Canopy soils form through the accumulation of decomposing organic matter on tree branches in tropical wet forests and temperate rainforests. These highly organic soils support hyper-diverse communities of epiphytic plants and represent a potentially important but unquantified component of the forest carbon stock. While a few studies have investigated the influence of small-scale factors such as branch angle and microclimate on canopy soil accumulation, knowledge about canopy soil distribution on a larger scale is lacking. Furthermore, the structure of canopy soil organic matter has never been investigated in tropical regions nor across large spatial scales. Patterns of canopy soil abundance and chemical structure may shed light on the factors driving its accumulation, as well as its vulnerability to climate change. We conducted the first survey of canopy soil distribution, abundance, and molecular composition in six sites across two Costa Rica mountain ranges which spanned large gradients of temperature (11 – 25°C ) and precipitation (2300 – 4000 mm). Canopy soil abundance was surveyed along a 100 m x 10 m transect on all trees over 50 cm DBH at each site and canopy soil was sampled from 6 trees within each site. Molecular composition was determined via high-resolution mass spectrometry.
Results/Conclusions
Canopy soil abundance exhibited a significant negative correlation with mean annual temperature (p<0.001), which means that warmer sites may have either higher canopy soil decomposition rates or lower rates of organic matter inputs to canopy soils compared to cooler sites. As temperatures rise in tropical montane sites where canopy soil is most abundant, canopy soil abundance may decrease due to either increased decomposition or decreased organic matter inputs. The number of aromatic and condensed aromatic compounds, an indicator of decomposability, did not change across the sites (with the exception of one site), suggesting that decomposition rates would be similar at equal temperature and moisture. The molecular diversity of compounds in canopy soils differed by site but showed no clear increasing or decreasing trend with elevation or mean annual temperature and precipitation. Canopy soil abundance increased significantly with tree diameter (p=0.003), which reflects the fact that canopy soil mats take a long time to develop and are found almost exclusively in old-growth forests. Together these findings highlight the vulnerability of canopy soils and the diverse epiphyte communities they support to warming temperatures and loss of primary forest in tropical regions.