Modelling data - tree mortality in response to invasive-insect induced defoliation at Reunion Island

Study area The study was conducted on Reunion Island (a French overseas territory) in the Mascarenes archipelago. Reunion Island is a tropical biodiversity hotspot thanks to its unique location, geology, and elevation gradient, and it displays one of the highest rates of endemism (Thébaut et al, 2009, Juillet et al, 2017), but also one of the highest rates of biological invasion (Turbelin et al, 2016, Fenouillas et al, 2020, 2021). Because of the two volcanic summits exceeding 2500 meters a.s.l., the island is characterised by a steep elevational gradient and a sharp difference in wind and rain exposure, creating a wide variety of habitats. Acacia heterophylla is a tree species endemic to Reunion Island. The species ecology covers a wide range of vegetation types along the elevation gradient between ~1300 and 2000 m a.s.l. Furthermore, the species also has great local importance as a part of the landscape and for its cultural and economic value. In this study, we focused on two major habitats of Reunion Island where the species structures the canopy: (i) Tamarinaie - the mesothermal mountain forest at Reunion Island, is found within the altitude range of 1300 to 2000 m a.s.l. It represents a form of pioneering vegetation that spontaneously colonises predominantly barren substrates, such as post-fire sites or recent geological formations (Cadet, 1977). This habitat corresponds to the central part of the distribution range of A. heterophylla along the altitudinal gradient, thus offering suitable ecological conditions for high-quality timber production from this species, as it can be found in high abundance with large individuals (Strasberg et al., 2005). (ii) Ericoide - the subalpine vegetation of the Reunion island is unique in the Mascarenes archipelago, harbouring a high level of endemism, making it a habitat of great importance for conservation. This shrubland vegetation type thrives at mountain summits, covering altitudes ranging from 1700 to 2000 m, depending on local ecological conditions. This particular habitat is the best-preserved ecosystem on the island, covering an area of approximately 20,000 hectares (Fenouillas et al., 2020), and is composed by two endemic species: Erica reunionesis E.G.H.Oliv. and Sophora denudata Bory. This habitat marks the upper limit of Acacia heterophylla’s altitudinal range, where the species occurs in low abundance and is represented by smaller individuals. Acizzia uncatoides was first described in New Zealand but originates from Australia and is naturally associated with the host plant Acacia melanoxylon. The species has been widely introduced around the world, living and feeding on the Acacia and Albizia genera (Dreistadt et al., 1994). It was first recorded in 2011 on Reunion Island, while the first important damage was recorded in 2017 (Ouvrard et al 2016). Both adults and larvae feed on phloem, primarily targeting young photosynthetic organs such as buds, young leaves, and green fruits. The honeydew they excrete promotes the development of sooty mould, which impairs host photosynthesis. During outbreaks, these psyllids induce defoliation, which can lead to the death of trees. Sampling DesignIn 2018, eight permanent plots (50 × 50 m) were established and monitored between March 2018 and June 2021 following A. uncatoides outbreaks. Six plots were located in ericoide subalpine vegetation - four on the Piton de la Fournaise plateau and two on the Maïdo massif - and two were located in tamarinaie on the Maïdo massif. In each plot, all A. heterophylla individuals taller than 20 cm were tagged, and their diameters were measured consistently at the same height during each survey (Table \ref{descrip}). For each tree, defoliation was estimated as the proportion of leafless branches within the crown, and tree mortality was recorded at each survey.