Analyzing long-term impacts of ungulate herbivory on forest-recruitment dynamics at community and species level contrasting tree densities versus maximum heights

Abstract

Herbivores are constitutive elements of most terrestrial ecosystems. Understanding effects of herbivory on ecosystem dynamics is thus a major, albeit challenging task in community ecology. Effects of mammals on plant communities are typically explored by comparing plant densities or diversity in exclosure experiments. This might over-estimate long-term herbivore effects at community levels as early life stage mortality is driven by a multitude of factors. Addressing these challenges, we established a set of 100 pairs of ungulate exclosures and unfenced control plots (25 m2) in mixed montane forests in the Alps in 1989 covering a forest area of 90 km2. Investigations ran until 2013. Analogous to the gap-maker–gap-filler approach, dynamically recording the height of the largest trees per tree species in paired plots with and without exclosures might allow for assessing herbivore impacts on those individuals with a high probability of attaining reproductive stages. We thus tested if recording maximum heights of regenerating trees would better reflect effects of ungulate herbivory on long-term dynamics of tree regeneration than recording of stem density, and if species dominance patterns would shift over time. For quantifying the effects of ungulate herbivory simultaneously at community and species level we used principle response curves (PRC). PRCs yielded traceable results both at community and species level. Trajectories of maximum heights yielded significant results contrary to trajectories of total stem density. Response patterns of tree species were not uniform over time: e.g., both Norway spruce and European larch switched in their response to fencing. Fencing explained about 3% of the variance of maximum tree heights after nine years but increased to about 10% after 24 years thus confirming the importance of long-term surveys. Maximum height dynamics of tree species, addressed in our study, can thus reflect local dominance of tree species via asymmetric plant competition. Such effects, both within and among forest patches, can accrue over time shaping forest structure and composition.

Interspecific Growth Reductions Caused by Wild Ungulates on Tree Seedlings and Their Implications for Temperate Quercus-Fagus Forests

“…Because of browsing, some species grow sparingly in height, become dominated by less browsing-sensitive species, and gradually disappear. This was observed for highlight species in our study (Figure 2) and supported our assumption that the browsing effect on seedling growth is a good indicator of future tree recruitment, according to [80].…”

Junipers enable heavily browsed rowan saplings to escape ungulates in boreal forest

“…However, the large increase in abundance over time when excluding deer and sheep to 0.44 rowans/m 2 opposed to only 0.07 rowans/m 2 in ungulate-access plots is perhaps more remarkable than shown in most previous studies. Although there are variation in plant species responses to excluding ungulates (Fortuny et al, 2020), other studies comparable in design, study period and ungulate species have typically found that exclosures exhibit around double the increase in rowan abundance over time compared to controls (Speed et al, 2013;Nopp-Mayr et al, 2020). The effect of excluding deer and sheep in our study is more equivalent to that found when excluding the most typical browser of northern ecosystems, the moose (Alces alces), from clear-cuts (Kolstad et al, 2018), a habitat that typically offers better conditions for germination of rowan than shaded old-growth forest.…”

Az erdei vadhatás és a természetes újulat térségi szintű vizsgálata az Északi-középhegységben

“…Táplálkozásuk és egyéb élőhelyhasználati aktivitásuk során kifejtett hatásaikat együttesen vadhatásnak nevezi a szakirodalom (vö. Velamazán et al 2018, Ramirez et al 2019, Nopp-Mayr et al 2020. Az élőhelyek szempontjából a vadhatást általában negatív előjellel (fékező, akadályozó, destruktív vonatkozásban) értelmezzük, de kisebb számban kedvező ökológiai hatásokra -így például magterjesztésben betöltött pozitív szerepre (Schmidt et al 2004, Mráz et al 2016) -is találunk példát.…”