Eucalyptus tree stockings effect on water balance and use efficiency in subtropical sandy soil

Abstract

Eucalyptus tree stocking can be adjusted to increase, depending on purpose, cellulose, timber or bioenergy production, but the effect of stocking on ecosystem water balance is still understudied. Studies on soil and catchment hydrology and forest management, encompassing distinct climatic years, are needed for understudied ecosystems to improve water use efficiency and streamflow regulation, and thus bridging forestry and ecosystem processes. For different Eucalyptus dunnii stockings, we measured rainfall dynamics to quantify rainfall partitioning, soil water balance, and eucalyptus water use efficiency in a sandy soil. Total rainfall, throughfall, stemflow, canopy interception, evapotranspiration, tree growth, and water use efficiency were evaluated over a 1-year study period (5th to 6th years after planting), in low (LS: 816 trees ha−1), medium (M: 1,633 trees ha−1), medium–high (MH: 3,265 trees ha−1), and high tree stockings (HS: 6,568 trees ha−1). Tree stocking had no clear relation with the partitioning of annual rainfall (P, 2583 mm) into canopy interception (34–37% of P) and effective rainfall (63–66% of P). By increasing tree stocking from 816 to 6,568 trees ha−1, throughfall decreased from 62 to 53% of P, and stemflow increased from 2.1 to 12.7% of P. Estimated deep drainage was <0.16% of P, even though the annual rainfall was almost double of historical rainfall and surface runoff was only 5.7% of P. Thus, groundwater recharge in Eucalyptus plantation may be potentially low even with high rainfall in a soil with high infiltration and low slope (about 4%). Cumulative evapotranspiration varied between 1304 and 1438 mm (50–56% of P), highest for high tree stocking, whereas daily evapotranspiration rates varied between 0.8- and 11.7-mm d−1. From the 5th to the 6th year after planting, the individual wood production varied from 4.9 (HS) to 51.8 kg−1 tree (LS), and stand wood production varied from 27.1 (MHS) to 42.2 (LS) Mg ha−1 yr−1. Water use efficiency varied from 2.1 to 3.1 g L−1, respectively for MHS and LS. Industrial eucalyptus planted in sandy soils at high stocking for energy production may overuse stored water, affecting long-term forest sustainability. Therefore, the challenge is decreasing evaporation from soil and canopy and increasing green-water use efficiency, while generating surface runoff for ecological flows and groundwater recharge (blue water flow).