Frederik Baumgarten

Global warming is affecting the phenological cycles of plants and animals, altering the complex synchronization that has co-evolved over thousands of years between interacting species and trophic levels. Here, we examined how warmer winter conditions affect the timing of budburst in six common European trees and the hatching of a generalist leaf-feeding insect, the spongy moth Lymantria dispar, whose fitness depends on the synchrony between egg hatch and leaf emergence of the host tree. We applied four different temperature treatments to L. dispar eggs and twig cuttings, that mimicked warmer winters and reduced chilling temperatures that are necessary for insect diapause and bud dormancy release, using heated open-top chambers (ambient or +3.5°C), and heated greenhouses (maintained at >6°C or >10°C). In addition, we conducted preference and performance tests to determine which tree species the larvae prefer and benefit from the most. Budburst success and twig survival were highest for all tree species at ambient temperature conditions, whereas it declined under elevated winter temperature for Tilia cordata and Acer pseudoplatanus, likely due to a lack of chilling. While L. dispar egg hatch coincided with budburst in most tree species within 10 days under ambient conditions, it coincided with budburst only in Quercus robur, Carpinus betulus, and, to a lesser extent, Ulmus glabra under warmer conditions. With further warming, we, therefore, expect an increasing mismatch in trees with high chilling requirements, such as Fagus sylvatica and A. pseudoplatanus, but still good synchronization with trees having low chilling requirements, such as Q. robur and C. betulus. Surprisingly, first instar larvae preferred and gained weight faster when fed with leaves of F. sylvatica, while Q. robur ranked second. Our results suggest that spongy moth outbreaks are likely to persist in oak and hornbeam forests in western and central Europe.

1. Phenological shifts in response to changing climatic conditions is a key acclimation process for the persistence of perennial plants in temperate and boreal climates. The optimal time to leaf-out is the result of an evolutionary process determined by the trade-off between minimizing the risk of freezing damages and herbivory pressure while maximizing resource uptake to increase competitiveness against the other plants.
2. We quantified the penalty exerted by frost damage at the time of leaf emergence on plant development (reduction in leaf area, canopy duration and growth) over the potential gains without frost (increased biomass and non-structural carbohydrate reserves), depending on when leaf-out occurs. To this purpose, we exposed 960 saplings of four temperate deciduous tree species with contrasting cold hardiness to two frost intensities shortly after leaf emergence, which was artificially induced at four occasions to reflect the whole range of natural leaf-out dates.
3. One year above-ground biomass (AGB) increments following the frost revealed a clear ranking among the species depending on their strategy to cope with damaging frosts. Prunus avium (−41% of AGB-increment compared to control saplings) resprouted from the stem base, Quercus robur (−62%) rapidly produced new leaves from dormant reserve buds, Fagus sylvatica (−98%) showed highest chlorophyll content in autumn and delayed senescence together with Carpinus betulus (−105%), which overcompensated NSC reserves after the growing season but showed highest mortality (up to 32%). In all species, NSC reserves recovered rapidly to control levels at the expense of growth.
4. The timing of leaf-out (advanced and delayed artificially) significantly affected the performance and recovery (regreening and growth) of both frozen and non-frozen saplings, with the lowest performance found at the most delayed leaf-out date. We propose that the potential to recover from frost damages is an important component of a tree's performance, particularly at the juvenile stage. The ability to recover may become even more decisive in the future with the predicted increase of false springs in many extratropical regions.

Winter chilling, spring forcing temperature and photoperiod are the most important drivers explaining the spatial and temporal variability of spring phenology in temperate trees. However, how these factors interact with each other on dormancy release and spring budburst date remains unclear and varies greatly depending on species. Our knowledge is also limited as to whether heat accumulation of forcing temperatures that trigger bud break in spring is a linear or non-linear process. Here, we aimed at experimentally quantifying the effect of chilling, forcing, photoperiod and their interactions on the budburst dates of nine different temperate tree species from East Asia (near Beijing, China) and Central Europe (near Zurich, Switzerland), including six phylogenetically related species (same genus). We conducted a full factorial experiment in climate chambers using two chilling (low and high, i.e., 0 vs. 56 days at 2°C after sampling at the end of December), four forcing (5, 10, 15, and 20°C), and two photoperiod (8 vs. 16 h) treatments simultaneously in Beijing and Zurich. We found that species growing near Beijing responded more readily to forcing conditions than species of the same genus growing near Zurich regardless of chilling treatment. Budburst timing of most species but European beech was marginally, if at all, affected by photoperiod. Furthermore, our results suggest that linear heat accumulation, as commonly used with the growing degree hours (GDH) model, could result in accurate prediction of budburst date depending on the temperature threshold used as a basis for heat accumulation. Our results also demonstrate the important role of chilling in shaping the sensitivity and rate of forcing accumulation to trigger budburst and suggest that species-specific sigmoid relationship for accumulating heat that accounts for prior chilling exposure may yield better predictions of budburst dates. Our results suggest that deciduous trees may have adapted their chilling and forcing requirements in regards to the predictability of winter-spring transition and late spring frosts. A less predictable winter-spring transition, as observed in Central Europe, could have driven species evolution towards higher chilling and forcing requirements compared to species growing in a more predictable climate of Northeastern Asia. Our cross-continental experiment therefore suggests that the spring phenology of East Asian species is tighter coupled to spring forcing temperature than Central European forests.

