A Review and Evaluation of Forest Canopy Epiphyte Roles in the Partitioning and Chemical Alteration of Precipitation

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Science of the Total Environment




Material and energy exchange at the Earth's surface is drastically altered by the presence of forest canopy cover. Canopy structures that control this exchange (leaves, branches, bark, epiphytes, etc.) differentially alter the amount, spatiotemporal patterning, and solute concentration of precipitation reaching the surface (Levia et al., 2011; Pypker et al., 2011). Precipitation over forest canopies is either intercepted and evaporated, or reaches the surface below via gaps and drips (throughfall, Levia and Frost, 2006) and concentrated flow down the tree trunk (stemflow, Levia and Frost, 2003). The sum of precipitation that reached the ground (as throughfall and stemflow) is called net precipitation. Amount, type, spatiotemporal configuration, and composition of canopy elements control the proportion of precipitation partitioned into interception or net precipitation, as well as the solute concentration and spatiotemporal patterning of net precipitation at the forest floor (Pypker et al., 2011). Of the various canopy surface types, epiphyte cover arguably has received less attention from the precipitation partitioning research community (Levia and Frost, 2003, 2006). This is surprising as epiphytes are ubiquitous across forest types (e.g., Hölscher et al., 2004; Husk et al., 2004; Zotz, 2005; Pypker et al., 2006a; Hauck, 2009; Lundström et al., 2013; Van Stan et al., 2015), and their coverage, patterns, and forms can significantly alter canopy structural attributes, by (1) closing canopy gaps and connecting edges across, and branches within, individual trees (Fig. 1a), (2) filling voids in branch crotches and tree holes (Fig. 1b), (3) increasing area of “stable” above-ground biomass structures (Fig. 1c), and (4) ultimately changing the vertical biomass distribution of the host forest (Fig. 1d). This, in conjunction with epiphytes' diversity and uniqueness of nutrient acquisition mechanisms (e.g., Pittendrigh, 1948; Madison, 1977; Martin, 1994), illustrates the need for work to evaluate epiphyte impacts on canopy precipitation partitioning and its solute dynamics.