Articles of Interest

ORGANON Update
David Hann, Oregon State University

A new edition of the SMC version of ORGANON has been developed and is now available on the ORGANON WEBB site, http://www.cof.orst.edu/cof/fr/research/organon. In this revision, changes were made in the following equations: height-to-crown-base (HCB) equations for predicting missing values in the “ORGANON INPUT DATA” component of the model; and the diameter growth equation for Douglas-fir and the mortality equations for Douglas-fir and western hemlock used in the “ORGANON MODEL EXECUTION” component of the model. Based upon validation results using two independent data sets, the original SMC HCB equations were found to predict crown lengths that were too long. This was probably due to the tendency to collect HCB data on those trees subsampled for height measurements, which concentrated the subsample in healthy, undamaged trees mainly from the dominant crown class. To remedy this problem, the Northwest Oregon HCB equations (which are based on measurements for all trees in a stand) were substituted for the SMC equations in the “ORGANON INPUT DATA” component of the model. These equations predict shorter crown lengths than the SMC equations. The user must be aware that, if subsampling height-to-crown-base in the tree list, it is critical that the subsample be selected randomly across all tree sizes, species and tree conditions (i.e., the subsample must represent all trees on the plot). Of course, measuring HCB (and total heights) on all sample trees avoids the problem completely.

As part of the project , the SMC database is currently being queried to provide evidence of positive Douglas-fir growth response as a result of nutrient management for different site classes. Preliminary data analysis suggests that tree volume, and thus yield, increases with increasing levels of fertilization, particularly for lower site classes. For example, the total volume increment for Douglas-fir stands were increased by as much as 60% by adding 800-1000 lbs N fertilizer per acre (Figure 1). Furthermore, there was no indication of volume increases leveling off after addition of 1000 lbs, suggesting that even higher yields may be possible with higher levels of nitrogen fertilization.

Back to SMC Newsletter