Driving factors behind the eutrophication signal in understorey plant communities of deciduous temperate forests

Abstract

Summary1. Atmospheric nitrogen (N) deposition is expected to change forest understorey plant community composition and diversity, but results of experimental addition studies and observational studies are not yet conclusive. A shortcoming of observational studies, which are generally based on resurveys or sampling along large deposition gradients, is the occurrence of temporal or spatial confounding factors. 2. We were able to assess the contribution of N deposition versus other ecological drivers on forest understorey plant communities by combining a temporal and spatial approach. Data from 1205 (semi-)permanent vegetation plots taken from 23 rigorously selected understorey resurvey studies along a large deposition gradient across deciduous temperate forest in Europe were compiled and related to various local and regional driving factors, including the rate of atmospheric N deposition, the change in large herbivore densities and the change in canopy cover and composition. 3. Although no directional change in species richness occurred, there was considerable floristic turnover in the understorey plant community and a shift in species composition towards more shade-tolerant and nutrient-demanding species. However, atmospheric N deposition was not important in explaining the observed eutrophication signal. This signal seemed mainly related to a *Correspondence author. E-mail: kris.verheyen@ugent.be Ó 2011 The Authors. Journal of Ecology Ó 2011 British Ecological Society Journal of Ecology 2012Ecology , 100, 352-365 doi: 10.1111Ecology /j.1365Ecology -2745Ecology .2011 shift towards a denser canopy cover and a changed canopy species composition with a higher share of species with more easily decomposed litter. 4. Synthesis. Our multi-site approach clearly demonstrates that one should be cautious when drawing conclusions about the impact of atmospheric N deposition based on the interpretation of plant community shifts in single sites or regions due to other, concurrent, ecological changes. Even though the effects of chronically increased N deposition on the forest plant communities are apparently obscured by the effects of canopy changes, the accumulated N might still have a significant impact. However, more research is needed to assess whether this N time bomb will indeed explode when canopies will open up again.

Microclimate moderates plant responses to macroclimate warming

“…In addition, current conservation actions in European forests are regularly directed toward restoring traditional management (e.g., coppicing in ancient forests), resulting in canopy opening. Such actions could not only result in soil nutrient depletion, lower biomass pools, and enhanced soil nitrogen release (28,31), but also, depending on the sylvicultural system applied, increase temperatures at the forest floor (12-15). Large-scale reopening of the canopy for woody biomass harvesting may thus hasten thermophilization of understory plant communities of temperate forests.…”

Unexpected long-range edge-to-forest interior environmental gradients

“…Other studies use plant indicator values to gain indirect metrics of environmental conditions (Diekmann 2003). This method has a wide applicability in biomonitoring, as it yields information on several environmental factors, including estimated soil, light and climate conditions in various communities (Diekmann 1995;Hawkes et al 1997), changes resulting from vegetation succession or past land use (Verheyen et al 2012), and soil acidification and eutrophication in forests (Thimonier et al and lowest soil moisture content in summer, in various ecosystems and several countries in Europe Ertsen et al 1998;Schaffers and Sykora 2000;Diekmann 2003;Wamelink et al 2002;Gégout et al 2003;Tichy et al 2010).…”

Temporal changes in vegetation of a virgin beech woodland remnant: stand-scale stability with intensive fine-scale dynamics governed by stand dynamic events

“…This can be a consequence of less intensive forest management or even abandonment, as well as the lack of natural disturbance including the activities of grazers (Brewer 1980, Davison and Forman 1982, Vera 2000, Verheyen et al 2012, Kopecky et al 2013. A drastic decrease in herbaceous species cover was found by Łysik (2008) in a primeval beech forest in Poland and by Ujházy et al (2007) in a primeval fir-beech forest in Slovakia.…”

“…During the last decade, remarkable changes in the herbaceous layer have been shown by several studies (e.g. reviewed in Verheyen et al 2012). Resurvey studies investigated the understorey to detect potential long-term changes and to assess the role of different environmental variables as driving forces in deciduous forests.…”

“…Resurvey studies investigated the understorey to detect potential long-term changes and to assess the role of different environmental variables as driving forces in deciduous forests. The observed changes were attributed to different background causes, such as combined effect of temperature increase and canopy opening (De Frenne et al 2013, abandonment of former forest management practices (Hédl et al 2010, Heinrichs et al 2014, lack of disturbance (Brewer 1980, Taverna et al 2005, deer herbivory (Rooney and Dress 1997, Taverna et al 2005, Wiegmann and Waller 2006, increased soil acidity (Falkengren-Grerup and Tyler 1991, Durak 2010, Šebesta et al 2011, nitrogen deposition (Verheyen et al 2012) and often the intermingled influence of several factors (Heinrichs et al 2014, Vanhellemont et al 2014, Bernhardt-Römermann et al 2015, Naaf and Kolk 2016. The majority of the observed changes in the herbaceous layer show an explicit trend, as a consequence of direct or indirect anthropogenic effects.…”