Autumn Leaf Research Articles

Warming does not delay the start of autumnal leaf coloration but slows its progress rate

AbstractAimInitiation of autumnal leaf senescence is crucial for plant overwintering and ecosystem dynamics. Previous studies have focused on the advanced stages of autumnal leaf senescence and reported that climatic warming delayed senescence, despite the fundamental differences among the stages of senescence. However, the timing of onset of leaf coloration (DLCO), the earliest visual sign of senescence, has rarely been studied. Here, we assessed the response of DLCO to temperature.Location30–75° N in the Northern Hemisphere.Time period2000–2018.Major taxa studiedDeciduous vegetation.MethodsWe retrieved DLCO from high‐temporal‐resolution satellite data, which were then validated by PhenoCam observations. We investigated the temporal changes in DLCO and the relationship between DLCO and temperature by using satellite and ground observations.ResultsDLCO was not significantly (p > .05) delayed between 2000 and 2018 in 94% of the area. DLCO was positively (p < .05) correlated with pre‐DLCO mean daily minimum temperature (Tmin) in only 9% of the area, whereas the end of leaf coloration (DLCE) was positively correlated with pre‐DLCE mean Tmin over a larger area (34%). Further analyses showed that warming slowed the progress of leaf coloration. Interestingly, DLCO was less responsive to pre‐DLCO mean Tmin in areas where daylength was longer across the Northern Hemisphere, particularly for woody vegetation.Main conclusionsThe rate of progress of coloration is more sensitive to temperature than its start date, resulting in an extension of the duration of leaf senescence under warming. The dependence of DLCO response to temperature on daylength indicates stronger photoperiodic control on initiation of leaf senescence in areas with longer daylength (i.e., shorter nights), possibly because plants respond to the length of uninterrupted darkness rather than daylength. This study indicates that the onset of leaf coloration was not responsive to climate warming and provides observational evidence of photoperiod control of autumnal leaf senescence at biome and continental scales.

Ecological Predictors of Pupal Survival in a Common North American Butterfly.

All holometabolous insects undergo a pupal life stage, a transformative period during which the insects are immobile and thus particularly vulnerable to both natural enemies and harmful abiotic conditions. For multivoltine species like the silver-spotted skipper [Epargyreus clarus (Cramer) (Lepidoptera: Hesperiidae)], which produces both diapausing and nondiapausing generations throughout much of its range, both the duration of the pupal stage and the ecological challenges faced by pupae can differ among generations. We conducted a set of field experiments to investigate the seasonal and annual variation in pupal mortality for E. clarus pupae experiencing different biotic and abiotic conditions. We also examined the behavioral and ecological factors influencing the construction and persistence of pupal shelters by prepupal larvae. Surprisingly, measures of both cumulative and daily pupal predation were significantly higher during the relatively short (10-14 d) nondiapausing (summer) generations, compared with the diapausing (winter) generations, despite a nearly 20-fold longer pupal duration recorded for the latter. Indirect evidence from field censuses suggested that this intergenerational difference in mortality was due to seasonal variation in consumption of pupae by generalist vertebrate predators. The presence of a shelter increased survival in summer, though not in winter, perhaps because winter pupae were likely to be buried under autumnal leaf litter, regardless of initial shelter status. When constructing their shelters, prepupal E. clarus larvae did not prefer host leaves over nonhost leaves, suggesting that induced preferences are unlikely to play an important role in this process. Despite finding marked differences in the decomposition rates of shelter leaves derived from host vs. nonhost plants, several lines of evidence suggest that these differences are unlikely to impact E. clarus pupal mortality during either the summer or winter generations.

Carbon Source Reduction Postpones Autumn Leaf Senescence in a Widespread Deciduous Tree.

