Connecting silvan and lacustrine ecosystems: transport of carbon from forests to adjacent water bodies

Abstract

The carbon cycle and hydrological cycle are closely connected and combine terrestrial and aquatic ecosystems. This study focuses on important processes of the carbon cycle at plant, ecosystem and landscape levels. Carbon allocation was investigated at the seedling scale with microcosm experiments, and carbon fluxes, especially the lateral carbon fluxes from soil to adjacent water bodies, at field sites. The carbon allocation pattern differed between typical boreal tree species, but an increase in temperature did not change the net growth of seedlings, because both photosynthesis and respiration increased similarly and compensated for each other. A higher temperature did not change the species composition of ectomychorrhizal fungi, but some symbiotic fungal species can alter carbon allocation at the plant scale. This study demonstrates that CO2 efflux from the soil is largely controlled by biological processes (i.e. the rate of photosynthesis and decomposition), whereas aquatic CO2 emissions are mostly affected by physical forces (i.e. convection controlling stratification). Lateral carbon flux from soil to the study lake and brook was regulated by hydrology and closely connected to the riparian zone. DOC concentrations in the brook were controlled by precipitation and DOC concentrations in the soil, and rain events increased CO2 concentrations both in the riparian zone and in the brook. The large water volume of the lake buffered it against changes. It is of crucial importance to consider terrestrial and aquatic ecosystems together, since lakes and rivers act as significant pathways for terrestrially bound carbon back to the atmosphere. In the natural old-growth forest of this study, lateral carbon transport accounted for 50% and brook discharge for 19% of the terrestrial net ecosystem exchange. Thus, exclusion of the lateral carbon flux would lead to overestimation of the role of the forest as a carbon sink. However, the role of lateral transport can be less important in younger or managed forests, which are faster growing.