Landscape ecological vegetation map of Sint Eustatius (Lesser Antilles) Freitas, J.A. de, A.C. Rojer, B.S.J. Nijhof & A.O. Debrot Report number ~nummer~ IMARES Wageningen UR Institute for Marine Resources & Ecosystem Studies Client: The Ministry of Economic Affairs, Agriculture and Innovation, P.O. Box 20401, 2500 EK The Hague, The Netherlands. Contact: Paul C. Hoetjes, Policy Coordinator Nature BO-11-011.05-004 Publication date: April 30, 2012 IMARES is: • an independent, objective and authoritative institute that provides knowledge necessary for an integrated sustainable protection, exploitation and spatial use of the sea and coastal zones; • an institute that provides knowledge necessary for an integrated sustainable protection, exploitation and spatial use of the sea and coastal zones; • a key, proactive player in national and international marine networks (including ICES and EFARO). “This research is part of the Wageningen University BO research program (BO-11-011.05-004) and has been financed by the Ministry of Economic Affairs, Agriculture and Innovation (EL&I) under project number 4308202004 ©Photographs titlepage: A.O. Debrot, IMARES P.O. Box 68 P.O. Box 77 P.O. Box 57 P.O. 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This report may not be reproduced and/or published partially or in its entirety without the express written consent of the client. A_4_3_2-V11.2 2 of 58 Report number ~nummer~ Abstract A semi-detailed landscape-based vegetation map (scale: 1: 37,500) is presented for the 21 km2 Lesser Antillean island of St. Eustatius, Netherlands Caribbean. The principal lower sections of the island possess a tropical savannah climate according to the Köppen (1931) classification system, and the documented flora of the island amounts to 505 species. Color aerial photographs (1: 8,000) taken in 1991 were used to produce the map. A total of 84 vegetation sample plots were analysed using a stratified random sampling design and TWINSPAN cluster analysis. A total of four main and 16 sub-landscape types were distinguished based on geology, geomorphology and different mixes and expressions of the component vegetation types. The five principal landscape types are in descending order of importance: H1, H2, M4, M9 and C, and covered some 67 % of the semi-natural habitat of the island. H1 and H2 are the Pisonia-Justicia hills and Pisonia-Bothriochloa hills and are limited to the Boven area. Analysis of the sampling data resulted in the distinction of 13 (semi- )natural vegetation types. The three principal vegetation types were, the Pisonia-Justicia type, Pisonia- Ayenia type and Bothriochloa-Bouteloua type which together accounted for 38 % of total (semi-)natural vegetation cover. The following well-developed vegetation types of St. Eustatius represent primary climax communities: Types 1, 2, 3, 5 and 7, all found in and around the Quill in the southwestern part of the island. A comparison of the vegetation types in the present study with those of Stoffers (1956) showed that only one vegetation type closely resembles one in Stoffers’ study. Major changes seem to have taken place in the natural vegetation of the island in the intervening five decades. The majority of the central sections of the island around Oranjestad the so-named “Cultuurvlakte”, amounting to approximately 25 % of the surface of the island, have suffered intensive disturbance due to past agriculture, livestock husbandry and invasive species and were not mapped. Only a small remnant portion of the semi-natural lowlands vegetation (L1 and L2) is left in the coastal reaches of Billy’s Gut. Nevertheless, this area is heavily affected by grazing and the invasive vine Antigonon leptopus. A comparison with the 1950s vegetation map by Stoffers shows that the rarest and most valuable elfin woodland vegetation of the rim of the Quill crater has been largely lost and that the areas he described as “Montane thickets” (Type 2) have declined and been degraded. We speculate that these losses may be most directly attributable to damage by a bush fire some 20 years ago and/or to the impact of recent hurricanes. Failure to regenerate after impact may be ascribed to the omnipresent feral grazers. On the lower slopes of the Quill, several areas mapped by Stoffers as farmland have been abandoned and have evidently regenerated into mixed deciduous and evergreen thorny woodlands. The vegetation of the Mountains area shows some recovery since the 1950s. There are more evergreen bushes, and less Acacia and Leucaena than Stoffers described. The vegetation Stoffers described for the lowlands had more Acacia than we found but the invasive Antigonon has since dramatically increased as a ubiquitous and often dominant species. The former importance of Opuntia prickly pear cacti in disturbed vegetations has dramatically declined since the 1950s. We ascribe this to the likely presence of the invasive parasitic insect Cactoblastis cactorum. In the 1980s and 1990s many Opuntia cacti were seen affected by this insect (G. Lopes, pers. comm.). Our field data show that all wilderness areas of St. Eustatius remain heavily affected by grazers. This reduces the resilience of natural vegetations and interferes with natural succession by imparting heavy losses to hardwood seedlings and saplings (see e.g. Melendez-Ackerman et al. 2008), by reducing plant biomass (which increases exposure to wind and sun), and by favoring hardy invasive plant species. In Curaçao, large scale reduction in densities of feral grazers in the Christoffelpark since 1993 has led to Report number ~nummer~ 3 of 58 rapid recovery of several rare plant species and vegetation types. Therefore the number one priority for terrestrial conservation in St. Eustatius will be to reduce feral grazer densities and impacts in key wilderness areas. This research is part of the Wageningen University BO research program (BO-11-011.05-004) and has been financed by the Ministry of Economic Affairs, Agriculture and Innovation (EL&I) under project number 4308202004. 4 of 58 Report number ~nummer~ Contents Abstract .................................................................................................................... 3 1 Introduction ..................................................................................................... 7 2 The Island of Sint Eustatius ................................................................................ 8 2.1 Geography .............................................................................................. 8 2.2 Climatic data ........................................................................................... 8 2.3 Geology and geomorphology ................................................................... 10 2.3.1 Geology ..................................................................................... 11 2.3.2 Geomorphology .......................................................................... 12 2.4 Soil- and landtypes of the geological formations ......................................... 13 2.4.1 Flora and vegetation.................................................................... 16 2.4.2 Human influence on flora and vegetation........................................ 17 3 Methods......................................................................................................... 19 3.1 Photo interpretation and fieldwork............................................................ 19 3.2 Data collection....................................................................................... 19 3.3 Data processing ..................................................................................... 20 3.4 Final map compilation............................................................................. 20 4 Results .......................................................................................................... 22 4.1 Vegetation types.................................................................................... 22 4.1.1 Myrcia splendens - Quararibea turbinata type (2 relevés) ................. 22 4.1.2 Coccoloba swartzii - Ardisia obovata type (3 relevés) ....................... 23 4.1.3 Chionanthus compacta - Nectandra coriacea type (9 relevés)............ 23 4.1.4 Pisonia subcordata - Eugenia axillaris type (7 relevés) ..................... 24 4.1.5 Pisonia subcordata - Justicia sessilis type (11 relevés) ..................... 25 4.1.6 Capparis cynophallophora - Gymnanthes lucida type (4 relevés)........ 25 4.1.7 Antirhea acutata - Dodanaea elaegnoides type (6 relevés)................ 26 4.1.8 Pisonia subcordata - Ayenia insulaecola type (12 relevés)................. 27