Forest Understorey Vegetation: Colonization and the Availability and Heterogeneity of Resources

Abstract

Understorey vegetation comprises a major portion of plant diversity and contributes greatly to nutrient cycling and energy flow. This review examines the mechanisms involved in the response of understorey vegetation to stand development and the overstorey canopy following disturbances. The overall abundance and diversity of the understorey is enhanced with the availability and heterogeneity of light, soil nutrients, soil moisture, and substrates. Vascular plants are positively impacted by the availability and heterogeneity of light and soil nutrients, whereas non-vascular vegetation is more strongly influenced by colonization time, soil moisture, and substrates, and is decreased with a higher proportion of broadleaf overstorey. The availability of resources is a prominent driver toward the abundance and diversity of understorey vegetation, from the stand initiation to stem exclusion stage under a single-species dominated overstorey. However, resource heterogeneity dominates at the later stages of succession under a mixed overstorey. Climate and site conditions modify resource availability and heterogeneity in the understorey layer, but the extent of their influences requires more investigation. Forest management practices (clearcutting and partial harvesting) tend to increase light availability and heterogeneity, which facilitates the abundance and diversity of understorey vascular plants; however, these factors reduce the occurrence of non-vascular plants. Nevertheless, in the landscape context, anthropogenic disturbances homogenize environmental conditions and reduce beta-diversity, as well, the long-term effects of anthropogenic disturbances on understorey vegetation remain unclear, particularly compared with those in primary forests.

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

“…Compared to other widely spread vegetation types (e.g., grasslands), forest understories are ecologically specific and most often resource-limited habitats, where the overstory tree layer induces both abiotic constraints (resource filtering, e.g., shading) and complex biotic interactions, as mature trees are strong competitors and major consumers of space, light, water and nutrients [30,37,38]. From an ecological perspective, the understory in mature, closed canopy forest stands can be seen as a highly selective environment, where shade tolerance affects the ability of plants to cope with other stressors [39,40].…”

“…In addition, soil characteristics play an important role in filtering plant species. Forest soils on calcareous bedrock (limestone and dolomite) normally have higher nutrient availability and soil pH, which is favorable for a larger set of species [38], thus increasing the potential for competitive interactions. By contrast, acidic and nutrient-poor soils usually harbor considerably lower species richness, as not many plant species are able to tolerate low pH values in the temperate forest understories [41,42].…”

Understorey changes after an extreme drought event are modulated by overstorey tree species mixtures in thermophilous deciduous forests

“…Hence, maintaining the integrity of the understorey is increasingly acknowledged as a major goal in sustainable forest management and biodiversity conservation programs (Canullo et al, 2016). Due to their often high sensitivity to environmental conditions, understorey plants form dynamic communities that respond in many ways to a broad range of habitat changes and disturbances over both short and long time scales (Ares et al, 2009;Su et al, 2019;Kutnar et al, 2019).…”

Trait-based responses to land use and canopy dynamics modify long-term diversity changes in forest understories

“…On the other hand, the tree-layer is a major modulator of resource availability in the understory, as light availability and the amount and quality of litter directly influence growing conditions in the field-layer (Maes et al, 2019) . More information is needed about how different forest management practices affect understories in the long-term (Su et al, 2019) , but differentiating between direct and canopy-mediated effects of management requires long-term concurrent monitoring of canopies and understories in both natural and managed forests.…”

“…In Fennoscandian boreal forests, forest management has intensified manyfold from low intensity wood harvesting to industrial-scale forestry after the 1950s with detrimental effects on forest biodiversity (Hyvärinen, Juslén, Kemppainen, Uddström, & Liukko, 2019) . More information is needed on how different forest management practices affect understories in the long-term (Su, Wang, Huang, Fu, & Chen, 2019) , especially since management effects on community composition can sometimes lag decades behind changes in management practices (Muurinen, Oksanen, Vanha-Majamaa, & Virtanen, 2019) .…”