Ongoing seasonally uneven climate warming leads to earlier autumn growth cessation in deciduous trees

Asymmetric Warming of Day and Night Benefits the Early Growth of Acer mono Seedlings More Than Symmetric Warming

Asymmetric warming refers to the difference between the increase in daytime maximum temperature and the increase in nighttime minimum temperature and has been documented in temperate regions. However, its impacts on seedling growth have been largely ignored. In this study, seedlings of a widely distributed tree species, Acer mono Maxim., were exposed to both symmetric warming (SW) and asymmetric warming scenarios (day warming [DW], night warming [NW] and diurnal asymmetric warming [DAW]). Compared to control, all warming scenarios were found to enhance belowground biomass. DW promoted the seedling growth, while NW reduced the stem biomass. DAW did not impact the total biomass relative to the control. Compared to SW, DAW advanced phenology, increased indole-3-acetic acid content and chlorophyll content, which enhanced total biomass and stored more NSC in the root. Future DAW would be not beneficial to the growth of A. mono seedlings by comparing with the control. This research encourages further exploration of tree growth experiments under asymmetric warming conditions, as most studies tend to underestimate the warming effects on plant growth by focusing on SW. Incorporating the responses of seedling physiology and growth to non-uniform diurnal warming into earth system models is crucial for more accurately predicting carbon and energy balances in a warmer world.

Variation in the timing and duration of autumn leaf phenology among temperate deciduous trees, native shrubs and non-native shrubs

The timing and duration of autumn leaf phenology marks important transitions in temperate deciduous forests, such as, start of senescence, declining productivity and changing nutrient cycling. Phenological research on temperate deciduous forests typically focuses on upper canopy trees, overlooking the contribution of other plant functional groups like shrubs. Yet shrubs tend to remain green longer than trees, while non-native shrubs, in particular, tend to exhibit an extended growing season that confers a competitive advantage over native shrubs. We monitored leaf senescence and leaf fall (2017-2020) of trees and shrubs (native and non-native) in an urban woodland fragment in Wisconsin, USA. Our findings revealed that, the start of leaf senescence did not differ significantly between vegetation groups, but leaf fall started (DOY 273) two weeks later in shrubs. Non-native shrubs exhibited a considerably delayed start (DOY 262) and end of leaf senescence (DOY 300), with leaf-fall ending (DOY 315) nearly four weeks later than native shrubs and trees. Overall, the duration of the autumn phenological season was longer for non-native shrubs than either native shrubs or trees. Comparison of the timing of spring phenophases with the start and end of leaf senescence revealed that when spring phenology in trees starts later in the season senescence also starts later and ends earlier. The opposite pattern was observed in native shrubs. In conclusion, understanding the contributions of plant functional groups to overall forest phenology requires future investigation to ensure accurate predictions of future ecosystem productivity and help address discrepancies with remote sensing phenometrics.

Drought shortens subtropical understory growing season by advancing leaf senescence

Subtropical forests, recognized for their intricate vertical canopy stratification, exhibit high resistance to extreme drought. However, the response of leaf phenology to drought in the species-rich understory remains poorly understood. In this study, we constructed a digital camera system, amassing over 360,000 images through a 70% throughfall exclusion experiment, to explore the drought response of understory leaf phenology. The results revealed a significant advancement in understory leaf senescence phenology under drought, with 11.75 and 15.76 days for the start and end of the leaf-falling event, respectively. Pre-season temperature primarily regulated leaf development phenology, whereas soil water dominated the variability in leaf senescence phenology. Under drought conditions, temperature sensitivities for the end of leaf emergence decreased from -13.72 to -11.06 days °C-1, with insignificance observed for the start of leaf emergence. Consequently, drought treatment shortened both the length of the growing season (15.69 days) and the peak growth season (9.80 days) for understory plants. Moreover, this study identified diverse responses among intraspecies and interspecies to drought, particularly during the leaf development phase. These findings underscore the pivotal role of water availability in shaping understory phenology patterns, especially in subtropical forests.

