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AbstractClimate change is expected to be heterogeneous across the world, with high impacts on the Himalayan ecosystems. There is a need to precisely document cambial phenology and wood formation in these regions to better understand climate-growth relationships and how trees face a warming climate. This study describes the dynamics of cambial phenology in pindrow fir (Abies pindrow) along its altitudinal gradient in the Himalaya. The stages of xylem phenology, and the duration and rate of wood formation were assessed from anatomical observations during the growing season from samples collected weekly from three sites at various altitudes (2392–2965 m a.s.l.) over two years. There were significant differences in the duration and rate of cell formation along the altitudinal gradient, which decreased at increasing altitudes. The growing season duration decreased by 5.2 and 3.7 days every 100 m of increase in altitude in 2014 and 2015, respectively, while the rate of cell formation decreased from 0.38 and 0.44 cells /day to 0.29 and 0.34 cells/day in 2014 and 2015, respectively. Cell production decreased from 63.3 and 67.0 cells to 38.3 and 45.2 cells with a decrease of 4.3 and 3.8 cells per 100 m increase in altitude in 2014 and 2015, respectively. The higher precipitation in 2015 increased the growth rate and resulted in a higher xylem production. Our findings give new insights into the dynamics of cambial phenology and help in better understanding of the potential impacts of climate change on tree growth and forest productivity of Himalayan forests.

Plasticity is vital for plants to rapidly acclimate to environmental changes, especially under the climate change. Global warming could advance bud break and extend the growing season, but it also increases the risk of frost damage to developing leaves. In this study, we explored the phenological plasticity of bud burst of half-sib family sugar maple (Acer saccharum Marsh.) seedlings from 11 seed origins in two common gardens at the center and the northern edge of the species distribution in Quebec, Canada. Results showed that the phenological plasticity of sugar maple originating from inland was significantly higher than those from coastal areas at the beginning of leaf development. This discrepancy may result from the long-term frost change frequency of seed origins. Our study suggests that in the context of climate warming, the higher plasticity observed in sugar maple originating from inland areas may benefit from the phenological adaptation of sugar maple and the survival of local populations. It also suggests that inland populations may have a higher potential regarding to assisted migration, but this needs to be confirmed for other functional traits than phenology.

AbstractUnder current global warming, high‐elevation regions are expected to experience faster warming than low‐elevation regions. However, due to the lack of studies based on long‐term large‐scale data, the relationship between tree spring phenology and the elevation‐dependent warming is unclear. Using 652k records of leaf unfolding of five temperate tree species monitored during 1951–2013 in situ in Europe, we discovered a nonlinear trend in the altitudinal sensitivity (SA, shifted days per 100 m in altitude) in spring phenology. A delayed leaf unfolding (2.7 ± 0.6 days per decade) was observed at high elevations possibly due to decreased spring forcing between 1951 and 1980. The delayed leaf unfolding at high‐elevation regions was companied by a simultaneous advancing of leaf unfolding at low elevations. These divergent trends contributed to a significant increase in the SA (0.36 ± 0.07 days 100/m per decade) during 1951–1980. Since 1980, the SA started to decline with a rate of −0.32 ± 0.07 days 100/m per decade, possibly due to reduced chilling at low elevations and improved efficiency of spring forcing in advancing the leaf unfolding at high elevations, the latter being caused by increased chilling. Our results suggest that due to both different temperature changes at the different altitudes, and the different tree responses to these changes, the tree phenology has shifted at different rates leading to a more uniform phenology at different altitudes during recent decades.

Global warming advances bud break, mismatches plant phenology from the local climate, and exposes the developing leaves to higher risks of frost damage. Bud break of sugar maple [Acer saccharum (Marsh.)], a species included in recent programs of assisted migration, is sensitive to nighttime spring temperatures. This suggests a link between frost events and leaf development. In this study, we raise the hypothesis that late frost is an evolutionary driver of growth reactivation in sugar maple provenances. We investigated the ecotypic variation of bud phenology in 30 provenances planted in two common gardens within and at the northern limit of the species range, in the Province of Quebec, Canada. Eight phases of bud break were assessed twice a week during 2020 on 252 and 272 seedlings in southern and northern sites, respectively. In the southern site, bud break occurred in May, starting on average 12 days earlier and ending 3 days earlier compared to the northern site. Logistic regression was used to estimate the probability of late frost and the results showed that regions located in the north, at higher elevations, and along the northeastern coast of the native maple range showed the latest occurrences of frost events in spring. This pattern mirrored the timing of bud break. When planted in the same common garden, provenances originating from sites with later spring frosts leafed out earlier. Such differences were maintained across the eight bud phenological phases and between the two common gardens, which indicates a similar response of the provenances to changing growing conditions. To avoid frost damage to sugar maple plantations, assisted migration should account for phenotypic traits in bud phenology, ensuring that the frost regime at the origin of the provenances is compatible with that of plantations.

