Global warming is increasing the discrepancy between green (actual) and thermal (potential) seasons of temperate trees

Abstract

Over the past decades, global warming has led to a lengthening of the time window during which temperatures remain favorable for carbon assimilation and tree growth, resulting in a lengthening of the green season. The extent to which forest green seasons have tracked the lengthening of this favorable period under climate warming, however, has not been quantified to date. Here, we used remote sensing data and long-term ground observations of leaf-out and coloration for six dominant species of European trees at 1773 sites, for a total of 6060 species-site combinations, during 1980-2016 and found that actual green season extensions (GS: 3.1 ± 0.1 day decade −1 ) lag four times behind extensions of the potential thermal season (TS: 12.6 ± 0.1 day decade −1 ). Similar but less pronounced differences were obtained using satellitederived vegetation phenology observations, that is, a lengthening of 4.4 ± 0.13 and 7.5 ± 0.13 day decade −1 for GS and TS, respectively. This difference was mainly driven by the larger advance in the onset of the thermal season compared to the actual advance of leaf-out dates (spring mismatch: 7.2 ± 0.1 day decade −1 ), but to a less extent caused by a phenological mismatch between GS and TS in autumn (2.4 ± 0.1 day decade −1 ). Our results showed that forest trees do not linearly track the new thermal window extension, indicating more complex interactions between winter and spring temperatures and photoperiod and a justification of demonstrating that using more sophisticated models that include the influence of chilling and photoperiod is needed to accurately predict spring phenological changes under warmer climate. They urge

Four ways to define the growing season

“…For 108 forests across eastern North America, dendrometers indicated an earlier start of the season in warmer springs (beginning of tree ring formation), but no effect on annual increment (Dow et al, 2022). A study across 1773 sites in central Europe found that the 'green season' (a phenological season) lags significantly behind the prolonging of the thermal season (T-threshold of 5 °C; Fu et al, 2022). Yet, in the Harvard forest, a longer and warmer meteorological season (1992-2015) revealed a significant increase of NEP (based on eddy covariance data) in this relatively young forest, explained by enhanced growth above, but not below the ground (Finzi et al, 2020).…”

Expansion of Treeline in North China and Its Relationship with Altitude Sensitivity Gradient of Larix gmelinii

“…The legacy effect on tree growth in the subtimberline forest was weakened, probably because warming lengthened the growing period. Under these conditions, trees would use more NSC for radial growth and sustenance in the current year, leaving less for the following year [54]. Modeling based on a remote sensing analysis found that climate warming extends the growing season of boreal forest plants [55].…”

A new temperature-photoperiod coupled phenology module in LPJ-GUESS model v4.1: optimizing estimation of terrestrial carbon and water processes

“…S2). Phenological extraction based on multiple methods consists of three steps: 1) smoothing and interpolating the NDVI date to obtain the smooth and continuous NDVI daily time series; 2) using the threshold value (0.5 for SOS and 0.2 for EOS) or the maximum rate of change to extract the vegetation phenology from each single method (Reed et al, 1994;White et al, 1997;White et al, 2009;Piao et al, 2006); 3) averaging the phenological results obtained by different extraction methods to reduce uncertainties associated with a single method (Due to the different fitting methods, interpolation methods and threshold settings of different extraction methods) (Fu et al, 2021;Fu et al, 2023).…”