The timing of seasonal biological events — such as when trees leaf out, flowers open, and leaves turn yellow — is called phenology. Phenology involves incredibly complex processes, but temperature and day length have been identified as critical environmental cues for trees to track time. I found warmer temperatures and artificial light in cities alter the timing of urban tree leafing in the U.S (Meng 2021, Science).
Research
My research aims to answer a key ecological question: How do climate change and human activities affect terrestrial ecosystems? I study the response of phenology to climate changes and urbanization. I am also interested in carbon and water cycles in terrestrial ecosystems using field measurements, satellite data, and Earth system models.
Cities and their associated urban heat islands are ideal natural laboratories for evaluating the response of plant phenology to warming conditions. In this project, I demonstrate that the satellite-derived start of season for plants occurred earlier but showed less covariation with temperature in most of the large 85 cities across the conterminous United States for the period 2001–2014. The results show a reduction in the response of urban phenology to temperature and imply that in non-urban environments the onset of spring phenology will likely advance but will slow down as the general trend toward warming continues (Meng et al, 2020, PNAS).
Night time is warming faster than daytime. Understanding how this asymmetric diurnal warming will affect phenology will improve future prediction of phenology. I found divergent phenological responses to daily maximum and minimum temperatures across Appalachian Trail regions in the Eastern United States between 2001 and 2013 using satellite images. I then proposed a new phenology framework considering such responses (Meng et al, 2020, AFM).
Besides temperature, day length — also known as photoperiod — is another key environment cue for phenology. To understand the photoperiod effect on phenology, it's needed to disentangle the effects of temperature and photoperiod, which change in a similar way across latitudes and days of the year. I found that the unique topography of the northern Alps of Europe (elevation decreases with the increase in latitude) provides a relatively uniform temperature distribution but changing daylengths across latitudes, which allowed me to separate the photoperiod effect. My work showed photoperiod decelerated the advance of spring phenology caused by climate warming (Meng et al, 2021, GCB).
Accurate mechanistic modeling of phenological processes is critical to understanding and correctly predicting terrestrial ecosystem feedbacks with changing climate conditions. In collaboration with Oak Ridge National Laboratory, I introduced new seasonal-deciduous phenology schemes into version 1.0 of the land model of the U.S. DOE Energy Exascale Earth System Model (ELM of E3SM). I also evaluated the model performance against the PhenoCam observations at the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment. This practice of model-experimental coupling highlights the importance of phenology in affecting complex terrestrial-climate interactions (Meng et al, 2021, AFM).
