Forest-air fluxes of carbon, water and energy over non-flat terrain

A field study of surface-air exchange of carbon, water, and energy was conducted at a mid-latitude, mixed forest on non-flat terrain to investigate how to best interpret biological signals from the eddy flux data that may be subject to advective influences. It is shown that during periods of Southwest winds (sector with mild topography), the eddy fluxes are well-behaved in terms of energy balance closure, the existence of a constant flux layer, consistency with chamber observations and the expected abiotic controls on the fluxes. Advective influences are evident during periods with wind from a steep (15%) slope to the Northeast of the tower. These influences appear more severe on CO2 flux, particularly in stable air, than on the energy fluxes. Large positive flux of CO2 (> 23 mumol m(-)2 s(-)1) occurs frequently at night. The annual sum of the carbon flux is positive, but the issue about whether the forest is a source of atmospheric carbon remains inconclusive. Attempts are made to assess vertical advection using the data collected on a single tower. Over the Southwest sector, vertical advection makes a statistically significant but small contribution to the 30-min energy imbalance and CO2 flux variations. Contributions by horizontal advection may be larger but cannot be verified directly by the current experimental method.