Forest structures and resource availability
Even-aged, monospecific stands that are favoured after clearcutting often have a homogeneous three-dimensional canopy structure which might, therefore, reduce the heterogeneity in resource availability for plants (Fedrowitz et al. 2014). In Central Europe, management has shifted to single tree harvesting since the 90’s to obtain more continuous cover forests (Gustafsson et al. 2019). This management regime affects light quantity (Forrester et al. 2018) and heterogeneity and, consequently, understory plant composition and density (Bengtsson et al. 2000, Getzin et al. 2012, Duguid and Ashton 2013). In addition, different tree species produce different crown structures (Ampoorter et al. 2015, 2016) and react differently to disturbances such as wind and snow damage, or pest outbreaks (Burton et al. 2014, Hilmers et al. 2018). More recently retention forestry has been promoted to enhance structural complexity in rather homogeneous, even-aged stands to favour forest biodiversity (Gustafsson et al. 2012, 2019). Understory plant species composition, species richness, and functional diversity have been shown to be affected by retention measures (Halpern et al. 2012, Lindenmayer et al. 2012). Retention forestry alters microclimate and resource availability across spatial and temporal scales, potentially affecting understory plant composition and density (Aubry et al. 2009, Kriebitzsch et al. 2013). Introducing structural diversity by creating gaps, or by leaving habitat trees, will affect light quality and quantity, wind speed and air humidity, soil temperature and moisture, litter input, and hence nutrient availability at the forest floor (Abd Latif and Blackburn 2010). In addition, these abiotic conditions will also vary across several temporal scales (daily fluctuations, seasonal changes, year-to-year variation, Leuschner et al. 2017). In particular, the spatio-temporal variability of light quantity at the forest floor is usually greater in structurally more complex forest stands (Liira et al. 2007). Thus the complex interplay of these changes results in an altered resource availability for plants (light, nutrients, water), and hence provides competitive advantages to certain species over others. Understory plant species composition, species richness, and functional diversity have been shown to be affected by retention measures (Halpern et al. 2012, Lindenmayer et al. 2012).