Characterizing spring phenology in a temperate deciduous urban woodland fragment: trees and shrubs

Phenological research in temperate-deciduous forests typically focuses on upper canopy trees, due to their overwhelming influence on ecosystem productivity and function. However, considering that shrubs leaf out earlier and remain green longer than trees, they play a pivotal role in ecosystem productivity, particularly at growing season extremes. Furthermore, an extended growing season of non-native shrubs provides a competitive advantage over natives. Here, we report spring phenology, budburst, leaf-out, and full-leaf unfolded (2017-2021) of a range of co-occurring species of tree (ash, American basswood, red oak, white oak, and boxelder) and shrub (native species: chokecherry, pagoda dogwood, nannyberry, American wild currant and Eastern wahoo, and non-native species: buckthorn, honeysuckle, European privet, and European highbush cranberry) in an urban woodland fragment in Wisconsin, USA, to determine how phenology differed between plant groups. Our findings show that all three spring phenophases of shrubs were 3 weeks earlier (p < 0.05) than trees. However, differences between shrubs groups were only significant for the later phenophase; full-leaf unfolded, which was 6 days earlier (p < 0.05) for native shrubs. The duration of the spring phenological season was 2 weeks longer (p < 0.05) for shrubs than trees. These preliminary findings demonstrate that native shrubs, at this site, start full-leaf development earlier than non-native species suggesting that species composition must be considered when generalizing whether phenologies differ between vegetation groups. A longer time series would be necessary to determine future implications on ecosystem phenology and productivity and how this might impact forests in the future, in terms of species composition, carbon sequestration, and overall ecosystem dynamics.

Climate-driven shifts in leaf senescence are greater for boreal species than temperate species in the Acadian Forest region in contrast to leaf emergence shifts

The Acadian Forest Region is a temperate-boreal transitional zone in eastern North America which provides a unique opportunity for understanding the potential effects of climate change on both forest types. Leaf phenology, the timing of leaf life cycle changes, is an important indicator of the biological effects of climate change, which can be observed with stationary timelapse cameras known as phenocams. Using four growing seasons of observations for the species Acer rubrum (red maple), Betula papyrifera (paper/white birch) and Abies balsamea (balsam fir) from the Acadian Phenocam Network as well as multiple growing season observations from the North American PhenoCam Network we parameterized eight leaf emergence and six leaf senescence models for each species which span a range in process and driver representation. With climate models from the Fifth Phase of the Coupled Model Intercomparison Project (CMIP5) we simulated future leaf emergence, senescence and season length (senescence minus emergence) for these species at sites within the Acadian Phenocam Network. Model performances were similar across models and leaf emergence model RMSE ranged from about 1 to 2 weeks across species and models, while leaf senescence model RMSE ranged from about 2 to 4 weeks. The simulations suggest that by the late 21st century, leaf senescence may become continuously delayed for boreal species like Betula papyrifera and Abies balsamea, though remain relatively stable for temperate species like Acer rubrum. In contrast, the projected advancement in leaf emergence was similar across boreal and temperate species. This has important implications for carbon uptake, nutrient resorption, ecology and ecotourism for the Acadian Forest Region. More work is needed to improve predictions of leaf phenology for the Acadian Forest Region, especially with respect to senescence. Phenocams have the potential to rapidly advance process-based model development and predictions of leaf phenology in the context of climate change.

Effect of climate warming on the timing of autumn leaf senescence reverses after the summer solstice

Climate change is shifting the growing seasons of plants, affecting species performance and biogeochemical cycles. Yet how the timing of autumn leaf senescence in Northern Hemisphere forests will change remains uncertain. Using satellite, ground, carbon flux, and experimental data, we show that early-season and late-season warming have opposite effects on leaf senescence, with a reversal occurring after the year's longest day (the summer solstice). Across 84% of the northern forest area, increased temperature and vegetation activity before the solstice led to an earlier senescence onset of, on average, 1.9 ± 0.1 days per °C, whereas warmer post-solstice temperatures extended senescence duration by 2.6 ± 0.1 days per °C. The current trajectories toward an earlier onset and slowed progression of senescence affect Northern Hemisphere-wide trends in growing-season length and forest productivity.

