Water balance of pine forests: Synthesis of new and published results

The forest hydrologic cycle is expected to have important feedback responses to climate change, impacting processes ranging from local water supply and primary productivity to global water and energy cycles. Here, we analyzed water budgets of pine forests worldwide. We first estimated local water balance of forests dominated by two wide-ranging species: Pinus taeda (36 °N) and Pinus sylvestris (64 °N). In these stands, growing season evapotranspiration (ET) was largely insensitive to inter-annual variation of precipitation (P), consistent with the insensitivity of canopy transpiration to P. Extending the analyses to include published data from 117 studies on 27 pine species, we found that pine forests annually use ∼66% ± 17% (SD) of P as ET, regardless of climatic regime, leaving a third of P as runoff to downstream aquatic ecosystems and users. However, during the growing season, pine forests used more water as ET than P in regions where P ≤ 326 ± 39 (SE) mm. Forests in regions of low growing season P exist in their current state only where the rooting depth is sufficient to supply trees with water from soil storage in addition to P, and these forests are likely to support only ephemeral streams that dry down during the growing season. Thus, globally, water use by pine forests is adapted to mean annual P, but shows a limited capacity to respond to inter-annual variability in P. Forests with a small buffer of growing season water availability (P + soil water storage - ET), are likely to be most sensitive to variation in P regimes, changing canopy leaf area, tree density, and species composition depending on the degree, direction and persistence of the change in P.