Reply on RC1

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Pine plantations are the dominant specie currently planted within the South African commercial forestry industry. Improvements in bioeconomy markets for dissolving wood pulp products have seen an expansion in fast-growing <em>Eucalyptus</em> plantations due to their higher productivity rates and better pulping properties than pine. This has raised concerns regarding the expansion of <em>Eucalyptus</em> plantations and how they will affect water resources as they have been reported to have higher transpiration (T) and total evaporation rates (ET) than pine. We compared T (mm), diameter at breast height (DBH, cm) and leaf area index (LAI) of an eight-year-old <em>Eucalyptus grandis</em> x <em>Eucalyptus nitens</em> clonal hybrid (<em>GN</em>) with twenty-year-old <em>Pinus elliottii</em>. Transpiration was measured for two consecutive seasons (2019&rsquo; 20 and 2020&rsquo; 21) using a heat ratio sap-flow method. The ET was calculated using published values of soil evaporation and rainfall canopy interception to quantify the impact of each species on water resources. In 2019&rsquo; 20 season, annual T for <em>P. elliottii</em> exceeded <em>GN</em> by 28 %, while 2020&rsquo; 21 season showed no significant differences. This was associated with 17 and 21 % greater LAI for <em>P. elliottii</em> than <em>GN</em> in 2019&rsquo; 20 and 2020&rsquo; 21 season, respectively. Dq increments were statistically similar (<em>p</em> &gt; 0.05) in 2019&rsquo; 20 season, whereas the 2020&rsquo; 21 season produced significant differences (<em>p</em> &lt; 0.05). Transpiration for <em>P. elliottii</em> showed a strong (R² &gt; 0.70) linear relationship with solar radiation, LAI and shallow soil matric potential, while <em>GN</em> had a good (R² &gt; 0.70) relationship with solar radiation only. The soil water potential was very low at the <em>GN</em> site, indicating that the site was water stressed, with trees competing for water as soon as it becomes available to sustain T, causing a rapid soil water depletion after rainfall, while <em>P. elliottii</em> used water more gradually. <em>P. elliottii</em> estimated ET was 18 % greater than <em>GN</em> in 2019&rsquo; 2020, with no significant differences in 2020&rsquo; 21 season. Results from this study indicated that on water limited sites, T and ET between <em>GN</em> and <em>P. elliottii</em> may not be different, however, in subtropical regions, <em>GN</em> T and ET have the potential to exceed <em>P. elliottii</em>, causing soil water depletion. Long-term total soil water balance studies in the same region would be beneficial to understand the impact of long-term commercial forestry on water resources.