A trigger may not be necessary to cause senescence in deciduous broadleaf forests

Plant phenological changes drive many ecosystem processes and are a key ecological indicator of climate change. Traditional models represent the onset of autumn leaf senescence, or color change, as a threshold response triggered by the accumulation of cold temperatures and declining day lengths, but the physiological mechanisms behind plant thermal memory and chilling thresholds remain elusive. Here we show that we can predict senescence in forest canopies by dynamically modeling daily greenness as the "memoryless" balance between chlorophyll synthesis, linearly-related to daily temperatures and day lengths, and breakdown. Indeed, summer-only data can be used to estimate breakdown and synthesis rates that in many cases successfully predict senescence at both calibration and validation sites. This mechanistic model shows that neither a trigger nor a physiological memory of coldness is necessary for senescence to start. These findings suggest that the start of senescence is not an irreversible transition, but a continuum of decreasing greenness where concurrent environmental conditions determine the rate of initial senescence. Furthermore, this emphasizes that in order to predict how senescence will shift in response to global change we likely need to focus on understanding the impacts on chlorophyll synthesis. Significance statementPlant phenology is a leading ecological indicator of climate change and has wide-ranging ecological and climatological impacts. Our findings here contradict the key assumption that senescence in deciduous broadleaf trees is actively triggered only when a threshold of cumulative cooling in combination of other stressors is reached. Instead we show that canopy greenness can be modeled as a passive process - balancing chlorophyll breakdown and temperature- and photoperiod-controlled synthesis - and still predict a rapid decline in greenness during senescence. This work is particularly important because it emphasizes that in order to understand climate change induced shifts in senescence, we need to focus on impacts on chlorophyll synthesis.