Late-season biosynthesis of leaf fatty acids and n-alkanes of a mature beech (Fagus sylvatica) tree traced via 13CO2 pulse-chase labelling and compound-specific isotope analysis

Leaf cuticular waxes play an important role in reducing evapotranspiration via diffusion. However, the ability of mature trees to regulate the biosynthesis of waxes to changing conditions (e.g., drought, light exposition) remain an open question, especially during the late growing season. This holds also true for one of the most widely distributed trees in Central Europe, the European beech tree (Fagus sylvatica L.). In order to investigate the ongoing formation of wax constituents like alkanes and fatty acids, we conducted a ¹³CO₂ pulse-chase labelling experiment on sun-exposed and shaded branches of a mature beech tree during the late summer 2018. The ¹³C-label was traced via compound-specific δ¹³C isotope analysis of n-alkanes and fatty acids to determine the de-novo biosynthesis within these compound classes. We did not observe a significant change in lipid concentrations during the late growing season, but we found higher n-alkane concentrations in sun-exposed compared to shaded leaves in August and September. The n-alkane and fatty acid composition showed ongoing modifications during the late growing season. Together with the uptake and following subsequent decrease of the ¹³C-label, this suggests ongoing de-novo biosynthesis, especially of fatty acids in European beech leaves. Moreover, there is a high variability in the ¹³C-label among individual branches and between sun-exposed and shaded leaves. At the same time, sun-exposed leaves invest more of the assimilated C into secondary metabolites such as lipids than shaded leaves. This indicates that the investigated mature beech tree could adjust its lipid production and composition in order to acclimate to changes in microclimates within the tree crown and during the investigated period.

Mowing mitigated the sensitivity of ecosystem carbon fluxes responses to heat waves in a Eurasian meadow steppe

The heat waves (HW) will be more frequent and intense in the future with increased human activity and uncertain implications for ecosystem carbon fluxes. The semi-arid Eurasian grassland is sensitive to climate change and under frequent HWs attacks. Mowing as one of the most common human practices in this region, combining with HW can have comprehensive effects on plant communities, biomass, and nutrient cycling. Hence, a 3-year (2019-2021) field manipulation experiment was conducted to assess how mowing influenced the carbon cycling under HWs, and the interactions between HWs and mowing on carbon fluxes at the community and ecosystem levels in a Eurasian meadow steppe. Over the three years, HW significantly reduced net ecosystem CO₂ exchange (NEE) and gross ecosystem production (GEP) by 28 % and 8 % (P < 0.05), respectively, whereas ecosystem respiration (Re) did not show significant changes. Moderate mowing (stubble height was set at 6-8 cm) for harvest effectively mitigated ecosystem sensitivity to HWs and significantly increased ecosystem carbon fluxes (NEE, Re, and GEP), biomass and the number of species. Mowing reduced the negative impact of HWs on ecosystem carbon fluxes by about 15 % compared to HWs alone, contributing to the invasion of species such as Thalictrum squarrosum and Vicia amoena, and increased the indirect effect of HW on NEE in the structural equation model. In addition, the higher soil water content (SWC) was another effective way to reduce the impact of HWs. Therefore, mowing and higher SWC would be effective ways to counteract the negative effects of HWs on carbon fluxes in future grassland management.

Low temperature and short daylength interact to affect the leaf senescence of two temperate tree species

Temperature and photoperiod are two major environmental cues shaping the leaf senescence of temperate tree species. However, how the control of leaf senescence is split between photoperiod and temperature is unknown for many ecologically important species. Here, we conducted a growth chamber experiment to test the effects of temperature (6, 9, 18 and 21°C) and photoperiod (8 and 16 h daylength) on leaf senescence of two temperate tree species (Quercus mongolica Fisch. and Larix principis-rupprechtii Mayr.) distributed in montane forest of China. The results showed that low temperature (LT) alone could induce leaf senescence of both species under long daylength (LD) conditions, but the leaf senescence of L. principis-rupprechtii was more sensitive to the decrease in temperature than that of Q. mongolica under the LD condition. Short daylength (SD) alone could only induce the leaf senescence of L. principis-rupprechtii, suggesting that the photoperiod sensitivity varies between species. SD could accelerate the LT-induced senescence, but the effect of SD reduced with the decrease in temperature. Based on these findings, we developed a new autumn phenology model by incorporating interspecific differences in the photoperiod sensitivity of leaf senescence. Compared with the three existing process-based autumn phenology models, the new model was more robust in simulating the experimental data. When employing these models to available long-term phenological data, our new model also performed best in reproducing the observed leaf senescence date of two closely related species (Quercus robur L. and Larix decidua Mill.). These results enhance our understanding of how LT and SD control leaf senescence. The prediction of the climate change impacts on forest carbon uptake could be improved by incorporating this new autumn phenological model into the terrestrial biosphere models.

