Climate, as well as branch-level processes, drive canopy soil abundance and chemistry

Abstract

Canopy soils can be important to forest nutrient cycling, hydrology, and biodiversity, but the factors determining their distribution and properties are largely unknown. We surveyed canopy soils across gradients of temperature and precipitation in six primary forests in Costa Rica. We used solid-state 13C nuclear magnetic resonance (NMR) and mass spectrometry (MS) to understand how the composition of canopy soil organic matter varies within and across sites. Climate, particularly fog, appears to drive canopy soil abundance across forests, while tree size determines canopy soil abundance within a forest. Canopy soil chemistry mostly varied within sites, though temperature was associated with the carbon (C) to nitrogen ratio, total dissolved nitrogen, and alkyl-C abundance, while fog explained some of the variation in dissolved organic carbon and O-alkyl C abundance. This study is the first-ever glimpse into large- and small-scale drivers of canopy soil abundance and biochemical composition. Our results highlight the importance of tree size and fog in determining the quantity and quality of canopy soil organic matter, suggesting that canopy soil stocks may be particularly vulnerable to climate and land use change. Identifying how multi-scale factors influence canopy organic matter processes will enhance our ability to identify and predict how environmental change might affect the abundance and chemistry of canopy soils and thus biodiversity that they support.