Oscar Briones

Spatial relationships between cacti and nurse shrubs in a semi-arid environment in central Mexico

. The Zapotitlan 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 Zapotitlan.

Correlation between water relations and within-canopy distribution of epiphytic ferns in a Mexican cloud forest

Abstract 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.

Competition intensity as a function of resource availability in a semiarid ecosystem

Abstract 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 m2), 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.

Competition between three Chihuahuan desert species: evidence from plant size-distance relations and root distribution

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.

Effect of predation and solar exposure on the emergence and survival of desert seedlings of contrasting life-forms

Abstract We evaluated the effect of solar exposure and predation on seedlings of six species of three life-forms. Columnar succulents and arborescent semi-succulents emerged faster under shrubs (24 and 55 days) than in open places (44 and 60 days). Succulents had a longer survival time (138 days) without predators and under shrubs. Predator exclusion led to a longer survival in semi-succulents under shrubs (237 days) and in open places (198 days). Predation did not affect the survival time of shrubs. Our results extend to the seedling stage, the hypothesis that in desert environments, each life-form adopts different strategies for survival and growth.

The significance of carotenoids and tocopherols in photoprotection of seven epiphytic fern species of a Mexican cloud forest

Epiphytes experience frequent and rapid changes in water availability and light intensity. The role of carotenoids and tocopherols in photoprotection of seven fern species (Asplenium cuspidatum Lam., Phlebodium areolatum (HB ex Willd.) Smith, Polypodium puberulum Schl. & Cham., Po. plebeium Schl. & Cham., Elaphoglossum glaucum Moode, E. petiolatum (Sw.) Urb., and Pleopeltis mexicana (Fee) Mickel & Beitel) with different adaptations against drought were investigated. The plants were sampled dark adapted (treatment I), and after light exposure with the substrate present (treatment II), with the substrate removed (treatment III) and with substrate and rhizomes removed (treatment IV) to induce increasing degrees of drought stress. The degree of dehydration reached does not result in permanent damage and was also observed in the field. While none of the treatments induced significant chlorophyll (chl) degradation, all plants showed strong de-epoxidation of the xanthophyll cycle on light exposure (up to a de-epoxidation state of 70%), but without an additional effect of desiccation. Most species showed a rapid increase (within hours) of carotenoids (mainly β-carotene and lutein) and α -tocopherol on exposure. In A. cuspidatum, a species with no apparent adaptations to drought, drought stress in combination with light resulted in an increase of tocopherols from 35 nmol µmol -1 chl (treatment I) to 400 nmol (treatment IV). This effect was not significant in the drought-deciduous species with succulent rhizomes, Po. puberulum (about 10 nmol µmol -1 chl) and Ph. areolatum (5 to maximum 40 nmol), which experience little desiccation under field conditions. This short-term induction of tocopherols and carotenoids has not been reported for other plants under light stress and is probably related to the epiphytic life form.

Effect of heat shock on germination of 23 plant species in pine–oak and montane cloud forests in western Mexico

We determined the effect of heat shock on seed germination of 23 species growing in fire-prone areas covered by pine–oak and montane cloud forests. Seeds of each species were exposed to 60, 80, 100 or 120°C for 5 or 60 min in dry and moist sand. Seeds of nine species were incubated for 15 h in moist sand and exposed to the temperature that best promoted germination. Eighteen species required heat shock for germination, and five tolerated it. Dry heat shock for 5 min promoted germination of these species; heat shock in moist sand at 100°C induced germination of 10 species. Exposure for 60 min to heat shock in dry and moist sand improved germination of 14 and 5 species respectively. Logistic models indicate that germination probabilities of most species are increased by temperatures of 70–120°C. Depending on substrate and seed hydration, heat shock increased or left unchanged the germination of most pine–oak and montane cloud forest species. Thus, montane cloud forest seeds can survive low-intensity surface fires. Most species with seeds hydrated before heat shock showed little or no germination. Prescribed burns at the end of the rainy season may therefore be harmful to pine–oak and montane cloud forest species.

Leaf physiological response to light environment of three tree fern species in a Mexican cloud forest

Abundance and physiology of three understorey tree fern species were compared in a Mexican cloud forest. We hypothesized that the distribution of species would be associated with canopy openness and leaf physiological characteristics. In gullies (1–2% full sun), Alsophila firma was abundant, Cyathea divergens was distributed in moderately open places (4–9%), and Lophosoria quadripinnata preferred more open canopy (9–30%). Although 11 leaf traits of five plants of each species growing under closed and open canopies over 1 y did not differ within species, there were significant interspecific differences. Alsophila firma had comparatively low maximum electron transport rate ETR max (26.8 ± 1.81 μmol m −2 s −1 ) and ETR light saturation point (ETR LSP : 261 ± 36.1 μmol m −2 s −1 ), high specific leaf area (SLA), thin leaves and decreased quantum yield during a leaf desiccation experiment. Cyathea divergens had relatively high maximum quantum yield (0.84 ± 0.004), ETR max (37.3 ± 1.8 μmol m −2 s −1 ) and ETR LSP (409 ± 40.0 μmol m −2 s −1 ). Lophosoria quadripinnata had comparatively thick leaves, low SLA, high predawn water potential, high density (606 ± 25.5 mm −2 ) and small length (0.026 ± 0.002 mm) stomata. The results support the hypothesis that light sensitivity shapes tree fern distribution in the cloud forest.

Germination response of the epiphytic cactus Rhipsalis baccifera (J. S. Miller) stearn to different light conditions and water availability.

In the forest canopy, seeds of epiphytic plants encounter heterogeneous environments created by a combination of factors such as solar radiation, humidity, and host characteristics. Germination requirements may explain the species distribution in the canopy; however, more knowledge is essential. Germination of Rhipsalis baccifera, a widespread tropical epiphytic cactus and representative of the humid montane forest in Mexico, was 80% or higher with far red, red, and white light and close to 0 in darkness. Germination was light saturated at very low photon flux density of only 13.5 μmol m−2 s−1. Germination decreased gradually at low water potentials and with increased storage time. After storage for 1 yr, no seeds germinated. Seeds have the ability to germinate in both the internal and external positions of the tree crowns. The germinative requirements of R. baccifera indicate that it could colonize a wide range of light conditions in the canopy; however, substrate humidity and seed age could limit germination.

Response of soil seed bank to a prescribed burning in a subtropical pine–oak forest

Prescribed burning is a management instrument applied to reduce the risk of fire and favour revegetation. Our objective was to generate information about the dynamics of post-fire regeneration via the soil seed bank (SSB), for fire management in subtropical forests. Samples taken at soil depths of 0–3cm, 3–6 cm and 6–10 cm before and 5 h after a prescribed burn showed that the fire immediately increased the number of germinable seeds and species in a Mexican pine–oak forest. Most of the germinable seeds were from species in genera with small seeds exhibiting physical or physiological dormancy, and that are tolerant or require fire for germination. Fire increased the number of germinable seeds during the wet season and the number of species was greater in the area control at 0–6-cm soil depth after 1 year; so that the fire modified the SSB seasonal pattern. Species diversity was not altered and was dominated by perennial herbaceous and shrub species both before and 2 years after the fire. Although fire completely eliminated the aboveground biomass of the understorey vegetation, the SSB can promote regeneration and persistence of understorey vegetation following a prescribed surface fire of low severity for the ecosystem studied.