The impact of the canopy structure on the spatial variability in forest floor carbon stocks

Forest ecosystems play an important role in the global carbon (C) cycle, acting as a carbon sink. Data on soil organic matter (SOM) stocks and turnover rates in relation to vegetation properties are important to understand carbon storage distribution mechanisms and to upscale carbon stocks to the stand scale. The canopy structure may influence the spatial variation of carbon input, firstly, by governing the throughfall and litterfall onto the organic layer, and, secondly, by influencing the climatic factors in the organic layer such as temperature and water content dynamics. Those climatic factors in turn have an impact on carbon mineralisation. In this study we investigated the influence of the canopy structure of a 55-year-old pine stand on the spatial distribution of forest floor carbon stocks (FFCS). The results showed that the spatial variation of FFCS was relatively low (cv 25%). The spatial variation of FFCS was partly caused by the small variation in needle litterfall (cv 8.8%) as a result of the heterogeneous canopy structure. The needle litterfall was significantly higher (7.5%) directly under the tree crowns compared with the areas without canopy cover. Also, the nutrient concentrations of the rain throughfall were significantly higher and its pH values were significantly lower under the crowns compared with the areas under the canopy gaps. However, the abiotic factors (pH, ion concentrations, N content, and C/N ratio) of the organic material sampled from the forest floor did not show any significant differences with respect to areas under tree crowns or in canopy gaps. Additionally, neither the carbon mineralisation nor enzyme activities of various steps of the mineralisation processes were related to the canopy structure. A geostatistical analysis revealed that the ranges of the FFCS and the thickness of the F layer were similar to the ranges of the crown structure. We concluded, firstly, that the needle litterfall is the main factor causing differences in FFCS and systematically varied as a function of tree canopy structure. Secondly, the reason for the small variations in litterfall and in carbon stocks was the high cover fraction of about 76% of the relatively young pine stand. We hypothesise that the litterfall variation will increase with the thinning of the stand. Finally, for upscaling the FFCS from the sample to the forest stand scale, the canopy structure can be neglected for pine stands with a cover fraction larger than 70–80%.