Effects of Disturbance on Understory Vegetation across Slovenian Forest Ecosystems

Abstract

The herbaceous understory represents a key component of forest biodiversity across temperate forests of Europe. Here, we quantified changes in the diversity and composition of the forest understory layer in representative Slovenian forest ecosystems between 2004/05 and 2014/15. In total, 60 plots were placed across 10 different managed forest types, ranging from lowland deciduous and mid-altitude mesic mixed forests to mountain conifer forests. This network is part of an international network of sites launched within the ICP Forests Programme aimed to assess the condition of forests in Europe. To examine how disturbance influenced understory dynamics, we estimated the disturbance impacts considering both natural and/or anthropogenic disturbances that cause significant damage to trees and to ground-surface layers, including ground-vegetation layers and upper-soil layers. Species richness across 10 sites (gamma diversity) significantly decreased from 272 to 243 species during the study period, while mean species richness per site did not significantly change. The mean value of site level Shannon diversity indices and evenness significantly increased. The cover of most common plant species increased during the monitoring period. The mean value of disturbance estimates per site increased from 0.8% in 2004/05 (ranging from 0% to 2.5%) to 16.3% in 2014/15 (ranging from 5.0% to 38.8%), which corresponded to a reduction in total vegetation cover, including tree-layer cover. More disturbed sites showed larger temporal changes in species composition compared to less disturbed sites, suggesting that forest disturbances caused understory compositional shifts during the study period. Rather than observing an increase in plant diversity due to disturbance, our results suggest a short-term decrease in species number, likely driven by replacement of more specialized species with common species.

Patterns of Understory Community Assembly and Plant Trait-Environment Relationships in Temperate SE European Forests

“…IM site 3-GB is located in the sub-Mediterranean phytogeographic region of Slovenia [54], where a warmer and drier regional climate prevails. It is characterized by secondary forest composed of Pinus nigra in the overstory and diverse ingrowth of thermophilous deciduous tree species (e.g., Ostrya carpinifolia, Quercus pubescens, Fraxinus ornus) in the lower tree layer.…”

“…During the period in which our study was conducted, the selected IM sites were managed with the same management practices and close-to-nature principles (e.g., small-scale interventions, sustainable use of forest resources). It is also important to note that the study sites were intentionally selected in homogeneous closed canopy stands lacking evidence of disturbance [54]. Consequently, the potential effects of disturbances on community assembly are difficult to explicitly assess with our results.…”

“…Consequently, the potential effects of disturbances on community assembly are difficult to explicitly assess with our results. However, future work will be dedicated to potential (long-term) temporal changes in understory vegetation in the studied forests and possible shifts in the drivers of community assembly caused by the increasing impacts of disturbance in recent years [54]. For instance, forest disturbances could prevent dominant species from competitively excluding others, thus favoring the coexistence of functional traits in understory plant communities [24,32].…”

“…The study sites differ significantly in altitude (ranging from 160 to almost 1400 m), climatic conditions (mean annual temperature, amount of precipitation), geology, soil characteristics, tree layer species and understory composition, and stand structural features. Detailed information on our study sites and their spatial distribution across Slovenia was given in Kutnar et al [54].…”

“…Large plots of 10 × 10 m (hereafter Lplots) were placed systematically across the core area, with a minimum distance between plots of 20 m. The number of Lplots per site (4 or 8) depended on the intensity and type of monitoring activities. In total, 60 Lplots were placed across all IM sites [54]. Additionally, 10 small plots of 2 × 2 m (hereafter Splots) were established at each IM site, i.e., 100 plots altogether.…”