Water-Holding Capacity of Canopy Soil Mats and Effects on Microclimates in an Old Growth Redwood Forest

Ambrose, Anthony

Abstract: "Ancient redwood (Sequoia sempervirens) tree crowns often contain large accumulations of organic matter, referred to as “canopy soil” mats, which provide substrate and habitat for a rich community of vascular epiphytes, salamanders, arthropods, and other desiccation-sensitive arboreal species. To help understand the ecological role of canopy soil mats in redwood forests, this study characterized environmental conditions associated with soil mats of various sizes within the crown of a large redwood tree growing in Prairie Creek Redwoods State Park, CA. An automated measurement system was installed in the tree that recorded photosynthetically active radiation, air temperature, relative humidity, soil temperature, soil water content, and rainfall in and around six soil mats and above the treetop every hour for one complete year. Atmospheric vapor pressure deficit was calculated using air temperature and relative humidity data. A water balance model was developed to partition water inputs to each sample mat into throughfall and stemflow, and to partition water losses from each mat into drainage and evapotranspiration. Measured rainfall during the study period was 804 mm, although the amount of water reaching the sample soil mats was typically much lower, ranging from 7% to 76% of treetop rainfall. A majority (ca. 72% to 100%) of water inputs to the sample mats arrived as throughfall. Light availability at the sample soil mats followed a strong vertical gradient, with the mats receiving only 1% to 11% of average treetop light levels. Redwood canopy soil mats were found to hold an enormous amount of water throughout the year. Whole-tree water content estimates ranged from almost 2,000 liters of water during the dry season to more than 4,500 liters of water during the wet season. The largest soil mats in the tree held a disproportionate amount of this stored water, although the smallest sample mats retained sufficient water year-round to support epiphytes. Drainage from the mats after rainstorms was responsible for rapid rates (ca. 6.5 mm/day average) of water loss, but a majority (ca. 64%) of the water lost from the mats was attributed to surface evaporation and plant transpiration. Results indicate that arboreal soil mats substantially ameliorated temperature and humidity extremes within the canopy at diurnal, seasonal, and annual time scales. However, the magnitude of this effect varied between seasons, and microclimatic gradients around soil mats primarily reflected the combined influence of mat size, crown position, architectural context, soil water content, and epiphyte abundance. By retaining large amounts of water and nutrients and by moderating microclimatic conditions, canopy soil mats thus appear to contribute to enhanced biodiversity and ecological functioning within old-growth redwood forest ecosystems."