Phenology of Photosynthesis in Winter-Dormant Temperate and Boreal Forests: Long-Term Observations From Flux Towers and Quantitative Evaluation of Phenology Models

We examined the seasonality of photosynthesis in 46 evergreen needleleaf (evergreen needleleaf forests (ENF)) and deciduous broadleaf (deciduous broadleaf forests (DBF)) forests across North America and Eurasia. We quantified the onset and end (StartGPP and EndGPP) of photosynthesis in spring and autumn based on the response of net ecosystem exchange of CO2 to sunlight. To test the hypothesis that snowmelt is required for photosynthesis to begin, these were compared with end of snowmelt derived from soil temperature. ENF forests achieved 10% of summer photosynthetic capacity similar to 3 weeks before end of snowmelt, while DBF forests achieved that capacity similar to 4 weeks afterward. DBF forests increased photosynthetic capacity in spring faster (1.95% d-1) than ENF (1.10% d-1), and their active season length (EndGPP-StartGPP) was similar to 50 days shorter. We hypothesized that warming has influenced timing of the photosynthesis season. We found minimal evidence for long-term change in StartGPP, EndGPP, or air temperature, but their interannual anomalies were significantly correlated. Warmer weather was associated with earlier StartGPP (1.3-2.5 days degrees C-1) or later EndGPP (1.5-1.8 days degrees C-1, depending on forest type and month). Finally, we tested whether existing phenological models could predict StartGPP and EndGPP. For ENF forests, air temperature- and daylength-based models provided best predictions for StartGPP, while a chilling-degree-day model was best for EndGPP. The root mean square errors (RMSE) between predicted and observed StartGPP and EndGPP were 11.7 and 11.3 days, respectively. For DBF forests, temperature- and daylength-based models yielded the best results (RMSE 6.3 and 10.5 days). We used records of forest-atmosphere carbon dioxide exchange and weather to determine when photosynthesis begins and ends each year in 46 northern hemisphere forests. We used observations of soil temperature to determine the timing of the end of the snowmelt period. We found that evergreen needleleaf forests began photosynthesis similar to 3 weeks before snowmelt ended, while deciduous broadleaf forests (DBF) waited until similar to 4 weeks after snowmelt ended. The DBF type ramped up photosynthesis in spring, and ramped down in autumn, faster than the ENF, and the length of the photosynthesis (or "growing") season was similar to 50 days shorter for DBF forests. Abundant evidence suggests that spring is occurring earlier in recent decades. We checked whether these forests are starting photosynthesis earlier by looking at forests with long-term records. We found minimal support for changes in photosynthetic phenology over time, but very strong connections between temperature and the timing of spring and autumn transitions. We tested 19 models that use weather data to predict plant phenological events. We used gridded weather data to drive the models, and the best models were able to predict the spring and autumn photosynthetic transitions to within similar to 10 days. Evergreen forests began photosynthesis in spring similar to 3 weeks before end of snowmelt, deciduous forests similar to 4 weeks after end of snowmelt There is little evidence for lengthening of the photosynthetic season in the northern hemisphere forest flux tower record Interannual variation in onset and end of photosynthesis was related to air temperature