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1. Indifferent to light (0) or positively photoblastic (+). References: Nolasco et al. (1996, 1997) and Rojas-Aréchiga et al. (1997).

2. Temperatures (°C) at which minimum, optimum, and maximum germination are observed. References: Godinez-Alvarez and Valiente-Banuet (1998), Nolasco et al. (1996), and Rojas-Aréchiga et al. (1998).

1. Indifferent to light (0) or positively photoblastic (+). References: Nolasco et al. (1996, 1997) and Rojas-Aréchiga et al. (1997).

2. Temperatures (°C) at which minimum, optimum, and maximum germination are observed. References: Godinez-Alvarez and Valiente-Banuet (1998), Nolasco et al. (1996), and Rojas-Aréchiga et al. (1998).

1966; Franco and Nobel 1989; Valiente-Banuet et al. 1991a; Arriaga et al. 1993; Suzán et al. 1996). Nurse plants can also provide protection against wintertime low temperatures, decreasing the susceptibility of seedlings to frost injury (Brum 1973; Steenbergh and Lowe 1969, 1977) and enhancing the establishment of C. gigantea and S. thurberi in marginal populations where seedlings are exposed to extreme low temperatures during the winter (Brum 1973; Steenbergh and Lowe 1969, 1977; Parker 1987).

Some nurse plants in arid ecosystems increase the nitrogen content of soils under their canopies compared to open spaces. This increment in soil nitrogen, which can reflect interactions with soil microorganisms, creates "islands of fertility" in which the growth rates of seedlings and their survival probability increase (García-Moya and McKell 1970; Franco and Nobel 1989; Valiente-Banuet and Ezcurra 1991; Godínez-Alvarez and Valiente-Banuet 1998). Nevertheless, the empirical evidence on the increase of soil nitrogen content is controversial, as some studies support this hypothesis (García-Moya and McKell 1970; Franco and Nobel 1989) whereas others do not (Valiente-Banuet et al. i99ia,b; Arriaga et al. 1993).

Protection against seed and seedling predators is another benefit associated with nurse plants (Steenbergh and Lowe 1969, 1977; McAuliffe 1984a; Hutto et al. 1986; Valiente-Banuet and Ezcurra i99i). Birds, mammals, and insects are among the main cactus predators; however, the protection provided by nurse plants against predators and therefore on the survival of seeds and seedlings depends on the foraging patterns of each predator (Hutto et al. 1986). In this respect, Valiente-Banuet and Ezcurra (1991) found that the seeds of Neobuxbaumia tetetzo are similarly predated by harvester ants and ground-foraging birds under nurse plants and in open spaces. On the other hand, McAuliffe (1984a) found that herbivores consume less Mammillaria microcarpa and Echinocereus engelmannii growing under the canopies of the tree-like cactus Opuntia fulgida, because spiny nurse plants inhibit the foraging beneath their canopies.

Among negative effects, nurse plants reduce the pho-tosynthetic photon flux (PPF) and increase the competition for limited supplies of water (Franco and Nobel 1989). For instance, the nurse plants of young C. gigantea and Ferocactus acanthodes reduce the total daily available PPF by about 70%. This reduction reduces net CO2 uptake, decreasing the growth rate compared to seedlings in open spaces. Moreover, the competition between seedlings and nurse plants for water can also affect seedling growth. However, such negative effects can be offset by increases in soil fertility under the canopies of nurse plants (Franco and Nobel 1989). Additionally, Altesor et al. (1992) indicate that during the first 10 to 20 weeks after germination, seedlings

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