Introduction Population Ecology Seed Germination Seedling Establishment and Growth Reproduction and Breeding Systems Seed Dispersal Population Dynamics Community Ecology Conclusions and Future Prospects Literature Cited
Cacti are a typical component of vegetation in tropical and subtropical America (Gibson and Nobel 1986; Barthlott and Hunt 1993; Valiente-Banuet et al. 1995a). Although some species reach high latitudes in British Columbia and Alberta in Canada and in the Patagonian region of Argentina, species diversity increases considerably toward the tropics, Mexico being the most important center with about 850 species and 54 genera (Bravo-Hollis 1978; Arias 1993). The tribes Opuntieae, Cacteae, and Echinocereeae, including species with laterally flattened stems in the genus Opuntia, globose or barrel cacti in the genera Coryphantha, Echinocereus, Mammillaria, and many monospecific genera such as Ariocarpus, Astrophytum, Aztekium, and Lophophora, are widely distributed in the Chihuahuan Desert of Mexico with almost 400 endemic species (Arias 1993). In the Sonoran Desert, seven species of columnar cacti (tribe Pachycereeae) with genera such as Carnegiea,
Pachycereus, and Stenocereus dominate the arid landscapes in northwestern Mexico, forming the sarcocaulescent desert of Shreve (1951), which also has many species of cylindropuntias and barrel cacti (Turner et al. 1995). In south-central Mexico in the Balsas River Basin and the Tehuacan Valley, the tribe Pachycereeae with its arborescent cacti is highly diversified; this tribe contains 45 of the 70 species of local columnar cacti in Mexico, constituting the principal structural and floristic elements of columnar-cactus forests (Valiente-Banuet et al. 1995a, 1996). Other tribes, such as Hylocereeae, are basically restricted to humid tropical regions in southeastern Mexico and Central America and are represented mostly by epyphitic cacti (Arias 1993). The genus Pereskia is an important component of the tropical deciduous forest of southern Oaxaca, Mexico (Bravo-Hollis 1978).
In these geographical areas, cacti show a considerable diversity of life form, and the different types of vegetation acquire their names depending on their physiognomic and/or structurai dominance (Miranda and Hernandez 1963; Rzedowski 1978; Osorio et ai. 1996). Spectacular examples of these plant associations include the Cardonales, dominated by columnar cacti called "cardones" since the Spanish conquest times, belonging to the genera Steno-cereus, Pachycereus, Cephalocereus, and Mitrocereus; the Nopaleras, a name derived from the term "nopal," which is the Nahuatl name for the flat-stemmed species of Opuntia, consist of dense stands of several species, and "Tetecheras" and "Giganteras," which are dominated by different species of Neobuxbaumia. The key aspects in the population and community ecology of these plants are considered in this chapter to determine how abiotic and biotic factors interact to influence the distribution and abundance of a particular species, leading to particular survivorship, fecundity, growth patterns, and groups of species, with an emphasis on the maintenance of biodiversity.
Cacti have lifespans of decades to hundreds of years (Steenbergh and Lowe 1977). Along their life cycle, the different stages, such as the seed and seedling, juvenile, mature, and senile plants, are exposed to different mortality factors related to high radiation levels, water stress, and bi-otic interactions such as predation and competition. Early stages of the life cycle are the most important for maintaining viable populations in the field (Steenbergh and Lowe 1969, 1977; Valiente-Banuet and Ezcurra 1991; Godinez-Alvarez et al. 1999). Therefore, the successful production of seeds, their dispersal and germination, seedling establishment, and the survivorship of seedlings and juveniles are essential for the maintenance of cactus populations under natural conditions. These stages constitute a link between the reproductive adults and the new individuals (Howe and Smallwood 1982); thus, the main goal of this section is to analyze their relative effect on the population dynamics of cacti.
Most studies analyzing seed germination of cactus species have been conducted under controlled conditions in the laboratory (Chapter 5) and few have considered seed germination under field conditions. Dubrovsky (1996, 1998) reported that the seeds of the Sonoran Desert cacti Car-negiea gigantea, Ferocactus peninsulae, Pachycereus pecten-aboriginum, Stenocereus gummosus, and S. thurberi, subjected to hydration-dehydration cycles of different lengths, germinated faster and accumulated higher biomass compared to untreated seeds. Thus, seeds apparently retain during dehydration the physiological changes promoted by seed hydration. This "seed hydration memory" may facilitate seed germination, increasing the survival of cacti in the field (Dubrovsky 1996).
Light stimulates seed germination for only some species (Table 6.1). Rojas-Aréchiga et al. (1997) divided cacti into two groups: (1) globose or barrel cacti such as Mammillaria, Echinocactus, and Ferocactus that require light for the seed germination (i.e., positively photoblastic), and (2) columnar cacti in the genera Neobuxbaumia, Cephalocereus, Pachycereus, and others for which seed germination is not affected by light. This correlation between light and lifeform in cacti may result from the environmental and maternal effects on seeds during their development (Rojas-Aréchiga et al. 1997). Light requirements are also affected by temperature regimes, washing of the seeds, and gibberellic acid (Rojas-Aréchiga and Vázquez-Yanes 2000). Ecologically, lack of light can inhibit germination when the seeds are deep in the soil, and light can stimulate germination when soil moisture is available (Kigel 1995). Germination occurs for a wide range of temperatures, 10 to 40°C (Nobel 1988; Rojas-Aréchiga and Vázquez-Yanes 2000). The optimal temperature for germination is 20 to 30°C. In addition to these responses, extreme temperatures and aging of seeds decrease germination (Nobel 1988; Rojas-Aréchiga and Vázquez-Yanes 2000).
The establishment phase and early seedling growth in deserts occurs under unpredictable conditions of precipitation and in soils with high temperatures and low water content. Under these circumstances, cacti are often established beneath the canopy of perennial "nurse plants" (Table 6.2), which modify the environment beneath their canopies. Nurse plant phenomenon have been addressed by determining the spatial relationships between cacti and perennial plants through nearest-neighbor analysis. Both positive and negative effects for cacti are derived from the association with nurse plants, as evidenced by means of a cost-benefit analysis (Franco and Nobel 1989). Among the positive effects of nurse plants on seeds and seedlings is protection against direct solar radiation, leading to a decrease in extreme soil temperatures and hence increasing the soil moisture available for seed germination and early seedling survival (Turner et al. 1966; Steenbergh and Lowe 1969, 1977; Franco and Nobel 1989; Valiente-Banuet and Ezcurra 1991; Nolasco et al. 1997). Surface temperatures in open spaces between nurse plants can be up to 65°C, whereas in the shaded microsites under nurse plants soil surface temperatures are 10 to 20°C lower (Turner et al.
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