The growing-season length of temperate and boreal trees has a strong effect on the global carbon cycle. Yet, a poor understanding of the drivers of phenological processes, such as autumn leaf senescence in deciduous trees, limits our capacity to estimate growing-season lengths under climate change. While temperature has been shown to be an important driver of autumn leaf senescence, carbon source–sink dynamics have been proposed as a mechanism that could help explain variation of this important process. According to the carbon sink limitation hypothesis, senescence is regulated by the interplay between plant carbon source and sink dynamics, so that senescence occurs later upon low carbon inputs (source) and earlier upon low carbon demand (sink). Here, we manipulated carbon source–sink dynamics in birch saplings (Betula pendula) to test the relevance of carbon sink limitation for autumn leaf senescence and photosynthetic decline in a widespread deciduous tree. Specifically, we conducted a gradient of leaf and bud removal treatments and monitored the effects on autumnal declines in net photosynthesis and the timing of leaf senescence. In line with the carbon sink limitation hypothesis, we observed that leaf removal tended to increase total leaf-level autumn photosynthesis and delayed the timing of senescence. Conversely, we did not observe an effect of bud removal on either photosynthesis or senescence, which was likely caused by the fact that our bud removal treatment did not considerably affect the plant carbon sink. While we cannot fully rule out that the observed effect of leaf removal was influenced by possible treatment-level differences in leaf age or soil resource availability, our results provide support for the hypothesis of carbon sink limitation as a driver of growing-season length and move the scientific field closer to narrowing the uncertainty in climate change predictions.

Nitrate fertilization may delay autumn leaf senescence, while amino acid treatments do not.

Fertilization with nitrogen (N)‐rich compounds leads to increased growth but may compromise phenology and winter survival of trees in boreal regions. During autumn, N is remobilized from senescing leaves and stored in other parts of the tree to be used in the next growing season. However, the mechanism behind the N fertilization effect on winter survival is not well understood, and it is unclear how N levels or forms modulate autumn senescence. We performed fertilization experiments and showed that treating Populus saplings with inorganic nitrogen resulted in a delay in senescence. In addition, by using precise delivery of solutes into the xylem stream of Populus trees in their natural environment, we found that delay of autumn senescence was dependent on the form of N administered: inorganic N (NO3−) delayed senescence, but amino acids (Arg, Glu, Gln, and Leu) did not. Metabolite profiling of leaves showed that the levels of tricarboxylic acids, arginine catabolites (ammonium, ornithine), glycine, glycine‐serine ratio and overall carbon‐to‐nitrogen (C/N) ratio were affected differently by the way of applying NO3 − and Arg treatments. In addition, the onset of senescence did not coincide with soluble sugar accumulation in control trees or in any of the treatments. We propose that different regulation of C and N status through direct molecular signaling of NO3 − and/or different allocation of N between tree parts depending on N forms could account for the contrasting effects of NO3 − and tested here amino acids (Arg, Glu, Gln, and Leu) on autumn senescence.

Evaluation of Land Surface Phenology for Autumn Leaf Color Change Based on Citizen Reports across Japan

Autumn foliage color is an important phenological characteristic associated with climate and appeals to populations as a cultural ecosystem service (CES). Land surface phenology (LSP) analyzed using time-series remotely sensed imagery can facilitate the monitoring of autumn leaf color change (ALCC); however, the monitoring of autumn foliage by LSP approaches is still challenging because of complex spatio-temporal ALCC patterns and observational uncertainty associated with remote sensing sensors. Here, we evaluated the performance of several LSP analysis approaches in estimation of LSP-based ALCCs against the ground-level autumn foliage information obtained from 758 sightseeing (high CES) sites across Japan. The ground information uniquely collected by citizens represented ALCC stages of greening, early, peak, late, and defoliation collected on a daily basis. The ALCC was estimated using a second derivative approach, in which normalized difference vegetation index (NDVI), kernel normalized difference vegetation index (kNDVI), enhanced vegetation index (EVI), two-band enhanced vegetation index (EVI2), and green red vegetation index (GRVI) were applied based on MODerate resolution Imaging Spectroradiometer (MODIS) MOD09A1 with four (Beck, Elmore, Gu, and Zhang) double logistic smoothing methods in 2020. The results revealed inconsistency in the estimates obtained using different analytical methods; those obtained using EVI with the Beck model estimated the peak stage of the ALCC relatively well, while the estimates obtained using other indices and models had high discrepancies along with uncertainty. Our study provided insights on how the LSP approach can be improved toward mapping the CESs offered by autumn foliage.

