Laura Arriaga

Association between cacti and nurse perennials in a heterogeneous tropical dry forest in northwestern Mexico

. The Sierra de La Laguna, Baja California Sur harbors the only tropical dry forest of the Peninsula. An inventory of the perennial plants was carried out at the eastern part of this mountain range. Plant spatial distribution was analyzed considering the species composition and vertical stratification of plant clusters in a study plot of 2 500 m2. A plant cluster was defined as a group of at least three individuals with a maximum distance between plants of 1 m. Soil physical-chemical characteristics and soil surface temperatures were compared between shade and sun conditions. The tropical dry forest of Baja California Sur presents a highly aggregated pattern of species distribution. Most of the perennial plants grow clumped, yet not all of the six cactus species recorded were significantly associated with them. Pattern analysis revealed that only Stenocereus thurberii and Ferocactus peninsulae were aggregated and associated to perennial plants. There is no replacement pattern between cacti and perennial plants. Nutrient content, including soil fertility, between shade and sun conditions does not seem to explain the patchy distribution of perennial plants, while the differences in solar radiation between these conditions appear to be restrictive and determinant of this spatial pattern of distribution.

Spatial variability in decomposition rates in a desert scrub of Northwestern Mexico.

The decomposition of leaf litter for five dominant plant species of a desert scrub in Baja California Sur, Mexico was investigated. We designed a factorial decomposition experiment using decomposition bags and the collected leaf-litter from Prosopis articulata, Jatropha cinerea, J. cuneata, Cyrtocarpa edulis, and Fouquieria diguetti. Factors, such as radiation exposure, rainfall, and the size of litter-consuming organisms were considered. The rates of litter decomposition were calculated for these plant species and the environmental conditions by using single exponential models. The initial concentration of nutrients (C, N, P, K, and Ca) and crude-fiber content of the leaf litter were determined. Our results show that the environmental heterogeneity generated by different conditions of radiation exposure and short-term rainfall patterns are the most relevant factors affecting decomposition processes in this Sonoran desert community. A species-specific pattern was observed in decay rates and mass-loss patterns. Decomposition rates varied from 0.0027 to 0.0201 depending on the species and exposure to different ecological conditions. The decay rates were higher under bare-soil conditions and during a wet year than under the shade provided by the canopy of nurse trees and during a dry year. The leaf litter of J. cuneata reincorporated to the soil more rapidly than that of P. articulata and C. edulis. Termites were the more important macroarthropods associated with litter decomposition, and their harvest distribution was independent of the resources distribution. The ecological significance of these results is discussed considering the extreme climatic conditions prevailing in this region.

Natural disturbance and treefalls in a pine-oak forest on the Peninsula of Baja California, Mexico

The magnitude of natural disturbances by treefalls and their spatial occurrence were studied in a pine-oak forest of Sierra de La Laguna, located at the southern part of the Peninsula of Baja California in Mexico. Twenty transects covering 4 ha, perpendicular to north- and south-facing slopes were sampled. The percentages of rocky outcrops, slope, orientation, and gap size created by treefalls were recorded. The mode of tree death, treefall direction, trunk length, and basal diameter were also measured. Data were analyzed using principal component analysis, one-way ANOVA, multiple comparison tests by contrasts, and chi-square independence tests. Results suggest that exogenous disturbances have an important effect in this community. A mean number of 80 gaps per ha was recorded comprising 18.1% of the observed area. Gap sizes were fitted to a lognormal distribution as 2.3±1.4 and patches were found to be created by single treefalls. Analysis of dead material shows that there are significant differences between numbers of fallen trees on N vs S facing slopes, and that forms of dead trees are significantly associated with species. Snapped trees represent 39.5% of treefalls, dead standing trees 26.4%, uprooted trees 20.5% and cut-down trees 0.9%. The frequency of occurrence of various forms of the dead trees suggests that the cause of mortality is primarily due to the high incidence of tropical hurricanes, and secondly to the combined effect of wind and natural fire which occurs during the rainy summer season.

The Mexican tropical deciduous forest of Baja California Sur: a floristic and structural approach

A quantitative study is presented of the tropical deciduous forest located in the Sierra de La Laguna in the southern part of the peninsula of Baja California, Mexico with data on structure, species composition, diversity, density, and abundance of perennial plants. 4 study plots were selected to represent the predominant geomorphologic units, and to include topographic and climatic variations reflected by the distribution of this vegetation on the lowlands of slopes facing the Gulf of California and the Pacific Ocean. 25 families containing 67 perennial species were found on the lowlands, with Leguminosae, Cactaceae, and Euphorbiaceae best represented. A high family diversity was found in the plots, but there was a low number of species per family. Dissimilarities between sites were found to be reflected significantly in growth-form abundances as well as in structural features and species diversity. Results show that the xeric environment, the low number of species, and the high incidence of dominant shrub species confer the vegetation of the lowlands simpler structural traits than those described for other tropical dry forests.

Seed bank dynamics and tree-fall gaps in a northwestern Mexican Quercus-Pinus forest

Abstract Questions: How does the seed bank respond to different types of tree-fall gaps and seasonal variations? How does the soil seed bank influence recovery of the standing vegetation in the mature forest and tree-fall gaps? Location: 1800 - 2020 m a.s.l., Quercus-Pinus forest, Baja California Sur, Mexico. Methods: Seed size, species composition and germination were estimated under different environmental conditions during dry and rainy seasons: a mature forest plot and gaps created by dead standing trees, snapped-off trees and uprooted trees. The soil seed bank was investigated using direct propagule emergence under laboratory conditions, from soil cores obtained during both seasons. Results: 21 species, 20 genera and 14 families constitute the seed bank of this forest community. Fabaceae, Asteraceae, Euphorbiaceae and Lamiaceae were the most frequently represented families in the seed bank. Floristic composition and species richness varied according to the different modes of tree death. Species composition of seed banks and standing vegetation had very low similarity coefficients and were statistically different. Seed bank sizes varied between 164 and 362 ind.m–2 in the mature forest plot for the dry and rainy seasons, respectively, while soil seed bank sizes for gaps ranged between 23–208 ind.m–2 for the dry season and between 81–282 ind.m–2 for the rainy season. Conclusions: Seed bank sizes and germination response were always higher in the rainy season under all the environmental conditions analysed. Results suggest that timing responses to gap formation of the soil seed bank could be more delayed in this temperate forest than expected. Nomenclature: Wiggins (1980).

AN INTERANNUAL CLIMATOLOGICAL ARIDITY SERIES FOR THE SIERRA DE LA LAGUNA, B.C.S. MEXICO

The Sierra de La Laguna is a group of mountains in the south of the Baja California Peninsula that, because of their height, show marked climatic differences. The life forms there are different from the rest of the Peninsula and the area is the site of many ecological studies. The objective of this paper is to construct a regional climatic series that can be useful for detecting annual dry and wet periods. An aridity index is calculated according to Martonne, using precipitation and temperature data from nine meteorological stations in the middle and lower ranges of the Sierra. When an auto-correction and Fourier analysis of the resultant series is made, a tendency to less aridity is observed with a fundamental frequency of 24 years, which approximates conformance to the double cycle of sunspots.