Comparisons of flux-based stemflow enrichment ratios for two Quercus spp. within the megalopolis of the eastern USA

Abstract

Throughfall and stemflow serve as two important transport mechanisms for water and solutes in urban forests, though these fluxes are seldom quantified within cities. This study is the first to utilize two flux-based enrichment ratios for stemflow to characterize spatial patterns in water and solute distribution in urban forest fragments. Using event-based, in situ sampling, this study quantified stemflow enrichment for Quercus rubra (northern red oak) and Quercus alba (white oak) trees relative to open precipitation (EP,B) and throughfall (ET,B) per unit trunk basal area for dissolved Ca, K, Mg, Mn, NO3-N, and S. The study investigated variability in nutrient enrichment at the fragment, municipal, and regional scales. Among all solutes, observations for EP,B and ET,B for Q. rubra and Q. alba were generally lowest for Mg and highest for Mn and K. Significant intra-urban variability in stemflow enrichment was limited to EP,B of K and ET,B of Ca (p < 0.05), while trans-regional variability in stemflow enrichment consistently indicated higher EP,B and ET,B in more highly developed portions of the study region. At the fragment scale, EP,B and ET,B for Q. rubra was consistently higher than for Q. alba, with variability in these observations significant for all solutes. For example, interspecific variability in EP,B was greatest for K, where median values ranged from 2.8 ± 29.7 in Q. alba to 87.1 ± 97.1 in Q. rubra. While observations for ET,B were generally lower than those for EP,B, observations for Q. rubra also consistently exceeded those for Q. alba, with median values for K ranging from 1.5 ± 0.5 to 21.9 ± 3.1 for Q. alba and Q. rubra, respectively. Findings were likely driven by variability in biophysical characteristics between the two species (e.g., bark morphology). Further, findings indicate that species heterogeneity within the urban forest contributes to significant variability in nutrient (and possibly pollutant) transport and fate via throughfall and stemflow below the canopy, with subsequent impacts on urban forest biogeochemistry.