Changes in current climatic conditions are well documented at both local and global scales.  While there is a general consensus on the causes and the expected trends for these changes, large uncertainties affect the predictions on the future roles of temperate forests in offsetting anthropogenic carbon emissions. This uncertainty mainly originates from the incomplete understanding of the mechanisms and controls that regulate the carbon cycle in forest, but also by a general lack of long-term measurements that could otherwise reveal trends in carbon, water and energy exchange between the ecosystem and the atmosphere.  The AmeriFlux site in the Morgan Monroe State Forest in Indiana (MMSF) has been collecting observations on net ecosystem exchange (NEE) of carbon and its environmental drivers since 1998.  Despite the large inter-annual variability in NEP, the observations show a significant increase in net forest productivity over the past 10 years (about 100 gC m²).  This increase in productivity is hypothesized to be caused by changes in phenology and carbon cycle occurring as a result of climate changes. Different independent techniques were used to determine the start, the end, and the length of the vegetative season, including ground-, eddy-covariance-, and remote sensing-based approaches.

Results/Conclusions

The majority of the methods show the same trends and they provide evidence for longer vegetative seasons that are caused by extension of the vegetative activity in the fall, not an earlier spring. Both phenological and flux observations indicate that the vegetative season extended later in the fall with an increase in length of about 3 days year^-1.  However, these changes are responsible for only 50% of the total annual gain in forest productivity in the past decade. A negative trend in air and soil temperature during the winter months may explain an equivalent increase in net uptake through a decrease in ecosystem respiration.  Our results confirm the great importance of long-term, high frequency records in unraveling the mechanisms and feedbacks that regulate the interaction between ecosystems and climate variability. They indicate that the sensitivity of the carbon cycle to climate change may not be assumed to be equal even among similar ecosystems, and caution should be used in extrapolating results from different forests. They remind us that the processes under investigation are extremely dynamic and long-term measurements are essential in order to make accurate predictions of the carbon dynamics of forest ecosystems.