Abstract
Future forest composition is uncertain in many areas due to climate change. The spread of non-native species adds to these uncertainties, particularly in forests recently colonised by novel tree species. To anticipate future forest composition, and thus the provision of ecosystem services, a thorough understanding of the factors influencing the establishment of non-native tree species is essential. We studied the presence and abundance of regeneration of Ailanthusaltissima (Mill.) Swingle in 89 plots on a regular grid in three sites in southern Switzerland to determine the main drivers of its establishment. All sites are located in abandoned, i.e. currently unmanaged stands of Castaneasativa Mill. Propagule pressure is caused by single mature A.altissima that colonised the area ca 40 years ago. We found high rock cover, low litter cover and high light availability to be the most important predictors for the presence of A.altissima regeneration, whereas its abundance was positively influenced by high light availability, low litter cover and high browsing on regeneration of competing species. However, the presence models performed much better than the abundance models. Interestingly, the most important factors favouring the establishment of A.altissima in recently undisturbed sites were found to be similar in a nearby site after a severe forest fire, which suggests a similar establishment strategy after a disturbance as in recently undisturbed forests. Based on our results we expect a further expansion of the species in lowland forests currently dominated by C.sativa, likely controlled primarily by light availability.
Highlights
Many forest ecosystems are expected to face compositional changes as a consequence of climate change, the introduction and spread of novel tree species, or interactions thereof (Lindner et al 2010; Hickler et al 2012)
A. altissima saplings often dominated the regeneration at the sampling point (57% ± 31.3% of all saplings, Table 2)
The occurrence of A. altissima saplings was mainly favoured by high rock cover, low litter cover and high light availability (Table 3)
Summary
Many forest ecosystems are expected to face compositional changes as a consequence of climate change, the introduction and spread of novel tree species, or interactions thereof (Lindner et al 2010; Hickler et al 2012). Knowledge on the possible future forest composition is a prerequisite for evidence-based, anticipatory management decisions (Lindner et al 2014), which may enable a continuous provisioning of forest ecosystem services (Gret-Regamey et al 2013; Seidl et al 2016). This is important in forests that have only recently started to be colonised by non-native trees, i.e. where the future development is often highly uncertain. Acacia dealbata Link, generally considered to be light demanding, was found to be able to grow below the canopy of native trees in south-central Chile (Fuentes-Ramírez et al 2011)
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