Larger diurnal temperature range undermined later autumn leaf senescence with warming in Europe

Aim

Climate change regulates autumn leaf senescence date (LSD), exhibiting a strong phenological control of plant carbon uptake. Unlike the delaying effect of daily mean temperature (T_mean) on LSD, the impact of warming asymmetry in daytime and nighttime, as evidenced by variations of the diurnal temperature range (DTR), remains elusive. The objectives of this study were to investigate physiological and ecological impacts of DTR on LSD using long-term in situ observations and to predict the future trends of LSD under warming.

Location

Europe.

Time period

1950–2015.

Major taxa studied

Plant phenology.

Methods

We used partial correlation analysis, multiple linear regression and ridge regression to explore the impacts of DTR on LSD. To quantify the importance of potential drivers of LSD, we trained random forest models and applied the SHapley Additive exPlanations method to isolate the marginal contributions of each predictor on LSD. For LSD modelling and projection, we first evaluated two temperature-driven LSD models [i.e., cooling-degree-day (CDD, without DTR effect) and day–night-temperature CDD (DN_CDD, with DTR effect)], then applied them to predict future LSDs.

Results

We found that observational increases in T_mean and DTR had contrasting effects on LSD. Increased T_mean delayed the LSD, whereas larger DTR overall had an advancing effect. Considering the DTR effect, the T_mean sensitivity of LSD was 14% lower than presently estimated (2.4 vs. 2.8 days °C⁻¹). Warming asymmetry-related drought stress and plant functional traits (i.e., plant isohydricity and water-use efficiency) potentially explained the advancing effect of DTR on LSD. We found that current projections of future LSD are overestimated because the DTR effect is discounted, suggesting the need for an adequate understanding of how plant phenology responds to warming asymmetry.

Main conclusions

Our findings highlight the importance of DTR in controlling LSD variations with an advancing-dominant effect and call for the improvement of phenology modelling incorporating the DTR effect. Given that DTR showed a globally narrowing trend over the last several decades, more efforts are needed to understand the potential ecological impacts of warming asymmetry and vegetation response to climate change.     

Thermal acclimation of leaf respiration of tropical trees and lianas: response to experimental canopy warming,and consequences for tropical forest carbon balance

Climate warming is expected to increase respiration rates of tropical forest trees and lianas, which may negatively affect the carbon balance of tropical forests. Thermal acclimation could mitigate the expected respiration increase, but the thermal acclimation potential of tropical forests remains largely unknown. In a tropical forest in Panama, we experimentally increased nighttime temperatures of upper canopy leaves of three tree and two liana species by on average 3  ° C for 1 week, and quantified temperature responses of leaf dark respiration. Respiration at 25  ° C (R₂₅) decreased with increasing leaf temperature, but acclimation did not result in perfect homeostasis of respiration across temperatures. In contrast, Q₁₀ of treatment and control leaves exhibited similarly high values (range 2.5–3.0) without evidence of acclimation. The decrease in R₂₅ was not caused by respiratory substrate depletion, as warming did not reduce leaf carbohydrate concentration. To evaluate the wider implications of our experimental results, we simulated the carbon cycle of tropical latitudes (24 ° S–24 ° N) from 2000 to 2100 using a dynamic global vegetation model (LM3VN) modified to account for acclimation. Acclimation reduced the degree to which respiration increases with climate warming in the model relative to a no‐acclimation scenario, leading to 21% greater increase in net primary productivity and 18% greater increase in biomass carbon storage over the 21st century. We conclude that leaf respiration of tropical forest plants can acclimate to nighttime warming, thereby reducing the magnitude of the positive feedback between climate change and the carbon cycle.