Human land use has perturbed the surface energy balance of temperate forests around the world by altering their biophysical traits. Such alterations in surface albedo, canopy roughness, and transpiration are most commonly associated with forest clearing. However, some historically cleared forests have returned over the past several decades, undergoing a ‘forest transition’ that is becoming increasingly common in temperate regions. Due in part to changing disturbance regimes, second growth forests often bear little resemblance to the historic forests that they replace. The resulting changes in species composition and stand structure alter canopy biophysical properties differently than forest clearing, but their effects remain understudied due to the multi-decadal and emergent nature of forest transitions. We address this gap by monitoring transpiration (via sap flux probes), canopy surface temperature, and energy fluxes in two forests representative of pre- and post-transition forests. We hypothesize that younger, more deciduous second growth forest would elicit cooler surface temperatures through greater canopy transpiration.
Results/Conclusions
Differences in canopy surface temperature were consistent with differences in transpiration but were also influenced by other biophysical differences among sites. Second growth forest transpired more and had cooler (relative to air) surface temperatures than primary forest in the growing season. Greater transpiration in second growth forest was largely attributable to vigorous water transport in sugar maple (Acer saccharum) and basswood (Tilia americana), the dominant species at the site. Cooler surface temperatures in second growth forest were enhanced by lesser net radiation, due in part to increased albedo, but offset by greater roughness and outgoing turbulent heat flux in pre-transition forest. When the impacts of net radiation, roughness, and transpiration were incorporated into a single model, transpiration was a small but significant driver of surface cooling. Our findings show that the shifts in composition and structure accompanying forest transition indeed influence surface energy balance, leading to cooler canopy surfaces in second-growth forests.