Effects of cattle grazing on young spruce trees in boreal production forest

Abstract

The compatibility of forest livestock grazing with timber production is disputed, as livestock can damage young trees through browsing and trampling. At the same time, livestock grazing might reduce the growth of competing vegetation and thereby enhance conifer forest regeneration. We investigated the effects of cattle (Bos taurus) on young spruce (Picea abies) trees in boreal production forest. We compared data from young spruce plantations within and outside of the grazing area, and during and off the cattle grazing season. Overall, stocking rates were 0.01 cows per hectare across the grazing areas. Cattle did not distribute equally, but concentrated their area use to young spruce plantations, leading to locally high densities during periods of the grazing season. Cattle removed vegetation competing with young spruce trees: Within the grazing area, the sward height of the field layer vegetation was lower, and young willow (Salix spec.) and downy birch (Betula pubescens) trees were browsed more than outside the grazing area. The annual height growth of spruce increased once the spruce grew above sward height. Spruce trees surrounded by many competing trees grew slower than those surrounded by few trees. Bent spruce trees grew slower than undamaged spruce trees. Yet, no difference in annual spruce growth was found between inside and outside the cattle grazing area. In contrast, we found a higher spruce mortality risk inside than outside the cattle grazing area. This mortality risk was very low and positively correlated with a given spruce tree's bark being damaged. Cattle did not increase the risk of damaged stem bark. This study did not succeed in pointing out the mechanism by which cattle might increase spruce mortality risk. Interestingly, the probability of being browsed and the probability of being bent were higher after winter than after summer, indicating that wild ungulates and snow might have a stronger effect on young spruce trees than summer grazing cattle at these stocking rates. In conclusion, cattle had minor effects on young spruce trees. This study suggests that at low stocking rates, forestry and livestock production in boreal production systems are compatible.