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71.
Experimental restoration may both accelerate and elucidate natural processes of succession on degraded agricultural land by offering insight into factors that influence rates of succession and the composition of resulting communities. A novel study in restoration of degraded tropical agricultural land in coexistence with cattle ranching activities was established in southern Veracruz, Mexico. The experimental planting of 16 mixed‐species stands of 18 pioneer and late‐successional tree species was established from September to November 2006 on an eroded hillside pasture with an elevational gradient from 182 to 260 m and heterogeneous soil depths. An unusually severe dry season in 2007 killed 72 per cent of the seedlings: least squares regression suggested that survival of six pioneer and 12 late‐successional species was mainly explained by initial basal diameter at planting followed by soil depth for pioneers and by elevational position on the hillside for late‐successional species. Individuals with larger initial size at planting (>4 mm basal diameter), regardless of germination size in a growing house, survived better probably because larger seedlings developed deeper roots that found fissures in substrate underlying thin soils. Interestingly, seedlings small at planting (<4 mm basal diameter) survived as well as large seedlings in deeper (>19 cm) soils, but virtually, all small seedlings died on thin soils (<18·5 cm). Mortality in restoration plantings can be reduced by planting large seedlings of a mix of pioneer and late‐successional species, recognizing that soil depth is a key criterion for survival of the smallest seedlings in a cohort. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   
72.

Clearing brush from rangeland and seeding it to buffelgrass (Cenchrus ciliaris L.) is a popular range improvement practice in Mexico, but no data are available on the effects of these practices on soil properties. Twenty-nine study sites were randomly selected across 3 major climatic regions in Mexico: 13 in the northwest, 11 in the northeast, and 5 in the southeast. Soils under buffelgrass stands more than 10 years old and on adjacent virgin rangelands were sampled at 0-10, 10-20, and 20-30 cm depths to quantify sand, silt, clay, organic C, total N, pH, EC, CEC, available P, and exchangeable Ca2+, Mg2+, Na+, and K+.Because soil variables were not significantly (P 0.05) different among depths, only data from the surface horizon received further analysis. Soils in the northwest had higher (P 0.05) sand content and were particularly low in total N (0.08%), Ca2+, and K+, while soils of wetter and warmer southeast Mexico were significantly (P 0.05) higher than the other two regions in measures of soil fertility such as organic C (10 times higher), total N, and CEC (both at least 3 times higher). Long-term buffelgrass establishment had the most effect on soils in the southeast and no significant effect (P0.1) on soils of northeast Mexico. Soil organic C and total N both declined (P 0.025) by about 40% under buffelgrass pastures in the southeast; excgangeable Ca2+ decreased (P=0.027) by 21% in the same region, and Mg2+ declined (P=0.03) by 36% under buffelgrass in the northwest. Soil P did not respond to buffelgrass establishment. The findings of this study suggest that high forage yields following conversion of rangeland to buffelgrass pasture will be less sustainable in subtropical regions with high rainfall (mean of 1070 mm in this study) than in the semiarid zone with inherently poorer soils.  相似文献   
73.
Native populations of the axolotl (Ambystoma mexicanum), a microendemic salamander from Central Mexico, have seen alarming decline in the last decades owing to habitat loss caused by urban growth. The last remnant of its distribution is in a highly heterogeneous urban-rural water system in the Xochimilco region, at the southern edge of Mexico City. We developed a model of the species local distribution based on its ecological niche, using occurrence data and ad hoc limnetic variables via the Genetic Algorithm for Rule-set Production (GARP), to identify suitable areas for the species and prioritize conservation efforts. Results indicated that potential distribution of the axolotl in Xochimilco is limited to 11 sites in six reduced, isolated, and scattered areas, located mostly in zones where traditional agriculture (chinampas) is the primary land use. Recent surveys found only a single organism in the whole study region, in one of the predicted sites, suggesting a critical situation for the long-term survival of the axolotl in the wild, and demanding urgent actions toward habitat and population restoration. This study also illustrates the utility of niche modeling approaches for aquatic systems at a fine scale.  相似文献   
74.
In this study we estimated the carbon content in vegetation, litter, and soil, under 10 different classes of land-use and land-cover classes (LU/LC) in the Purepecha Region, located in the Central Highlands of México. Forests in this area are representative of the montane forests of Central and Southern Mexico and are subject to rapid degradation and deforestation by human pressure.  相似文献   
75.
[目的]发掘利用墨西哥牧草生长繁茂和抗逆能力强等优良遗传特性,改良普通玉米材料的抗逆性和创制新型饲草材料。[方法]以墨西哥牧草与4个普通玉米自交系杂交后,研究了墨西哥牧草和4个杂交材料的植株生长发育和生物产量及饲用品质的差异。[结果]杂交材料植株生长加快,叶龄增长速度和植株高度生长明显快于墨西哥牧草,生物鲜产量和干产量明显增加,饲用品质更好,但平均单株分蘖数减少。[结论]墨西哥牧草的生长繁茂和生物产量高的特性能转育到普通玉米中,培育出新型高产饲草玉米新材料。  相似文献   
76.
