Predicting spring green-up across diverse North American grasslands

Vegetation phenology influences many ecosystem and climate processes, such as carbon uptake and energy and water cycles. Thus, understanding drivers of vegetation phenology is crucial for predicting current and future impacts of climate change on ecological systems. Existing models can accurately predict the date of spring green -up in temperate forests but tend to perform poorly in grassland systems. We hypothesize this is because most do not incorporate water availability, a primary limiting factor for grassland plants. In this study, we used long-term datasets of digital imagery from the PhenoCam Network of 43 diverse North American grassland sites (195 site -years) to test existing spring phenology models, as well as develop several new models that incorporate pre-cipitation or soil moisture (53 models). Most of the new models performed substantially better, with the best model requiring sufficient accumulated precipitation followed by warm temperatures to trigger spring onset (root mean square error, RMSE, between predicted and observed dates = 16.0 days). Importantly, the best model performed well across all grassland types using a single set of parameters, from temperate to arid grasslands. Since plants are adapted to their local climates, model performance was further improved when parameters were independently optimized for four separate climate regions (RMSE = 10.4 days). Therefore, both sufficient pre-cipitation and temperature are required for grassland green-up, but optimal thresholds vary by region. Running the top model with projected climate data (representative concentration pathway 8.5) suggests that, depending on the climate region, spring onset will occur up to 12 days earlier within 100 years in temperature-limited sites, but the trend is unclear for precipitation-limited sites (3.5 +/- 8.0 days later). This new phenology model improves our ability to understand and predict grassland dynamics, with implications for both current and future ecosystem processes related to carbon and water cycling.

Automated summary

Vegetation phenology plays a crucial role in ecological systems and climate processes. Existing models for predicting spring green-up date in temperate forests perform poorly in grassland systems. This study uses long-term datasets to test existing models and develop new ones that consider water availability. The best model performs well across all grassland types, with parameters optimized for each climate region. Running the model with projected climate data suggests changes in spring onset in temperature-limited sites, but the trend is unclear for precipitation-limited sites. This new phenology model improves our understanding and prediction of grassland dynamics, with implications for carbon and water cycling.