Seasonal Variation in Water Status and Growth of Pinus taeda L., Under Different Silvicultural Management in Northeastern Uruguay

Abstract

Thinning and pruning modify water availability, and therefore growth of forest plantations, although it is not clear that the silvicultural management would affect equally stands with different levels of soil water availability. The effect of first thinning (40 %) and pruning (50 %) (May/2008) on water potential and growth were evaluated in a five-year-old stand of Pinus taeda L. in Tacuarembó, Uruguay (31º 38’S, 55º 41’W). Experimental and control plots were installed at three sites, two with deep soils and one with shallow soils. Predawn (Ψpd) and midday (Ψmd) needle water potential, diameter at breast height, and stem height were measured in each plot every six weeks, from July/2008 to January/2010. Vapor pressure deficit (Vpd) on stand basis and relative evapotranspiration (RET) on site basis were calculated. Treatment reduced Ψpd during the highest RET-deficit period (autumn/2009) and its effect lasted longer on plots with shallow soils. Ψmd was lower on deeper-soil plots (-2.51 vs -2.09 ± 0.34 MPa) probably due to higher water availability and therefore, higher transpiration rates (ΔΨw). Ψpd and Ψmd were linearly related up to Ψpd values near -1.00 MPa but there was no relationship during the highest RET-deficit period. Relative growth rate (RGR) and ΔΨw were linearly related but RGR was not modified either by site or silvicultural treatment. These results suggest that thinning and pruning increased transpiration activity.