Interactive climate factors restrict future increases in spring productivity of temperate and boreal trees

Abstract

Climate warming is currently advancing spring leaf‐out of temperate and boreal trees, enhancing net primary productivity (NPP) of forests. However, it remains unclear whether this trend will continue, preventing for accurate projections of ecosystem functioning and climate feedbacks. Several ecophysiological mechanisms have been proposed to regulate the timing of leaf emergence in response to changing environmental cues, but the relative importance of those mechanisms remains unclear. Here, we use 727,401 direct phenological observations of common European forest trees to examine the dominant controls on leaf‐out. Using the emerging mechanisms, we forecast future trajectories of spring arrival and evaluate the consequences for forest carbon dynamics. By representing hypothesized relationships with autumn temperature, winter chilling, and the timing of spring onset, we accurately predicted reductions in the advance of leaf‐out. There was a strong consensus between our empirical model and existing process‐based models, revealing that the advance in leaf‐out will not exceed 2 weeks over the rest of the century. We further estimate that, under a ‘business‐as‐usual’ climate scenario, earlier spring arrival will enhance NPP of temperate and boreal forests by ~0.2 Gt per year at the end of the century. In contrast, previous estimates based on a simple degree‐day model range around 0.8 Gt. As such, the expected NPP is drastically reduced in our updated model relative to previous estimates—by a total of ~25 Gt over the rest of the century. These findings reveal important environmental constraints on the productivity of broad‐leaved deciduous trees and highlight that shifting spring phenology is unlikely to slow the rate of warming by offsetting anthropogenic carbon emissions.

Chilled to be forced: the best dose to wake up buds from winter dormancy

“…Based on recent evidence from phenological modelling (Wang et al, 2020;Zohner et al, 2020), we expected the temperature range of efficient chilling for dormancy release to be wider than commonly assumed, including subzero temperatures. We further expected that key parameters of dormancy progression show substantial interspecific variation depending on species' phenological strategy.…”

A simple explanation for declining temperature sensitivity with warming

“…We show this by deriving the relationship between a biological response and temperature using a simple stochastic model, which describes the first time a random process hits a threshold (see 'A first-hitting-time model of leafout' in Supplementary Information). Our model holds the temperature threshold for leafout constant (Hunter and Lechowicz, 1992;Zohner et al, 2020).…”

Warming sensitivity of spring phenology of deciduous species lies in their bud energy budget

“…Recent evidence, though, suggests that the sensitivity of spring phenology to warming is decreasing in northern forests 16 and that the rate of change in plant productivity does not match that of air temperature 17 . Indeed, plant phenology may be acclimated to long-term biogeographical constraints [18][19][20] and may be co-limited by several other factors, such as light 21 , water 9,22,23 and nutrients 24 . These observations suggests that warming does not have the same effect everywhere 25 , which has increased interest in other environmental drivers in recent decades, especially illustrated by multiple debates about the specific role of light (and photoperiodism) in spring phenology 21,[26][27][28][29][30][31][32][33] .…”