Phenology of forest-atmosphere carbon exchange for deciduous and coniferous forests in southern and northern New England variation with latitude and landscape position

We used ecosystem carbon exchange measurements at five sites in New England to examine how interannual variation in leaf development and leaf abscis-sion, as well as latitude and landscape position, affected the phenology of carbon exchange in recent years. We studied three deciduous forest sites, two in southern and one in northern New England, at latitudes of about 42.54 and 44.28°N with carbon exchange records of 3-15 years, and also two coniferous forests, one also at about 42.54°N and the other at 45.25°N, with records of 4 and 11 years, including 3 years of concurrent data. In the southern New England deciduous forest with 15 years of data, the time at which carbon uptake increased in spring was significantly correlated with observed leaf development, but the cessation of carbon uptake was not significantly correlated with observed leaf abscission, which does not quickly follow leaf senescence in the dominant species, red oak (Quercus rubra). A measure of canopy greenness appears necessary for accurately estimating or predicting cessation of carbon uptake by this species. Differences between two southern New England deciduous forests in landscape position, slope aspect (northwest vs. east), and the degree of dominance by red oak vs. other deciduous and coniferous trees had effects on the annual time course of carbon exchange which were similar in magnitude to the effects of the 1.75° difference in latitude between these two deciduous forests and one further north. We hypothesize that a change in dominant tree species from red oak, which has ring-porous wood and must form new xylem each year prior to leaf growth, to diffuse porous trees (Acer, Betula and Fagus spp.), which lack this requirement, further north could enable the timing of leaf development to remain relatively early in the more northerly location, despite a cooler climate. A coniferous forest in southern New England (latitude 42.54°) showed two annual peaks in carbon uptake: a large one in spring before maximum carbon uptake by deciduous forests, and a smaller peak in autumn. In contrast, in a more northerly coniferous forest (45.25°N), the autumn peak was not observed. Significant late-winter (March) carbon uptake also occurred only in the more southerly conifer forest when early soil thawing occurred in 2006.