Spatial patterns of accumulated browsing and its relevance for management of red deer Cervus elaphus

Abstract

AbstractThe substantial increase of deer populations in Europe and North America in recent decades has led to concerns regarding the long‐term sustainability of current management from an ecosystem perspective. Key questions to be answered are how herbivore density relates to available resource levels, e.g. food availability and browsing pressure, and how this relation should be monitored and included in management. In Norway, the harvest of red deer Cervus elaphus has increased from 1,479 to 35,700 during 1960‐2008. Current monitoring programmes focus on deer body mass and number of seen deer rather than on the state of the vegetation communities. In our study, we quantify browsing frequency on common shrubs and trees in two municipalities, Gloppen and Flora, in the county of Sogn and Fjordane, Norway, to document current browsing pressure levels and to evaluate the potential of using browsing frequency of indicator species as a tool in the monitoring of red deer populations. We found that several species were heavily browsed (median browsing frequency > 60%). Due to their wide availability, we analysed the spatial patterns of accumulated browsing of the highly selected rowan Sorbus aucuparia compared to the less selected bilberry Vaccinium myrtillus and birch Betula sp. We predicted less spatial variation in browsing frequency of the highly selected rowan. However, we found that the best model predicting browsing on rowan (forest type, habitat productivity, canopy cover, aspect, slope and distance to arable land) based on habitat variables was more complicated than for birch (habitat productivity and altitude) and bilberry (forest type, habitat productivity, altitude and distance from the coast). This suggests large spatial variation in browsing frequency of rowan even though the average browsing frequency of rowan was higher than for bilberry and birch. Browsing frequency for all species was positively correlated with faeces counts, but only bilberry showed additional correlation with red deer (harvest) density at the local management unit scale. Due to its wide distribution and promising link to local red deer density, bilberry stands out as the most promising species to monitor among winter browse species. However, browsing frequency on different species was not always correlated. It remains to be determined to which extent browsing on bilberry can be used as an indicator for the browsing pressure on the whole vegetation community. The documentation of high browsing frequencies clearly suggests that further focus on these issues is warranted.