Seedlings of a native shrub can establish under forests dominated by an alien tree, Casuarina equisetifolia, on subtropical oceanic islands

Abstract

We investigated the effects of litter accumulation by an alien tree, Casuarina equisetifolia, on the germination and seedling establishment of Rhaphiolepis wrightiana, a shrub native to the Ogasawara (Bonin) Islands, in the northwestern Pacific Ocean, in a field experiment. We compared the emergence of seedlings in forests dominated by C. equisetifolia with that in native forests, with and without litter removal. More than 75% of seedlings emerged during a year except at the C. equisetifolia sites with litter removal (approximately 45%). Thus, seeds of R. wrightiana can germinate under C. equisetifolia if they are dispersed on the ground.

Abiotic barriers limit tree invasion but do not hamper native shrub recruitment in invaded stands

“…This is the main reason why the absence of litter also reduced seedling emergence of native species in the field. Similarly, Hata et al (2012) observed that removal of litter reduced seedling establishment of a native shrub in an area invaded by C. equisetifolia.…”

“…The likelihood of a dispersed M. obtusifolia seed becoming an established seedling was almost five times higher in the Casuarina stand below litter than inside vegetation patches. This helps to drive recolonization of native shrubs in areas invaded by this alien species (Hata et al 2012). In addition, seedlings of M. obtusifolia survived longer than 24 months in both vegetation types, probably due to seedling morphology.…”

“…Plant litter is another key factor in the recruitment, and may have positive (by decreasing the amplitude of fluctuations in soil temperature and evaporation; Eckstein and Donath 2005;Hovstad and Ohlson 2008) or negative effects (by creating physical and/ or chemical barriers-the former generally stronger the latter; Rotundo and Aguiar 2005;Hovstad and Ohlson 2008) during the early life-cycle stages of the plants (Facelli and Pickett 1991;Xiong and Nilsson 1999). Although invasive species can alter abiotic factors, such as light availability, soil factors, and features of the litter layer, some native species have the capacity to re-colonize invaded areas (Guerrero and Bustamante 2007;Fuentes-Ramírez et al 2011;Hata et al 2012). As the removal of an invasive species is usually difficult and expensive (Pimentel et al 2000;van Wilgen et al 2016), a better understanding of the factors that limit establishment of native species is required to implement effective recovery efforts in areas affected by invasive species (Ens and French 2008;Fuentes-Ramírez et al 2011).…”

Secondary invasion by an invasive shrub limits native tree seedling growth in a size‐dependent manner following the eradication of a different invasive species in an island ecosystem

“…The density and size of L. leucocephala seedlings in the seed sowing experiment confirmed that this species can invade and dominate forest gaps following the death of C. equisetifolia if there are sufficient amounts of dispersed seeds. In nature, there are few L. leucocephala seedlings and saplings in the understory of forests dominated by C. equisetifolia (Hata et al 2009), probably because L. leucocephala cannot invade undisturbed forests with closed canopies (Yamamura et al 1999). L. leucocephala seeds are supplied by both mature trees and the soil seed bank.…”

“…Both species have aggressively invaded and caused serious problems in tropical and subtropical ecosystems, especially on islands (Weber 2017). On the Ogasawara Islands, C. equisetifolia dominates secondary forests, where there are fewer seedlings and saplings of native trees (Hata et al 2009) due to the inhibition of seed germination and initial growth of native tree seedlings (Hata et al 2010a). Therefore, C. equisetifolia has been controlled on these islands to restore native forest ecosystems (Japan Ministry of the Environment 2011).…”