Negative effects of brown bear digging on soil nitrogen availability and production in larch plantations in northern Japan: Their potential role as an agent of bioturbation

Brown bear digging decreases tree growth: Implication for ecological role of top predators in anthropogenic landscapes

“…We evaluated 40 years of radial growth of larches through tree ring analysis, but soil nutrients were measured in only 2019. We detected the negative effects of digging on soil nutrients in 2017 and 2018 (Tomita & Hiura, 2022; Tomita, unpublished data), suggesting that their effects on soil are consistent across years as long as bears dig in the larch plantations every year. Although bears may cease digging for cicada nymphs in poor emergence years, the occurrence frequency of digging in larch plantations remains high even in poor emergence years (Tomita, 2021; Tomita & Hiura, 2021a).…”

“…Dwarf bamboo is not likely a confounding factor for digging effects on soil and tree growth because its fine root biomass had no effects on soil nitrogen availability (Figure 1), and soil nutrients were higher in undug plots than in dug plots. Moreover, our previous paper showed brown bear digging decreased nitrogen availability in larch plantations of the SWH site irrespective of dwarf bamboo (Tomita & Hiura, 2022). Environmental differences associated with a light environment before the beginning of this digging behavior may confuse our results because dwarf bamboo prefers a light environment (Lei & Koike, 1998).…”

“…This is because bears are motivated to dig up shallow soil in search of cicada nymphs due to their preference for the final instar nymphs, which typically reside in shallow soil in preparation for emergence. Tomita and Hiura (2022) proposed a potential mechanism for the negative effect of digging on nitrogen availability in surface soil: mixing of organic and mineral soil by digging decreases organic nitrogen content and thereby inorganic nitrogen concentration. If this mechanism dominates, nitrogen concentrations in deep mineral soil in the dug plots will be higher than those in the undug plots.…”

“…Brown bear digging may affect the nutrient status of canopy trees via direct and indirect pathways. Digging can (1) decrease root biomass and nutrient uptake by mechanical damage, and (2) decrease soil nutrients such as water contents and inorganic nitrogen concentration (Tomita & Hiura, 2022). Nutrient deficiency of trees due to brown bear digging may subsequently decrease the foliar photosynthesis rate, which is strongly determined by soil nutrient availability, and thereby decreases tree growth (Shen et al, 2014).…”

“…The brown bears Ursus arctos , among the most widespread large carnivores in the Northern Hemisphere, play major ecological roles such as predators, seed dispersers, and ecosystem engineers (Steyaert et al, 2019; Tomita & Hiura, 2022). Owing to their high niche breadth and recent conservation efforts, the global population of brown bears remains stable or is even showing signs of increase, despite their typical habitat being dominated by human activities (Ripple et al, 2014; Takinami et al, 2021) and changing behavior (Wheat & Wilmers, 2016).…”