The absorption and evaporation of water vapor by epiphytes in an old‐growth Douglas‐fir forest during the seasonal summer dry season: Implications for the canopy energy budget

Abstract

Our goal was to determine how epiphytic lichens and bryophytes affect canopy latent heat fluxes in an old‐growth Douglas‐fir forest when the canopy was dry. The epiphyte water content (WCe expressed as a percent of dry weight) of representative epiphytic foliose lichens, fruticose lichens, and bryophytes was measured in the laboratory after 1 to 12 hr of exposure at five different values of vapor pressure deficit (VPD). After 12 hr of exposure, WCe increased fivefold to sixfold as VPD decreased from 1849 to 132 Pa. In addition, we measured WCe in the field using strain gauges. These field measurements were used to calibrate the models described below. Two models were created to estimate the potential latent heat flux from epiphytes at the canopy scale (LEe). The first model combined measured total biomass of epiphytes with a model that estimated the laboratory determined VPD‐dependent changes in WCe of the lichens/bryophytes (VPD method). The second model estimated LEe by scaling the change in WCe of epiphyte‐laden branches that were continuously monitored in situ in the canopy by a strain gauge (SG method). Both methods showed a strong diurnal trend in LEe when VPD was less than 645 Pa. Prior to sunrise, the epiphytes absorbed water, corresponding to a latent heat flux of 5 to 15 W/m2 per unit ground area, whereas after sunrise, the epiphytes lost water at a rate of −10 to −20 W/m2. For short periods, epiphytes may contribute a significant portion of the latent heat flux from Douglas‐fir forests.

Stand-Level Variation Drives Canopy Water Storage by Non-vascular Epiphytes Across a Temperate-Boreal Ecotone

“…Forest age is also likely to be an important factor. In contrast to many prior studies in older forests (e.g., Liu et al, 2000;Köhler et al, 2007;Pypker et al, 2017), only one site in the present study was >100 years (Cloquet Forestry Center Pinus resinosa forest). This age bias reflects both natural disturbance from fire and windthrow and a history of logging in Minnesota, such that few centennial stands of forest persist (Friedman and Reich, 2005;Vogeler et al, 2020).…”

“…However, despite their potential importance, studies of the hydrological role of epiphytes have been geographically patchy, focusing on ecosystems with very large amounts of vascular or non-vascular epiphytes such as old growth wet temperate (Pypker et al, 2006(Pypker et al, , 2017, tropical montane forests (Chang et al, 2002;Villegas et al, 2008;Ah-Peng et al, 2017) or other exceptionally epiphyte-rich ecosystems (Stanton et al, 2014). For example, while Porada et al (2018) cite a number of studies of epiphyte biomass and water storage from boreal and temperate ecosystems, the majority of these were conducted in old-growth forests in humid coastal or mountainous areas: ideal conditions for the development of dense non-vascular epiphyte cover (Gehrig-Downie et al, 2011).…”

Macroclimate drives growth of hair lichens in boreal forest canopies

“…Epiphytic lichens are important components of forest canopies world‐wide and contribute substantially to ecosystem function (Asplund & Wardle, 2017; Porada et al., 2018). They act as microclimate buffers by modifying temperature and evaporation from their host plants (Pypker et al., 2017; Stanton et al., 2014), offer fodder and habitat for animals (Seaward, 1988) and are indicators of forest ecosystem integrity (Nimis et al., 2002). Hair ( Alectoria , Bryoria) and beard lichens ( Usnea , hereafter also referred to as hair lichen) form conspicuous epiphytic communities in boreal forests (Ahti, 1977; Boudreault et al., 2015; Hauck, 2011).…”

Precipitation Partitioning—Hydrologic Highways Between Microbial Communities of the Plant Microbiome?

“…Microbiota may rely on dissolved nutrients in net precipitation waters, having been found to consume ~90% of dissolved organic matter transported by throughfall and stemflow within 1-4 days, for example (Howard et al 2018). Finally, evaporation from wet canopy and litter surfaces exerts a significant influence over humidity dynamics throughout the plant microbiome (Pypker et al 2017;Van Stan et al 2017b). Pathogenic disease susceptibility depends on a "triangular A c c e p t e d m a n u s c r i p t interaction" between microbial pathogens, plants and environmental factors, called the "disease triangle" (Stevens et al 1960), within which the role of humidity has been well researched (Colhoun 1973;Huber and Gillespie 1992).…”