Disentangling the effects of climate variability and herbivory on spatial and temporal changes in northern vegetation greening

Understanding ongoing vegetation dynamics is essential to disentangle ecosystem response to climate variability and identify threats and management opportunities. The basic distribution of vegetation cover and activity is largely determined by climatic conditions and management. Recent field studies suggest that herbivore-induced disturbances are important factors in vegetation dynamics, but analyses relying on long timeseries of empirical observations investigating the combined effect on vegetation of changing climate and herbivore densities are limited. In this study, the impact of temperature, precipitation, and large ungulates (both livestock and wildlife) on vegetation greenness (assessed with the enhanced vegetation index) from 2003 to 2021 in Norway are reconstructed for the growing season (May to October) and elucidated using multiple linear regression models. Vegetation greenness shows a widespread dominant increase in all months, with temperature being the dominant driver. Vegetation greenness is mostly affected by climate and ungulates in May, June and October (i.e., the start of growing season and the senescence), and at relatively high elevations (above 1000 m), where temperature is a limiting factor to vegetation growth. Precipitation can further contribute to explain the changes, especially in June and July. Generally, the positive temperature effect is amplified by increasing precipitation, but it has been hampered in places where precipitation was insufficient. Livestock mainly affect vegetation greenness negatively in northern grassland/shrubland ecosystems due to semi-domestic reindeer (Rangifer tarandus) grazing, while wild cervids (mostly moose Alces alces) negatively affect vegetation greenness in southeastern deciduous forests. The impact of livestock grazing on vegetation greenness is usually stronger than that of wildlife, which leads to a slightly more prevalent negative temperature-livestock interaction than temperature-wildlife interaction. Improving our understanding of these fundamental mechanisms is vital to support informed management of livestock, wildlife and natural vegetation under changing environmental conditions.