designed to test the effects of chilling temperatures on dormancy release of 9 temperate woody species in Beijing, China. A total of 1620 twigs were first exposed to a wide range of temperatures (-10 to 10 °C) with different durations and then moved to growth chambers. Based on budburst data in experimental conditions, we examined whether freezing temperatures are effective on dormancy release. We also developed a new framework for constructing chilling functions based on the curve between chilling duration and forcing requirement (FR) of budburst. The chilling function derived from this framework was not affected by experimental forcing conditions. KEY RESULTS: We demonstrated that freezing temperatures down to -10°C were effective in dormancy release. The rate of dormancy release, indicated by the rate of decay in chilling duration-FR curve, did not differ significantly between chilling temperatures in most cases, although it exhibited a maximum value at 0 or 5°C. The chilling function-associated phenological models could simulate budburst date from independent experimental and observational data with a mean RMSE of 7.07 days. CONCLUSIONS: The effective freezing temperatures found here are contrary to the well-known assumption of <0°C temperature generally not contributing to accumulated chilling in many previous chilling functions. A chilling function assuming that temperature below an upper-temperature threshold has the same effects on dormancy release could be adopted to calculate chilling accumulation when using experiments to develop spring phenological models based on the chilling-forcing relationship.

Spring wood phenology responds more strongly to chilling temperatures than bud phenology in European conifers.

A comparative assessment of bud and wood phenology could aid a better understanding of tree growth dynamics. However, the reason for asynchronism or synchronism in leaf and cambial phenology remains unclear. To test the assumption that the temporal relationship between the budburst date and the onset date of wood formation is due to their common or different responses to environmental factors, we constructed a wood phenology dataset from previous literature, and compared it with an existing bud phenology dataset in Europe. We selected three common conifers (Larix decidua Mill., Picea abies (L.) H. Karst. and Pinus sylvestris L.) in both datasets and analyzed 909 records of the onset of wood formation at 47 sites and 238,720 records of budburst date at 3051 sites. We quantified chilling accumulation (CA) and forcing requirement (FR) of budburst and onset of wood formation based on common measures of CA and FR. We then constructed negative exponential CA-FR curves for bud and wood phenology separately. The results showed that the median, variance and probability distribution of CA-FR curves varied significantly between bud and wood phenology for three conifers. The different FR under the same chilling condition caused asynchronous bud and wood phenology. Furthermore, the CA-FR curves manifested that wood phenology was more sensitive to chilling than bud phenology. Thus, the FR of the onset of wood formation increases more than that of budburst under the same warming scenarios, explaining the stronger earlier trends in the budburst date than the onset date of woody formation simulated by the process-based model. Our work not only provides a possible explanation for asynchronous bud and wood phenology from the perspective of organ-specific responses to chilling and forcing, but also develops a phenological model for predicting both bud and wood phenology with acceptable uncertainties.

Magnitude and direction of green-up date in response to drought depend on background climate over Mongolian grassland.

Increasing drought is one major consequence of ongoing global climate change and is expected to cause significant changes in vegetation phenology, especially for naturally vulnerable ecosystems such as grassland. However, the linkage between the response characteristic of green-up date (GUD) to drought and background climate remains largely unknown. Here, we focused on how the GUD of Mongolian grassland responds to extreme drought events (EDE). We first extracted the GUD from the MODIS Enhanced Vegetation Index data during 2001-2020 and identified the preseason EDE using the standardized precipitation evapotranspiration index data. Subsequently, we quantified the response of GUD to preseason EDE (DGUD) in each pixel as the difference in GUD between drought and normal years. The effect of 12 factors on DGUD was analyzed using the random forest algorithm. The results showed that the GUD under EDE may delay or advance by > 20 days compared to normal years. For the regions with mean annual temperature > -2 °C, the GUD was delayed under EDE due to the dominant role of water restriction on GUD, while the GUD was advanced under EDE in colder areas due to the warmer temperature during drought. However, the magnitude of delay in GUD under drought was greater in regions with less precipitation and more severe droughts. Our results could help to develop appropriate management strategies to mitigate the impacts of drought on grasslands.

Modeling the effect of adaptation to future climate change on spring phenological trend of European beech (Fagus sylvatica L.).