· Over the last decades, spring leaf‐out of temperate and boreal trees has substantially advanced in response to global warming, affecting terrestrial biogeochemical fluxes and Earth’s climate system. However, it remains unclear whether leaf‐out will continue to advance with further warming because species’ effective chilling temperatures as well as the amount of chilling‐time required to break dormancy are still largely unknown for most forest tree species.
· Here, we assessed the progress of winter dormancy and quantified the efficiency of different chilling temperatures in six dominant temperate European tree species by exposing 1’170 twig cuttings to a range of temperatures from ‐2° to 10°C for 1, 3, 6, and 12 weeks.
· We found that freezing temperatures were most effective for half of the species or as effective as chilling temperatures up to 10°C, i.e. leading to minimum thermal time to and maximum success of budburst. Interestingly, chilling duration had a much larger effect on dormancy release than absolute chilling temperature.
· Our experimental results challenge the common assumption that optimal chilling temperatures range around 4‐6°C, instead revealing strong sensitivity to a large range of temperatures. These findings could be valuable for improving phenological models and predicting future spring phenology in a warming world.

• Microclimatic effects (light, temperature) are often neglected in phenological studies and little is known about the impact of resource availability (nutrient and water) on tree’s phenological cycles.
• Here we experimentally studied spring and autumn phenology in four temperate trees in response to changes in bud albedo (white- vs. black-painted buds), light conditions (non-shaded vs. c. 70% shaded), water availability (irrigated, control and reduced precipitation) and nutrients (low vs. high availability).
• We found that higher bud albedo or shade delayed budburst (up to +12 days), indicating that temperature is sensed locally within each bud. Leaf senescence was delayed by high nutrient availability (up to +7 days) and shade conditions (up to +39 days) in all species, except oak. Autumn phenological responses to summer droughts depended on species, with a delay for cherry (+ 7 days) and an advance for beech (-7 days).
• The strong phenological effects of bud albedo and light exposure reveal an important role of microclimatic variation on phenology. In addition to the temperature and photoperiod effects, our results suggest a tight interplay between source and sink processes in regulating the end of the seasonal vegetation cycle, which can be largely influenced by resource availability (light, water and nutrients).

Im Wald treiben und blühen Pflanzen in unterschiedlichen Schichten. Das Zusammenleben der Arten auf kleinem Raum und in verschiedenen Höhen ist eingespielt, wird aber von Änderungen des Klimas neu herausgefordert.

Die Phänologie der Pflanzen wird in verschiedenen Beobachtungsnetzen in der Schweiz erfasst. Sie ist stark von Umwelteinflüssen abhängig, wie der schnelle Klimawandel immer deutlicher zeigt. Die Reaktionen vieler Waldbaumarten über verschiedene Höhenstufen und über die Zeit sind grossen Veränderungen ausgesetzt. Bekannte Gesetzmässigkeiten der vergangenen Jahrzehnte verändern sich, was die Projektionen in die Zukunft erschwert.

Vapour pressure deficit is a major driver of seasonal changes in transpiration, but photoperiod also modulates leaf responses. Climate warming might enhance transpiration by increasing atmospheric water demand and the length of the growing season, but photoperiod-sensitive species could show dampened responses. Here, we document that day length is a significant driver of the seasonal variation in stomatal conductance. We performed weekly gas exchange measurements across a common garden experiment with 12 oak species from contrasting geographical origins, and we observed that the influence of day length was of similar strength to that of vapour pressure deficit in driving the seasonal pattern. We then examined the generality of our findings by incorporating day-length regulation into well-known stomatal models. For both angiosperm and gymnosperm species, the models improved significantly when adding day-length dependences. Photoperiod control over stomatal conductance could play a large yet underexplored role on the plant and ecosystem water balances.

Late-spring frosts (LSFs) affect the performance of plants and animals across the world's temperate and boreal zones, but despite their ecological and economic impact on agriculture and forestry, the geographic distribution and evolutionary impact of these frost events are poorly understood. Here, we analyze LSFs between 1959 and 2017 and the resistance strategies of Northern Hemisphere woody species to infer trees' adaptations for minimizing frost damage to their leaves and to forecast forest vulnerability under the ongoing changes in frost frequencies. Trait values on leaf-out and leaf-freezing resistance come from up to 1,500 temperate and boreal woody species cultivated in common gardens. We find that areas in which LSFs are common, such as eastern North America, harbor tree species with cautious (late-leafing) leaf-out strategies. Areas in which LSFs used to be unlikely, such as broad-leaved forests and shrublands in Europe and Asia, instead harbor opportunistic tree species (quickly reacting to warming air temperatures). LSFs in the latter regions are currently increasing, and given species' innate resistance strategies, we estimate that ∼35% of the European and ∼26% of the Asian temperate forest area, but only ∼10% of the North American, will experience increasing late-frost damage in the future. Our findings reveal region-specific changes in the spring-frost risk that can inform decision-making in land management, forestry, agriculture, and insurance policy.