GEOÖKO VOLUME/BAND XXXI, 28-49 GÖTTINGERN 2010 FOREST STRUCTURES OF THE JUAN FERNANDEZ ISLANDS, CHILE: SIGNIFICANCE FOR BIRD COMMUNITY AND CONSERVATION INGO HAHN, PABLO VERGARA & UWE RÖMER ZUSAMMENFASSUNG Drei Grundtypen der Waldstruktur werden auf dem Juan Fernandez-Archipel identifiziert und analysiert: Kulturwald/Forst der basalen Stufe, nativer Wald der montanen Stufe und nativer Baumfarn-Wald der subalpinen Stufe. Für den Kulturwald/Forst auf Robinson Crusoe sind die eingeschleppten Eucalyptus und Cupressus charakteristisch. Der montane Wald wird durch endemische Myrtenbäume der Gattung Myrceugenia auf beiden Hauptinseln dominiert. Der subalpine Baumfarnwald ist durch ein Mosaik der Gattungen Dicksonia und Drimys auf die Insel Alejandro Selkirk beschränkt, denn auf Robinson Crusoe wird diese Höhenstufen nicht erreicht. Die strukturellen Parameter divergieren stark sowohl zwischen autochthonen und allochthonen Waldtypen als auch zwischen autochthonen Wäldern unterschiedlicher Höhenstufen. Der montane Wald auf Robinson Crusoe und Alejandro Selkirk zeigt ähnliche strukturelle Muster, aber die Bestände auf Selkirk sind stärker verinselt und artenärmer. Die deutlichen strukturellen Unterschiede der analysierten Waldtypen spiegeln sich in der Vogeldiversität und -dichte wider. Native Wälder bevorteilen ein gleichmäßigeres Vorkommen von Arten und liefern einen diverseren Vogellebensraum; Kulturwälder/Forste können nur für einzelne heimische Vogelarten aufgrund bestimmter ökologischer Gegebenheiten von Bedeutung sein. Endemische Vögel hängen hochgradig von nativen Waldstrukturen ab, weil sie angepasst sind an spezifische Muster der Nahrungssuche, der Nistplatzwahl und des Schutzes von Fressfeinden. Anthropogen verursachte Zerstörung solch nativer Waldstrukturen hat mehrere Vogelarten an den Rand des Aussterbens gebracht. Naturschutz-Management muss die Wiederherstellung des gesamten Inselökosystem zum Ziel haben, beginnend mit Ausrottungsprogrammen für die eingeschleppten Säugetiere. Schlüsselworte: gefährdete Endemiten, Inselökologie, Pflanze-Tier-Interaktionen, Naturschutz, Ökosystem-Management, Vogelanpassung. SUMMARY Three principal forest structure types were identified and analysed on the Juan Fernandez Archipelago: cultivated forest of the basal region, native montane forest, and native sub- 28 alpine tree-fern forest. In the cultivated forest of Robinson Crusoe the introduced Eucalyptus and Cupressus are characteristic. The montane forest is dominated by endemic myrtle trees Myrceugenia and found on both major islands. The sub-alpine tree-fern forest is a Dicksonia- Drimys mosaic restricted to Alejandro Selkirk. Structural parameters between autochthonous and allochthonous forest types diverge strongly as well as between autochthonous forests of different altitude levels. The montane forests of Robinson Crusoe and Alejandro Selkirk show similar structural patterns, but stands are more scattered and species-poor on Selkirk. The clear structural differences between the analysed forest structures are reflected in bird diversity and abundance. Native forests favour a more equal occurrence of species and prepare a more diverse bird habitat; cultivated forests may be of importance for single species in regard to a specific ecological feature. Endemic birds highly depend on native forest structures, as they are adapted to specific patterns of forage, nest site, and shelter from predation. Human-related destruction of such native forest structures has brought several bird species close to extinction. Conservation management should restore the island ecosystem, starting with an eradication program of the introduced herbivores. Keywords: animal-plant interaction, bird adaptation, ecosystem management, island ecology, nature conservation, threatened endemics. 1 INTRODUCTION The structure of forests generally depends on the plant species present and the environmental impacts (Korpel 1995, Haberle 2003, Stuessy et al. 2005). In turn, forest structures have been identified to be a decisive factor for the presence of a certain animal community or occurrence of a specific bird species (e.g. Mattes 1988). On the Juan Fernandez Islands birds are the sole native land vertebrates, and the only animal group yet investigated in regard to its community ecology (Hahn et al. 2005, 2009). However, several of the endemic bird species are critically endangered and about to disappear from the community. According to King (1980, 1985), Simberloff (1978), and Wiens (1994) the main reason for extinction in island birds is the destruction and degradation of vegetation. Besides the bird community, a great number of other plant and animal species directly depends on the specifity of forest. Thus, the presentation of detailed forest structure