Tree diameter growth after root trenching in a mature mixed stand of Norway spruce (Picea abies [L.] Karst) and European beech (Fagus sylvatica [L.])

Root trenching of Norway spruce and European beech caused no long-term decrease of stem growth and only temporal growth reductions in European beech. Root volume reduction is a common occurrence in silvicultural, urban, and scientific environments, often the direct result of trenching applications. Although this interference is widespread near forest roads and in rural and urban areas, trees' growth reactions to these limitations have rarely been analysed. Here, we compare the growth of 81 mature Norway spruce (Picea abies [L.] Karst) and European beech (Fagus sylvatica [L.]) trees, trenched for an experiment down to 1 m depth in a Luvisol soil (Parabraunerde) in 2010, and compared with 69 untrenched trees. The trenching was conducted prior to a long-term precipitation-exclusion treatment in a temperate mixed forest in Southern Germany. Stem and crown sizes, tree positions, distance to the trench, and annual stem growth of the 150 trees were measured from 2006 through 2013. Statistical analysis revealed that (1) stem growth reduction does not correlate with the loss of rooting area by trenching, (2) trenching does not significantly decrease the stem growth in the long-term post-trenching application, and (3) European beech showed a greater reaction in the first years after trenching as indicated by a greater decrease in stem growth. The relationship between stem growth reduction and the loss of rooting area arises during the first year post-trenching, but diminishes by the third year. The trenching effect on stem growth was strongest in trees at the plot edge and diminished towards the centre of the plots, where the actual study trees of the precipitation-exclusion experiment were located. The relevance of trenching for growth reduction is discussed, including potential underlying mechanisms and conclusions about experimental interferences.