Plant Ecology & Diversity ISSN: 1755-0874 (Print) 1755-1668 (Online) Journal homepage: https://www.tandfonline.com/loi/tped20 Invasive plant species thresholds in the forests of Robinson Crusoe Island, Chile Rodrigo Vargas-Gaete, Christian Salas-Eljatib, Stefanie M. Gärtner, Osvaldo J. Vidal, Jan R. Bannister & Aníbal Pauchard To cite this article: Rodrigo Vargas-Gaete, Christian Salas-Eljatib, Stefanie M. Gärtner, Osvaldo J. Vidal, Jan R. Bannister & Aníbal Pauchard (2018) Invasive plant species thresholds in the forests of Robinson Crusoe Island, Chile, Plant Ecology & Diversity, 11:2, 205-215, DOI: 10.1080/17550874.2018.1444109 To link to this article: https://doi.org/10.1080/17550874.2018.1444109 Accepted author version posted online: 08 Mar 2018. Published online: 09 Mar 2018. Submit your article to this journal Article views: 96 View Crossmark data Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=tped20 PLANT ECOLOGY & DIVERSITY 2018, VOL. 11, NO. 2, 205–215 https://doi.org/10.1080/17550874.2018.1444109 Invasive plant species thresholds in the forests of Robinson Crusoe Island, Chile Rodrigo Vargas-Gaete a, Christian Salas-Eljatiba, Stefanie M. Gärtnerb, Osvaldo J. Vidalc, Jan R. Bannisterd and Aníbal Paucharde,f aDepartamento de Ciencias Forestales, Universidad de La Frontera, Temuco, Chile; bBlack Forest National Park, Seebach, Germany; cInstituto de la Patagonia, Universidad de Magallanes, Punta Arenas, Chile; dInstituto Forestal, Valdivia, Chile; eFacultad de Ciencias Forestales, Universidad de Concepción, Concepción, Chile; fInstituto de Ecologia y Biodiversidad, Santiago, Chile ABSTRACT ARTICLE HISTORY Background: Invasion by exotic plants worldwide can lead to the loss of native species, (Received 17 February 2017; particularly on islands with a high proportion of endemic plants, such as Robinson Crusoe Accepted 20 February 2018) Island (RCI). KEYWORDS Aims: We studied the two most invasive exotic plant species occurring in the forest of RCI: Aristotelia; canopy gaps; Aristotelia chilensis and Rubus ulmifolius. We aimed at establishing thresholds for environ- Juan Fernández Islands; mental and microsite variables related to invasion. restoration; regression trees; Methods: Environmental and forest understorey variables, including canopy gaps and inva- Rubus sive species cover were measured in non-invaded and invaded forest sites. We expected more invasion in plots located close to invasive shrublands, and in large gaps with high solar radiation. Results: We found no relationship between the distance to invasive shrublands and invasion probability. Solar radiation tended to be slightly related with a higher cover of R. ulmifolius, the most abundant invasive exotic plant in RCI forests. Overall, the cover of native ferns appeared to inhibit invasion. Conclusions: The identification of variable thresholds related to invasion can be useful for guiding management decisions. Our results suggest that management should consider monitoring forest canopy gap formation and promote the establishment of ferns to reduce the probability of invasive species establishing. Introduction the loss of species dependent on natural forest characteristics and composition, such as birds The ecological impact of invasive plant spe- (Minden et al. 2010) and smaller vascular cies on native communities they invade has species. These impacts are particularly strik- been well documented, especially on islands ing on island ecosystems where invasive plant (Hejda et al. 2009;Vilàetal.2011). Invasive species can be particularly successful due to plant species are those introduced by humans the high availability of resources and niches into a region in which they did not evolve and and the fact that native species are usually wheretheycancompetewithnativespecies, specialists that are easily displaced by exotics alter natural habitats and affect nutrient (Daehler et al. 2004). cycling and/or interfere with native food Most plant invasions occur in forests altered webs, impacting on the natural resources and by human disturbances, open canopy and/or may affect economic activities (Heleno et al. canopy gaps (Totland et al. 2005; Baret et al. 2010). Invasive plants may prevent tree seed- 2008). Canopy gaps caused by the death and ling recruitment in forest by direct competi- fall of trees or large branches are the dominant tion for light, water, nutrients and/or space small-scale disturbance in temperate forests (Burnham and Lee 2010). Furthermore, inva- (Oliver and Larson 1990; Schliemann and sive species may persist for several years in Bockheim 2011). Such disturbances increase seedbanks or as seedlings (Vilà et al. 2011), resource availability, and facilitate the coloni- and they can promote