Modelling the influence of stand structural, edaphic and climatic influences on juvenile Pinus radiata fibre length

The objectives of this study were to determine the influence of site, fertilisation and age on fibre length and develop predictive models of fibre length from a comprehensive set of climatic, edaphic and stand variables. Data were collected from a nationwide set of 22 site quality plots where Pinus radiata D. Don was established at high stand densities (40 000 stems ha−1) and grown over a period of 4 years. The main environmental drivers of fibre length were identified by assessing the strength of bivariate correlations and use of multiple regression. Path analysis was used as an extension to multiple regression to separate cause from effect and quantify the direct influence of variables significantly related to fibre length.Fibre length exhibited significant variation between sites. When averaged across fertiliser treatments, site variation for fibre length in rings 1–4 ranged from 1.08 to 1.76 mm. Fertilisation did not significantly affect fibre length. Fibre length significantly increased with age from an average of 1.10 mm in rings 1–2 to 1.62 mm in rings 3–4. The interaction between age and fertilisation was insignificant.In total 16 variables were significantly related to fibre length. For rings 1–4, fibre length exhibited the strongest significant relationships with stem slenderness (r = 0.69), mean minimum air temperature (r = 0.67), green crown height (r = 0.66), tree height (r = 0.64) and average air temperature (r = 0.62). The best multiple regression models for both age classes and the whole stem included mean minimum air temperature and stem slenderness in a linear formulation. Path analysis indicated that the direct positive contribution of both these variables to fibre length was of a similar magnitude (path coefficients of 0.52 versus 0.49).Significant determinants of P. radiata fibre length in our study were similar to those identified in trees grown at conventional stockings across a similar environmental gradient in New Zealand. Although these studies did not examine how stem slenderness influences fibre length, they found average air temperature to be the strongest environmental determinant of fibre length, and that soil fertility, as indexed by Olsen phosphorus and total nitrogen and pH had little influence on fibre length. Further research is needed in conventionally stocked stands to confirm the link found here between stem slenderness and fibre length.