Effects of Planting Density on Transpiration, Stem Flow and Interception for Two Clones Differing in Drought Tolerance in a High Productivity Eucalyptus Plantation in Brazil

Abstract:

Eucalypt plantations cover about 20 M hectares worldwide and expansion is expected to mainly occur in marginal growing areas where dry conditions may lead to water conflicts. One of the principal reasons for the expansion of Eucalyptus plantations is rapid wood growth but these forests also transpire large amounts of water. Genotype selection and planting density, are key factors regulating carbon and water tradeoffs at a stand scale, but few studies have examined these simultaneously especially in highly productive clonal plantations. Our goal in this study was to examine the effects of planting density on carbon and water interactions using a drought tolerant and drought sensitive eucalyptus clone. This work is part of a larger study (TECHS project - Tolerance of Eucalyptus Clones to Hydric and Thermal Stresses) and is located in a flat Oxisol in southeast of Brazil. A drought tolerant (E. grandis x E. camaldulensis (Grancam) and drought sensitive clone E. grandis x E. urophylla (Urograndis) were planted at four densities ranging from 600 to 3.000 stem ha^-1. We measured transpiration using thermal heat dissipation probes, wood growth, canopy interception and stemflow during a full year (21 to 33 months old). Precipitation during the study period was 738 mm. Independently of genetics, growth increased with increasing density. Transpiration also increased with planting density and ranged from 515-595 mm at wider spacing to 735-978 mm at tighter spacing. Interception increased with planting density representing 18-22% of precipitation versus 13-14% in wider spacing while stem flow represented 2-5% in denser spacing and 1-2% at broader spacing. When density was higher than 1.250 and 1.750 stems ha^-1 in Urograndis and Grancam clones, respectively, the water balance were negative. On a stand scale, results show both genetics and spacing can be used as silvicultural tools to better manage the tradeoff between wood growth and water consumption.