Investigating Infiltration Areas and Infiltration Funneling Ratios for American Beech Trees

Abstract:

Twelve stemflow simulation runs were conducted on four American beech trees (Fagus grandifolia Ehrh.) to investigate the size of stemflow infiltration areas and corresponding infiltration funneling ratios. The American beech trees were situated within the Fair Hill Natural Resources Management Area in north-eastern Maryland (39° 42’ N, 75° 51’ W) and had a mean diameter at breast height (DBH) of 28.9 ± 0.6 cm. Simulations were run using the volume of stemflow expected from a rainfall of 12 mm but under different intensities. Stemflow input rates were 31, 72, and 290 L h^-1, corresponding to rainfall intensities of 10, 24, and 96 mm h^-1, respectively. Infiltration areas (dye area on the ground) were always < 0.1 m² tree^-1, averaging 0.0288 m² tree^-1 (range = 0.0035 m² tree^-1 to 0.0950 m² tree^-1). With a 10% increase in initial soil moisture content, infiltration areas decreased by -0.005 to -0.03 m² tree^-1. Median saturated hydraulic conductivity of the surface soil proximal to the boles on a per tree basis ranged from 131 mm h^-1 to 214 mm h^-1 using the ROSETTA pedotransfer function taking soil texture and bulk density into account. Saturated hydraulic conductivity values were much greater when measured in situ with a single-ring infiltrometer, with median values per-tree ranging from 3441 mm h^-1 to 18520 mm h^-1. Simulated infiltration areas were greatly overestimated (averaging 2570%) when dividing the stemflow input rates by the median saturated hydraulic conductivity values derived using the Rosetta model, while infiltration areas simulated using the median single-ring infiltrometer conductivity values were underestimated by an average of 35%. The better agreement between simulated and observed infiltration areas using the single-ring infiltrometer derived conductivities is attributed to soil macropore and preferential flow being taken into account, while the ROSETTA model values are for matrix flow only. The observed infiltration areas resulting from the simulated stemflow volumes corresponded to infiltration funneling ratios of 32, 258 and 76 for the 290, 72 and 31 L h^-1 stemflow rate simulation runs, respectively. As such, the depth of stemflow water infiltrating into the proximal area around the tree bole was found to be one to two orders of magnitude greater than the depth of rainfall incident on the canopy.