The physiological traits associated with water relations of eight common epiphytic ferns in a Mexican cloud forest were investigated in relation to the distribution of these species within the canopy. Fern distribution was significantly correlated with the relative water content at which stomata close, leaf thickness, stomatal density and size. Trichomanes bucinatum desiccated completely within hours in moderately dry air and was confined to the stem bases, and Asplenium cuspidatum, with no evident adaptations to cope with drought, grew in the second most shaded zone within the tree crowns. Despite growing in a humid cloud forest, all other species had xeric adaptations including coriaceous leaves (Pleopeltis mexicana, Elaphoglossum glaucum), succulent rhizomes (Polypodium puberulum, Phlebodium areolatum), low rates of uncontrolled water loss (all species except P. puberulum), leaf scales (Elaphoglossum petiolatum, Polypodium plebeium), and high cell wall elasticity (all species). P. plebeium and Pl. mexicanum, which grow in the most exposed locations, tolerated water loss beyond the turgor loss point before the stomata closed and appear to be poikilohydric or at least to tolerate high water deficits.

Abstract. The Zapotitlán de las Salinas valley, central Mexico, harbours a high diversity of cacti. Pattern analysis indicated that the establishment of two columnar cacti, Neobuxbaumia tetetzo and Cephalocereus hoppenstedtii, and of three small globose cacti, Coryphantapallida, Mammillaria colina andM casoi, is aggregated and associated with perennial nurse shrubs. Some nurse species, Castela tortuosa, Caesalpinia melanadenia and Eupatorium spinosarum have a higher number of cacti beneath their canopies than would be expected by chance. A replacement pattern was found between the columnar cacti and their nurses, an aspect which was not found with the globose cacti. Following the assumption that protection against excessive radiation is the main factor determining the nurse effect, the azimuth orientation of the cacti with respect to their nurses was evaluated. Only Coryphanta pallida presented a non‐random distribution with a tendency towards the North and West. The difference in maximum temperature between the soil surface under the different nurse species and of open spaces, which is reached at midday, was 16 °C. No significant differences were found in beneath‐canopy temperatures for the three nurse species considered. Soil nitrogen levels were significantly lower beneath the different nurse plants than in open spaces. This result suggests that soil fertility is not an important factor in the nurse‐plant phenomenon in Zapotitlán.

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Abstract. Using size‐distance data we tested the intensity and importance of competition between Hilaria mutica (a tussock grass), Larrea tridentata (a microphyllous shrub) and Opuntia rastrera (a succulent) in the Chihuahuan desert. We also compared the vertical and horizontal distribution of roots to assess the potential degree of overlap in the use of soil resources. The relationships between sizes and distances of nearest‐neighbour plants suggested that intraspecific competition is generally more important than interspecific competition. However, evidence of stronger inter than intraspecific competition was found in some cases. Species combinations showing significant interspecific competition involved always Opuntia, whereas Larrea and Hilaria did not influence each other. The analysis of the symmetry of competition showed that Opuntia was adversely affected by the presence of Hilaria or Larrea. Although differences were found in the distribution of roots, the results of the size‐distance study support the idea that, (particularly) Opuntia, below‐ground niche differentiation is not sufficiently important to totally avoid the negative effects of plant competition.

Two field experiments were conducted using three dominant perennial species of the Chihuahuan Desert: Hilaria mutica (a tussock grass), Larrea tridentata (a microphyllous shrub) and Opuntia rastrera (a flat-stemmed succulent cactus). Two hypotheses concerning competition in arid plant communities were tested. (1) Marked resource partitioning with no interspecific competition could be expected since the three species belong to different life-forms, and that plant growth in deserts is basically limited by harsh environmental conditions. (2) Alternatively, resource scarcity (particularly water) will result in strong plant competition. In a 1-year removal experiment, water status and plant growth of the three species were monitored in twelve 10 m × 10 m plots randomized in three blocks and assigned to the following treatments: (a) removal of all species, except H. mutica; (b) removal of all species, except L. tridentata; (c) removal of all species, except O. rastrera, and (d) control without any manipulation. In a watering experiment, under two neighbourhood conditions (growing isolated or in associations of plants of the three species in plots of 20 m), the water status of the three species and the growth of H.mutica and L.tridentata were studied for 32 days after an irrigation equivalent to 30 mm of rain, similar to a strong storm event at the site. In the removal experiment, where plants were free to capture water, no evidence of competition was observed. However, during the watering experiment, in which water was forced into the soil, competitive effects were observed. Associated individuals of L. tridentata had lower xylem water potentials and osmotic potentials (OPs) and produced shorter twigs and less leaves and nodes. Although less pronounced, neighbours also had a negative effect on the OP in O. rastrera. According to these results, the intensity of the interspecific competition for water seems to depend on the level of resource availability in the soil. Thus, the validity of the two hypotheses tested in this study also depends on the level of resources. Competition could be absent or very low in years of low precipitation, as in the year of this study (173 mm against a 25-year average of 264 mm). However, when soil water availability is high, e.g. following heavy rain, the negative interactions between species could be more intense.

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