Some autoecological aspects of Opuntia imbricata (Haw.) D.C. (cholla)
Opuntia imbricata (Haw.) D. C. (â€œcholla") is a native cactus species common on semiarid and arid ranges of the southwestern U. S. and northern Mexico. For reasons not well understood, the species sometimes increases its density and becomes a range weed.
The general objective of this study was to collect autecological information on cholla so that management or control plans can be developed. The research was divided in two parts: 1) the individual plant, and 2) the population. At the individual level, the architecture, phenoiogy and growth of O.imbricata were studied at Post, Garza Co., Texas. Two types of cladodes, long plagiotropic and long orthotropic, were identified by subjective (tropism, position in the plant) and objective (diameter, length, dry matter content and tubercle index) features as the main structural units of a plant. Differences within parameters measured suggest that the long piagiotropic cladodes may have a dominant synthesis function while the long orthotropic ciadodes have a support and transport function. Both types of shoots possess determinate growth: each growing season a new cohort of shoots is produced. The long plagiotropic cladodes terminate usually with flower buds, while long orthotropic cladodes primarily bear vegetative meristems. Analysis of growth data suggest that the differences between cladode types may be present from their inception. The long plagiotropic cladodes arouse in a starlike or crownlike structure with a long orthotropic ciadode in the center, somewhat resembling a palm. Old long plagiotropic cladodes are dehisced after some seasons, but the long orthotropic cladodes remain on the plant, forming an axis that keeps the starlike structure on the upper part of the plant. Based on the growth form of piagitropic and orthotropic cladodes, the plant architecture of O.imbricata conforms to the Schoute architecture model.
The architectural unit formed by the long plagiotropic and long orthotropic cladodes was used as the basis for the physiologicai study of O. imbricata,, also conducted at Post, Garza Co., Tx. Trends of photosynthesis and total nonstructurai carbohydrates were monitored from March 198 9 to June 1990 in flowers/fruits, current and past seasons' long plagiotropic and long orthotropic ciadodes, oid long orthotropic cladodes, and roots. Under west Texas weather conditions, air temperatures were the dominant factor in dividing the year into two periods, winter and spring-summer fall, unfavorable and favorable, respectively, for the physioiogical activity of O.imbricata. During the favorable season, photosynthetic activity of cholla is relatively constant, irrespective of short-term droughts caused by the interaction of high temperatures and lack of rainfall. However, the phenological stage of the plant may affect the characteristic acid fluctuation by its influence in carbohydrate reserves, a substrate providing CO2 during unfavorable periods of short duration. The information collected also confirms the hypothesis that the long piagiotropic cladodes of cholla act as the main source of photosynthates and can be considered as equivalent to â€œleaves" of other species. Despite some morphoiogical similitude at first, the group of plant organs composing the starlike structure are physiologicaiiy specialized with the long orthotropic cladodes being the support and transport unit of the plant. The size of the long plagiotropic cladodes also makes them a large water reservoir. This fact, coupled with their photosynthetic activity, suggests that they are the organs most likely to act as vegetative disseminules for the species.
During the favorable period for photosynthesis, low TNC concentrations were observed during spring, when the new cohorts of reproductive and vegetative organs were developing, and gradually increased to reach a maximum during late summer-fall when the growth activity was minimal.
Information collected at the plant level permitted an estimation of recruitment age for an O,imbricata individual, defined as the year when a plant is able to surpass the height of the grass, and begins to be an hindrance for livestock grazing and handling. In order to assess the temporal dynamics of cholla, dispersion on time of waves of recruitment of different pastures, located in west Texas and New Mexico, were analyzed under an invasion model for the species. The rates of addition of population growth curves based on age structure data and density, modeled by the logistic equation were also tested for similarity. Similarity among rates suggests that invasion of O.imbricata is repeatable, and not much can be done in order to prevent the species' encroachment.
Pastures were classified according to climate and to management (proper and improper) managed, based on the appropriate stocking rate for the range sites where cholla occurs. A constant supply of disseminules, either sexual (seeds) or asexual (cladodes) was assumed to be produced every year based on phenological and growth data. Waves of recruitment of cholla throughout time were related to negative departures of the annual rainfall, during the previous 4 to 5 years before the recruitment has occurred. Lack of moisture can stop the growth of grass potentially causing overgrazing, even if the pasture is stocked at the appropriate rates, subsequently opening up of the grass sward. A close contact between cladode and soil is necessary to promote rooting and start a new individual of cholla, therefore a recruitment wave of 0.imbricata is likely to be caused by the conjunction of drought, wildlife or livestock grazing and the openness of the grass sward. This modei is based on asexual reproduction, rather than sexual, as the main mechanism by which O.imbricata increases on rangelands.
The hypothesis of repeatability of invasion was accepted. The only exception was a pasture located toward the colder edge of the range of distribution of the species, with deep and fertile soils, and where the slow speed of invasion can be attributed to the resilience of the site to overgrazing. From a practical standpoint, results of this research indicate that once O.imbricata is in the pasture, the basic range management practice of keeping an appropriate stocking rate in relation to the range site wiii not prevent cholla spread. Cholla is eaten by both livestock and wildlife, dispersing seed via their feces and ciadodes via clinging to their hide. On the other hand, disturbance caused by livestock grazing around the plant will eventually cause the invasion to be a self-sustaining process.
In order to assess the importance of the "'passive" (cladodes and fruits falling to the ground closer to the mother plant) versus the '"active" (transport of disseminules by livestock or wildlife) mechanism of spread, probability density functions based on a diffusion model of spread was applied to field data of distance of offspring from a mother plant. An Index T, based on the radial distance within which 99% of the offspring were located, was used to evaluate the amount of time, expressed in favorable years, required for an offspring to eventuality reach 100 m from the original parent plant. Results suggest that cholla disperses by the passive and active mechanisms simultaneously. If passive and active mechanisms are compared using the index T, the observed densities of O.imbricata could not have been possible with only a passive method of dispersion. Concurring with previous conclusions, it seems that the mere presence of grazing livestock, irrespective of management type, will cause O.imbricata to spread and invade the pasture. Once O Imbricata is present above a specific density threshold, either by integrating the diet of wildlife and livestock, or simply by being in the way of grazing or wandering animals, interaction with the animals will cause dispersion.