Abstract High mountains around the globe are some of the most vulnerable ecosystems to climate change and of great concern for conservation. The Himalayan Mountains are experiencing a higher warming than average global warming, which can significantly impact their biodiversity, vegetation distribution and ecosystem structure. There is a need to study the process of wood formation in Himalayan conifers to have a better understanding of their growth responses to predicted climate change. Variations in the climatic factors regulating cambial activity would result in changes in the timing of cambial phenology. In this study, the variations in the timing of different stages of cambial phenology (cell enlargement stage, wall-thickening stage and cell maturation stages) in pindrow fir (Abies pindrow) were investigated from anatomical observations of wood microcores collected during 2014-15 along an elevation range of c.2300−3000 m asl in the north-western Himalaya. The onset of all three cambial phenological stages was significantly correlated with elevation, with onset of cambial activity happening more than a week earlier at the lowest elevation than at the highest elevation site. Although the termination of wall-thickening and maturation stage appeared minimally related to elevation, the cell-enlargement stage showed significant correlation with elevation, with tracheid formation ceasing approximately three weeks earlier in trees at the highest elevation. The timing of these phenological stages did not show strong variations between the two study years. Our findings provide new data on the timings of cambial phenophases and help to understand tree growth response to ongoing changing climate in the Himalayan region.

In Mediterranean mountains, Pinus sylvestris L. is expected to be displaced under a warming climate by more drought-tolerant species such as the sub-Mediterranean Quercus pyrenaica Willd. Understanding how environmental factors drive tree physiology and phenology is, therefore, essential to assess the effect of changing climatic conditions on the performance of these species and, ultimately, their distribution. We compared the cambial and leaf phenology and leaf gas exchange of Q. pyrenaica and P. sylvestris at their altitudinal boundary in Central Spain and assessed the environmental variables involved. Results indicate that P. sylvestris cambial phenology was more sensitive to weather conditions (temperature at the onset and water deficit at the end of the growing season) than Q. pyrenaica. On the other hand, Q. pyrenaica cambial and leaf phenology were synchronized and driven by photoperiod and temperatures. Pinus sylvestris showed lower photosynthetic nitrogen-use efficiency and higher intrinsic water-use efficiency than Q. pyrenaica as a result of a tighter stomatal control in response to summer dry conditions, despite its less negative midday leaf water potentials. These phenological and leaf gas exchange responses evidence a stronger sensitivity to drought of P. sylvestris than that of Q. pyrenaica, which may therefore hold a competitive advantage over P. sylvestris under the predicted increase in recurrence and intensity of drought events. On the other hand, both species could benefit from warmer springs through an advanced phenology, although this effect could be limited in Q. pyrenaica if it maintains a photoperiod control over the onset of xylogenesis.

Global changes affect the growing conditions of terrestrial ecosystems, causing a mismatch between plant phenology and local climates in Northern regions. Due to their long lifespan and irregular regeneration periods, trees cannot respond quickly enough to climate variability through long-term genetic adaptation. In this study, we explored the phenological plasticity and genetic variation among populations of bud burst in sugar maple (Acer saccharum Marsh.) seedlings from 30 Canadian provenances with contrasting climates planted in two common gardens near and at the northern limit of the species’ range. We tested the hypothesis that phenotypic plasticity and genetic variation among populations affect bud phenology. We expect that phenotypic plasticity is more important in regulating bud phenology due to the high variability in short-term weather events characterizing this part of North America. Bud development and leafing occurred in April–May, with complete bud burst lasting between 21 and 29 days. On average, bud swelling differed by 12 days between common gardens. Both factors site (common gardens) and provenance significantly affected bud burst, demonstrating phenological plasticity and genetic variation of sugar maple, respectively. A significant interaction between site and provenance was also found. Overall, the site (11.8–90.3%) contributed more than provenance (0–3.1%) to the variance in timings of bud burst, indicating a dominant role of plasticity in regulating spring phenology. Our study demonstrated the concurring effects of genetic variation and phenological plasticity of sugar maple and revealed the dominant role of the latter factor. The high plasticity observed in sugar maple has a crucial role in the phenological adaptation of maple and the survival of its local populations in a context of changing climate.

AbstractThe interaction between xylem phenology and climate assesses forest growth and productivity and carbon storage across biomes under changing environmental conditions. We tested the hypothesis that patterns of wood formation are maintained unaltered despite the temperature changes across cold ecosystems. Wood microcores were collected weekly or biweekly throughout the growing season for periods varying between 1 and 13 years during 1998–2014 and cut in transverse sections for assessing the onset and ending of the phases of xylem differentiation. The data set represented 1321 trees belonging to 10 conifer species from 39 sites in the Northern Hemisphere and covering an interval of mean annual temperature exceeding 14 K. The phenological events and mean annual temperature of the sites were related linearly, with spring and autumnal events being separated by constant intervals across the range of temperature analysed. At increasing temperature, first enlarging, wall‐thickening and mature tracheids appeared earlier, and last enlarging and wall‐thickening tracheids occurred later. Overall, the period of wood formation lengthened linearly with the mean annual temperature, from 83.7 days at −2 °C to 178.1 days at 12 °C, at a rate of 6.5 days °C−1. April–May temperatures produced the best models predicting the dates of wood formation. Our findings demonstrated the uniformity of the process of wood formation and the importance of the environmental conditions occurring at the time of growth resumption. Under warming scenarios, the period of wood formation might lengthen synchronously in the cold biomes of the Northern Hemisphere.