Soil-site productivity relationships of central Oregon ponderosa pine

The relationship between annual foliage production and nitrogen and water stress was examined in 14 naturally regenerated, mid-rotation ponderosa pine stands in central Oregon. Current-year and older foliage masses, and foliage nitrogen contents and concentration were estimated from 132 destructively sampled trees. Litterfall mass and nitrogen retranslocation rates from senescing foliage were estimated, and site water stress was indexed with stable carbon isotope ratios (δ13C) of current-year foliage. Models based on the nitrogen productivity concept of Ingestad (1981) and Agren (1983) were shown to accurately predict annual foliage production. Current-year foliage production was strongly related to older foliage nitrogen content (R 2 = 0.82). Models including water stress (δ13C) as an additional covariate explained 95% of the variability, and showed that foliage production decreased with increasing water stress. Older foliage mass and δ13C were weakly correlated (r = 0.11), suggesting that they acted independently on current-year foliage growth. Foliage relative growth rates were strongly related to both water stress and nitrogen concentration (R2 = 0.84). Retranslocated nitrogen accounted for 47–116% of current-year foliage nitrogen, and annual nitrogen uptake (from the soil) was 0–11% of total canopy-N. Annual nitrogen uptake rates in mature ponderosa pine stands were small relative to nitrogen retained in the canopy, and nitrogen uptake rates may be poorly correlated with annual productivity for this reason. Foliage macro- and micro-nutrient ratios relative to nitrogen were very consistent across sites suggested possible P, and S deficiencies.; Annual soil nitrogen mineralization and uptake rates varied widely among sites (1–52 and 5–58 kg ha−1 yr−1 ,respectively), estimated using in situ incubations. Current-year foliage production and canopy N-contents were not significantly correlated with nitrogen uptake rates. N-retranslocation rates were shown to be similar in magnitude to soil N-uptake, and substantially increased N-availability. Foliar nitrogen was highly conserved with a mean retention time of 10.5 yrs, which averaged 2.2 times longer than foliage retention. Nitrogen retention was correlated with an index of site water stress (r = 0.43). The relationship between relative growth rate of foliage and relative uptake rate of nitrogen was non-significant (p = 0.27). A close linear relationship between these was predicted by the nitrogen productivity concept, but was not confirmed.