In a temperate climate, understory trees leaf out earlier than canopy trees, but the cause of this discrepancy remains unclear. This study aims to investigate whether this discrepancy results from ...ontogenic changes or from microclimatic differences.
Seedlings of five deciduous tree species were grown in spring 2012 in the understory and at canopy height using a 45-m-high construction crane built into a mature mixed forest in the foothills of the Swiss Jura Mountains. The leaf development of these seedlings, as well as conspecific adults, was compared, taking into account the corresponding microclimate.
The date of leaf unfolding occurred 10–40 d earlier in seedlings grown at canopy level than in conspecific adults. Seedlings grown in the understory flushed c. 6 d later than those grown at canopy height, which can be attributed to the warmer temperatures recorded at canopy height (c. 1°C warmer).
This study demonstrates that later leaf emergence of canopy trees compared with understory trees results from ontogenic changes and not from the vertical thermal profile that exists within forests. This study warns against the assumption that phenological data obtained in warming and photoperiod experiments on juvenile trees can be used for the prediction of forest response to climate warming.
One hundred years ago, Andrew D. Hopkins estimated the progressive delay in tree leaf-out with increasing latitude, longitude, and elevation, referred to as “Hopkins’ bioclimatic law.” What if global ...warming is altering this well-known law? Here, based on ∼20,000 observations of the leaf-out date of four common temperate tree species located in 128 sites at various elevations in the European Alps, we found that the elevation-induced phenological shift (EPS) has significantly declined from34 d·1,000m−1 conforming to Hopkins’ bioclimatic law in 1960, to 22 d·1,000 m−1 in 2016, i.e., −35%. The stronger phenological advance at higher elevations, responsible for the reduction in EPS, is most likely to be connected to stronger warming during late spring as well as to warmer winter temperatures. Indeed, under similar spring temperatures, we found that the EPS was substantially reduced in years when the previous winter was warmer. Our results provide empirical evidence for a declining EPS over the last six decades. Future climate warming may further reduce the EPS with consequences for the structure and function of mountain forest ecosystems, in particular through changes in plant–animal interactions, but the actual impact of such ongoing change is today largely unknown.
Global warming has strong impacts on snow cover, which in turn affects ecosystems, hydrological regimes and winter tourism. Only a few long-term snow series are available worldwide, especially at ...high elevation. Here, we analyzed several snowpack characteristics over the period 1970–2015 at eleven meteorological stations, spanning elevations from 1139 to 2540 m asl in the Swiss Alps. Snow cover duration has significantly shortened at all sites, on average by 8.9 days decade
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. This shortening was largely driven by earlier snowmelt (on average 5.8 days decade
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) and partly by later snow onset but the latter was significant in only ~30 % of the stations. On average, the snow season now starts 12 days later and ends 26 days earlier than in 1970. Overall, the annual maximum snow depth has declined from 3.9 to 10.6 % decade
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and was reached 7.8 ± 0.4 to 12.0 ± 0.4 days decade
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earlier, though these trends hide a high inter-annual and decadal variability. The number of days with snow on the ground has also significantly decreased at all elevations, in all regions and for all thresholds from 1 to 100 cm. Overall, our results demonstrate a marked decline in all snowpack parameters, irrespective of elevation and region, and whether for drier or wetter locations, with a pronounced shift of the snowmelt in spring, in connection with reinforced warming during this season.
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•Maximum temperatures have increased more than minimum temperatures.•Spring phenology has advanced at a faster rate than the date of the last frost.•The risk of frost injury to trees ...has increased at higher elevations in Switzerland.•The risk of frost injury to trees has remained unchanged at lower elevations.•Planting summer-adapted trees should be carefully considered regarding frost risk.