Stimulation, Reduction and Compensation Growth, and Variable Phenological Responses to Spring and/or Summer–Autumn Warming in Corylus Taxa and Cornus sanguinea L

Understanding species-specific responses to climate change allows a better assessment of the possible impact of global warming on forest growth. We studied the responses of the shrub species Corylus avellana L., Corylus maxima Mill. and intermediate forms, together stated as the Corylus taxa, and Cornus sanguinea L. upon periodically elevated temperatures in spring and/or in summer–autumn. Experiments were performed in a common garden, with Belgian and Pyrenean provenances for Corylus avellana and Cornus sanguinea. In the Corylus taxa, a warmer spring resulted in a reduction in height and diameter growth. Remarkably, the reduced diameter increment was restored with full compensation in the following year. The height increment for Cornus sanguinea was larger upon a warmer summer–autumn, concurring with a later leaf senescence. Our results suggest that Corylus is more sensitive to spring warming, influencing growth negatively, whereas Cornus is more sensitive to summer–autumn warming, influencing height growth positively. These deviating responses can be explained, at least partly, by their diverging ecological niches, with the Corylus taxa being more shade-tolerant compared to Cornus sanguinea. The warm spring conditions advanced bud burst in all studied taxa, whereas the warm summer–autumn advanced leaf senescence but prolonged its duration in the Corylus taxa, as well as delayed this phenophase in Cornus sanguinea. Little to no after-effects of the temperature treatments were detected. Although Corylus avellana and Cornus sanguinea plants originated from similar origins, their growth and phenological responses in the common garden diverged, with Corylus being more stable and Cornus displaying more variation between the Belgian and Pyrenean provenances.

Spring and autumn phenology in an understory herb are uncorrelated and driven by different factors

PREMISE

Climate warming has altered the start and end of growing seasons in temperate regions. Ultimately, these changes occur at the individual level, but little is known about how previous seasonal life-history events, temperature, and plant-resource state simultaneously influence the spring and autumn phenology of plant individuals.

METHODS

We studied the relationships between the timing of leaf-out and shoot senescence over 3 years in a natural population of the long-lived understory herb Lathyrus vernus and investigated the effects of spring temperature, plant size, reproductive status, and grazing on spring and autumn phenology.

RESULTS

The timing of leaf-out and senescence were consistent within individuals among years. Leaf-out and senescence were not correlated with each other within years. Larger plants leafed out and senesced later, and size had no effect on growing season length. Reproductive plants leafed out earlier and had longer growing seasons than nonreproductive plants. Grazing had no detectable effects on phenology. Colder spring temperatures delayed senescence in two of three study years.

CONCLUSIONS

The timing of seasonal events, such as leaf-out and senescence in plants can be expressed largely independently within and among seasons and are influenced by different factors. Growing season start and length can often be dependent on plant condition and reproductive status. Knowledge about the drivers of growing season length of individuals is essential to more accurately predict species and community responses to environmental variation.

