400 years for long-distance dispersal and divergence in the northern Atacama desert – Insights from the Huaynaputina pumice slopes of Moquegua, Peru
Introduction
Four hundred years ago one of the most violent volcanic eruptions in recorded history and the largest in the Andes took place in southern Peru (Adams et al., 2001, Thouret et al., 2002). Between the 16th of February and the 6th of March 1600 the volcano Huaynaputina (Departamento Moquegua) erupted, covering an estimated area of 95 000 km2 with pumice deposits and several ash layers, ranging from 1 to 1200 cm in depth. The area of Omate and the Río Tambo valley (Departamento Moquegua, Prov. Sánchez Carrión) are situated only some 10 km from the crater of the volcano and were covered in >2 m of pumice. The Omate region and the Río Tambo valley south and west of the volcano were particularly heavily affected, since they additionally received heavy pyroclastic flows amounting to an estimated 1.5–2 km3 of volume. The water of the Río Tambo below Omate was temporarily dammed by the deposits to a lake an estimated 28 km long and heated to near boiling temperatures. This temporary dam broke and washed the entire valley with a lahar (Adams et al., 2001, Thouret et al., 2002). The area of Omate and the neighboring Río Tambo was thus burnt and buried by pumice and ashes and boiled by the waters of the Río Tambo. It was likely denuded of all vegetation and the seed bank was either destroyed through the heat and the chemical changes and/or buried under several meters of pumice and ashes. The pumice layers deposited by the eruption in 1600 now cover much of the region and dominate its current aspect. Pumice can be a very difficult substrate to colonize for plants and the Newbury pumice (Oregon, USA) remains largely devoid of vegetation after 200 years (Gay, 1979). Pumice soils are characterized by high albedo and net radiation and very low thermal conductivity (Cochran et al., 1967, Gay, 1979), all of which may compromise plant establishment by, e.g., exacerbating temperature extremes.
According to the most recent studies on the vegetation of the departments Arequipa and Moquegua (Galán de Mera and Gómez Carrión, 2001, Galán de Mera et al., 2003, Galán de Mera et al., 2009), the Omate region should belong to the “Weberbauerocereo weberbaueri – Corryocactetum brevistyli” or “Weberbauerocereo torataensis-Corryocactetum brevistyli” associations at elevations around and above 2000 m a.s.l., a vegetation type dominated by the columnar cacti for which it is named. At elevations of 1000–2000 m a.s.l. there is a very little vegetation in southern Peru and this is therefore generally referred to as the “piso desertico” in the literature (Weberbauer, 1945, Ferreyra, 1960, Ferreyra, 1961, Koepcke, 1961, Rundel et al., 1991). The lowest, coastal parts (0–1000 m), are locally covered by a mostly ephemeral Lomas vegetation, composed mostly of annuals and occurring during the southern winter. This vegetation type is found from northern Peru into central Chile (Koepcke, 1961, Rundel et al., 1991, Galán de Mera et al., 2009).
However, when we visited the Omate region in 2004, 2006 and 2009 we found that columnar cacti, which should dominate this region, are rare and restricted to some parts of the valley bottoms and rock outcrops. The largest part of this area is covered in deep pumice layers from the Huaynaputina eruption on which cacti are entirely absent. Conversely, these slopes are covered by a rich, albeit short-lived vegetation during the rainy season (December–March). The bulk of the species we found is not reported by Galán de Mera and Gómez Carrión (2001) and Galán de Mera et al., 2003, Galán de Mera et al., 2009 from Arequipa and Moquegua. This, largely herbaceous vegetation, thus presents an intrazonal vegetation type restricted to the pumice layers from the 1600 AD Huaynaputina eruption.
Numerous recent studies indicate that long-distance dispersal (LDD) is a much more common mechanism than previously thought (Nathan, 2006, Zhou et al., 2006, Dick et al., 2007), and many disjunctions thought to represent cases of vicariance can now be clearly ascribed to incidents of LDD (Renner et al., 2001, Givnish et al., 2004, Zhou et al., 2006). Numerous examples from historical times are, e.g., summarized in Sauer (1988). Long-distance dispersal appears to have played an important role across angiosperm clades and at a range of geographical and time scales, from the Tertiary (Renner et al., 2001) to the Holocene (Kropf et al., 2006). However, the majority of studies on past LDD events have focussed on intercontinental dispersal and dispersal to islands, and most studies focussed on dispersal in the time scale of hundreds of thousands or millions of years. Recent publications emphasize the importance of the “tail end” of the dispersal curves (Cain et al., 2000, Levin et al., 2003) and non-standard means of dispersal (Higgins et al., 2003). They argue, that mean dispersal distances of propagules and the “standard” dispersal modes inferred from propagule morphology do not accurately reflect the actual likelihood of LDD. Disjunctions are also found on a continental scale and several disjunct populations of species otherwise known from Chile or Argentina have been reported from southern Peru (Departments Arequipa, Moquegua and Tacna; Brako and Zarucchi, 1993). The taxa concerned are, e.g., Parkinsonia aculeata L. (Fabaceae), Bougainvillea spinosa (Cav.) Heimerl (Nyctaginaceae), Larrea divaricata Cav. and Bulnesia retama (Gill ex Arnott & Hooker) Griseb. (Zygophyllaceae, Sarmiento, 1975, Brako and Zarucchi, 1993, Prado and Gibbs, 1993, Zuloaga et al., 2008). Different hypotheses have been put forward for the causes of these disjunctions. They might be the result of (recent) long-distance dispersal, or relics of formerly continuous distributions either before the Andean uplift (Rauh, 1958) and/or under mostly drier conditions during the Pleistocene (Prado and Gibbs, 1993, Pennington et al., 2000, Pennington et al., 2004, López et al., 2006).
