Estimating the integrated degradation rates of woody debris at the scale of a Mediterranean coastal catchment

Woody debris is found in all habitats of the land-sea continuum. While isolated experimental studies of wood degradation exist, few programs have observed the dynamics of wood degradation, in situ across this gradient. Since 2014, we have been conducting a series of long-term observations of wood decay in three characteristic areas of a Mediterranean Sea coastal watershed: forest leaf litter ('Forest'), river bed ('River') and the near-shore marine environment ('Sea'). The study sites are within the Massane River watershed (France) whose headwaters are in a protected beech tree (Fagus sylvatica) dominated forest. Branch sections from a recently fallen beech tree and standardized blocks of Norway spruce (Picea abies) were installed in all three environments. The proportion of remaining mass and volumetric mass of the individual wood samples were determined periodically over 4.2 years. Regardless of wood type, there were marked differences in the decay dynamics. Mass losses at the Forest and River sites were well-described by continuous negative exponential models. At the Sea site, there was a short latency period followed by rapid degradation for the wood fraction exploited by shipworms; in this case, a Weibull-type function was fitted to the data. Integrated mass loss rates at the coastal location were 6 to 20 times faster than in the other two environments. Our study suggests that the early dynamics of wood degradation in a land-sea meta-ecosystem are dominated by the marine invertebrate community. This means woody debris, once it reaches the sea, is likely to break down rapidly within near shore coastal habitats. These results highlight the need to quantify the mass transport dynamics between local ecosystems.