the expansion of other sation by species with greater competitive abil- exotic invasive species (Vargas et al. 2013) ities than the natives (Prieur-Richard and leading to changes in forest structure and Lavorel 2000). Canopy gaps provide safe sites CONTACT Rodrigo Vargas-Gaete [email protected] © 2018 Botanical Society of Scotland and Taylor & Francis 206 R. VARGAS-GAETE ET AL. for tree species to germinate, particularly light- and biotic factors promoting invasion at the demanding taxa (Yamamoto 2000). microsite scale, remained unclear. Consequently, invasive species may take Understanding which environmental and micro- advantage of these areas with higher radiation site variables are important for invasion success to establish (Funk 2013;Driscolletal.2016). is fundamental for scientific and management/ This process occurs in several island ecosys- conservation purposes, including forest restora- tems worldwide where invasive plants establish tion in the Juan Fernandez Archipelago, as well in canopy gaps, affecting forest regeneration, as on other islands with natural forest remnants. leading to fragmentation, loss of forest area We regard microsite as the space where a plant is and eventually to species extinctions (Baret rooted, above which its foliage develops and et al. 2008;Vargasetal.2013;Denslow expands, i.e., a space of similar environmental 2003). Thus, it is relevant to understand conditions such as: vegetation cover, sunlight, which environmental and microsite variables temperature, soil and physical characteristics promote plant invasions on island ecosystems (Whittaker and Levin 1977). Identifying envir- so that their impact on biodiversity could be onmental and microsite variables could help to reduced. understand the biotic resistance to exotic species Robinson Crusoe Island (RCI) (33° 38` 29” S invasion within natural communities (Levine 78° 50` 28” W, Juan Fernández Archipelago) has et al. 2004). Some variables have been considered the highest rate of plant endemism in the world important to explain invasion in forest commu- − (1.9 species km 2) Bernadello et al. 2006). nities worldwide. They include the distance to Canopy gaps affect, on average, 22 ± 15.8% of roads, distance to the nearest large source of the native forests (Smith-Ramírez et al. 2017). propagules of invasive species (Von Holle and The montane forests of RCI (Greimler et al. Simberloff 2005) and the forest canopy cover, 2002), referred to as Myrtisylva due to the dom- which relates with solar radiation that usually inance of Nothomyrcia fernandeziana of the correlates with invasive species presence Myrtaceae (Danton 2006), covers an area of (Pauchard and Alaback 2004; Reinhart et al. 1,015 ha (Smith-Ramírez et al. 2013). This forest 2006). Similarly, the proportion of forest gaps contains around 50 to 60 endemic vascular plant and the time since gap formation (i.e., gap age) species (ca. 70% endemism) and is the main have also been found to be related to invasion, habitat of critically endangered plant and bird because the probability of invasion tends to species (Hahn et al. 2011; Vargas et al. 2011). increase with time (Burnham and Lee 2010). Plant invasion in the forests of RCI mainly Defining thresholds of ecological variables involves two exotic plant species: Aristotelia chi- related with the occurrence of a given species lensis and Rubus ulmifolius, that arrived from can be very useful to guide management activ- continental Chile around 1864 and 1927, respec- ities (Hothorn and Müller 2010). Thresholds go tively (Dirnbörk et al. 2003). The fleshy fruits of a little further than just identifying a positive or these invasive plants are principally spread into negative relationship of a predictor variable with, the forest gaps by gravity, wind and the native for example, the presence of an endangered spe- austral thrush (Turdus falcklandii) (Mora and cies (Müller and Butler 2010). They provide a Smith-Ramírez 2016). Once established, both limit to the relationship found, hence an invasive species spread vegetatively, often by sto- improvement in actions can be used to promote lons (R. ulmifolius), quickly occupying the dis- or prevent certain species (e.g., number of stand- turbed area. In canopy gaps, where invasive ing dead trees snags for birds, microsites for tree species dominate, the abundance (−69%) and regeneration; Müller and Bütler 2010; Vargas richness (−28%) of native plants are reduced et al. 2013). Although there exists a large body (Lara et al. 2014) and can even lead to the local of knowledge about environmental and microsite extinction of some native species in some characteristics that invasive species generally instances (Vargas et al. 2013). prefer (e.g., vacant