Recent phylogenetic studies have tried to unravel the evolution of several plant groups typical of this region and other (semi-) arid regions of South America in order to understand biogeographical hierarchies (Gengler-Nowack, 2002, Moore et al., 2006, Simpson et al., 2005, Lia et al., 2001). In general terms, evolution and divergence appear to show a south-to-north pattern in the Atacama Desert (Larrea, Nolana, Malesherbia, Prosopis), but the phylogenies also demonstrate that (long-distance) dispersal events within South America, to the Galapagos Islands and to North America have played a major role in the establishment of present-day distribution patterns (Moore et al., 2006, Lia et al., 2001, Bessega et al., 2006). However, the time frame of dispersal events between Peru, Chile, Bolivia and Argentina is not resolved in these studies.
For a better understanding of migrations and disjunctions in arid South America it would be interesting to know how frequently LDD (and subsequent establishment) of plant species occurs in the Atacama Desert. This could answer the question in how far isolated populations can really be seen as relics of formerly wider distributions or whether they are more likely to reflect dispersal events with subsequent local establishment.
One possible approach is the study of a flora of known age to see how fast and from where plant species colonized newly available habitats in the Atacama Desert. Ozinga et al. (2005) argue that abiotic factors are relatively good at predicting species composition of a given habitat, but that predictability is constrained by the unpredictability of colonization via dispersal. Therefore, an additional understanding of soil characteristics and local climate would help to better understand the role of specific habitat properties (filters) for the establishment of species. The present-day flora of the Omate pumice slopes represents an ideal study object to investigate these dynamics. The area was likely denuded of all vegetation during the 1600 AD eruption and the deep, largely undegraded pumice layers from this eruption represent a substrate different from that found at similar elevations in neighboring regions. Plant species now growing on these slopes must have colonized them since 1600 AD, either as the result of immigration from the neighboring, surviving vegetation types, or by long-distance dispersal.
The present study describes the flora of the pumice slopes of the Omate region and attempts to investigate its phytogeographical affinities by comparing it to floristic inventories from various other sites in Peru, Chile and Argentina. Climate and soil data are compared, for a better understanding of the abiotic conditions. The aim is to identify from which source areas the plant species now present dispersed into this area in the past 400 years and how the present floristic composition can best be explained in the light of dispersal and environmental filters.
Section snippets
Study areas
Five study sites were selected in the area immediately below Omate and along the Río Tambo (Fig. 1), all of them situated in the area most heavily affected by the Huaynaputina eruption (Adams et al., 2001, Thouret et al., 2002). Field studies were carried out by three of the authors (FCH, MW, CS). All sites were located at detrital slopes composed of pumice material, exhibiting slope angles between 15° and 50°. Study sites along the road from Omate (Prov. General Sánchez Cerro) to Moquegua
Climatic conditions
The climate diagrams of the areas included in this study are shown in Fig. 2. Climate data were obtained for Omate and Arequipa for the period 1997 to 2006 (i.e., including the 97/98 El Niño event) and then again for 2008/2009 (Appendix 4). In Omate and Arequipa the Sierra climate is characteristically developed, showing mean annual temperatures around 15 °C, with remarkably little variation over the year (mean variation 2 °C between summer and winter).
Precipitation falls in the austral summer
Abiotic conditions
The climate of Omate is similar to that of Arequipa and is a typical Sierra climate in its seasonality, form and amount of precipitation and relative air humidity, but Omate receives considerably more precipitation compared to Arequipa. Microclimate on the Omate pumice slopes may be additionally influenced by the physical properties of the pumice, i.e., the very low-thermal conductivity and net radiation (Gay, 1979, Cochran et al., 1967), which likely increase irradiation, temperature extremes
Conclusion
The Omate pumice slopes have a comparatively rich flora of 59 angiosperm taxa. The flora of the pumice slopes falls outside the vegetation types classified for the region by Galán de Mera et al., 2003, Galán de Mera et al., 2009 and includes several species not previously reported from Peru. The pumice slopes show a peculiar combination of abiotic (climatic and edaphic) factors not paralleled in other southern Peruvian habitats. In relation to the large distance there is a considerable
Acknowledgements
We want to express our sincere gratitude to F. Luebert (Berlin), M. Lavin (Montana, USA), M. Dillon (Chicago, USA) and O. Tovar (Lima) for help with plant determinations and other helpful comments. We thank three anonymous reviewers for their suggestions.
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