Peculiarities of the phenological development of <i>Malus</i> P. Mill species in the conditions of the steppe zone of the southern urals

The article presents the results of phenological observations of five-year-old trees of introduced species and forms of apple trees. The research was conducted in the Orenburg branch of the Federal Horticulture Research Center for Breeding, Argotechnology and Nursery in 2019-2021. Phenological studies were carried out according to the methodology of G. N. Zaitsev, statistical data processing was carried out using the methods of G. N. Zaitsev and B. A. Dospekhov. Data on the passage of 10 phenological phases were used (bud opening, beginning of flowering, end of flowering, end of shoot growth, lignification of shoots, beginning of fruit ripening, end of fruit ripening, autumn leaf coloring, beginning of leaf fall, end of leaf fall). In variable weather and climatic conditions during the growing season, it was found that the timing of the onset of the main phenological phases directly depends on the amount of accumulated positive temperatures, in particular, for the beginning of the growing season, this parameter is 245.2-304.7°C; for the beginning of the flowering phase — 630-609.4°C; the end of shoot growth was accompanied by the accumulation of positive temperatures from 2 152.8 to 2 398°C; at the end of fruit ripening, the sum of positive temperatures was 2 736.3-3 386°C; the beginning of leaf fall corresponds to the sum of positive temperatures from 2 873.9 to 3 200.8°C. In the conditions of the study, the duration of the growing season is 169-176 days, which is a short growing season. Weather and climatic conditions had a significant impact on the processes of active growth and flowering, the genotype of the studied species and forms caused the processes of fruit ripening, lignification of shoots, leaf fall. The duration of the shoot growth period is in direct correlation (r = 0.74-0.81) with the hydrothermal coefficient. The main decorative periods, including flowering, fruit ripening, and the period of autumn coloring, depend to varying degrees on both genotypic characteristics and meteorological conditions in the growing area.

Delayed autumnal leaf senescence following nutrient fertilization results in altered nitrogen resorption.

Increased atmospheric nitrogen (N) deposition could create an imbalance between N and phosphorus (P), which may substantially impact ecosystem functioning. Changes in autumnal phenology (i.e., leaf senescence) and associated leaf nutrient resorption may profoundly impact plant fitness and productivity. However, we know little about how and to what extent nutrient addition affects leaf senescence in tree species, or how changes in senescence may influence resorption. We thus investigated the impacts of N and P addition on leaf senescence and leaf N resorption in 2-year-old larch (Larix principisrupprechtii) seedlings in northern China. Results showed that nutrient addition (i.e., N, P or N+P addition) significantly delayed autumnal leaf senescence, and decreased leaf N resorption efficiency (NRE) and proficiency (NRP), particularly in the N and N+P treatments. Improved leaf N concentrations were correlated with delayed leaf senescence, as indicated by the positive relationship between mature leaf N concentrations and the timing of leaf senescence. Following nutrient addition, larch seedlings shifted toward delayed onset, but more rapid, leaf senescence. Additionally, we observed an initial negative correlation between the timing of leaf senescence and NRE and NRP, followed by a positive correlation, indicating delayed and less efficient remobilization during the early stages of senescence, followed by accelerated resorption in the later stages. However, the latter effect was potentially impaired by the increased risk of early autumn frost damage, thus failed to fully compensate for the negative effects observed during the early stages of senescence. Improved soil P availability increased leaf N resorption and thus weakened the negative impact of delayed leaf senescence on leaf N resorption, so P addition had no significant impact on leaf N resorption. Overall, our findings clarify the relationship between nutrient addition-resorption and the linkage with leaf senescence, and would have important implications for plant nutrient conservation strategy and nutrient cycling.

Severe drought can delay autumn senescence of silver birch in the current year but advance it in the next year

Historically, the autumn dynamics of deciduous forest trees have not been investigated in detail. However, autumn phenological events, like onset of loss of canopy greenness (OLCG), onset of foliar senescence (OFS) and cessation of wood growth (CWG), have an important impact on tree radial growth and the entire ecosystem's seasonal dynamics. Here, we monitored the leaf and wood phenological events of silver birch (Betula pendula) at four different sites in Ås, southeastern Norway: (a) a natural mature stand, (b) a plantation on former agricultural ground, (c) young natural trees, and (d) young trees in pots under different fertilization levels. The study took place over four consecutive years (from 2017 to 2020), with a particular focus on 2018, a year in which there was a severe summer drought, and the next year, 2019, which featured more normal conditions. First, we provided a description of birch phenology within its mid-north distributional. Second, we showed that drought advanced CWG by about 5 to 6 weeks and it delayed OLCG and OFS up to 30 days. Third, we observed an unexpected advance in OLCG in 2019 compared to 2018 (30 days) and 2020 (14 days). OFS presented similar dynamics as OLCG, whereas CWG was advanced only in 2018. These findings might indicate lag-effects of severe drought on the next year autumn leaf phenology but not on wood growth. On the other hand, the comparison between the natural stand and the plantation showed that, under drought conditions, wood growth is more sensitive to site fertility than autumn leaf phenology. In summary, our study elucidated the autumn dynamics of an important deciduous forest species in the northern temperate zone and showed unexpected impacts of a severely dry and warm summer on the current and next year leaf phenology.

