The effect of understory invasive species removal on transpiration, soil moisture, and forest canopy leaf area in a northeastern U.S. forest.

Mechanical removal of invasive species is a common management technique to promote growth of native tree species and seedling establishment. Plant removal can alter forest transpiration and canopy structure, leading to changes in forest water and carbon cycling. We examined the influence of mechanical removal of common buckthorn (Rhamnus cathartica) and bittersweet vine (Celastrus orbiculatus) in a green ash (Fraxinus pennsylvanica) dominated suburban forest in central New York State. We measured impacts on plant transpiration, leaf area, and soil microclimate. Sap flow, soil moisture (0-14 cm), and soil temperature (6 cm) sensors were established in a removal and control plot. Leaf allometry and multispectral images from a small Uncrewed Aerial System (sUAS) were used to quantify changes to canopy indices and leaf area. Soil was drier and warmer following the reduction in leaf area in the removal plot compared to the control plot. Green ash daily transpiration normalized by canopy leaf area (L m-2 day-1) did not differ between the control and removal plots. Understory leaf area was reduced as a result of buckthorn removal. The average Normalized Difference Vegetation Index (NDVI) in the removal plot slightly declined, and NDVI decreased by up to 0.4 (-) in regions of the removal plot previously dominated by buckthorn. The negligible impact of invasive species removal on green ash transpiration suggests competition for water between ash and understory species may be minimal. The loss of leaf area from understory plant removal reduces stand scale transpiration. We show that indices commonly associated with leaf area derived from sUAS can be limited to the dominant canopy overstory, potentially underestimating reductions in the area of leaves responsible for transpiration. Although impacts on adult ash transpiration were minimal, changes in soil microclimate suggest that invasive understory species removal affects soil water balance and biogeochemical cycles.