Analysis of fine rooting below skid trails using linear and generalized additive models

• Published in: Canadian journal of forest research Vol. 39; no. 11; pp. 2047 - 2058
• Main Authors: Schäffer, Jürgen, Wilpert, Klaus von, Kublin, Edgar
• Format: Journal Article
• Language: English
• Published: Ottawa, ON National Research Council of Canada 01.11.2009; NRC Research Press; Canadian Science Publishing NRC Research Press
• Subjects: Analysis; Biological and medical sciences; compacted soils; data analysis; density; forest roads; Forest soils; Forestry; Fundamental and applied biological sciences. Psychology; linear models; logging; Machinery; Poisson distribution; root growth; roots; Roots (Botany); soil compaction; Soil science; Soil sciences; statistical analysis; statistical models; wheel tracks; Germany; Additive; Analysis; Use; Forestry; Rooting; Generalized; Modeling
• ISSN: 0045-5067; 1208-6037
• DOI: 10.1139/X09-125

Soil compaction caused by forest machinery changes the basic conditions for root propagation below skid trails. In consequence, lower fine-root densities have to be expected under wheel tracks compared with other skid trail strata that experience no direct traffic. Explorative data analysis of fine-root densities below a skid trail revealed that the fundamental assumptions for linear modelling were violated. Using a generalized linear model following a Poisson distribution with a log link function for the predictor variables together with an exponential covariance function to cope with spatial autocorrelation, the formal model criteria were met. In contrast to the linear models, generalized additive models provide flexible surface estimators that enable us to model continuous response surfaces. In addition, generalized additive models allow for the calculation of confidence intervals for the estimated density surface and for the use of inferential statistics, such as comparisons between depth gradients of fine rooting at distinct transect locations or depth layers. These model characteristics improve the possibility to recognize differences and to evaluate fine-root disturbances below skid trails without integrating uncertain strata information. They also enhance the options for determining the duration of time that is necessary to restore the rooting capacity on formerly compacted soils.