Long-term effect of elevated air humidity on seasonal variability in diurnal leaf conductance and gas exchange in silver birch

Environmental conditions and photoperiod length drive the seasonal variability of gas exchange in plants. Still, little is known about trees’ adaptation to climate change, expressed as a delay in decreasing photosynthetic capacity at the end of the growing season. We investigated the effect of elevated air humidity (RH) and sampling period (from July to September) on the variability of net photosynthesis (An), dark respiration (R), daytime (gl_day) and night-time (gl_night) leaf conductance, an index of leaf chlorophyll content (SPAD), and An:SPAD ratio in cut shoots of silver birch (Betula pendula Roth). Measurements of cut shoots were conducted in a climate chamber to eliminate the direct effect of field conditions. The An, An:SPAD ratio, and gl_day were higher in the humidification treatment (H) than in the control (C) (P < 0.05) in autumn. The R was higher in the control than in the humidification treatment across the study period. The gl_night increased considerably in September in both treatments (P < 0.05) and was significantly correlated with R. Our findings suggest that autumnal leaf conductance and gas exchange in silver birch are considerably influenced by long-term exposure to elevated RH and are probably related to a complex of leaf senescence processes, including nitrogen retranslocation.

Remote Sensing of Autumn Phenology by Including Surface Soil Temperature: Algorithm Development, Calibration, and Validation

Phenology exercises a critical control on annual terrestrial ecosystem carbon uptake and indicates interaction between climate and vegetation. Solely vegetation index (VIs) is insufficient to accurately detect the end of growing season (EOS). Soil temperature (Ts) plays a modulating role in soil microbial functioning and plant growth, while its impact on EOS remains largely unknown. Hence, we compared the potential between Ts and air temperature (Ta) as indicators of EOS by using flux data from 14 deciduous broadleaf forests (DBF), 24 evergreen needleleaf forests (ENF), 7 mixed forests, and 23 none-forests (NF) over Northern temperate and boreal regions (30°-60°N) for 2001-2014. The widely used NDVI-based double logistic approach failed to capture EOS variability for these ecosystems, and we derived a new EOS algorithm with a soil temperature-based scaler, which improved the EOS modeling for all plant functional types. We found that Ts at different depths showed varied abilities for EOS modeling, and Ts at the 0-10 cm depth provided the best estimates of EOS in terms of both numbers of significant sites and the correlation coefficients (R). Estimated EOS occurred earlier by on average 2.9 days than the current MODIS phenology product for ∼56.5% pixels, especially for the ENF ecosystems (∼5.5 days). Our study suggests the usefulness of surface soil temperature for autumn leaf senescence phenology modeling, and that combination of environmental variables with the current modeling strategy can improve our understanding of autumn phenology with future climate change.

Exploring Populus phenological response to climate change using observational data and ecosystem modelling

The length of the growing season for deciduous trees in temperate and boreal forests is determined by the timing of bud burst and autumn senescence. It is generally assumed that a warmer climate leads to a longer growing season due to earlier bud burst and delayed autumn senescence and thereby increased gross primary production (GPP) of forests. In this study, we analysed past (1873–1951) and current (2008–2020) phenological observations on bud burst and senescence from aspen trees (Populus tremula) grown in Sweden. The observations indicated a reduction in temperature sensitivity of bud burst between the time periods, likely associated with warmer winters and reduced exposure to chilling. The phenological observations were used in the evaluation of an ecosystem model. Biases in modelling spring and autumn leaf cover development influenced the seasonal and annual GPP estimates. The overestimation of the modelled GPP was more pronounced in spring than in autumn, reflecting the GPP limitations by leaf cover development in spring, and by daylength and temperature conditions in autumn. Calibration of the spring phenology parameters, using the accumulated temperature sums and chilling days at the observed timing of bud burst, significantly improved model performance compared to the original parameterisation. The calibrated ecosystem model projections representing RCP8.5 suggested 15 days earlier timing of bud burst and enhanced mean annual GPP by the end of the century compared with current climate conditions in Sweden.