Temperate trees could cope with climate change through phenotypic plasticity of phenological key events or adaptation in situ via selection on genetic variation. However, the relative contribution of local adaptation and phenotypic plasticity to phenological change is unclear for many ecologically important tree species. Here, we analyzed the leaf-out data of European beech (Fagus sylvatica L.) from 50 provenances planted in 7 trial sites. We first constructed a function between chilling accumulation (CA) and photoperiod-associated heat requirement (PHR) of leaf-out date for each provenance and quantified the relationship between parameters of the CA-PHR function and climatic variables at provenance origins by using the random forest model. Furthermore, we used the provenance-specific CA-PHR function to simulate future leaf-out dates under two climate change scenarios (RCP 4.5 and 8.5) and two assumptions (no adaptation and adaptation). The results showed that both CA, provenance, and their interactions affected the PHR of leaf-out. The provenances from southeastern Europe exhibited a stronger response of PHR to CA and thus flushed earlier than northwestern provenances. The parameters of the CA-PHR function were connected with climatic variables (e.g., mean diurnal temperature range, temperature seasonality) at the originating sites of each provenance. If only considering the phenotypic plasticity, the leaf-out date of European beech in 2070-2099 will advance by 6.8 and 9.0 days on average relative to 1951-2020 under RCP 4.5 and RCP 8.5, respectively. However, if F. sylvatica adapts to future climate change by adopting the current strategy, the advance of the leaf-out date will weaken by 1.4 and 3.4 days under RCP 4.5 and RCP 8.5, respectively. Our results suggest that the European beech could slow down its spring phenological advances and reduce its spring frost risk if it adopts the current strategy to adapt to future climate change.

Stronger Spring Phenological Advance in Future Warming Scenarios for Temperate Species With a Lower Chilling Sensitivity.

Spring warming could induce earlier leaf-out or flowering of temperate plant species, and decreased chilling in winter has a delaying effect on spring phenology. However, the relative contribution of the decreased chilling and increased forcing on spring phenological change is unclear. Here, we analyzed the experimental data for 14 temperate woody species in Beijing, China and quantified the forcing requirements (FR) of spring phenology and chilling sensitivity (the ratio of the FR at the low chilling condition to the FR at the high chilling condition) for each species. Furthermore, using species-specific functions between the amount of chilling and FR, we established a phenological model to simulate the annual onset dates of spring events during the past 69 years (1952-2020) and in the future (2021-2099) under RCP 4.5 and RCP 8.5 climate scenarios. We also developed a novel method to quantitatively split the predicted phenological change into the effects caused by changes in forcing and those caused by changes in chilling. The results show that the FR of spring events decreased with the increase in the amount of chilling, and this relationship could be described as an exponential decay function. The basic FR (the FR at the high chilling condition) and chilling sensitivity varied greatly among species. In the 1952-2020 period, the advancing effect of increased forcing was stronger than the effect of chilling, leading to earlier spring events with a mean trend of -1.96 days/decade. In future climate scenarios, the spring phenology of temperate species would continue to advance but will be limited by the decreased chilling. The species with lower chilling sensitivities showed stronger trends than those with high chilling sensitivities. Our results suggested that the delaying effect of declining chilling could only slow down the spring phenological advance to a certain extent in the future.

Spatiotemporal variations in leaf-out phenology of typical European tree species and their responses to climate change.

Over the past decades, global warming significantly affected the spring phenology of plants. Many studies have reported the temporal and spatial patterns of spring phenological changes in China, but relatively less is known for that in Europe, which is also located in the temperate area of the Northern Hemisphere. To facilitate the regional comparison of phenological change and understand its response to climate change, we used the data of first leaf date (FLD) in Europe (1980-2014) and the corresponding meteorological data to examine the spatiotemporal variations in leaf-out phenology of four typical tree species (Aesculus hippocastanum, Betula pendula, Fagus sylvatica, and Quercus robur), and to identify the major climatic factors driving such variations. The results showed that the FLD of the four species in the study area advanced by 3.3-7.5 d·10 a-1 during 1980-2014. The FLD was delayed at a rate of 2.03-3.19 d per degree of latitude from south to north, of 0.19-0.80 d per degree of longitude from west to East (except for Fagus sylvatica), of 2.25-3.44 d·100 m-1 from low to high elevation. The advances in the FLD were mainly attributed to the increases of temperature in spring and the increases of precipitation in spring and winter. The rise of temperature in autumn and winter would delay FLD.