Transpiration rates from mature <i>Eucalyptus grandis</i>&thinsp; × &thinsp;<i>E. nitens</i> clonal hybrid and <i>Pinus elliottii</i> plantations near the Two Streams Research Catchment, South Africa

<p>Pine plantations are the dominant species currently planted within the South African commercial forestry industry. Improvements in bio-economy markets for dissolving wood pulp products have seen an expansion in fast-growing <i>Eucalyptus</i> plantations due to their higher productivity rates and better pulping properties than pine. This has raised concerns regarding the expansion of <i>Eucalyptus</i> plantations and how they will affect water resources as they have been reported to have higher water use (quantified using transpiration rates) than pine. We measured transpiration rates (mm yr<span class="inline-formula">⁻¹</span>), diameter at breast height (quantified as quadratic mean diameter, <span class="inline-formula"><i>D</i>_q</span>, m) and leaf area index of an 8-year-old <i>Eucalyptus grandis</i> <span class="inline-formula">×</span> <i>Eucalyptus nitens</i> clonal hybrid (<i>GN</i>) and a 20-year-old <i>Pinus elliottii</i>. Transpiration rates were measured for two consecutive hydrological years (2019/20 and 2020/21) using a heat ratio sap-flow method, calibrated against a lysimeter. In the 2019/20 year, annual transpiration for <i>P. elliottii</i> exceeded <i>GN</i> by 28 %, while for the 2020/21 hydrological year, there was no significant difference between the transpiration of the two species, despite a 17 % and 21 % greater leaf area index for <i>P. elliottii</i> than <i>GN</i> in 2019/20 and 2020/21 measurement years respectively. Quadratic mean diameter increments were statistically similar (<span class="inline-formula"><i>p</i></span> <span class="inline-formula">&gt;</span> 0.05) in 2019/20, whereas the 2020/21 year produced significant differences (<span class="inline-formula"><i>p</i>&lt;0.05</span>). Tree transpiration is known to be influenced by climatic variables; therefore, a random forest regression model was used to test the level of influence between tree transpiration and climatic parameters. The soil water content, solar radiation and vapour pressure deficit were found to highly influence transpiration, suggesting these variables can be used in future water-use modelling studies. The profile water content recharge was influenced by rainfall events. After rainfall and soil profile water recharge, there was a rapid depletion of soil water by the <i>GN</i> trees, while the soil profile was depleted more gradually at the <i>P. elliottii</i> site. As a result, trees at the <i>GN</i> site appeared to be water stressed (reduced stem diameters and transpiration), suggesting that there was limited access to alternative water source (such as groundwater). The study concluded that previous long-term paired catchment studies indicate that eucalypts use more water than pine; however, periods of soil water stress and reduced transpiration observed in this study must be accommodated in hydrological models. Long-term total soil water balance studies are recommended in the same region to understand the long-term impact of commercial plantations on water resources.</p>