Herbivory in a changing climate-Effects of plant genotype and experimentally induced variation in plant phenology on two summer-active lepidopteran herbivores and one fungal pathogen

With climate change, spring warming tends to advance plant leaf-out. While the timing of leaf-out has been shown to affect the quality of leaves for herbivores in spring, it is unclear whether such effects extend to herbivores active in summer. In this study, we first examined how spring and autumn phenology of seven Quercus robur genotypes responded to elevated temperatures in spring. We then tested whether the performance of two summer-active insect herbivores (Orthosia gothica and Polia nebulosa) and infection by a pathogen (Erysiphe alphitoides) were influenced by plant phenology, traits associated with genotype or the interaction between these two. Warm spring temperatures advanced both bud development and leaf senescence in Q. robur. Plants of different genotype differed in terms of both spring and autumn phenology. Plant phenology did not influence the performance of two insect herbivores and a pathogen, while traits associated with oak genotype had an effect on herbivore performance. Weight gain for O. gothica and ingestion for P. nebulosa differed by a factor of 4.38 and 2.23 among genotypes, respectively. Herbivore species active in summer were influenced by traits associated with plant genotype but not by phenology. This suggest that plant attackers active in summer may prove tolerant to shifts in host plant phenology-a pattern contrasting with previously documented effects on plant attackers active in spring and autumn.

Demographic Consequences of Phenological Shifts in Response to Climate Change

When a phenological shift affects a demographic vital rate such as survival or reproduction, the altered vital rate may or may not have population-level consequences. We review the evidence that climate change affects populations by shifting species’ phenologies, emphasizing the importance of demographic life-history theory. We find many examples of phenological shifts having both positive and negative consequences for vital rates. Yet, few studies link phenological shifts to changes in vital rates known to drive population dynamics, especially in plants. When this link is made, results are largely consistent with life-history theory: Phenological shifts have population-level consequences when they affect survival in longer-lived organisms and reproduction in shorter-lived organisms. However, there are just as many cases in which demographic mechanisms buffer population growth from phenologically induced changes in vital rates. We provide recommendations for future research aiming to understand the complex relationships among climate, phenology, and demography, which will help to elucidate the extent to which phenological shifts actually alter population persistence.

Plant phenological responses to experimental warming-A synthesis

Although there is abundant evidence that plant phenology is shifting with climatic warming, the magnitude and direction of these shifts can depend on the environmental context, plant species, and even the specific phenophase of study. These disparities have resulted in difficulties predicting future phenological shifts, detecting phenological mismatches and identifying other ecological consequences. Experimental warming studies are uniquely poised to help us understand how climate warming will impact plant phenology, and meta-analyses allow us to expose broader trends from individual studies. Here, we review 70 studies comprised 1226 observations of plant phenology under experimental warming. We find that plants are advancing their early-season phenophases (bud break, leaf-out, and flowering) in response to warming while marginally delaying their late-season phenophases (leaf coloration, leaf fall, and senescence). We find consistency in the magnitude of phenological shifts across latitude, elevation, and habitat types, whereas the effect of warming on nonnative annual plants is two times larger than the effect of warming on native perennial plants. Encouragingly for researchers, plant phenological responses were generally consistent across a variety of experimental warming methods. However, we found numerous gaps in the experimental warming literature, limiting our ability to predict the effects of warming on phenological shifts. In particular, studies outside of temperate ecosystems in the Northern Hemisphere, or those that focused on late-season phenophases, annual plants, nonnative plants, or woody plants and grasses, were underrepresented in our data set. Future experimental warming studies could further refine our understanding of phenological responses to warming by setting up experiments outside of traditionally studied biogeographic zones and measuring multiple plant phenophases (especially late-season phenophases) across species of varying origin, growth form, and life cycle.