Herbaria Reveal Herbivory and Pathogen Increases and Shifts in Senescence for Northeastern United States Maples Over 150 Years

Recent studies suggest climate-related delays in the timing of leaf coloration and abscission in maple trees but lack baseline data prior to the late 20th century. To better understand how autumn foliar phenology and late-season damage risks have changed for this genus over the past century, we evaluated 2,972 digitized herbaria specimens of red and sugar maple collected between 1826 and 2016 for the presence of leaves, autumn leaf coloration, and pathogen or herbivore damage. We found that the onset (first appearance) of colored leaves has shifted 0.26 days later each year, leading to a delay of more than a month in autumn phenology since 1880. We find that these shifts are related to precipitation regimes in both the fall and summer seasons and that more severe droughts are associated with higher probabilities of colored leaves. Moreover, we found that the probability of both herbivory and pathogen damage has increased significantly over the study period. In particular, we find a strong association between increasing summer drought conditions and increased probability of herbivory. Furthermore, the presence of foliar damage increased the probability of leaf coloration on herbaria specimens. However, the end-of-season abscission date (last appearance of leaves) was strongly associated with herbivory and climate in a contrary direction: Increasing yearly drought, higher fall temperatures, and the presence of herbivory were associated with earlier abscission. In fact, the last leaf dates for specimens with herbivory were nearly 2 weeks earlier than specimens without herbivore damage. Our study documents significant changes in maple senescence over the last 150 years and suggests that incorporating herbivory into models may improve our ability to predict forest responses to climate shifts.

Pathogen infection influences the relationship between spring and autumn phenology at the seedling and leaf level.

Seasonal life history events are often interdependent, but we know relatively little about how the relationship between different events is influenced by the abiotic and biotic environment. Such knowledge is important for predicting the immediate and evolutionary phenological response of populations to changing conditions. We manipulated germination timing and shade in a multi-factorial experiment to investigate the relationship between spring and autumn phenology in seedlings of the pedunculate oak, Quercus robur, and whether this relationship was mediated by natural colonization of leaves by specialist fungal pathogens (i.e., the oak powdery mildew complex). Each week delay in germination corresponded to about 2days delay in autumn leaf senescence, and heavily shaded seedlings senesced 5-8days later than seedlings in light shade or full sun. Within seedlings, leaves on primary-growth shoots senesced later than those on secondary-growth shoots in some treatments. Path analyses demonstrated that germination timing and shade affected autumn phenology both directly and indirectly via pathogen load, though the specific pattern differed among and within seedlings. Pathogen load increased with later germination and greater shade. Greater pathogen load was in turn associated with later senescence for seedlings, but with earlier senescence for individual leaves. Our findings show that relationships between seasonal events can be partly mediated by the biotic environment and suggest that these relationships may differ between the plant and leaf level. The influence of biotic interactions on phenological correlations across scales has implications for understanding phenotypic variation in phenology and for predicting how populations will respond to climatic perturbation.

Influence of Scale Effect of Canopy Projection on Understory Microclimate in Three Subtropical Urban Broad-Leaved Forests

The canopy is the direct receiver and receptor of external environmental variations, and affects the microclimate and energy exchange between the understory and external environment. After autumn leaf fall, the canopy structure of different forests shows remarkable variation, causes changes in the microclimate and is essential for understory vegetation growth. Moreover, the microclimate is influenced by the scale effect of the canopy. However, the difference in influence between different forests remains unclear on a small scale. In this study, we aimed to analyze the influence of the scale effect of canopy projection on understory microclimate in three subtropical broad-leaved forests. Three urban forests: evergreen broad-leaved forest (EBF), deciduous broad-leaved forest (DBF), and mixed evergreen and deciduous broad-leaved forest (MBF) were selected for this study. Sensors for environmental monitoring were used to capture the microclimate data (temperature (T), relative humidity (RH), and light intensity (LI)) for each forest. Terrestrial laser scanning was employed to obtain the canopy projection intensity (CPI) at each sensor location. The results indicate that the influence range of canopy projection on the microclimate was different from stand to stand (5.5, 5, and 3 m). Moreover, there was a strong negative correlation between T and RH, and the time for T and LI to reach a significant correlation in different urban forests was different, as well as the time for RH and LI during the day. Finally, the correlation between CPI and the microclimate showed that canopy projection had the greatest effect on T and RH in MBF, followed by DBF and EBF. In conclusion, our findings confirm that canopy projection can significantly affect understory microclimate. This study provides a reference for the conservation of environmentally sensitive organisms for urban forest management.

