Rain Forest Islands in the Chilean Semiarid Region: Fog-Dependency, Ecosystem Persistence and Tree Regeneration
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Rain Forest Islands in the Chilean Semiarid Region: Fog-dependency, Ecosystem Persistence and Tree Regeneration Ek del-Val,1*à Juan J. Armesto,1,2,3 Olga Barbosa,1 Duncan A. Christie,1,2 Alvaro G. Gutie´rrez,2 Clive G. Jones,3 Pablo A. Marquet,1 and Kathleen C. Weathers3 1Center for Advanced Studies in Ecology and Biodiversity, Departamento de Ecologı´a, Facultad Ciencias Biolo´ gicas, Pontificia Universidad Cato´ lica de Chile, Casilla 114-D, Santiago, Chile; 2Centro milenio para Estudios Avanzados en Ecologia e Investigacio´ n en Biodiversidad (CMEB), Departamento de Ciencias Ecolo´ gicas, Facultad de Ciencias Universidad de Chile, Casilla 653, Santiago, Chile; 3Institute of Ecosystem Studies, Box AB, Millbrook, New York 12545, USA. ABSTRACT Tree presence in semiarid ecosystems is generally edges (WE), leeward edges (LE), and the interior constrained by insufficient annual rainfall. How- (center) of rainforest patches varying in area from ever, in semiarid Chile, rainforest patches domi- 0.2 to 22 ha. Mean fog-water input was estimated nated by Aextoxicon punctatum are unexpectedly from passive collectors over 1 year in WE and LE of found on coastal mountaintops (450–600 m) at patches. Tree regeneration was greater in the WE 30°S, surrounded by a xerophytic vegetation ma- and forest interior (FI) and decreased toward the LE trix that receives only 147 mm of annual precipi- of patches, following a marked pattern of decline in tation. It has been proposed that these forests fog inputs. Older trees and coarse woody debris persist as a result of fog-water inputs. If so, then were concentrated in the FI and LE of patches. Tree because fog-water deposition is spatially heteroge- regeneration and patch structure appear to be lar- neous and shows strong edge effects, the potential gely controlled by fog-input direction and edge ef- environmental gradient created by the direction of fects. We propose that forest patches may be slowly fog input should determine forest structure and growing toward the incoming fog edge, while dying tree regeneration patterns. To investigate this at the opposite edge. hypothesis, we measured fog inputs, forest struc- tural attributes (age and size distribution, basal Key words: Aextoxicon punctatum; edge effects; area, and coarse woody debris), and tree regener- forest structure; tree regeneration; mortality; water ation in three different habitats: the windward limitation; Chile. INTRODUCTION *Corresponding author; e-mail: [email protected] Trees are generally considered to be excluded from Current address: FORECOS, Facultad de Ciencias Forestales, Universidad semiarid regions by insufficient rainfall (Grace Austral de Chile, Casilla 567, Valdivia, Chile 1997). For example, in the steppe–forest border of àCurrent address: Centro de Estudios Ecosistemas, inversidad Nacional tuto´ noma de Me´xico, Ap. 27–3, Santa Maria de guido Morelia, Michoa- southern Patagonia, trees are replaced by shrubs ca´n, Me´xico, 58089 when rainfall is less than 400 mm per year (Veblen Fog-dependent Forests in Semiarid Chile and Markgraf 1988). Worldwide, shrubs replace Corte´s and Lo´ pez 2004), and coincidently Olivillo trees in semiarid regions when precipitation is less forests are restricted to this altitudinal range in this than 300–400 mm (Holdridge 1947). In the semi- northern location. Recent experimental studies arid region of Chile (30°S), where annual precipi- using stable isotopes indicate that Olivillo trees tation (150 mm, Lo´ pez-Corte´s and Lo´ pez 2004) is obtain a major part of their water from fog (P. well below that usually associated with tree Vidiella and T. E. Dawson unpublished data). Fog growth, temperate rainforest patches nonetheless, occurrence and fog water deposition are not persist on coastal mountaintops, surrounded by a homogeneous in the coastal landscape. Fog-water matrix of xerophytic vegetation (Squeo and others deposition depends on fog frequency, liquid water 2004). The ecological and historical factors that content, wind speed and direction, and most could explain the persistence of these isolated importantly, on the presence of plant surfaces rainforest patches in this northern outpost have capable of intercepting fog (Weathers and others long been a subject of inquiry among botanists, 1992, 2000). To be a source of water for plants, ecologists, and biogeographers (Philippi 1884; wind-driven fog droplets must be intercepted by Mun˜ oz and Pisano 1947; Skottsberg 1948). Despite the tree canopy (for example, Azevedo and Morgan their improbable location, these forests bear a 1974; Dawson 1998; Schemenauer and Cereceda striking floristic similarity to Valdivian temperate 1994). ‘‘Captured’’ fog-water is then redistributed rainforests found 1,000 km to the south (Villagra´n by the stems and leaves of trees. Although fog- and Armesto 1980), where rainfall far exceeds water can be