Bridging empirical and carbon-balance based forest site productivity - Significance of below-ground allocation

In forestry, site productivity is commonly described using the concept of site index (SI) and its effect on the potential mean annual increment. Although a unified causal explanation between SI and productivity is still missing, operational methods have been developed to determine SI on the basis of the existing tree crop or other indirect indicators of environmental drivers. In ecology, progress has been made in deriving productivity from the carbon balance, including photosynthesis, respiration and tissue longevity, but considerable gaps remain in quantifying the impact of soil processes on below-ground carbon allocation which would be crucial for closure of the total carbon balance. Recent empirical studies in Finland have found significant and consistent differences in the foliage to fine-root ratio and fine-root lifetime between the conventional SI based on field layer vegetation in geographical vegetation zones. In this study, we utilised this information to bridge the gap between the large body of empirical information on SI and productivity, and productivity as derived from the carbon balance. We incorporated assumptions concerning the climate and soil response of the carbon balance components in a process-based model, PipeQual, and applied the approach to Norway spruce stands in Fenno-Scandian conditions. The results agreed well with an empirical growth and yield model and provided fairly accurate projections for two contrasting test sites. The study provided insights into the role of the component processes of productivity. Importantly, the north-south gradient of productivity was equally attributed to a northward decrease in temperature and an increase in carbon allocation to fine roots. This cautions against climate change projections on the basis of current growth variation with temperature and calls for improved mechanistic models incorporating plant, soil and microbial interactions in regulating the soil-forest carbon and nitrogen fluxes.