The annual carbon and water flux dynamics of two Eastern North American temperate forests were compared over a six-year period from 2012 to 2017. The geographic location, forest age, soil characteristics, and climate were similar between two stands, however, the species composition varied in terms of tree leaf-retention and shape strategy, where one was a broadleaf deciduous forest, while the other an evergreen needleleaf conifer forest. The six- year mean annual net ecosystem productivity (NEP) of the conifer forest (218 ± 109 g C m-2 yr-1) was slightly higher than that of the deciduous forest (200 ± 83 g C m-2 yr-1). Similarly, the six-year mean annual evapotranspiration (ET) of the conifer forest (442 ± 33 mm yr-1) was higher than that of the deciduous forest (388 ± 34 mm yr-1). While both forests showed similar interannual variability in ET, interannual variability in NEP was more pronounced in the conifer forest. Summer meteorology greatly impacted carbon and water fluxes in both forests, however the degree of response varied among two forest species. In general, warm temperatures caused higher ecosystem respiration (RE), resulting in reduced annual NEP values. This impact was more pronounced in the deciduous forest as compared to the coniferous forest. However, during warm and dry years, the conifer forest experienced larger reduction in annual NEP as compared to the deciduous forest. Variability in annual ET at both forests was related mostly to variability in annual temperature, with the largest annual ET was observed in the warmest years in the deciduous forest. Additionally, ET was sensitive to prolonged dry periods that caused reduction in ET at both stands, although the ET reduction at the conifer forest was relatively larger than that of the deciduous forest. Our study suggests that under the future warmer climate that is expected to have greater precipitation variability, the deciduous forest will remain a strong carbon sink, while the carbon sink-source status of the conifer forest will be uncertain in this region of Eastern North America.