taken up by leaves (Burgess and 2,000 mm per year. Dawson 2004), it also drips from the canopy and is These rainforests are dominated by the tree Aex- routed to the forest floor via throughfall and toxicon punctatum (Olivillo), the only member of the stemflow (Hutley and others 1997; Dawson 1998). endemic family Aextoxicaceae. Evergreen Olivillo In addition, fog-water deposition is spatially het- forests are distributed continuously along the erogeneous; where edges exist, it has been shown ocean-facing slopes and deep ravines of the Coastal to decrease predictably from the windward (WE) to mountain range from about 33–43°S, but isolated, the interior of forests because of interception by remnant patches occur north of 33°S, on coastal trees (Beier and Gunderson 1989; Erisman and mountaintops in the semiarid region, where coastal others 1997; Weathers and others 1995, 2000; fogs are prevalent (Kummerow 1966). Within this 2001). broad latitudinal range, annual rainfall varies from Based on the above, we hypothesized the exis- 2,657 mm at the southern limit of Olivillo distri- tence of a ‘‘fog shadow’’ associated with distinct bution (Isla Guafo 43°40¢ S) (Smith-Raminez et al rainforest patches such that the windward forest 2005), to less than 147 mm at Fray Jorge National edges would receive greater fog inputs that would Park (FJNP) (30°40’ S) (Armesto and others 1996; impact a wide range of other patterns in this Lo´ pez-Corte´s and Lo´ pez 2004). landscape. Because the trees in these forest pat- In this northern outpost, Olivillo forests consist ches may largely depend on fog as a water source, of a mosaic of small, isolated patches found only we propose that the forest would create a wind- on coastal mountaintops. A number of in- ward-to-leeward environmental gradient orien- vestigators have proposed that Olivillo trees are tated toward the fog-input direction, and that this able to exist in a semiarid area where rainfall is gradient may be one of the main factors deter- insufficient for tree growth and survival by using mining patch structure as well as tree regeneration water from frequent coastal fogs, thereby enabling and mortality patterns within these forest patches. the persistence of an associated rich assemblage of We tested three specific hypotheses about the rainforest species (Philippi 1884; Looser 1935; correlation between fog inputs and forest patch Mun˜ oz and Pisano 1947; Skottsberg 1948; structure in this northern outpost of temperate Schmithusen 1956; Kummerow 1966; Villagra´n rainforest and predicted that (1) fog-water depo- and Armesto 1980). sition is heterogeneous between patches and Fog moves inland from the Pacific Ocean on higher at the WE than at the legward edge (LE) of westerly winds – the prevailing wind system along forest patches; (2) tree regeneration is concen- the west coast of southern South America since the trated toward the edge of the incoming fog, Holocene (Heinz 1998; McCulloch and others whereas basal area, tree mortality, and coarse 2000). Fog is frequent between 500- and 700-m woody debris increase away from it; and (3) dif- elevation on the ocean-facing, coastal mountains of ferent tree species are distributed within forest the semiarid region of Chile (Rundel and Mahu patches in relation to their different drought tol- 1976; Cereceda and Schemenauer 1991; Lo´ pez- erances. E. del-Val and others Figure 1. Location of study site showing the rainforest patch mosaic in Fray Jorge National Park. Modified from Novoa-Jerez and others (2004). STUDY SITE facing the ocean, thus having a larger zone of fog influence (Figure 1). It is important to note that fog The study was carried out at Fray Jorge National is patchy and that due to the complex topography, Park (FJNP), (30°40¢S, 71°30¢W), the northernmost delivery of fog is not uniform across the landscape. location of Olivillo forest. Here, a mosaic of 180 In large patches, fog can follow different directions rainforest patches, ranging in size from 0.1 to 22 as it moves down ravines, for example. The re- ha, occurs on the summits of coastal mountains gional climate is characterized as mediterranean- (450–660 m a.s.l.) within a matrix of semiarid scrub arid with hot-dry summers and cool winters (Di vegetation (Novoa-Jerez and others 2004). Smaller Castri and Hajek 1976). Mean annual temperature patches tend to be located on the mountain pla- is 13.6°C and mean annual precipitation is 147 mm teau, whereas larger patches are on steeper slopes (from 21-year records, 1983 – 2003) (Lo´ pez-Corte´s Fog-dependent Forests in Semiarid Chile and Lo´ pez 2004), falling mostly (around 95 % of two passive fog collectors (PFC) outside five of the annual rainfall) in the austral winter, between patches sampled for structure (Table 1). The PFC June and August. There is a substantial interannual measured fog-water inputs to the WE (facing variability in rainfall coefficient of variation approximately west) and LE (facing approximately (CV = 81%), with wet pulses every 3–5 years, east) edges of each patch.