Basal area growth response of Scots pine to drainage: An analysis using a mixed-effects modelling approach

Abstract

Wood production in peatland forests is often associated with management of site hydrology that aims to improve soil moisture status and stand growth. Silvicultural outcomes of ditching activities vary greatly among drained sites, therefore new data and modelling approaches are necessary for better understanding of forest growth dynamics on a tree-level and a long-term scale. A mixed-effects model estimating growth response to initial ditching performed 40 years ago in Ongassaare region (northeast Estonia) was developed using basal area increment data derived from ring-width series of 287 Scots pine trees from 39 stands. The model indicated that ditching response was up to around 800 mm2 year−1 on average; this level of drainage-induced additional increment was reached 16 years after treatment. Pre-drainage tree diameter, mean stand diameter and radial growth rate were among the significant predictors, therefore the results suggest that tree size, stand growing stock and site quality or microsite conditions before the ditching are affecting post-drainage tree growth. Smaller trees experienced greater growth response; however larger trees responded to the treatment faster than smaller ones. Ditching therefore induced changes in stand structure by decreasing tree size variation. Ditching response was more pronounced in stands with smaller pre-drainage diameter, faster growing trees reached the maximum response values quicker. Trees closer to the drainage ditch responded slightly more intensively to changing site conditions, however, this relationship was significant in the case of a reference ditch (one of the ditches surrounding a forest stand, selected according to the topography of the landscape) but not the nearest one. Drained peatlands are very diverse in terms of pre-treatment site conditions, stand history and ditch network design, therefore additional data from extended network of sample plots would be necessary for updating the developed model to make it more applicable for further use (e.g., predicting forest growth and yield).