Long-term Wood Production in Water-Limited Forests: Evaluating Potential CO₂ Fertilization Along with Historical Confounding Factors

Increased aridity may have severe effects on productivity of dry forests. However, it remains unclear to what degree the positive effects of elevated CO₂ (both increased carboxylation rates and enhanced water-use efficiency) may offset the negative effects of drought and climate warming. In forest ecosystems, it is particularly challenging to evaluate CO₂ effects on productivity because the impacts of climate variability, competition, and management, combine to have longlasting effects on stand-level productivity. Here we address this problem using a unique long-term database containing repeated inventories of wood biomass for every decade from 1912 to 2002 in a pine forest (Pinus pinaster Ait.) in central Spain (≈7,500 ha.). The approach is based upon a combination of statistical analyses of long-term historical management data and mechanistic modeling which allows us to evaluate the effects of potential CO₂ fertilization, climate, and stand structure on woody net primary production (W-NPP). We found a significant negative effect of drought on W-NPP during the first half of the twentieth century that diminishes at the turn of the century. Simulations with a process-based ecosystem model, ORCHIDEE, suggest that wood production under conditions that included CO₂ fertilization produced a more highly correlated long-term W-NPP than simulations keeping CO₂ values in preindustrial levels. Interestingly, however, the CO₂ effect was only apparent when accounting for confounding factors such as competition and management legacies. Identifying CO₂ fertilization on forest growth is a critical issue, and requires partitioning CO₂ effects from confounding factors that have jointly shaped stand dynamics and carbon balance during the twentieth century.