Shifting Trends in Phenology of a Temperate Deciduous Forest in the US Midwest

Phenological changes in deciduous forests induced by short- and/or long-term climate variability have been shown to influence the ecosystem carbon fluxes. The most oft-reported change in vegetation phenology is the early onset of spring green-up due to the warming climates and a few studies have also reported changes in autumn phenology. Our long-term eddy-covariance observations at the Morgan Monroe State Forest - AmeriFlux site in south-central Indiana (MMSF) 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 longer vegetative seasons caused by extension of the vegetative activity in fall (about 3 days year-1 for the past 10 years). Using satellite-derived greenness index data, we extended our phenological analysis to a larger contiguous area of the deciduous forest surrounding our site for the period 1989-2008. Our results indicate that even though the spring green-up was occurring early in this forest in the 1990s, that trend has essentially stopped, and it may have reversed for some of the areas. Contrarily, the autumn senescence is occurring later in this forest, and that trend has accelerated during the 2000s (2 days year-1), compared to that of the 1990s (1.6 days year-1). This is a new finding in contrast to the other published studies of temperate deciduous forests’ phenology. We also found that these phenological changes can be partially explained by the changes in the temperature regimes. As phenology is strongly associated with ecosystem productivity, our results may lead to improved ecosystem models that can incorporate remotely sensed data to better assess spatially distributed estimates of ecosystem carbon fluxes under the changing climates.