The understorey, a forest layer with an underestimated importance for ecosystem functioning

The major forest types in Europe, from the Boreal to the Mediterranean region, all include a variable number of native vascular plant species in the understorey layer (1.3 m height), both herbaceous and woody. In spite of its small stature and biomass this layer can contain 90% or more of the plant species of the forest. The functional role of the understory vegetation has already been shown in previous studies on temperate forests. They can serve as:

  • Food plants – Several understorey species produce berries or vegetative parts that represent an important and/or popular food for local people. Examples are the bluberries (Vaccinium myrtillus) and other dwarf Ericaceae growing diffusely in the boreal forests, or the wild Asparagus in the thermophilous deciduous forests of southern Europe.
  • Medicinal plants – Some species are known in the local traditional medecine for their therapeutic properties
  • Ornamentals – Several understorey species are used as ornamentals Cyclamen or are wild relatives of cultivated varieties Viola Helleborus.
  • Understorey species may also be involved in Carbon fixation and sequestration: The understorey contributes significantly to carbon sequestration, and can account for up to 5% of the net primary production of the forest.
  • Source of nectar for pollinators and seeds/fruits for small mammals. Many species produces nectar-rich flowers that provide source of energy to a wide range of pollinators and seeds or fruits that serve as food for forest animals. An example are the seeds of species such as Viola that have a lipid-rich elaiosome for dispersal by ants.
  • Decomposition and nutrient cycling. The herb layer contribute to up 15% of the foliar litter to the forest floor. Understorey litter has usually an elevated concentration of nutrients compared to trees (especially N and K). The rapid decomposition and high turnover rate of the herb-layer facilitates efficient recycling of  nutrients in the forest soil.
  • Tree regeneration. Understorey species may facilitate regeneration of tree species by positively influencing microclimatic conditions at the ground level and by sheltering (hiding) the seedlings against browsing by herbivores.
  • Mechanisms of resistance to invasion by alien organisms. Understorey vegetation often includes highly adapted and competitive species that are likely to reduce the chances for alien, exotic species to establish in the community and become invaders.
  • Habitat provision for a multitude of small mammals and insects.

Establishing universal priorities among these services is very difficult, as the relative importance of each of them is strongly context-dependent.  First, the region and type of ecosystem determines the pool of understorey species  present, and the services provided are mostly species-specific.  Secondly, the cultural and historical local contexts vary to the same extent as the type of forest ecosystem, so that a species that is not associated to a service in a given region may be considered of high interest in another region. Moreover, Hector & Bagchi (2007) and Isbell et al. (2011) indicated that different species promote different functions in different years, at different locations and under different environmental conditions. This promotes the precautionary principle of conserving maximum biodiversity in order to obtain long-term forest ecosystem stability and provisioning of ecosystem services. Trees have a species-specific impact on local environmental conditions: the mean amount, variability and nature of the light reaching the forest floor (Messier et al. 1998), soil water content via differences in throughfall, transpiration and water uptake by roots (e.g. Barbier et al. 2008, 2009; Geiβler et al. 2012), soil, nutrient availability and acidity via differences in litter quality and quantity, nitrogen fixation, nutrient uptake and atmospheric deposition on leaves and needles (e.g. Augusto et al. 2003; Hagen-Thorn et al. 2004), phytotoxic compounds, thickness litter layer (Rodríguez-Calcerrada et al. 2011). As a consequence of these tree species identity effects, the composition of the tree layer may substantially affect herb layer composition and productivity as the germination and performance of herb layer species depends on the environmental conditions (e.g. Ellenberg & Leuschner 2010).

Plants include a number of familiar organisms including trees, forbs, shrubs, grasses, vines, ferns, and mosses. The term plant implies a taxon with characteristics of multicellularity, cell structure with walls containing cellulose, and organisms capable of photosynthesis.
This includes the vast range of food products derived from plants, animals, and microbes. (MA 2003)
The processes by which elements are extracted from their mineral, aquatic, or atmospheric sources or recycled from their organic forms, converting them to the ionic form in which biotic uptake occurs and ultimately returning them to the atmosphere, water, or soil. (MA 2003). the exchange or transformation of elements among the living (organic and biotic) and nonliving (inorganic and abiotic) components (SAFnet, 2008)
The unconsolidated mineral or organic material on the immediate surface of the earth that serves as the natural medium for the growth of land plant ( SAF dictionary, 2008).
The capacity of an ecosystem to withstand the impacts of drivers without displacement from its present state. ( MA 2003)
Any biological entity capable of replication or of transferring genetic material. (European Union 2001).
Biological diversity, or the shorter "biodiversity," means the diversity, or variety, of plants and animals and other living things in a particular area or region. It describes the variability among living organisms from all sources, including terrestrial, marine, and other aquatic ecosystems and the ecological complexes of which they are part. Biodiversity includes diversity within species, between species, and between ecosystems. (MA, 2003)
The amount of water lost from a soil when it is dried to constant mass at 105 degree celicus, is expressed as the mass of water per unit mass of dry soil of the volume of water per unit bulk volume of soil (Sage Publicaation, 2014).
Kris Verheyen - tree species and understorey vegetation

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