A Simple Method of Predicting Autumn Leaf Coloring Date Using Machine Learning with Spring Leaf Unfolding Date

A Simple Method of Predicting Autumn Leaf Coloring Date Using Machine Learning with Spring Leaf Unfolding Date

Molecular and metabolic insights into anthocyanin biosynthesis during leaf coloration in autumn

Pistacia chinensis is a native plant of China and is used for the production of biodiesel. Certain species of this plant accumulate anthocyanin in the leaves and turn red during autumn. The mechanism of anthocyanin biosynthesis has been well studied in model and horticultural plants. However, the main factors that induce anthocyanin accumulation during leaf senescence in autumn are still obscure, and the internal mechanism is poorly understood. Here, we performed multi-omic analysis and identified 33 metabolites and 6379 transcripts with different abundance levels during autumn leaf coloration in P. chinensis. The transcriptomic data suggested that 31 structural genes in the anthocyanin biosynthesis pathway were upregulated during leaf coloration. Several genes involved in light and low temperature responses were significantly induced or reduced during autumn leaf coloration. ELONGATED HYPOCOTYL 5 (HY5) is considered as an integrator of light and low temperature signals and facilitates anthocyanin accumulation in the autumn leaves. Interestingly, jasmonic acid content was significantly enriched during leaf coloration, thereby indicating a consistent change with anthocyanin content. Furthermore, under shading treatment, the expression of the structural genes was maintained at a low level, consistent with less anthocyanin accumulation. Notably, the expression of MYB113 was consistent with anthocyanin content under both natural and shading conditions. Combining our data with previous reports, we proposed an anthocyanin biosynthesis model of P. chinensis during autumn leaf coloration. Our findings provide new insights into the preliminary molecular mechanism of autumn leaf coloration and potential target genes for molecular breeding.

Phenological shifts induced by climate change amplify drought for broad-leaved trees at low elevations in Switzerland

Climate change alters the bioclimatic conditions during the growing period of trees directly, but also indirectly by causing shifts in spring and autumn leaf phenology that lead to changes in the timing and length of the growing period. Several studies researched the ecological consequences of direct climate change effects on bioclimatic conditions during the growing period of trees. However, the complementary and indirect effects through phenological shifts on these conditions have been insufficiently investigated. We analysed 49088 leaf unfolding and leaf colouring dates of six major European tree species from Switzerland, observed between 200 and 1900 m a.s.l. during 1961–2018. We estimated phenological trends, the resulting changes in bioclimatic conditions during the growing period, and the relative contributions of phenological shifts towards these changes. Our results show that climate change advanced leaf unfolding by up to –3.0 days/decade since 1985. Leaf colouring was mainly delayed at low elevations and was advanced or delayed at high elevations with species-specific rates between –3.1 and +4.0 days/decade. While the length of the growing period and growing degree-days increased for most species after 1985, precipitation during the growing period predominantly decreased by up to –43.6 mm/decade. Furthermore, drought intensity during the growing period (based on the number of days with negative water balance) increased significantly for most species, reaching +6.7 days/decade at low elevations. Phenological shifts amplified the trends towards drier conditions by up to +81% at low elevations for beech, rowan, and sycamore, but weakened them by up to –84% at high elevations for beech, rowan, sycamore, and larch. These findings indicate widely increased drought stress, especially at lower elevations. Further, we conclude that future forest net ecosystem productivity in Central Europe will depend strongly on elevation and species composition, despite a general lengthening of the growing period for trees.

Does drought advance the onset of autumn leaf senescence in temperate deciduous forest trees?

