Environmental basis of soil–site productivity relationships in ponderosa pine

Abstract

Understanding the environmental basis for soil–site quality relationships requires that we connect the environmental factors important to resource availability to the physiological processes influencing tree productivity. The nitrogen productivity concept provides this link by relating nitrogen uptake rate to plant growth, although the concept has been verified almost exclusively by laboratory experiments on tree seedlings. We tested the nitrogen productivity concept in a field setting by relating foliage production to nitrogen mineralization rate in 19 mature ponderosa pine (Pinus ponderosa) stands across a moisture gradient in central Oregon, USA. Models developed following the nitrogen productivity concept predicted annual foliage production precisely and adequately represented the different influences of nitrogen and water stress. Current‐year foliage production was proportional to older foliage nitrogen content (R2 = 0.82), and a model including a water stress index (stable carbon isotope ratio, δ13C) further explained 95% of the variability.A direct link between soil nitrogen availability and canopy nutrition was less clear. Annual foliage production was positively, but weakly, correlated with soil‐estimated N‐uptake (estimated in situ), likely because annual nitrogen uptake was small relative to nitrogen retained in the canopy. Foliage nitrogen was highly conserved with a mean retention time of 10.5 yr, which was 2.2 times longer than foliage retention. Annual nitrogen uptake amounted to 0–11% of total canopy N. Multiyear estimates of cumulative N fluxes are needed to adequately assess N availability. Soil nutrient pools were poorly correlated with nutrient uptake and were not useful for predicting stand productivity.