Lauro López-Mata

Vegetation ordination at the southern Chihuahuan Desert (San Luis Potosi, Mexico)

Cover data for 93 perennial plant species from fifty 1 ha sites, were used to ordinate desert vegetation in relation to 50 environmental variables at El Huizache Corridor. Cumulative variance recovered in the Bray and Curtis variance-regression ordination was substantial (80%). Community structure of desert plant communities at El Huizache Corridor may be influenced primarily by a combination of landscape and edaphic variables, which in turn may determine the distribution and abundance of moisture and nutrients, and perhaps promote habitat specialization and or competitive exclusion. Secondly, to a lesser extent, climate variables could be influencing community organization at small scale gradients, the longer the gradient the more relevant climatic factors become. First axis represented a landscape gradient; it was positively correlated to exposure, geology, slope angle, rocks, stoniness, iron, January mean temperature, and organic matter content; it was negatively correlated with latitude, longitude, soil depth, and potassium content. The second axis represented mainly a climatic gradient; it was positively correlated with mean precipitation of January, February, July, August, September, November, December, annual mean precipitation, Langs Index, organic matter content, and stoniness. The third axis represented an edaphic gradient; it was positively correlated with electrical conductivity, Mn, Zn and elevation, and negatively correlated with pH, nitrates, Ca, and disturbance. These findings should guide conservation efforts to maintain species diversity and endemism at this area.

Demography of the seedling bank of Manilkara zapota (L.) Royen, in a subtropical rain forest of Mexico

Selection of individuals in tropical trees, occurs mainly in the seedling phase, which in part explains the low densities of most species. The main objective of this work was to gain an insight into those factors that influence growth and survival of Manilkara zapota seedlings, one of the most abundant species in the lowland forests of Mexico and Central America. Eight 10-m2 rectangular plots (5m×2m) were established, in which all < 35-cm-high M. zapota seedlings were marked, measured and enumerated at bimonthly intervals. In each census seedling height, number of leaves, length of largest leaf, type of damage and seedling death were registered. Seedling survival during 2 yr was high, reaching 82% including newly emerged seedlings. A maximum likelihood regression analysis showed that both number of leaves and length of largest leaf had a direct influence on seedling survival; however, neither type of damage nor seedling density and height had significant effect. Seedling height growth averaged 2.8 cm in the 2 yr of study. The combined effect of high seedling survival, reduced growth, and impact of physical damage mainly due to falling branches and leaves reveals the occurrence of a persistent seedling bank. Such a bank would contribute to recruitment of individuals in the juvenile and eventually in the adult stages. This seedling bank could explain the high density of individuals of M. zapota in the tropical forests of Mexico.

A comparison of traditional elasticity and variance-standardized perturbation analyses: a case study with the tropical tree species Manilkara zapota (Sapotaceae)

Knowledge of the population dynamics of tropical trees has expanded considerably in the past 20 years. An important observation deriving from these investigations is the confirmation that population behaviour varies both in time and space. A tool recently developed to evaluate the potential for variation in vital rates, and therefore in population growth rate, is variance-standardized perturbation analysis (VSPA). In this paper we report the results of a 2-y demographic analysis of a population of the tropical tree Manilkara zapota in a subtropical rain forest in the Mexican state of Veracruz, in which variance-standardized perturbation analysis was applied and compared with the results of the traditional elasticity analyses. To build population projection matrices, we tagged and followed a sample of 91 juvenile and adult individuals, and 635 seedlings. We subdivided the sample in nine size classes (defined by tree height and dbh; as well as leaf size, in the case of seedlings) and estimated transition probabilities and fecundity for each class. The demography of M. zapota varied greatly from the first to the second year of study (in 1998-1999, λ = 0.987, while in 1999-2000, λ = 1.038) due to negligible seed production during the first year and a massive reproductive event during the second. The largest elasticity values for both years corresponded to persistence of large juveniles and adults. Although the fecundity entries showed very low elasticity values, the variance-standardized perturbation analysis revealed the importance of these matrix entries; transition to larger categories and retrogression to smaller ones of saplings and juveniles were also important demographic processes contributing to variation in λ according to the VSPA. Thus, although the results of elasticity analysis and VSPA were similar for the 1998-1999 matrix, they differed substantially for the 1999-2000 matrix. In the latter, the VSPA enhanced the importance of demographic processes that are intuitively relevant for the population studied. This points toward the necessity of further exploring the use of VSPA, since it offers several advantages over the traditional elasticity analysis: it concentrates on the impact on λ of vital rates that actually vary, and the interpretation of the results is more realistic and straightforward.

Structure, species composition and diveristy of woody species in a cloud forest in the Sierra de Manantlan, Jalisco

Resumen es: Se describe la estructura, la composicion floristica y la diversidad de especies lenosas (arboles y arbustos) de un bosque mesofilo de montana en Cuzala...