Fernando Vite

Interaction between the cactus neobuxbaumia tetetzo and the nurse shrub mimosa luisana

. Pattern analysis and association analysis showed that recruitment of the giant columnar cactus Neobuxbaumia tetetzo in the semi-arid valley of Zapotitlan is largely limited to areas beneath the canopies of perennial shrubs, acting as nurse plants. Chi-square and Haberman tests revealed that young cacti were more frequently found beneath canopies of leguminous shrubs, especially Mimosa luisana, than were older cacti. Segregation analysis indicated a positive association (= negative segregation) of young cacti and M. luisana canopies, but older cacti were randomly distributed with respect to M. luisana. This, and the height class distributions of cacti associated withM luisana or not, suggested the replacement of M. luisana by N. tetetzo. Mimosa luisana plants in association withiV. tetetzo had greater amounts of dead basal area thanM luisana lacking associated N tetetzo. This suggested competition from JV. tetetzo promotes the replacement process. The roots of N tetetzo are ≤ 30 cm deep. Interception of soil water before penetration to deeper roots of M luisana may be the mode of competition between the two species.

STEM TILTING AND PSEUDOCEPHALIUM ORIENTATION IN CEPHALOCEREUS COLUMNA-TRAJANI (CACTACEAE): A FUNCTIONAL INTERPRETATION

This paper analyzes the functional implications of stem tilting and pseudocephalium orientation in the giant columnar cactus Cephalocereus columna-trajani. This species shows a consistent northern orientation of its pseudocephalium (a nonphotosynthetic hairy structure where flowers are produced) and stem tilting in the same direction. Analysis of pseudocephalium orientation was made on field data gathered from subpopulations of C. columna-trajani from slopes with different exposures. Additionally, from morphometric characteristics measured in the field, a model cactus was constructed with the purpose of simulating radiation interception by different morphologies. Variations of this model cactus allowed the simulation of irradiance on erect and tilted cacti, as well as on plants with varying pseudocephalium orientation. Results of irradiance interception by different morphologies were related to actual data of growth rates, flowering period, and rainfall and temperature patterns on the study zone. Sampled individuals of C. columna-trajani showed a significant north-northwest pseudocephalium orientation (angular mean = 339° ± 22°). Simulations showed that tilted cacti with pseudocephalia facing northwards increase yearly interception of direct solar radiation by the whole plant compared to erect cacti with or without a pseudocephalium (2 and 7% increase, respectively), and with tilted cacti with the pseudocephalium facing away from the north (9–10% increase). Additionally, the observed morphology decreases radiation interception during the hottest and driest period of the year. From our results, pseudocephalium orientation and stem tilting in C. columna-trajani appear to be morphological adaptations that allow the fine-tuning of a columnar morphology to its thermal and radiation environment. However, the cost of tilting in this giant columnar cactus is that branching (which increases photosynthetic area and reproductive output) appears to be almost impossible without serious risk of stem breakage.

Stem tilting, pseudocephalium orientation, and stem allometry in Cephalocereus columna-trajani along a short latitudinal gradient

Cephalocereus columna-trajani is a giant columnar cactus endemic of the Tehuacán-Cuicatlán Valley in Central Mexico. Stem tilting and northward pseudocephalium azimuth in C.␣columna-trajani have functional advantages in terms of interception of direct solar radiation at the northernmost portions of its range. Since the success of both characters strongly depends on the apparent position of the sun during the growing season, in this paper we test the hypothesis that the occurrence of such columnar morphology is restricted geographically and imposes mechanical restrictions that limit column height. Following a latitudinal gradient along the Tehuacán-Cuicatlán Valley, we selected five populations, recorded tilting angle and pseudocephalium azimuth, and carried out allometric and biomechanical analyses of height–diameter relationships. Northern populations showed higher tilting angles. Pseudocephalium azimuth significantly differed among populations, and pseudocephalium orientation was consistently North-Northwestern. Stem allometry showed that the stems of the southern populations increased in height at a far greater rate with respect to diameter than the northern populations. The southernmost population showed the lowest safety factor. These results support the hypothesis that stem tilting in C.␣columna-trajani is functionally advantageous in a restricted geographical range, and imposes mechanical restrictions to column height.

Distribution of Peucetia viridans (Araneae: Oxyopidae) on Croton ciliatoglandulifer

ABSTRACT Peucetia viridans (Hentz) lives almost exclusively on Croton ciliatoglandulifer (Ortega) in the dry forests of western Mexico. This spider is usually found on pubescent shrubs. Within their host plants, P. viridans has been associated with plant height and cover, as well as with number of flowers or inflorescences in anthesis. Flowers can be used as cues of good habitat conditions or because they attract prey detected by the spider. In this study, we assessed the importance of flowers, plant cover, and plant exposure (sun/shade) on the spider distribution in five 50-plant transects. In a field experiment, we also compared the number of spiders between controls, plants from which inflorescences were removed, and plants with artificial inflorescences. The results from the transects indicate that, when the number of flowers per spider is high, spiders were more abundant in exposed locations, which presumably offer better microclimatic conditions; when flowers become scarce, food may be more difficult to find and the spider distribution become strongly associated with the number of flowers, where they are more likely to find prey. Spider abundances on the experimental plot decreased on plants from which flowers were removed in comparison to control plants. Spider abundance increased on those in which artificial inflorescences were added. The similarity between plants with natural and artificial inflorescences suggests that spiders use flowers as cues of good microhabitats instead of prey visitors, which are significantly less abundant on artificial inflorescences.

Spatial Variation in the Community of Insects Associated with the Flowers of Pachycereus weberi (Caryophyllales: Cactaceae)

ABSTRACT The positive relationship between productivity and species diversity is well-known. Insect communities associated with the flowers of Cactaceae species represent an interesting system to explore the productivity-diversity relationship because branches facing the equator receive more photosynthetically active radiation and have higher productivity. Thus, flowers with contrasting orientations within an individual, and even within a single branch, might differ in productivity. Therefore, higher abundance, species richness, and diversity are expected for the insect communities associated with south-facing flowers. This hypothesis was tested in Pachycereus weberi (J.M. Coulter) Backeberg (Cactaceae). Insects within flowers with contrasting orientations were collected and its abundance, richness, and diversity were estimated. We also asked if insects prefer big flowers. Thus, flower volume was estimated and regression analyses were conducted to test if there is a positive relationship between flower size and insect abundance. Flower orientation did not affect species richness. However, species abundance and diversity were different in flowers with contrasting orientations. In general, species abundance was higher in flowers facing southwards than in north-facing flowers. On the contrary, species diversity was higher in north-facing flowers. Abundance of Coleoptera was explained by flower volume in south-facing flowers. Contrary to our hypothesis, total diversity was greater in the less productive oriented flowers. Three possible explanations are discussed to explain the low diversity found in the highly productive, south-facing flowers. Our study provides evidence for the effects of productivity on the structure of insect communities at a very small-scale.