Effects of species and environmental factors on browsing frequency of young trees in mountain forests affected by natural disturbances

https://doi.org/10.1016/j.foreco.2020.118364Get rights and content

Highlights

  • Browsing by ungulate herbivores is concentrated upon deciduous tree short saplings.

  • The most preferred tree species are: rowan, sycamore maple and goat willow.

  • Browsing frequency is positively correlated with stand openness.

  • Browsing frequency is not related to environmental variables.

  • CWD accumulation affects the browsing frequency in a non-linear way.

Abstract

We analyzed the effects of natural disturbances on relationships between forest regeneration and ungulate browsing. Specifically, we asked if natural disturbances affect preferences of ungulate herbivores towards tree species and size classes; and how local and landscape-scale environmental factors affect spatial patterns of ungulate browsing in disturbed forest ecosystems. Our research was conducted in the Tatra National Park, a mountain range affected by large-scale intense disturbances over the last few decades. We used field data collected in a network of regularly distributed 600 circular sample plots where young trees were recorded and examined for browsing signs. Using forage ratio, the Ivlev’s electivity index and chi square tests, we found that the preferences of wild ungulates were similar to those known from undisturbed forests. Our analyses showed preference for deciduous species, especially rowan, sycamore maple and goat willow. The most common species, Norway spruce was strongly avoided by browsing ungulates. Short saplings from 0.5 m to 1.3 m tall were the most browsed size class. Effects of environmental variables on browsing were analyzed with zero-inflated negative binomial models. The occurrence of browsing was positively related to the total number of young trees and mature tree mortality. This suggests the influence of disturbances on spatial patterns of browsing pressure. The browsing frequency (number of browsed individuals per plot) was affected by mature tree mortality and densities of young palatable trees, including silver fir, sycamore maple and rowan. Elevation, geology, soil type, and potential solar radiation did not show any significant influence on the browsing frequency. Browsing frequency was also positively related to the distance to roads and hiking trails, indicating the role of human pressure upon the behaviour of ungulate herbivores. In addition, we analyzed the effects of accumulation of coarse woody debris (CWD) on browsing frequency and found this relationship to be nonlinear. Analysis for all tree species revealed that at low levels of CWD, the browsing frequency increased with CWD accumulation as a consequence of forest regeneration. However, a large amount of CWD (>25 m3) resulted in a decrease in browsing frequency, and this relationship was species-specific.

Introduction

The browsing of young trees by ungulate herbivores is a widespread phenomenon, affecting tree regeneration in most forests of the temperate zone (Ammer, 1996, Rooney, 2001, Takatsuki, 2009, Côté et al., 2004, Russell et al., 2017). It is considered one of the main driving forces in forest dynamics (Bradshaw et al., 2003, Didion et al., 2009, Herrero et al., 2016) and it plays a major role both in natural forests (Kuijper et al., 2010, Beschta and Ripple, 2016) and managed stands with various management intensities (Klopčič et al., 2010, Kupferschmid et al., 2013, Pendergast et al., 2016, Boulanger et al., 2018). With growing deer populations, the intensity of browsing increases, making the natural regeneration of some highly palatable species very difficult (Kupferschmid and Bugmann, 2005, Boulanger et al., 2009, Ramirez et al., 2018).

Browsing pressure is uneven at both local and landscape spatial scales. Wild ungulates prefer some plant species more than others, and therefore tree species can be browsed selectively (Boulanger et al., 2009, Kupferschmid, 2018). The strength of ungulate pressure upon certain tree species varies geographically, depending strongly on the local composition of tree stands and forage availability (Tripler et al., 2005, Boulanger et al., 2009, Gerhardt et al., 2013). In Central-European temperate forests, silver fir (Abies alba), sycamore maple (Acer pseudoplatanus) and oaks (Quercus spp.) are among the most preferred by wild ungulates, while Norway spruce (Picea abies) and European beech (Fagus sylvatica) are usually avoided (Senn and Suter, 2003, Götmark et al., 2005, Boulanger et al., 2009, Didion et al., 2009, Klopčič et al., 2010, Nagel et al., 2015). Locally, ground vegetation and management practices may affect browsing habits of wild ungulates (Partl et al., 2002, Bakker et al., 2004, Rammig et al., 2007, Ewald et al., 2014, Möst et al., 2015).

Although numerous studies have shown that the amount of forage was probably the most important factor determining browsing intensity (Russell et al., 2017), landscape-scale factors matter, including surrounding land cover types, management activities, vegetation structure and preferences of wild browsers in terms of habitat and food sources (Sabo, 2019). Analyses of the browsing behaviours of most important European wild ungulates revealed their preferences for browsing in gaps rather than in closed forests (Kuijper et al., 2009, Tahtinen et al., 2014). The amount of open spaces, forest edges and fields with palatable crops in the surrounding landscape is also positively correlated with the browsing activity of wild ungulates in forests (Staines and Welch, 1984, Tufto et al., 1996, Nagel et al., 2015). Proximity to roads and trails significantly reduces browsing by wild herbivores (Möst et al., 2015). Some studies also reported relationships between browsing pressure and topography, including elevation, slope and aspect (Mysterud et al., 2010; Heurich at al., 2015), snow cover (Månsson, 2009) as well as habitat productivity and habitat type (Mysterud et al., 2010). However, relationships between all the above-mentioned factors and the habits of wild browsers may be even more complex. Whether the effects of some of these factors are strong or modest, it will depend mainly on the ungulate densities (Ward et al., 2008, Möst et al., 2015). The relationship between the ungulate density and browsing is not a linear one (Tremblay et al. 2007).

