Project Title: Hydrology of the vegetation on vegetation: Comparison and scaling of rainfall interception and solute alteration by common arboreal epiphytes.
*This research is being conducted in collaboration with the research groups of John Van Stan (Georgia Southern) and Clifton Buck (University of Georgia)*
Forests cover ~1/3 of earth’s land surface and 100% of some critical watersheds. Forest cover itself, is also often covered—by lichens, ferns, bromeliads, and other epiphytes. Epiphytes can enable forests to intercept up to 50% of annual rainfall & significantly enrich or deplete rain chemistry, altering water & nutrients available for runoff & ecosystem functions. Despite its importance, epiphyte-rainfall interactions are rarely measured & not well represented in hydrologic models—especially compared to other canopy elements (like leaf and woody area indices). We will address this knowledge gap by examining, comparing and scaling hydrologic & solute dynamics for three epiphyte groups that have dramatically contrasting water exchange mechanisms-lichens, ferns, and bromeliads. Despite substantial differences in form and function, these epiphyte groups are common across a number of temperature and tropical forest ecosystems and therefore have the potential to have a large impact on canopy water storage and cycling. This information is not only hydrologically valuable, but would provide critical insights about the degree to which climate change (i.e., rainfall frequency/magnitude) will affect the epiphyte communities themselves, their host forests via canopy microclimatological modifications, and, ultimately, the ecosystem services they provide.
Arboreal epiphytes have a diversity of water and nutrient exchange mechanisms that differentially impact rain partitioning and chemical alteration. Nevertheless, no study has simultaneously examined epiphytes of differing common paraphyletic groups for comparison of canopy ecohydrological impacts during rainfall. This research is taking place on Skidaway Island in Georgia (USA), and in the Plant Growth Facility at Franklin and Marshall College. The forests of Skidaway are dominated by Live Oak, Quercus virginiana, which hosts the three epiphyte groups mentioned above in high abundance (see photo above). These epiphytes provide the unique opportunity to investigate canopy ecohydrological processes during storms for epiphytes (a) that lack the ability to regulate internal water content (poikilohydric lichen, Fig. 1-left), (b) that contain mechanisms to control water exchange (homoiohydric bromeliad, T. usneoides, Fig. 1-right), and (c) that have limited water loss control (partially poikilohydric fern, P. polypodioides, Fig. 1-center). Lacking comparisons of rain absorption, storage, evaporation, drainage and chemical alteration dynamics across a continuum of epiphytes’ water control strategies introduces significant uncertainty at the start of the terrestrial hydrologic pathway.
We propose to address this uncertainty through monitoring, analytical, & scaling activities under 4 objectives:
Objective 1) Assess/compare storage, evaporation and drainage for epiphytes of differing ecohydrological traits.
Objective 2) Evaluate ecohydrological traits that underlie epiphyte water balance and determine how drying events will affect the survival and interception capacity of the canopy community.
Objective 3) Quantify/compare alterations to rainfall solutes by epiphytes from major paraphyletic groups.
Objective 4) Scale findings to estimate relative stand-level influence of epiphytes on net rainwater/solute fluxes.
This work will provide much needed information to extend hydrological (and related biogeochemical/meteorological) theory by improving estimation of critical parameters at the start of the rainfall-to-runoff pathway. We will employ hydrochemical and ecohydrological techniques alongside interstorm sampling and in-canopy epiphyte monitoring efforts to capture spatiotemporal relationships between storm-specific biotic and abiotic variables affecting epiphytes’ rainfall interception, redistribution and chemical alteration. An estimate of impacts for epiphytes along the range of water control strategies (Fig. 1) on coupled canopy water-nutrient budgets answers long-standing calls from the forest ecohydrological community to better constrain major climate-vegetation feedbacks (regional-to-global temperatures) and vegetation-soil interactions (above-to-belowground nutrient returns).
Figure 1. (top) Study epiphytes, (middle) their leaf anatomy (hand-sectioned with safranin stain for contrast, and viewed 100x magnification), and a comparison of their water relations (poikilohydry-homoiohydry), relevant anatomy, and water absorption capability. *preliminary data.
Figure 2. Hypothesized canopy (a) water and (b) solute alterations during a rain event with and without epiphytes.