Patterns of moisture and temperature in canopy and terrestrial soils in a temperate rainforest, Washington

Abstract

The temperate rainforest on the Olympic Peninsula represents a biome that is characterized by unusually high epiphyte biomass and accompanying canopy soils. These canopy communities form thick mats that play important ecological roles: increasing ecosystem nutrient capital, fostering abundant invertebrates, and enhancing moisture retention. Little is known about the physical properties and microenvironmental conditions of these mats. We investigated seasonal patterns in temperature and moisture of canopy soils and compared them to O- and A-horizon terrestrial soils. Temperature tended to fluctuate more in canopy soils than in terrestrial soils. During the 4 months of highest precipitation, canopy soils also showed higher maximum saturation levels than terrestrial soils. Both soil types displayed sharp “dry-downs” during the summer dry season, which contrasts with results from similar research in a tropical cloud forest, where only canopy soils dried significantly during the dry season. In the late summer in the Olympic Rainforest, canopy soils remained dry until the first rainfall, whereas terrestrial soils began to rehydrate a month earlier, possibly through hydraulic redistribution. Regional climate models predict increased winter precipitation but drier summers for this area. Our results suggest that extended summer droughts may increase canopy drying, which may have negative impacts on epiphyte communities.