Increasing aridity will not offset CO2 fertilization in fast‐growing eucalypts with access to deep soil water

Rising atmospheric [CO2] (Ca) generally enhances tree growth if nutrients are not limiting. However, reduced water availability and elevated evaporative demand may offset such fertilization. Trees with access to deep soil water may be able to mitigate such stresses and respond more positively to Ca. Here, we sought to evaluate how increased vapor pressure deficit and reduced precipitation are likely to modify the impact of elevated Ca (eCa) on tree productivity in an Australian Eucalyptus saligna Sm. plantation with access to deep soil water. We parameterized a forest growth simulation model (GOTILWA+) using data from two field experiments on E. saligna: a 2‐year whole‐tree chamber experiment with factorial Ca (ambient =380, elevated =620 μmol mol−1) and watering treatments, and a 10‐year stand‐scale irrigation experiment. Model evaluation showed that GOTILWA+ can capture the responses of canopy C uptake to (1) rising vapor pressure deficit (D) under both Ca treatments; (2) alterations in tree water uptake from shallow and deep soil layers during soil dry‐down; and (3) the impact of irrigation on tree growth. Simulations suggest that increasing Ca up to 700 μmol mol−1 alone would result in a 33% increase in annual gross primary production (GPP) and a 62% increase in biomass over 10 years. However, a combined 48% increase in D and a 20% reduction in precipitation would halve these values. Our simulations identify high D conditions as a key limiting factor for GPP. They also suggest that rising Ca will compensate for increasing aridity limitations in E. saligna trees with access to deep soil water under non‐nutrient limiting conditions, thereby reducing the negative impacts of global warming upon this eucalypt species. Simulation models not accounting for water sources available to deep‐rooting trees are likely to overestimate aridity impacts on forest productivity and C stocks. To anticipate Eucalyptus saligna responses to increasing aridity and rising atmospheric CO2 we trained and validated a simulation model with data from two field experiments. We then used the calibrated model to evaluate E. saligna behavior under different climate scenarios. Our results suggest that (1) the ability of E. saligna to access deep soil water buffers the impacts of increased aridity for this tree species; (2) thus, under no nutrient limitation, aridity increases will not offset rising CO2 fertilization on E. saligna; (3) vapor pressure deficit increases will potentially li