Tejocote (Crataegus spp.) is a genus of fruit-bearing trees distributed widely throughout Mexico; 13 species are reported for the north and central zones and two or more species may be present in southern Mexico. Accessions of this genus are safeguarded in the Germplasm Bank of Tejocote at the Autonomous University of Chapingo, mainly from three regional sources, i.e. the states of Puebla, Mexico, and Chiapas, including five different species that belong to series Mexicanae and series Crus-galli. They can be morphologically characterized by leaves from different shoot types, flowers and fruits. Multivariate statistical methods were used to elucidate patterns of variation in each of these regional sources. The sources displayed very low intra-source variability. The source from Chiapas showed significant statistical differences in all morphological variables evaluated, as a result, this genetic pool is considered as different from the other two sources. The sources from the states of Puebla and Mexico only differed by 22.79% (with P ≤ 0.05), and thus they could be considered as components of a single genetic pool. The most highly discriminant variables were from the leaf, such as basal angle, petiole length/major axis length ratio, minor axis length/major axis length ratio, and number of veins. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   
77.
Lethal yellowing disease has drastically affected the coconut populations on the east coasts of Mexico and is threatening to invade all the production areas in Mexico. For the purpose of defining the existence of genetically differentiated coconut populations that eventually could have a differential response to this disease, we studied the germination patterns in 20 representative coconut populations using 90 nuts per population in three lots under similar growing conditions. Following the emergence of the leaf through the plumuler fissure, the following were calculated weekly: (1) percent emergence of the first leaf, percent mortality between germination and emergence, and percent germination capacity; (2) time required to reach 25%, 50%, 75%, and 100% of germination; (3) mean time to complete germination, coefficient of the rate of germination, and uniformity of germination; and (4) sigmoid curve parameters of accumulative percent germination, adjusted to a model of nonlinear regression (log scale). The results showed three population groups: (a) with early and uniform germination; (b) with early and heterogeneous germination, and (c) with late and heterogeneous germination. The same population groups were previously observed using fruit morphological traits. This suggests the presence in Mexico of three tall coconut genotypes. Late and heterogeneous populations have been almost totally eliminated by the disease and some precocious populations have been shown very low levels of mortality. This suggests a correlation between precocity and resistance. The geographical distribution of the ecotypes suggest that the mortality will be deferential when the disease arrives to the other coconut producing areas in Mexico.  相似文献   
78.
Long-term, fisheries-independent bag seine surveys conducted in TX, USA estuaries from 1977 to 2009 were used to develop a spatio-temporal species-environment model for juvenile spotted seatrout, Cynoscion nebulous. Relationships between environmental predictors and juvenile spotted seatrout distribution were investigated using boosted regression trees (BRTs). Results showed good model performance and suggested that, in relation to environmental factors, juvenile spotted seatrout distribution is most closely linked to salinity, temperature, and distance from tidal inlets. There was also a strong seasonal pattern where abundance increased from May to October and declined precipitously after November. Juveniles were rarely captured between January and April. By interpolating the environmental predictors, monthly maps of the probability of capture were produced using ordinary kriging. Spatial patterns were also evident. Probability of occurrence began increasing first in Upper Laguna Madre peaking in Baffin Bay. Probability of occurrence was consistently higher in this region than other regions within the study area. Predicted occurrence was also high in portions of Corpus Christi, Aransas, San Antonio Bays, and the southern portion of Galveston Bay. Overall, probability of occurrence increased with increasing distance from tidal inlets. The development of spatially explicit models allows for prioritization and conservation of areas in a region that has great potential for human disturbance and climate change impacts.  相似文献   
79.
Relationships between abundance of post‐larval and juvenile carangid (jacks) fishes and physical oceanographic conditions were examined in the northern Gulf of Mexico (GoM) in 2011 with high freshwater input from the Mississippi River. Generalized additive models (GAMs) were used to explore complex relationships between carangid abundance and physical oceanographic data of sea surface temperature (SST), sea surface height anomaly (SSHA) and salinity. The five most abundant carangid species collected were: Selene setapinnis (34%); Caranx crysos (30%); Caranx hippos (10%); Chloroscombrus chrysurus (9%) and Trachurus lathami (8%). Post‐larval carangids (median standard length [SL] = 10 mm) were less abundant during the spring and early summer, but more abundant during the late summer and fall, suggesting summer to fall spawning for most species. Juvenile carangid (median SL = 23 mm) abundance also increased between the mid‐summer and early fall. Most species showed increased abundance at lower salinities and higher temperatures, suggesting entrainment of post‐larval fishes or feeding aggregations of juveniles at frontal convergence zones between the expansive river plume and dynamic mesoscale eddy water masses. However, responses were species‐ and life‐stage specific, which may indicate fine‐scale habitat partitioning between species. Ordination methods also revealed higher carangid abundances at lower salinities for both post‐larval and juvenile life stages, with species‐ and life‐stage specific responses to SST and SSHA, further suggesting habitat separation between species. Results indicate strong links between physical oceanographic features and carangid distributions in the dynamic northern GoM.  相似文献   
80.
The protection conferred by the reduced-dose Rev 1 Brucella melitensis vaccine in goats that had been immunized 5 years previously was evaluated. Sixteen goats vaccinated 5 years before with Rev 1 (1 x 10(5) cfu) and 5 non-vaccinated goats were challenged with B. melitensis 16M (4 x 10(5) cfu) using the conjunctival route. After giving birth or aborting, the goats were sacrificed and tissue samples were taken for bacteriological study. The challenge strain was recovered in 12%, of the animals from the vaccinated group, and in (80% of the control group. It is concluded, therefore that the use of reduced-dose Rev 1 protects goats vaccinated in endemic areas for at least 5 years after immunization.  相似文献   
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