Divergent responses of phenology and growth to summer and autumnal warming

Plant phenology is highly sensitive to climate change, and shifts in autumnal foliar senescence are critical for plant productivity and nutrient cycling. Global warming has delayed the timing of foliar senescence, but the response of autumnal foliar senescence to nonuniform seasonal warming remains poorly understood, with experimental evidence in trees especially scarce. We therefore conducted a field experiment on seasonally asymmetric warming on 2-year-old larch (Larix principis-rupprechtii) seedlings in two hydrologically contrasting years (wet 2018 and dry 2019). Autumnal and year-round warming significantly delayed the timing of foliar senescence by 6 and 7 d in 2018, the wet year, with corresponding temperature sensitivities of 6.73 ± 1.47 and 8.26 ± 1.00 d/°C, respectively. Interestingly, the dates of senescence did not change across the warming treatments in 2019, the dry year. However, there was no significant effect of summer warming on the timing of foliar senescence neither in the wet nor dry year. The delayed autumnal foliar senescence was responsible for an increase in biomass only in the wet year, 2018. In contrast, summer warming, but not autumnal warming, increased the mortality of the seedlings in both 2018 and 2019. These results suggest that the hydrological conditions substantially modify the response of autumnal phenology and growth to seasonal warming. Autumnal warming increases growth, whereas summer warming could cause carbon starvation/hydraulic failure, reduce growth, and lead to higher mortality. Our results suggest that the functioning, ecosystem services, and sustainability of forests in the future depend on the strength and pattern of nonuniform seasonal warming. This study can inspire new research in phenology and tree growth in experiments with asymmetric warming.

Increased growing-season productivity drives earlier autumn leaf senescence in temperate trees

Changes in the growing-season lengths of temperate trees greatly affect biotic interactions and global carbon balance. Yet future growing-season trajectories remain highly uncertain because the environmental drivers of autumn leaf senescence are poorly understood. Using experiments and long-term observations, we show that increases in spring and summer productivity due to elevated carbon dioxide, temperature, or light levels drive earlier senescence. Accounting for this effect improved the accuracy of senescence predictions by 27 to 42% and reversed future predictions from a previously expected 2- to 3-week delay over the rest of the century to an advance of 3 to 6 days. These findings demonstrate the critical role of sink limitation in governing the end of seasonal activity and reveal important constraints on future growing-season lengths and carbon uptake of trees.

Climate change fingerprints in recent European plant phenology

A paper published in Global Change Biology in 2006 revealed that phenological responses in 1971-2000 matched the warming pattern in Europe, but a lack of chilling and adaptation in farming may have reversed these findings. Therefore, for 1951-2018 in a corresponding data set, we determined changes as linear trends and analysed their variation by plant traits/groups, across season and time as well as their attribution to warming following IPCC methodology. Although spring and summer phases in wild plants advanced less (maximum advances in 1978-2007), more (~90%) and more significant (~60%) negative trends were present, being stronger in early spring, at higher elevations, but smaller for nonwoody insect-pollinated species. These trends were strongly attributable to winter and spring warming. Findings for crop spring phases were similar, but were less pronounced. There were clearer and attributable signs for a delayed senescence in response to winter and spring warming. These changes resulted in a longer growing season, but a constant generative period in wild plants and a shortened one in agricultural crops. Phenology determined by farmers' decisions differed noticeably from the purely climatic driven phases with smaller percentages of advancing (~75%) trends, but farmers' spring activities were the only group with reinforced advancement, suggesting adaptation. Trends in farmers' spring and summer activities were very likely/likely associated with the warming pattern. In contrast, the advance in autumn farming phases was significantly associated with below average summer warming. Thus, under ongoing climate change with decreased chilling the advancing phenology in spring and summer is still attributable to warming; even the farmers' activities in these seasons mirror, to a lesser extent, the warming. Our findings point to adaptation to climate change in agriculture and reveal diverse implications for terrestrial ecosystems; the strong attribution supports the necessary mediation of warming impacts to the general public.

Early spring warming may hasten leaf emergence in Erythronium americanum

PREMISE

Climate change is making spring arrive earlier than in the past, causing some species to alter the timing of their spring activities. This study addressed whether Erythronium americanum Ker Gawl. (trout lily), a common spring ephemeral, can emerge earlier if exposed to early spring warming.

METHODS

I collected corms of Erythronium americanum in the fall, overwintered them in soil, and exposed them to warming in either mid (early treatment) or late (late treatment) February. The timing of leaf emergence was monitored and compared between treatments.

RESULTS

Leaves exposed to early warming emerged earlier than those in the late treatment. Bud break happened closer to date of exposure to warming in the late treatment than in the early treatment.

CONCLUSIONS

Spring ephemerals may be able to produce leaves early in response to early spring warming induced by climate change. Risk of late frost and eventual shading by the canopy may limit the duration of a potentially extended growing season.