Abstract. Severe droughts are expected to become more frequent and persistent. However, their effect on autumn leaf senescence, a key process for deciduous trees and ecosystem functioning, is currently unclear. We hypothesized that (I) severe drought advances the onset of autumn leaf senescence in temperate deciduous trees and (II) tree species show different dynamics of autumn leaf senescence under drought. We tested these hypotheses using a manipulative experiment on beech saplings and 3 years of monitoring mature beech, birch and oak trees in Belgium. The autumn leaf senescence was derived from the seasonal pattern of the chlorophyll content index and the loss of canopy greenness using generalized additive models and piecewise linear regressions. Drought and associated heat stress and increased atmospheric aridity did not affect the onset of autumn leaf senescence in both saplings and mature trees, even if the saplings showed a high mortality and the mature trees an advanced loss of canopy greenness. We did not observe major differences among species. To synthesize, the timing of autumn leaf senescence appears conservative across years and species and even independent of drought, heat and increased atmospheric aridity. Therefore, to study autumn senescence and avoid confusion among studies, seasonal chlorophyll dynamics and loss of canopy greenness should be considered separately.

How changes in spring and autumn phenology translate into growth‐experimental evidence of asymmetric effects

Abstract Earlier leaf‐out and later autumn leaf senescence under climate warming have been linked to increases in plant productivity and ecosystem carbon uptake. Yet, despite the potential implications of shifting phenology for plant carbon uptake, the degree to which phenological changes affect overall plant growth and the partitioning between above‐ and below‐ground biomass remains unclear. Here we use a 3‐year experiment to quantify changes in root and shoot growth of three woody plant species (two common European tree species, Fagus sylvatica and Quercus robur, and one shrub Lonicera xylosteum) under spring and autumn warming. In both tree species, the magnitude—and in Quercus even the direction—of the effects of growing‐season length on growth depends on whether the warming happened in spring or in autumn. Each day earlier leaf‐out in response to warming resulted in total biomass increases of 0.8%–2.5%, whereas delayed senescence led to reductions of 0.2%–2.1%. Advances in leaf‐out also led to increased root‐to‐shoot biomass ratios because root growth was proportionally more stimulated than shoot growth. In the shrub species, earlier leaf‐out had no effect, while delayed senescence led to increases in root, but not shoot, biomass. Synthesis. The strong asymmetry between growth responses to spring versus autumn phenology demonstrates that growing‐season length per se is a weak indicator of individual‐level tree productivity. The results further imply that phenological shifts are reshaping the functional balance between above‐ and below‐ground growth, which is critical for quantifying forest carbon dynamics under climate change.

VITALITY AND DECORATIVENESS OF PLANTS OF THE GENUS SPIRAEA L IN THE ARCTIC ZONE OF RUSSIA (КОLА PENINSULA)

The article contains а hst of introduced specimens of the genus Spiraea L. in the Polar-Alpine Botanical Garden-Institute located abovethe Arctic Circle. The collection fund ofthe Polar-Alpine Botanical Garden-Institute contains 51 Spiraea specimens belonging to 12 species, 1 subspecies, 3 varieties, 4 cultivars and 2 hybrids. Most of the introduced accessions аге of seed origin. The age of the collection spireas is from 4 to 79 years. The article gives а comprehensive assessment of the viability and decorativeness of introduced plants of the genus Spiraea in the Kola Subarctic. Evaluated the degree of hgnification of the annual shoot, winter hardiness, retention of gmwth fonn, shoot-fonning ability, height gain, abihty to generative development, methods of reproduction in culture, crown architectonics, duration and abundance of flowering, color and size of flowers and inflorescences, attractiveness of fmits, color range of autumn leaf color, leafing duration, plant агата, damage, correlation between viability, decorative effect and the period of being in the experiment. Observations for 2000-2020 were used in the work. А verage score of winter hardiness of introduced S. betuhfoha, S. chamaedryfolia, S. nipponica, S. media 1-3. S. japonica, S. japonica 'Norroboth', S. х bumalda, S. х bumalda 'Shraederii', S. х cinerea 'Grefsheim', S. х rosalba аге frozen to the snow cover and soil level. Winter-hardy species retain their vital fonn, weak-hardy ones are аЫе to restore their growth foпn Most of the studied samples Ыооm and bear fmit not annually, reproduction is possiЫe with artificial sowing, using cuttings. The studied samples can Ье divided into groups: quite viaЫe and viaЫe high and medium decorative; less and less viaЬle high and medium decorative; non-viaЫe and absolutely non-viaЫe medium and low decorative. The most viaЫe and decorative under conditions of introduction to the Kola Peninsula are S. betuhfolia, S. chamaedryfoha, S. nipponica, S. media. S. japonica, S. japonica 'Norroboth', S. х bumalda, S. х bumalda 'Shraederii', S. х cinerea 'Grefsheim', S. х rosalba have low decorative qualities due to low viability.