Dominant clonal Eucalyptus grandis × urophylla trees use water more efficiently

• Are dominant trees more or less efficient at using water to produce wood?
• Dominant trees grew more, transpired more water and were more water use efficient.
• This pattern was the same in irrigated and rainfed stands for the same clone.
• Manipulating stand density may minimize the tradeoffs between growth and water use.

Wood growth in trees depends on the acquisition of resources, and can vary with tree size leading to a variety of stand dynamics. Typically, larger trees obtain more resources and grow faster than smaller trees, but while light has been addressed more often, few case studies have investigated the contributions of water use and water use efficiency (WUE) within stands to isolate the tree-size dominance effect. Our sites were located near the cities of Aracruz and Eunapolis in Northeastern Brazil. We measured tree biomass growth, water use and WUE to explore patterns of growth among dominant and non-dominant trees in rainfed (1350 mm yr−1) and irrigated experimental stands in two high productivity tropical clones of Eucalyptus grandis × urophylla growing in clayey Ultisol soils. During the study period, irrigation supplied an additional 607 mm and 171 mm at the Aracruz and Eunapolis sites respectively. We tested two hypotheses; (1) larger trees transpire more water, and produce more wood per water used (higher water use efficiency, WUE) than smaller trees of the same clone and; (2) this pattern also applies if a water surplus is added via irrigation to alleviate water stress.Across both sites, we measured stand water use using sap flow sensors from August to December, and quantified wood growth on a tree-basis and then derived WUE, in kg wood per m3 of water transpired. Dominant trees showed higher rates of tree growth, water use and WUE than dominated trees for the two sites-clones and under both water supply regimes. Using the rainfed trees at Aracruz as an example, 50-kg trees grew 1.0 kg month−1 compared with growth of 100-kg trees of 3.8 kg month−1. The smaller trees would use water in a rate of 2.1 m3 month−1, compared with 3.1 m3 month−1 for the larger trees, demonstrating a higher WUE for the larger tree (1.2 kg m−3 versus 0.5 kg m−3). Our results suggest that manipulating stand density on heterogeneous stands, e.g. thinning, has the potential to minimize the tradeoffs between wood growth and tree water use in Eucalyptus grandis × urophylla plantations, mainly in tropical regions with seasonal water deficit. However, more research is needed to discern the underlying mechanisms responsible for higher WUE exhibited by dominant trees and distinct clones.