Changes in energy balance and total evaporation with age, and between two commercial forestry species in South Africa

Expansion of the area planted to eucalypts has been observed in the last two decades due to an improvement in markets for products from this tree species. This has raised concerns over the management of freshwater resources as other species are replaced by Eucalyptus, which has been shown to use more water than other commercial forestry species. The energy balance (EB) and total evaporation (ET) over Acacia mearnsii was previously monitored at the Two Streams research catchment, and the site harvested in 2018 with subsequent re-planting of E. dunnii. This presented an opportunity to measure the two-year-old E. dunnii (Edun(2)) EB and ET for comparison on the same site with the previously planted A. mearnsii with results from two-year-old A. mearnsii (Amear(2)) and six-year-old A. mearnsii (Amear(6)) crops. ET and EB measurements on Amear(2) were obtained using a large aperture scintillometer, while eddy covariance was used for Amear(6) and Edun(2). Measurements were conducted in October 2007 to September 2008, October 2012 to September 2013 and October 2019 to September 2020 for Amear(2), Amear(6) and Edun(2). The leaf area index (LAI) was measured using a LAI 2200 plant canopy analyser for all crops. The annual plantation water productivity (PWPWOOD) was calculated as a ratio of productive stand volume to ET for Amear(2), Amear(6) and Edun(2). Results showed that latent energy fluxes dominated the EB in all crops for both summer and winter seasons, indicating a possibility that trees were not limited by plant available water in winter (dry season). The Edun(2) and Amear(2) annual ET was statistically (p > 0.05) similar, while ET of the younger crops (Amear(2) and Edun(2)) was 12% greater than Amear(6). High ET in Edun(2) was caused by high LAI while Amear(2) was caused by high transpiration per unit leaf area in young trees than in mature trees. Monthly crop factors were derived from FAO ETo and ET for all three crops, providing a convenient and transferable method of estimating ET from meteorological data over a large scale. The Edun(2) PWPWOOD was greater than Amear(2), while Amear(6) was greater than both the young crops. This study provides insight into the total water-use by different species at different stages of growth at the same site. It is recommended that catchment water balance measurements be continued on the current E. dunnii crop for the full crop rotation to assess the long-term impact of E. dunnii on streamflow.