Temporal Dynamics in the Concentration, Flux, and Optical Properties of Tree‐Derived Dissolved Organic Matter in an Epiphyte‐Laden Oak‐Cedar Forest

Abstract

Studies on the fate and transport of dissolved organic matter (DOM) along the rainfall‐to‐discharge flow pathway typically begin in streams or soils, neglecting the initial enrichment of rainfall with DOM during contact with plant canopies. However, rain water can gather significant amounts of tree‐derived DOM (tree‐DOM) when it drains from the canopy, as throughfall, and down the stem, as stemflow. We examined the temporal variability of event‐scale tree‐DOM concentrations, yield, and optical (light absorbance and fluorescence) characteristics from an epiphyte‐laden Quercus virginiana‐Juniperus virginiana forest on Skidaway Island, Savannah, Georgia (USA). All tree‐DOM fluxes were highly enriched in dissolved organic carbon (DOC) compared to rainfall, and epiphytes further increased concentrations. Stemflow DOC concentrations were greater than throughfall across study species, yet larger throughfall water yields produced greater DOC yields versus stemflow. Tree‐DOM optical characteristics indicate it is aromatic‐rich with fluorescent DOM dominated by humic‐like fluorescence, containing 10–20% protein‐like (tryptophan‐like) fluorescence. Storm size was the only storm condition that strongly correlated with tree‐DOM concentration and flux; however, throughfall and stemflow optical characteristics varied little across a wide range of storm conditions (from low magnitude events to intense tropical storms). Annual tree‐DOM yields from the study forest (0.8–46 g C m−2 yr−1) were similar to other yields from discrete down‐gradient fluxes (litter leachates, soil leachates, and stream discharge) along the rainfall‐to‐discharge flow path.

Interstorm Variability in the Biolability of Tree-Derived Dissolved Organic Matter (Tree-DOM) in Throughfall and Stemflow

“…However, rain event intensities for sampled storms (0.7-1.8 mm h −1 ) were low compared to previous studies at the site (1-31 mm h −1 [11]). Throughfall water yields for these storms were comparable to previous work on the same cedar species [11,28,29].…”

“…However, rain event intensities for sampled storms (0.7-1.8 mm h −1 ) were low compared to previous studies at the site (1-31 mm h −1 [11]). Throughfall water yields for these storms were comparable to previous work on the same cedar species [11,28,29]. Stemflow water yields from epiphyte-covered cedar trees were similar to past studies; however, bare stemflow volumes were higher, >10% for all storms (Table 1), than reported elsewhere, Forests 2018, 9, 236 7 of 10 5-7% of rainfall [28,29], and for two previously-monitored cedar trees at this site, 4.2% of rainfall [11].…”

“…A previous, more comprehensive sampling of storms at the study site, but for fewer cedar trees indicates that the sampled storms (Table 1) nearly represent the full range of typical rain event magnitudes and durations (7-74 mm [11]). However, rain event intensities for sampled storms (0.7-1.8 mm h −1 ) were low compared to previous studies at the site (1-31 mm h −1 [11]).…”

“…Cedars on site host substantial epiphyte biomass, Tillandsia usneoides L. (Spanish moss) (Figure 1c), yet some individual trees were bare. Further site information and a complete site inventory can be found in [11]. …”

“…Sampling of rainfall, throughfall and stemflow was performed for five storms during the 2017 growing season: 14 May, 3 June, 30 July, 28 August and 23 October following published methods [11]. Three bulk rainfall and twenty throughfall samplers consisting of 0.18 m 2 high density polyethylene (HDPE) bins were deployed immediately before each storm.…”