Moose browsing effects on volume production in Scots pine
Märtha Wallgren1
Skogforsk (Forestry Research Institute of Sweden), Uppsala Science Park, 751 83 Uppsala, SE
Abstract The issue of moose browsing on Scots pine has been discussed for decades in the Nordic countries and while the effects on timber quality have been long known, effects on stem volume production have received little attention. In 2010 a report from a long-term study of heavy browsing on Scots pine in a then 30-year exclosure experiment in central Sweden revealed that the effects of browsing on volume production may be much more severe than previously assumed. Because that study had targeted an area with extreme, albeit not unique for Sweden - neither then nor now, browsing pressure on the pines, the results called for another more representative assessment of the effects from current browsing levels on Scots pine across Sweden. A national exclosure study with fenced and unfenced plots was established during 2012-2014, with 21 replicates across a 1000 km north-south gradient. Yearly surveys of browsing and growth parameters have been conducted from start, generating detailed and individual-based data for >16 000 Scots pines plus >6 000 Norway spruces, and a basis for quantifying volume and quality effects derived from different types and levels of browsing. The study will run for at least 7 more years, but there are results available, especially from the southern area where the pines have reached “safe” heights >4 m in several of the stands. The overall effects from browsing on the growth of Scots pines vary considerably between different stands and age/height stages. In some study stands there is no detectable difference between fenced and unfenced plots, while in others the effect of pine protection by fencing is highly conspicuous. Changing production tree species from palatable Scots pine to the mainly avoided Norway spruce so far indicates even larger volume loss, as the latter grow poorly in these sites suited for pine production. Several of the unfenced plots are lingering within browsing height of moose (<2.5 m) and there are no signs of compensatory growth in the browsed plots which have reached >4 m. These results emphasize that reduced volume production should be assessed alongside timber quality reduction as consequences of browsing.