EFFECTS OF THINNING ON TRANSPIRATION BY RIPARIAN BUFFER TREES IN RESPONSE TO ADVECTION AND SOLAR RADIATION

Upwind advective energy occurring in edge environments may increase tree water use. In humid agricultural landscapes, advection-enhanced transpiration in riparian buffers may provide hydrologic regulation. The objectives of this study were to determine how water use by trees growing in a riparian buffer in central Iowa is influenced by 1) advective energy, 2) tree position, and 3) thinning (40% LAI reduction). We measured meteorological variables and sap flux density from July to September in 13 trees (2009) and 12 trees (2010) in one thinned plot (TP, treated in Aug 2010) and one untreated (control) plot (UP). The difference in Q(s) between edge and interior trees (39% higher in 2009) was attributed to the advective energy at the buffer edge. After thinning, maximum Q(s) increased was greater in TP compared to UP, explained primarily by solar radiation (R-2=0.7, p<0.05), since meteorological conditions were not optimal for advection. However, the LAI reduction counteracted the increase in whole tree Q(s), such that post-treatment plot transpiration per LAI measured from TP (0.4 mm day(-1)) was less than for UP (0.5 mm day(-1)).