Mechanisms Affecting the Fate of Prosopis Flexuosa (Fabaceae
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Austral Ecology (2002) 27, 416–421 Mechanisms affecting the fate of Prosopis flexuosa (Fabaceae, Mimosoideae) seeds during early secondary dispersal in the Monte Desert, Argentina PABLO E. VILLAGRA,1 LUIS MARONE2* AND MARIANO A. CONY2 1Departamento de Dendrocronología e Historia Ambiental, IANIGLA, CONICET, Mendoza, Argentina and 2Instituto Argentino de Investigaciones de las Zonas Áridas (IADIZA), CONICET, Casilla de Correo 507, 5500 Mendoza, Argentina (Email: [email protected]) Abstract The fate of seeds during secondary dispersal is largely unknown for most species in most ecosystems. This paper deals with sources of seed output of Prosopis flexuosa D.C. (Fabaceae, Mimosoideae) from the surface soil seed-bank. Prosopis flexuosa is the main tree species in the central Monte Desert, Argentina. In spite of occasional high fruit production, P. flexuosa seeds are not usually found in the soil, suggesting that this species does not form a persistent soil seed-bank. The magnitude of removal by animals and germination of P. flexuosa seeds was experimentally analysed during the first stage of secondary dispersal (early autumn). The proportion of seeds removed by granivores was assessed by offering different types of diaspores: free seeds, seeds inside intact endocarps, pod segments consisting of 2–3 seeds, and seeds from faeces of one herbivorous hystricognath rodent, the mara (Dolichotis patagonum). The proportion of seeds lost through germination was measured for seeds inside intact endocarps, seeds inside artificially broken endocarps, and free seeds. Removal by ants and mammals is the main factor limiting the formation of a persistent soil seed-bank of P. flexuosa: >90% of the offered seeds were removed within 24 h of exposure to granivores in three of four treatments. Seeds from the faeces of maras, on the other hand, were less vulnerable to granivory than were other types of diaspores. These results suggest that herbivory might be an indirect mechanism promoting seed longevity in the soil (and likely germination) by discouraging granivore attack. On the other hand, germination did not seem to have an important postdispersal impact on the persistence of P. flexuosa seeds in the soil. Both direct and indirect interactions between vertebrate herbivores and plants may foster P. flexuosa’s seed germination in some South American arid zones. Key words: germination, granivory, seed predation, indirect interactions, mutualism, Argentina, arid land. INTRODUCTION soil (Marone et al. 1998a) seems to be a consequence of bird and, secondarily, mammal consumption In most ecosystems, a major proportion of newly pro- (approximately 50% of grass seeds are lost to grani- duced seeds are lost from the surface soil seed-bank vores), deep burial (30% are lost to burial, including all during secondary dispersal or redistribution (Cham- tiny seeds), germination (<5%), and pathogen attack bers & MacMahon 1994). Major mechanisms that (Marone et al. 2000b). The postdispersal fate of the account for such losses are germination, consumption larger shrub and tree seeds in the Monte Desert by animals, attack from microorganisms and deep remains largely unknown, despite the strong decline burial. Hence, for a plant species to form persistent soil seeds suffer as soon as they land on the ground seed-banks, its seeds must not only be capable of some (Marone et al. 1998a). type of dormancy, but also be able to cope with the Prosopis flexuosa D.C. (Fabaceae, Mimosoideae) is several postdispersal challenges imposed by mechan- the most abundant tree species in the central Monte isms other than germination. Desert, and its seeds are among the biggest in this In the central Monte Desert of Argentina, annual ecosystem (24–40 mg). Prosopis flexuosa production is forb seeds appear to form persistent soil seed-banks, highly variable from year to year, with records of whereas perennial grass seeds, shrub seeds and tree 80 000–800 000 seeds per hectare in different areas of seeds form transient soil seed-banks, with most seed the Monte Desert (Ffolliot and Thames 1983; output occurring 1–6 months after production (i.e. in Dalmasso and Anconetani 1993). Despite such a autumn and winter; Marone et al. 1998a). The strong high potential input, only a few P. flexuosa seeds were decline of grass seeds in the top 2 cm of Monte Desert found in the soil seed-bank of the central Monte Desert during the winters and springs of 1993 through 1998 *Corresponding author. (Marone et al. 1998a; L. Marone, unpubl. data). This Accepted for publication January 2002. period had at least two events of high fruit production FATE OF TREE SEEDS IN THE MONTE DESERT 417 (the summers of 1995 and 1998; L. Marone, pers. energy and foster germination capacity by breaking obs.). Occasional high seed production along with the dormancy in the digestive tract), some indirect con- absence of a persistent soil seed-bank suggests that at sequences of seed consumption and dispersal may be least some mechanisms