Can Species Richness Be Maintained in Logged Endemic Acacia Heterophylla Forests (Réunion Island, Indian Ocean)?
Total Page：16
File Type：pdf, Size：1020Kb
View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by I-Revues CAN SPECIES RICHNESS BE MAINTAINED IN LOGGED ENDEMIC ACACIA HETEROPHYLLA FORESTS (RÉUNION ISLAND, INDIAN OCEAN)? Stéphane BARET1,2*, Thomas LE BOURGEOIS2,3, Jean-Noël RIVIÈRE2,3, Thierry PAILLER2,4, Jean-Michel SARRAILH2,3 & Dominique STRASBERG2,4 RÉSUMÉ. — La richesse spécifi que peut-elle être conservée dans des forêts exploitées d’Acacia hete- rophylla, espèce endémique de l’île de la Réunion (océan Indien) ? – Il est globalement considéré que les forêts peuvent assurer des usages multiples, incluant la production de bois et le maintien d’un niveau élevé de biodiversité. Dans le but de tester cette hypothèse, nous avons étudié les méthodes d’éradication de Rubus alceifolius actuellement utilisées au sein d’une forêt exploitée d’Acacia heterophylla, espèce endémique de l’île de la Réunion. Nos résultats montrent que tous les individus de l’espèce envahissante la plus répandue dans la zone -Rubus alceifolius-, proviennent de bouturage. Nous avons aussi quantifi é la forte capacité de croissance de cette espèce au travers d’une comparaison de croissance avec l’Acacia heterophylla mais aussi avec un congénère indigène -Rubus apetalus var. apetalus. Ces capacités de multiplication et de croissance végétatives importantes permettent à Rubus alceifolius de former des massifs monospécifi ques en seulement 2,5 ans. La richesse spécifi que de la forêt d’Acacia heterophylla exploitée en est alors largement affectée. En effet, de manière générale, même si la richesse spécifi que semble augmenter légèrement au cours du temps, les espèces observées sont généralement des espèces exotiques envahissantes. Comparant l’extraction du bois à un haut niveau de perturbation, les auteurs sont très pessimistes sur la possibilité de maintenir une diversité spécifi que élevée au sein d’une forêt exploitée. Néanmoins ces résultats apportent une lueur d’es- poir et un programme d’éradication mieux adapté au type de forêt étudié est proposé. SUMMARY. – It is assumed that forests can serve multiple uses, including wood production and main- tenance of high biodiversity level. We tested this hypothesis by studying eradication methods of invasive plants currently implemented in exploited endemic Acacia heterophylla forests located in a tropical highland forest region on the island of Réunion. We also compared species richness in logged (over time) and natural forests. Our results show that all individuals of the widespread alien invasive plant Rubus alceifolius were generated from cuttings. We quantifi ed the high growth capability of this species by comparing with those of Acacia heterophylla along with Rubus apetalus var. apetalus, a close non-invasive congener. The substantial multiplicative and vegetative growth ability of R. alceifolius enabled it to form monospecifi c patches in only 2.5 years. The species richness of the exploited Acacia heterophylla forest was thus very affected. Although the species richness increased over time after logging, the extent of the resulting richness was not as great as that in natural forests which, moreover, included numerous exotic plants. The authors assess the impact of highly disturbing logging operations and, based on the results of the comparisons, are very pessimistic about the possibility of maintaining species richness in logged forests. Nevertheless, the results indicate that the situation is not beyond hope and an eradication program specifi cally tailored to this kind of forest is proposed if exploitation will stop. 1 Conservatoire Botanique National de Mascarin, 2 rue du Père Georges. F-97436 Saint-Leu, La Réunion, France 2 UMR CIRAD/Université de la Réunion, Peuplements Végétaux et Bioagresseurs en Milieu Tropical * author for correspondence: Tel.: +262 (0) 262247926. Fax: +262 (0) 262248563. E-mail: [email protected] 3 Cirad, Pôle de Protection des Plantes, Ligne Paradis. F-97410 Saint-Pierre, La Réunion, France 4 Université de la Réunion, BP. 7151. F-97715 Saint-Denis Messag. Cedex 9, La Réunion, France Rev. Écol. (Terre Vie), vol. 62, 2007. – 273 – Biological invasions are well known to contribute to biodiversity loss (Vitousek, 1988; Wilcove et al., 1998). Once introduced in a new area, the most suitable way to control alien invasive plants is eradication, while facilitating spontaneous indigenous regeneration in order to maintain species richness (Prach et al., 2001 a,b). This is the strategy adopted by forestry services. For many years they have been striving to associate wood production with the main- tenance of species richness (e.g. Work et al., 2003). However, the idea that forests can serve multiple uses, including wood production and maintenance of high biodiversity level, is an untested hypothesis (Simberloff, 1999). The endemic Acacia heterophylla Willd. forests of Réunion (Mascarene archipelago, Indian Ocean) offer a good opportunity to test this hypothesis with an experimental fi eld approach. Although it belongs to one of the world biodiversity