Winters and early springs are predicted to become warmer in temperate climates under continued global warming, which in turn is expected to promote earlier plant development. By contrast, there is no consensus about the changes in the occurrence and severity of late spring frosts. If the frequency and severity of late spring frosts remain unchanged in the future or change less than spring phenology of plants does, vulnerable plant organs (dehardened buds, young leaves, flowers or young fruits) may be more exposed to frost damage. Here we analyzed long-term temperature data from the period 1975–2016 in 50 locations in Switzerland and used different phenological models calibrated with long-term series of the flowering and leaf-out timing of two fruit trees (apple and cherry) and two forest trees (Norway spruce and European beech) to test whether the risk of frost damage has increased during this period. Overall, despite the substantial increase in temperature during the study period, the risk of frost damage was not reduced because spring phenology has advanced at a faster rate than the date of the last spring frost. In contrast, we found that the risk of frost exposure and subsequent potential damage has increased for all four species at the vast majority of stations located at elevations higher than 800m while remaining unchanged at lower elevations. The different trends between lower and higher elevations are due to the date of the last spring frost moving less at higher altitudes than at lower altitudes, combined with stronger phenological shifts at higher elevations. This latter trend likely results from a stronger warming during late compared to earlier spring and from the increasing role of other limiting factors at lower elevations (chilling and photoperiod). Our results suggest that frost risk needs to be considered carefully when promoting the introduction of new varieties of fruit trees or exotic forest tree species adapted to warmer and drier climates or when considering new plantations at higher elevations.
A considerable number of studies have investigated the phenology of European beech using models, experimental controlled conditions, or descriptive surveys of patterns in situ. In spite of this ...interest, there is no consensus about the environmental factors controlling bud burst in beech, especially about the role of photoperiod and chilling temperature (cold temperature effective to release bud dormancy). However, recent experimental and modelling studies provide new insights into the means by which these environmental factors control beech phenology. This present contribution aims to reconcile contradictory hypotheses about the main environmental factors controlling bud burst date of European beech. First, we review the main published results on the environmental control of beech phenology both in controlled and in natural conditions. Second, supported by the findings of recent studies, we propose a new theory for the role of photoperiod during the chilling phase for explaining spatial and temporal variations in bud burst phenology of European beech. Examples using long-term data from the Swiss Alps and Germany are presented to support this theory. The possible impacts of future and ongoing climate warming on beech phenology are discussed. Finally, due to interactions between chilling, forcing temperature, and photoperiod, we assert that beech phenology follows a nonlinear trend across biogeographical gradients such as changes in elevation or latitude and that the bud burst date of beech is expected not to undergo significant changes in response to global warming, especially in warmer climates.
We investigated how deciduous trees can adjust their freezing resistance in response to temperature during the progress of the ecodormancy phase, from midwinter to budburst.
We regularly sampled ...twigs of four different temperate deciduous tree species from January to the leaf-out date. Using computer-controlled freezers and climate chambers, the freezing resistance of buds was measured directly after sampling and also after the application of artificial hardening and dehardening treatments, simulating cold and warm spells. The thermal time to budburst in forcing conditions (c. 20°C) was also quantified at each sampling as a proxy for dormancy depth.
Earlier flushing species showed higher freezing resistance than late flushing species at either similar bud development stage or similar dormancy depth. Overall, freezing resistance and its hardening and dehardening potential dramatically decreased during the progress of ecodormancy and became almost nil during budburst.
Our results suggest that extreme cold events in winter are not critical for trees, as freezing resistance can be largely enhanced during this period. By contrast, the timing of budburst is a critical component of tree fitness. Our results provide quantitative values of the freezing resistance dynamics during ecodormancy, particularly valuable in process-based species distribution models.
In deciduous trees growing in temperate forests, bud break and growth in spring must rely on intrinsic carbon (C) reserves. Yet it is unclear whether growth and C storage occur simultaneously, and ...whether starch C in branches is sufficient for refoliation. To test in situ the relationships between growth, phenology and C utilization, we monitored stem growth, leaf phenology and stem and branch nonstructural carbohydrate (NSC) dynamics in three deciduous species: Carpinus betulus L., Fagus sylvatica L. and Quercus petraea (Matt.) Liebl. To quantify the role of NSC in C investment into growth, a C balance approach was applied. Across the three species, >95% of branchlet starch was consumed during bud break, confirming the importance of C reserves for refoliation in spring. The C balance calculation showed that 90% of the C investment in foliage (7.0-10.5 kg tree(-1) and 5-17 times the C needed for annual stem growth) was explained by simultaneous branchlet starch degradation. Carbon reserves were recovered sooner than expected, after leaf expansion, in parallel with stem growth. Carpinus had earlier leaf phenology (by ∼25 days) but delayed cambial growth (by ∼15 days) than Fagus and Quercus, the result of a competitive strategy to flush early, while having lower NSC levels.