of P. flexuosa seed loss were equally important. For example, Lerner and Peinetti effective during that period. (1996), following Janzen (1969), suggested that dis- Until now it has been generally assumed that germin- persal of P. caldenia seeds by vertebrates might favour ation and predation by insects of the family Bruchidae seed longevity in the soil because the passage of are the main fates of seeds of the genus Prosopis seeds through the intestinal tract of vertebrates (Tschirley & Martin 1960; Smith & Ueckert 1974; would eliminate previous infection or discourage Lerner & Peinetti 1996; Ortega Baez et al. 2001). new infection by beetles of the family Bruchidae. However, germination is probably not an important Likewise, a plausible hypothesis is that seed consump- cause of seed loss during redistribution. For example, tion and dispersal by vertebrate herbivores favours seed Lerner and Peinetti (1996) reported that less than 10% longevity in the soil by reducing seed vulnerability to of P. caldenia seeds germinated under natural con- granivores. ditions in central Argentina, whereas Tschirley and We measured the seed loss of P. flexuosa due to Martin (1960) reported that 16% of hulled P. velutina germination and predation during the early post- seeds and 45% of seeds planted in pod segments dispersal stage (a few weeks after primary dispersal) in germinated after the first year and a few more during the central Monte Desert of Argentina. We investigated the second and the third years. Regarding seed preda- which mechanism is more important in preventing this tion, there is convincing evidence of a high predispersal species from forming a persistent soil seed-bank. We impact of bruchids, which can decrease Prosopis seed also assessed whether seeds coming from the faeces of production by 25–70% (Smith & Ueckert 1974; Solbrig the hystricognath rodent Dolichotis patagonum (mara) & Cantino 1975; Kingsolver et al. 1977; Agrawal are as vulnerable to granivory as are seeds coming 1996). On the other hand, the impact of post- directly from the parent plant, thus testing the hypo- dispersal predators on Prosopis seeds is largely thesis of an indirect positive interaction between unknown. Lerner and Peinetti (1996) found that herbivores and plants. bruchids infested approximately 35% of the P. caldenia seeds they arranged experimentally on the soil (pro- tected against predation) during the spring following METHODS production. Ortega Baez et al. (2001) found that 99% of P. ferox seeds were predated by bruchids after 6 years on the soil in a subtropical mountain desert. Solbrig Study site and Cantino (1975) suggested that ant predation would not be high for P. flexuosa seeds in the northern The Biosphere Reserve of Ñacuñán (67Њ58ЈW, Monte Desert, although they did not report figures to 34Њ02ЈS) is located in the central part of the Monte support their assertion. The great number of animals Desert, Argentina (Morello 1958). Grazing has been that are capable of eating Prosopis fruits and seeds excluded from the reserve since 1972. One of its most (foxes, medium-sized and small mammals, ants) indi- conspicuous communities is an open woodland, with a cates, however, that predation might be an important sparse tree layer dominated by Prosopis flexuosa within force in preventing such tree species from forming a shrub matrix of Larrea divaricata, Condalia micro- persistent soil seed-banks in arid areas of southern phyla and Capparis atamisquea (tall shrubs), and South America. Lycium spp., Verbena spp. and Accantolippia seriphioides Prosopis seeds are frequently dispersed by domestic (low shrubs). Grass cover, composed almost exclu- and wild animals (Peinetti et al. 1993; Campos & Ojeda sively of perennial species, reaches 25–50% (Pappo- 1997). Its fruits have morphological and biochemical phorum spp., Trichloris crinita, Aristida spp., Digitaria adaptations to animal dispersal. These adaptations californica, Setaria leucopila, Sporobolus cryptandrus). include indehiscent pods with a thin exocarp, a spongy The climate in the central Monte Desert is dry and and sweet mesocarp (with a high proportion of sugars temperate, with cold winters. On average, 75% and proteins), and a hard and leathery endocarp. At (250 mm, n = 25 years) of the annual rainfall occurs in maturity, the seed is free inside a cavity in the endocarp spring and summer (October–March), when seeds are and a hard seed coat prevents water uptake (Kingsolver produced (Marone et al. 1998a). The present study et al. 1977). The endocarp and seed coat seem to was carried out during the exceptionally wet spring– prevent mortality of embryos when seeds pass through summer of 1997–1998 (453 mm), associated with a the digestive tracts of herbivores (Peinetti et al. 1993; strong El Niño/Southern Oscillation event. Fruit Campos & Ojeda 1997). Although seed dispersal by production by P. flexuosa was relatively high in the animals is often interpreted as a classical model of summer of 1998, with most dispersal occurring during direct animal–plant mutualism (e.g. herbivores obtain January–February (L.