Evidence of increased net ecosystem productivity associated with a longer vegetated season in a deciduous forest in south-central Indiana, US

Observations of net ecosystem exchange (NEE) of carbon and its biophysical drivers have been collected at the AmeriFlux site in the Morgan Monroe State Forest (MMSF) in Indiana, USA since 1998. Thus, this is one of the few deciduous forest sites in the world, where a decadal analysis on net ecosystem productivity (NEP) trends is possible. Despite the large inter-annual variability in NEP, the observations show a significant increase in forest productivity over the past 10 years (by an annual increment of about 10 gC m^-2 yr^-1). This trend can partially be attributed to changes in phenology. In particular, there is evidence for longer vegetative seasons caused by extension of the vegetative activity in the fall. 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 for the past 10 years. A strong negative correlation is found between the onset of senescence and the total cold degree-days during the vegetative season, with the latter showing a significant negative trend in the past decade. 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 NEP through a decrease in ecosystem respiration. Our results provide evidence of increased NEP associated with changes in late-summer phenology and confirm the importance of long-term measurements in enhancing our understanding of the response of deciduous forests to long-term climate variability. Furthermore, they result essential in correctly quantify the intra- and inter-annual variability in NEP, which directly link with the ecosystem response to short-term climate variability.