Reply to C.W. Ramm about analytical techniques employed for the Nelders: Thanks for your interest. My objective was to determine when competition began to affect the growth of young crops, and whether it might be possible to represent this in terms of age, stocking and site quality, although I did play with some growth and yield modelling of older-age data from two of the Nelders. For the purposes of determining the inception of competition, I calculated mean dbhob, mean height, and initial stocking for each ring in each Nelder for each year of measurement. I then plotted mean dbhob and mean height against stocking by age. Results were as outlined in my posts, that there was liitle evidence of a relationship between dbhob and stocking until age 5, when the trees were 4-8 metres tall (varying with site quality). Unfortunately, on the best sites, there were measurements at age two, and then none until age 5. In all cases where measurements were made in the years prior to and after the onset of competition as expressed by dbhob, there was no evidence of "flattening" of the dbhob vs stocking relationship at wider spacings. Height increased with stocking between 500 and 2000 stems per hectare in two out of five Nelders where appropriate measurements had been conducted. Above 2000 stems per hectare there was no evidence of a correlation. I fitted a variety of traditional G&Y functions to a dataset which included measurements from older ages taken in two of the Nelders. Gross basal area was modelled in both difference and yield form, but no functional form was free from bias. Bias was particularly evident close the ages when basal area was zero, and an adjustment was made to the yield form of the functions to remove the false assumption that dG/dT=0 when G=0. This adjustment certainly improved the fits, but no single fit was perfect, none-the-less. Gross basal area was used because of the high levels of mortality at older ages. Mortality is a problem in the analysis of Nelders, to put it mildly! Implications of these studies for the purposes of modelling initial growth and yield were that anamorphic mortality functions were more likely to fit than polymorphic ones in very young stands; exponential functions were likely to provide a good representation of initial basal area and height growth, so long as the basal area functions were properly adjusted to allow for growth when G=0; and that stocking could be a useful independent variable in models of initial height growth. In the events that followed, all except the latter proved to be the case. A publication is in prep covering the initial growth modelling studies. I have not yet decided whether the analyses of the Nelders are worth enough to others to warrant formal publication. Regards, Euan ================================================================== Dr Euan G. Mason Silviculture, Modelling and Lecturer in Silviculture Decision-support systems School of Forestry University of Canterbury New Zealand's professional Christchurch, New Zealand Forestry School ==================================================================
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