Earlier spring leaf unfolding is a frequently observed response of plants to climate warming. Many deciduous tree species require chilling for dormancy release, and warming-related reductions in ...chilling may counteract the advance of leaf unfolding in response to warming. Empirical evidence for this, however, is limited to saplings or twigs in climate-controlled chambers. Using long-term in situ observations of leaf unfolding for seven dominant European tree species at 1,245 sites, here we show that the apparent response of leaf unfolding to climate warming (ST, expressed in days advance of leaf unfolding per °C warming) has significantly decreased from 1980 to 2013 in all monitored tree species. Averaged across all species and sites, ST decreased by 40% from 4.0 ± 1.8 days °C(-1) during 1980-1994 to 2.3 ± 1.6 days °C(-1) during 1999-2013. The declining ST was also simulated by chilling-based phenology models, albeit with a weaker decline (24-30%) than observed in situ. The reduction in ST is likely to be partly attributable to reduced chilling. Nonetheless, other mechanisms may also have a role, such as 'photoperiod limitation' mechanisms that may become ultimately limiting when leaf unfolding dates occur too early in the season. Our results provide empirical evidence for a declining ST, but also suggest that the predicted strong winter warming in the future may further reduce ST and therefore result in a slowdown in the advance of tree spring phenology.
Temperate climates are defined by distinct temperature seasonality with large and often unpredictable weather during any of the four seasons. To thrive in such climates, trees have to withstand a ...cold winter and the stochastic occurrence of freeze events during any time of the year. The physiological mechanisms trees adopt to escape, avoid, and tolerate freezing temperatures include a cold acclimation in autumn, a dormancy period during winter (leafless in deciduous trees), and the maintenance of a certain freezing tolerance during dehardening in early spring. The change from one phase to the next is mediated by complex interactions between temperature and photoperiod. This review aims at providing an overview of the interplay between phenology of leaves and species-specific freezing resistance. First, we address the long-term evolutionary responses that enabled temperate trees to tolerate certain low temperature extremes. We provide evidence that short term acclimation of freezing resistance plays a crucial role both in dormant and active buds, including re-acclimation to cold conditions following warm spells. This ability declines to almost zero during leaf emergence. Second, we show that the risk that native temperate trees encounter freeze injuries is low and is confined to spring and underline that this risk might be altered by climate warming depending on species-specific phenological responses to environmental cues.
•Warmer winters significantly delayed budburst and flowering.•Winter warming that counteracts the advancing effect of preseason warming.•The effect of winter warming was 2.3 times lower than the ...effect of spring warming.•Warmer winter temperature conditions have a significantly larger effect at lower elevations.
Mountain regions are particularly susceptible and influenced by the effects of climate change. In the Alps, temperature increased two times faster than in the Northern Hemisphere during the 20th century. As an immediate response in certain tree species, spring phenological phases, such as budburst and flowering, have tended to occur earlier. However, recent studies have shown a slowing down of phenological shifts during the last two decades compared to earlier periods, which might be caused by warmer winters. Indeed, cold temperatures are required to break bud dormancy that occurs in early fall; and dormancy break is a prerequisite for cell elongation to take place in spring when temperature conditions are warm enough.
Here we aimed at evaluating the effects of winter warming vs. spring warming on the phenological shift along mountain elevation gradients. We tested the hypothesis that a lack of chilling temperature during winter delayed dormancy release and subsequently spring phenological phases. For this, we used eight years of temperature and phenological records for five tree species (Betula pendula, Fraxinus excelsior, Corylus avellana, Picea abies and Larix decidua) gathered with the citizen science program Phenoclim (www.phenoclim.org) deployed over the French Alps.
Our results showed that for similar preseason (i.e. after dormancy break) temperatures, warmer winters significantly delayed budburst and flowering along the elevation gradient (+0.9 to +5.6 days °C−1) except for flowering of Corylus and budburst of Picea. For similar cold winter temperatures, warmer preseasons significantly advanced budburst and flowering along the elevation gradient (−5.3 to −8.4 days °C−1). On average, the effect of winter warming was 2.3 times lower than the effect of spring warming. We also showed that warmer winter temperature conditions have a significantly larger effect at lower elevations.
As a consequence, the observed delaying effect of winter warming might be beneficial to trees by reducing the risk of exposure to late spring frost on a short term. This could further lead to partial dormancy break at lower elevations before the end of the 21st century, which, in turn, may alter bud development and flowering and so tree fitness.
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