hotspots within the Malagasy region (Myers et al., 2000), the last 30% of intact virgin plant cover of Réunion are nowadays threatened by invasive alien plants (Strasberg et al., 2005; Baret et al., 2006). Since the fi rst human settlement on Réunion in the 17th century, more than 2000 plant species have been introduced. Most of them are only cultivated as agricultural or ornamental in town and in small gardens but many species became subspontaneous and 62 species are now considered very invasive (Macdonald et al., 1991). The national forestry offi ce tries to control these invasive alien plants, particularly Rubus alceifolius Poiret, the most widespread of these plants (Mac- donald et al., 1991). For many years, at great expense, the forestry service has focused on eradicating this weed. The annual control expenses devoted solely to Rubus in 1983 (mainly manual control) were about 160 000 €, for an area of 345 ha (ONF, 1983). In the last survey, 1 500 ha of exploited Acacia forests (logged since 1947, Miguet, 1957) were included in the last 5 500 ha of well preserved endemic Acacia heterophylla forests (Michon, 1998). Rubus alceifolius control expenses are particularly high in this type of habitat (particularly the exploited part), which is currently highly invaded (Strasberg et al., 2005). As exploited A. heterophylla forests are located on the border of the last natural 4 000 ha of preserved virgin forest, it is essential to control this invasive plant in exploited areas to keep it from spreading further and endangering the natural remnant forest patches. This has been the goal of the forestry services for more than 15 years (see review in Hivert, 2003). In 1990, a grey report estimated expenses linked to manual and chemical R. alceifolius control in this unique forest at 2 240 €/ha (Parret, 1990). Despite this fi nancial output, this alien plant continues to invade and threaten A. heterophylla forests. In order to determine whether wood production and biodiversity maintenance are compat- ible, we need to better understand eradication methods implemented in exploited A. hetero- phylla forests and compare species richness within logged forests and similar natural habitats. The goal of this study was therefore to gain insight into why Rubus alceifolius (alien plant which requires sustained intensive and costly eradication programs) continues to fl ourish in exploited A. heterophylla forests and, secondly, to determine whether species richness is main- tained in these forests. The following questions were thus addressed: (1) What is the reason for new R. alceifolius infestations after eradication? (2) Do the growth ability and seed germina- tion rate of R. alceifolius explain its invasiveness? and (3) What are the species richness differ- ences between exploited and natural A. heterophylla forests? In order to answer these questions, we focused our study on gaining insight into why only a single total cut (the main method used against R. alceifolius) of the invasive plant for eradication is not suffi cient. We thus fi rst aimed to confi rm Miguet’s hypothesis (1980), who considered that R. alceifolius invasion resulted from the growth of numerous seedlings after patches eradication. We also compared R. alceifolius and A. heterophylla growth. The growth and seed germination rates of both Rubus species (R. alceifolius and R. apetalus var. apetalus, an indigenous congener) were also compared. In order to assess the maintenance of species richness in exploited forests (for wood production), we also compared plant diversity between a logged A. heterophylla forest and a similar natural habitat. The answers to these questions could help to orient eradication schemes in endangered endemic Acacia heterophylla forests and indicate whether or not wood production is compat- ible with species richness maintenance in forests. – 274 – MATERIAL AND METHODS STUDY SITE This study was carried out in Réunion (2 512 km2), a volcanic island located in the southwestern Indian Ocean region (21°06’S, 55°32’E). Measurements were carried out at Bélouve (889 ha), at 1 500 m of elevation, with abundant precipitation (3 346 mm.year-1) and with a mean annual temperature of 13.6°C. This locality was totally covered by Acacia heterophylla forest before human settlement in the 17th century. This habitat type includes a natural plant association dominated by A. heterophylla (an endemic species) associated with different endemic trees, ferns and orchids (consisting of 96 fl owering plant species, including 41 endemic species, Strasberg et al., 2005) STUDY SPECIES Acacia heterophylla Willd.– This endemic species is taxonomically close to another phyllodinous species, Acacia koa A. Gray, endemic to Hawaii, and heterophyllous species do not occur on continents except Australia where they arose (Pedley, 1975; Coulaud et al., 1995). In Réunion, this endemic tree grows (in pure or mixed stands) in a vast forest surrounding the island between 1 400 and 1 800 m of elevation, excluding areas where rainfall is heavy (more than 6 m precipitation per year). Before human settlement in the 17th century, this habitat area was estimated at 8 000 ha.