Browsing pressure is of vital importance for both forest management and biodiversity conservation, and therefore great insights into the relationships between forest ecosystems and wild ungulates have been gained. Most studies on browsing pressure are usually focused on undisturbed forest ecosystems affected by high levels of ungulate densities. However, large intensity disturbances can dramatically change the availability of resources and vegetation structures, and therefore usually affect the behaviour of wild ungulates (Widmer et al., 2004). For example, a large number of fallen logs after natural disturbances decrease the browsing intensity as the area is less available for browsing herbivores and there is an increasing risk of predation (Rammig et al., 2007, Hagge et al., 2019, Whyte and Lusk, 2019). The presence of large tip-up windthrow mounds may decrease the ungulate browsing pressures and provide good conditions for regeneration of palatable species (Long et al., 1998, Krueger and Peterson, 2006). In addition, rapid forest regeneration following some large-scale disturbances results in an enormous increase in the available forage (Rammig et al., 2007, Pajtík et al., 2015), which can potentially affect previously recognized browsing behaviour.

The aim of this study was to analyse the patterns of browsing pressure in the Tatra Mts. (southern Poland) after large scale intense disturbances. Specifically, we aimed to answer the following questions:

  • (1)

    Do the effects of natural disturbances change the already known preferences of ungulate herbivores towards certain species and size classes of young trees?

  • (2)

    Is the browsing frequency influenced more by the effects of recent disturbances on stand structure than by the environmental factors, like elevation, solar radiation, bedrock and soil type?

  • (3)

    Do the landscape-scale effects of natural disturbances modify the influence of human pressure on the browsing patterns of ungulate herbivores?

Section snippets

Study area

The research was carried out in the Tatra National Park (TNP) covering an area of 21,197 ha. The TNP covers the entire area of the Polish Tatra Mountains, which in the southern part borders the Slovak Tatra National Park. Depending on the location, the bedrock consists of two distinct various rocks formations; limestones and dolomites in the northern part, ans. gneisses and granites in the south. The climate in the lower mountain zone is moderately cool, with an annual temperature up to 5 °C

Differences in browsing pressure among tree species

Of the 40,707 young trees recorded in sample plots, 5193 individuals (12.8%) showed signs of herbivore browsing. The percentage of young trees that had been browsed by ungulates varied from 6.8% in seedlings, through 11.6% in tall saplings to 21.2% in short saplings. The browsing pressure was very unevenly distributed among the tree species. Among the seedlings, extremely high levels of browsing were recorded in the case of species represented by only a few individuals; therefore, in the

Differences in browsing pressure among species and size classes

Wild herbivores have specific preferences with regards to their forage and therefore exert a higher browsing pressure on some tree species than on other species. Large differences in browsing pressure upon various tree species have been frequently documented in the literature (Motta, 2003, Boulanger et al., 2009, Čermák et al., 2009, Klopčič et al., 2010). In this study, we partially corroborated the previous findings of other authors. In the Tatra Mts., rowan is the most heavily browsed

Conclusion

Browsing pressure in the Tatra N.P. is strongly concentrated upon the shorter saplings of the highly palatable species, especially rowan and sycamore maple. It is highly variable among the sample plots, and these differences cannot be explained by environmental factors, like elevation, slope, geology or soil type. Canopy tree mortality contributes to increased levels of browsing, mostly due to the larger amount of young trees under partly open forest canopies. However, because of the ample

CRediT authorship contribution statement

Jerzy Szwagrzyk: Conceptualization, Funding acquisition, Methodology, Project administration, Writing - original draft, Writing - review & editing. Anna Gazda: Data curation, Formal analysis, Funding acquisition, Methodology, Writing - original draft, Writing - review & editing. Elżbieta Muter: Writing - review & editing. Remigiusz Pielech: Formal analysis, Methodology, Writing - original draft, Writing - review & editing. Janusz Szewczyk: Data curation, Writing - original draft, Writing -

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was financially supported by the Polish National Science Foundation (NCN) grant No. 2018/31/B/NZ8/02786 (project title: “How do large-scale disturbances influence the relationship between ungulate herbivory and natural regeneration in temperate forests?”), Ministry of Science and Higher Education of the Republic of Poland in frame of statutory activities SUB/040011-D019/2020 of Department of Forest Biodiversity, University of Agriculture in Krakow and by “Forest Found” (resources

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