patterns would enable a comparison of the demands of endemic birds, helping to identify significances for the community, and finally provide a basis for conservation management (Ricci 2006, comp. also Gerold & Markussen 2007). Skottsberg (1953) has given a provisory but comprehensive description of plant communities, including the first data on height and coverage of several of the archipelago’s forests. However, since his field studies in the years 1916 to 1918 introduced numerous species invaded forests, changed plant composition and structure, or even formed complete allochthonous stands. In the second half of the last century further aspects of forest ecology or selected units have been the matter of investigation (Kunkel 1956, Schwaar 1979, Stuessy 29 1992, Stuessy & Ono 1998), and advances in description and classification of the FLORA FERNANDEZIANA were made (Marticorena et al 1998, Bernadello et al 2006, Baeza et al. 2007, Wheeler 2007). In the year 2002 Greimler et al. (2002a) presented a detailed description of vegetation distribution on Robinson Crusoe Island (comp. also Moreira-Muñoz 2007). However, all these valuable surveys did not prepare recent information on structural patterns: and from the ornithological view specific scales not congruent with the present results derived from botanical methodology are favourable. The main question of this survey is: Which forest structures exist on the Juan Fernandez Islands, and which importance do they have for endemic versus invasive landbirds? No such investigations yet exist, but it can be hypothesised that endemic landbirds are more bound to native forest structures on islands than invasive landbirds. Therefore a comparative avifauna-focussed analysis of the forest environments of the Juan Fernandez Archipelago is aimed at, combining methodological approaches and data from vegetation structure research, phyto-coenology, and ornithology. 2 METHODS The Juan Fernández Archipelago is of volcanic origin and situated in the south-east Pacific Ocean off the coast of Chile (33° 28' 48'' S to 33° 47' 57'' S and 78° 47' 12'' W to 80° 47' 44'' W). It consists of the islands Robinson Crusoe (formerly Másatierra), Alejandro Selkirk (formerly Másafuera), Santa Clara, and some small rocks (Fig. 1). The easternmost Robinson Crusoe (47.11 km², 915 m high) is located 567 km away from the continent, the westernmost Alejandro Selkirk (44.64 km², 1320 m) another 167 km west of Robinson Crusoe, the smaller Santa Clara (2.23 km², 375 m) only 1.5 km south-west of Robinson Crusoe. The whole archipelago is a Chilean national park since 1935 and UNESCO Biosphere Reserve since 1977, except from the San Juan Bautista settlement. More detailed (bio)geographical descriptions may be taken from Castilla (1987), Skottsberg (1920-1956), and Stuessy & Ono (1998). 30 Fig. 1. Geographical position of the Juan Fernandez Archipelago National Park and its single islands Alejandro Selkirk, Robinson Crusoe and Santa Clara in the south-eastern Pacific Ocean off mainland Chile. The cold Humboldt Current causes a similar temperate oceanic climate in the islands like in southern Chile between 38° and 41° S latitude. 31 From 1992 to 2009 five field campaigns were carried out to study the ecology of the Juan Fernández Archipelago. All three major islands were visited during a total of 315 days. Composition and distribution of vegetation types may be compared to Skottsberg (1953), Greimler et al. (2002a), Hahn et al. (2005), and Hahn (2006). Forest structures are described on the base of structural vegetation analyses. These were made by measuring any plant individual in regard to its position, height, thickness of the stem, and crown extension to the directions within a sample plot. Data registration was completed by drawing rough sketches and taking photo shots. Field data then were transferred drawing on millimetre scaled paper. To also document the entire plant species composition in selected sample plots, vegetation relevees were carried out using the method of Braun-Blanquet (1964). These relevees were useful for comparisons to the results of Greimler et al. (2002a) as well as Skottsberg (1953), the latter following the Swedish school. Completing tabular data of species name, stem diameter, storey coverage, and height are given in the appendices. 3 RESULTS 3.1 FOREST STRUCTURE ANALYSES Principally the forests can be distinguished between types of the islands Robinson Crusoe and Alejandro Selkirk as well as between allochthonous and autochthonous types. Native forests once covered major parts of both these islands, as illustrated and reported from the 17th and 18th century (e.g. Walter & Robins 1748, comp. Danton et al. 1999). Through man-made fires, selective cutting, impact of introduced herbivores,