Soil and plant water ages in two Pinus radiata forests with markedly different annual precipitation amounts

Pinus radiata, which grows under a wide range of climate conditions, makes up the majority of the planted forests in New Zealand leading to concerns over the impact of this non-native species on water availability and quality. Transit times of water through the vadose zone reflect water fluxes and affect runoff chemistry. However, little is known on how vadose zone transit times differ for forests of the same species under different precipitation regimes. Our goal here is to evaluate how root water uptake (RWU), water transit times, and groundwater recharge in Pinus radiata plantations differ under variant precipitation. We investigated soil water fluxes and RWU in nine soil profiles in two Pinus radiata forests with greatly differing annual precipitation amounts (2934 mm vs. 725 mm) in New Zealand’s South Island. We estimated water age of vadose zone and xylem water using an isotope-enabled version of the one dimensional hydrological model Hydrus 1-D. We inversely derived the model parameters using a Monte-Carlo simulation with Latin hypercube sampling with times series of soil moisture and soil and xylem water stable isotopes. 
At the dry forest site, we found that transpiration and recharge accounted for over 80%, and around 10% of annual precipitation, respectively. At the wet forest site transpiration accounted for 24% and recharge 70% of annual precipitation. RWU at the dry forest site was nearly constantly soil moisture-limited while vapour pressure deficit was the limiting factor at the wet forest site. At the dry forest site, a large range of water ages contributed to RWU. During dry periods water age of RWU was high, but dropped sharply in surface soil layers and RWU following intense precipitation events. This was not observed at the wet forest site where soil water age and xylem water age were less variable. While at the wet forest site Pinus radiata almost exclusively relied on water from the current season for RWU, at the dry forest site significant amounts of RWU in the summer stemmed from winter precipitation.   Our results demonstrate not only greater impacts of these plantation forests on soil water balances in more arid climates, but also suggest greater susceptibility of dryland forests to variation in precipitation regimes.