AFFINITY AMONG MOUNTAIN-RANGES IN MEGAMEXICO - A PHYTOGEOGRAPHICAL SCENARIO

The concept of Megamexico as a phytogeographic unit has been introduced by Rzedowski (1991) on the basis of distribution patterns of genera. Until now precise information on the resemblance between plant communities in the proposed Megamexico was scarce. This phytosociological study documents the affinities between plant communities within Megamexico, based on studies from mountain ranges in Mexico (Sierra Chichinautzin, Sierra Nevada) and Guatemalan mountain ranges (Sierra de los Cuchumatanes, Cadena Volcanica). Data, collected in the style of Braun-Blanquet during extensive fieldwork, were pooled into a single data set, and analyzed by using (1) TWINSPAN, for plant community classification, (2) ordination (DCA), (3) alpha log series biodiversity-index to measure intergroup diversity, and (4) information on the altitudinal distribution of the vegetation belts. Classification showed that four communities were common to the two groups of mountain ranges, namely, alpine bunchgrassland, pine forest, fir forest and mixed forest. Along the altitudinal gradient Mexican communities are distributed 200 m higher than their Guatemalan equivalents. This appears to be an illustration of the 'Massenerhebungseffekt'. DCA showed that the first axis represents a set of minor differences of closely related ecological factors (e.g., temperature, precipitation) and the second a humidity gradient. The biodiversity index showed that the Mexican mixed forest was significantly more diverse than the Guatemalan mixed forest. From the present results, ecological conditions among mountain ranges in Megamexico differed significantly. To conclude, phytogeographical units in Megamexico can better be defined on basis of both historical and ecological characteristics of the communities

HIGH-ELEVATION CONIFEROUS VEGETATION OF GUATEMALA - A PHYTOSOCIOLOGICAL APPROACH

A phytosociological study of the juniper (locally called huito), pine (locally called sacch), pine-alder and fir forests of the Sierra de los Cuchumatanes and Cadena Volcanica in Guatemala was carried out. The Zurich-Montpellier approach was followed. In total 119 releves were sampled and the data were organised in phytosociological tables to distinguish vegetation clusters. TWINSPAN was used to evaluate major differences among plant communities. Seven zonal plant communities were distinguished and described, namely: (1) Relbunium microphyllum-Agrostis tolucensis, (2) Werneria nubigena-Agrostis exserta, (3) Lachemilla vulcania-Pinus hartwegii, (4) Holodiscus argenteus-Pinus hartwegii, (5) Hypnum cypressiforme-Juniperus standleyi, (6) Agave hurtei-Alnus firmifolia and (7) Sabazia pinetorum-Abies guatemalensis. This paper provides a thorough floristic characterisation of each community and outlines the major anthropogenic activities. To conclude, ecologic and floristic (dis)similarities between plant communities of the study area and those of Central Mexico, like the different altitudinal distribution of fir forests and the establishment of mid-successional communities such as the Agave hurteri-Alnus firmifolia were discussed

Chloroplast DNA Microsatellites Reveal Contrasting Phylogeographic Structure in Mahogany (Swietenia macrophylla King, Meliaceae) from Amazonia and Central America

Big-leaf mahogany (Swietenia macrophylla) is one of the most valuable and overharvested timber trees of tropical America. A description of the organization of genetic variation across its broad range would be useful for management of genetic diversity and for understanding its demographic history. Here we report on a phylogeographic analysis of mahogany based on six polymorphic cpDNA simple sequence repeat loci (cpSSRs) genotyped in 16 populations distributed across the Brazilian Amazon and Mesoamerica (N = 245 individuals). Of the 31 cpDNA haplotypes identified, 15 occurred in Amazonia and 16 in Mesoamerica with no single haplotype shared between the two regions. The populations from Central America showed moderate differentiation (FST = 0.36) while within population genetic diversity was generally high (mean Nei's HE = 0.639). In contrast, the Amazonian populations were strongly differentiated (FST = 0.95) and contained low haplotype diversity (mean HE = 0.176), with the exception of the highly diverse Marajoara population from the Eastern Amazon (HE = 0.925). SAMOVA identified a single Mesoamerican phylogroup and four Amazonian phylogroups, indicating stronger phylogeographic structure within Amazonia. The results demonstrate high levels of cpDNA variation and differentiation of regional S. macrophylla populations, and provide the first evidence of a major phylogeographic break between Mesoamerican and South American mahogany populations

Rapid climate changes in the tropical Atlantic region during the last deglaciation

The climate system is capable of changing abruptly from one stable mode to another(1-3). Rapid climate oscillations--in particular the Younger Dryas cold period during the last deglaciation--have long been recognized from records throughout the North Atlantic region(4-14), and the distribution of these records at mostly high latitudes suggests that the changes were caused by rapid reorganizations of the North Atlantic thermohaline circulation(6,8,10,15). But events far from the North Atlantic region that are synchronous with the Younger Dryas(16-19) raise the possibility that a more global forcing mechanism was responsible(20). Here we present high-resolution records of laminated sediments of the last deglaciation from the Cariaco basin (tropical Atlantic Ocean) which show many abrupt sub-decade to century-scale oscillations in surface-ocean biological productivity that are synchronous with climate changes at high latitudes. We attribute these productivity variations to changes in or duration of upwelling rate (and hence nutrient supply) caused by changes in trade-wind strength, which is in turn influenced by the thermohaline circulation through its effect on sea surface tempersture(6,21). Abrupt climate changes in the tropical Atlantic during the last deglaciation are thus consistent with a North Atlantic circulation forcing mechanism