Abstract
Abstract. The annual carbon and water dynamics of two eastern North American temperate forests were compared over a 6-year period from 2012 to 2017. The geographic location, forest age, soil, and climate were similar between the two stands; however, stand composition varied in terms of tree leaf-retention and shape strategy: one stand was a deciduous broadleaf forest, while the other was an evergreen needleleaf forest. The 6-year mean annual net ecosystem productivity (NEP) of the coniferous forest was slightly higher and more variable (218±109 g C m−2 yr−1) compared to that of the deciduous forest NEP (200±83 g C m−2 yr−1). Similarly, the 6-year mean annual evapotranspiration (ET) of the coniferous forest was higher (442±33 mm yr−1) than that of the deciduous forest (388±34 mm yr−1), but with similar interannual variability. Summer meteorology greatly impacted the carbon and water fluxes in both stands; however, the degree of response varied among the two stands. In general, warm temperatures caused higher ecosystem respiration (RE), resulting in reduced annual NEP values – an impact that was more pronounced at the deciduous broadleaf forest compared to the evergreen needleleaf forest. However, during warm and dry years, the evergreen forest had largely reduced annual NEP values compared to the deciduous forest. Variability in annual ET at both forests was related most to the variability in annual air temperature (Ta), with the largest annual ET observed in the warmest years in the deciduous forest. Additionally, ET was sensitive to prolonged dry periods that reduced ET at both stands, although the reduction at the coniferous forest was relatively larger than that of the deciduous forest. If prolonged periods (weeks to months) of increased Ta and reduced precipitation are to be expected under future climates during summer months in the study region, our findings suggest that the deciduous broadleaf forest will likely remain an annual carbon sink, while the carbon sink–source status of the coniferous forest remains uncertain.
Highlights
Temperate forests play a significant role in the global carbon and water cycles through their photosynthetic CO2 uptake and through their evapotranspiration (ET) (Huntington, 2006; Houghton, 2007)
The geographic location, forest age, soil characteristics, and climate were similar in both stands, where one was an evergreen needleleaf conifer plantation while the other was a naturally regenerated deciduous broadleaf forest
Due to the different leaf strategies, the mean start of the growing season for our conifer forest began over a month earlier than the deciduous forest, with greater interannual variability observed in the conifer forest, especially in years with warm spring conditions
Summary
Temperate forests play a significant role in the global carbon and water cycles through their photosynthetic CO2 uptake and through their evapotranspiration (ET) (Huntington, 2006; Houghton, 2007). In eastern North America, temperate forests are a significant sink of carbon and are an important element of future climate mitigation strategies; these forests have been going through transformations due to both natural and anthropogenic impacts for quite some time (Bonan, 2008; Cubasch et al, 2013; Weed et al, 2013). Beamesderfer et al.: Multiyear carbon and water flux responses to meteorology and phenology reforested through natural regrowth and afforestation practices (Canadell and Raupach, 2008). Much of the forested area within the mixed-wood plains ecozone in the Great Lakes region of Canada and the USA is comprised of reforested or plantation stands which are in different stages of growth (Wiken et al, 2011)
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