Responses of bud-break phenology to daily-asymmetric warming: daytime warming intensifies the advancement of bud break

Bud structure, time of budbreak and crown architecture in woody species from Cerrado and seasonal forests of Brazil

Different light intensities in cerrado stricto sensu (CSS) and semideciduous seasonal forests (SSF) can result in distinct morphological responses among woody species. This research evaluated the size and bud composition, budbreak time, and crown architecture of woody species in response to precipitation and varying light intensities in these two environments. The study was conducted in CSS (19°57'29″ S and 44°25'29″ W) and an SSF fragment (19°53'84″ S and 44°25'56″ W) in Minas Gerais, Brazil. The research focused on four species: Miconia albicans and Xylopia aromatica, which occur in both environments, as well as Bauhinia cfr. ungulata (CSS) and B. cfr. rufa (SSF). Shoots from the main axis were manually dissected, and budbreak times were recorded. Crown architecture was evaluated based on diagrams of the vegetative above-ground structure, excluding leaves. Light intensities was measured with a luxmeter at the crown's apex, interior, and base. Shoots were larger and had more preformed organs in CSS than in SSF trees. Trichomes were observed on shoots of all CSS and SSF trees. The number of cataphylls varied: B. cfr. ungulata had one, B. cfr. rufa had 1-3, while compound buds of X. aromatica averaged 5.4 in CSS and 3.7 in SSF. Simple buds of X. aromatica and all M. albicans buds lacked cataphylls. Budbreak occurred in September for M. albicans (CSS and SSF), October for B. cfr. ungulata, Juy-October for X. aromatica and August-October for B. cfr. rufa. A positive correlation between budbreak and rainfall was recorded only for B. cfr. ungulata. Despite differences in bud size and composition between environment, these didn't result in distinct crown architectures. The findings highlight that tropical woody species with cataphyll-protected buds are as common as in temperate regions. Further research is needed to explore phylogenetic traits and the ecological role of cataphylls in tropical species.

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.

Accurate phenology analyses require bud traits and energy budgets

Spring phenology is mainly driven by temperature in extratropical ecosystems. Recent evidence highlighted the key role of micrometeorology and bud temperature on delaying or advancing leaf unfolding. Yet, phenology studies, either using ground-based or remote sensing observations, always substitute plant tissue temperature by air temperature. In fact, temperatures differ substantially between plant tissues and the air because plants absorb and lose energy. Here, we build on recent observations and well-established energy balance theories to discuss how solar radiation, wind and bud traits might affect our interpretation of spring phenology sensitivity to warming. We show that air temperature might be an imprecise and biased predictor of bud temperature. Better characterizing the plants' phenological response to warming will require new observations of bud traits and temperature for accurately quantifying their energy budget. As consistent micrometeorology datasets are still scarce, new approaches coupling energy budget modelling and plant traits could help to improve phenology analyses across scales.