Carbon, water, and energy dynamics of a temperate pine forest during the first decade since plantation on a former cropland

Abstract

This study presents the energy, carbon (C), and water exchange dynamics of a recently afforested temperate white pine (Pinus strobus L.) forest, established on former agricultural land in 2002, in southern Ontario, Canada during the initial thirteen years (2003–2015). Our observations show that the forest became a consistent sink of C after only 5 years of its establishment (ranging from 105 g C m–2 to 216 g C m–2 between 2008 to 2015), owing to sandy soils and low residual soil organic matter from prior agricultural activities. This region frequently experiences low precipitation (P) and soil moisture (VWC) limitations and/or heat stress in late summer, causing a reduction in net ecosystem productivity (NEP). Seasonal and annual dynamics of NEP showed reduced C uptake during years with heat and/or drought events (i.e. 2007 and 2012). In 2007, the impact of a seasonal drought was much more exacerbated when combined with a heatwave, resulting in a strong C source. Similarly, the inter-annual variability of evapotranspiration (ET) gradually increased with stand age (mean 370 mm yr–1) and water use efficiency (WUE) consistently increased (mean 2.65 g C kg–1 H2O). Quantum yield, α (0.019 to 0.045) and maximum photosynthetic capacity, Amax (4.37 to 33.6 µmol m–2s–1) increased steadily as the size and density of the canopy increased with stand age. Energy fluxes were influenced by canopy development as net radiation (Rn), latent heat (LE), and sensible heat (H) flux increased, while ground heat flux (G) peaked in 2007 and then gradually declined. Our analysis showed that daily C fluxes are primarily driven by Rn and temperature (Ts, Ta) which explained 47%, 61%, 52%, and 68% of the variability in gross ecosystem productivity (GEP), ecosystem respiration (RE), NEP, and ET. This study is a significant contribution to our understanding of the energy, C, and water dynamics of young planted conifer forests and controls on their growth and C uptake. Our findings demonstrate the potential of utilizing white pine as a means to sequester atmospheric CO2 in southern Ontario and other regions of North America with similar climate and site history.