Low litter cover, high light availability and rock cover favour the establishment of Ailanthus altissima in forests in southern Switzerland

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.

Between approval and disapproval: Citizens’ views on the invasive tree Ailanthus altissima and its management

“…Some rural populations also exist in forest gaps (e.g. Knüsel et al 2019;Lapin et al 2019;Wagner et al 2020), but these are usually less abundant in central Europe than urban populations. While there is little evidence of negative effects of Ailanthus on urban biodiversity, wild urban populations can be challenging due to the vigorous clonal growth and regeneration of the tree, necessitating increased efforts for maintaining green spaces, transportation corridors and built structures (Kowarik and Säumel 2007;Sladonia et al 2017), including ancient monuments in southern Europe (Celesti Grapow and Ricotta 2020;Trotta et al 2020).…”

Spatial patterns of native Robinia pseudoacacia and invasive Ailanthus altissima and their influence on regeneration, abundance, and diversity of neighboring trees at local and regional scales

“…Some A. altissima populations may exhibit greater shade-tolerance than once thought with evident growth under light conditions below typical of shade-intolerant species (Martin et al 2010;Knüsel et al 2017). In fact, the microsite conditions that favor A. altissima colonization of forests once dominated by Castanea sativa were the same for recently undisturbed forests or post-re forests (Knüsel et al 2019). Such a capacity has not been established for R. pseudoacacia.…”

Similar Impacts of Alien and Native Tree Species on Understory Light Availability in a Temperate Forest

“…It is also one of the most important drivers of plant species composition [24,29,30], as well as soil biota [21]. It is also crucial for regeneration of canopy species [31][32][33]. Usually minimum light availability for sapling survival is higher for pioneer tree species than mid-and late-successional tree species [25,34,35].…”

“…This value varies light availability under open sky by around 5%-10% [32,36,37].Despite numerous studies confirming light availability limitation by alien species, few studies compare their influence with functionally analogous native competitors. Most studies on invasive plants focused rather on conditions favoring their establishment than their impacts [33,38,39]. For that reason we aimed to assess how much leaf area index (LAI, m 2 of leaves m −2 stand area) determines light availability on the forest floor and whether alien, invasive trees contribute more to canopy foliage area (expressed by LAI) than native species, assuming similar contributions to stem quantity (expressed as basal area, BA, m 2 ha −1 ).…”