The effect of nitrogen fertilization, rock type, and habitat type on individual tree mortality

An individual tree mortality model for nitrogen fertilized Douglas-fir (Pse udotsuga menziesii var. glauca [Beissn.] France) stands was developed using data from permanent research plots located throughout the inland Northwest . The proposed linear logistic model included the following independent var iables: a dummy variable for the two habitat types, a set of dummy variable s for the five rock types, a set of dummy variables for the three N fertili zer treatments, diameter at breast height, crown ratio, and crown competiti on factor. The results show that N fertilization, rock type, and habitat ty pe significantly affect individual tree mortality. The probabilities of tre e mortality on fertilized plots were greater than those on control plots an d increased with increasing N fertilizer application rates. Trees growing o n granitic and metasedimentary rocks had lower foliar potassium concentrati on and exhibited greater probabilities of mortality than did those growl ng on other rocks. The probabilities of mortality for trees growing on sedime ntary rocks were very low. Moist sites had lower soil fertility and produce d higher mortality rates than dry sites. Furthermore, the N fertilization r esponse ratio, defined as the annual mortality probability of a fertilized tree over the annual mortality probability of a unfertilized tree with iden tical tree and stand characteristics, was estimated based on the mortality model. The response ratios were nearly constant (about 1.4) across a range of tree diameters for all rock types with the 224 kg N treatment. The respo nse ratios were also nearly constant (about 2.1) across a range of tree dia meters for all rock types with the 448 kg N treatment. Finally, the mortali ty prediction model passed a validation test on independent data not used i n model development.