Location and General Description
The Sonoran-Sinaloan ecoregion occurs entirely in Mexico, from the Pacific coasts of the states of Sonora and Sinaloa, and north into the foothills of the Sierra Madres Occidental of central Sonora. Bio-climatically, the ecoregion is classified as a dry steppe life zone, in contrast to the more humid seasonal forests to the south, and arid deserts to the north (Rzedowski 1994). Like neighboring regions, rainfall predominates in the summers (Rzedowski 1994). Annual rainfall is approximately 10-20 cm. Because of its proximity to the coast, fluctuations in annual temperatures are only on the order of 10-15° C (difference between median monthly high and low temperature). Frost and temperatures below freezing are rare, in contrast to the Sonoran desert, to the north. Unlike the distinctly xeric desert vegetation to the north, and the tropical deciduous forest to the south, the vegetation of the Sonoran-Sinaloan transition dry forest is dominated by a deciduous thorn forest or "selva espinosa". Pockets of semiarid mattoral as well as thorn scrub are also present (Instituto de Geografia 1990, Rzedowski 1994, Stattersfield et al. 1998).
Topography is quite variable in this ecoregion. Elevation ranges from approximately sea level near coastal mangroves to roughly 2000 m in the foothills of the Sierra Nevada. Moving north and east, relatively flat coastal areas give way to rolling country with arroyos. Steep canyons, cliffs, and waterfalls characterize the foothills of the sierras. Similarly, geomorphology varies from a low-gradient slope along the alluvial coastal plain, composed primarily of unconsolidated sediments to steeper gradients of acidic rocks of igneous origin and clastic sediments as one moves up into the mountains (FAO 1998).
Dominant trees in this forest include many species from the families Acaciaceae, Burseraceae and Leguminosae. Cacti, such as organ pipe cactus (Stenocereus thurberi), are often conspicuous and abundant. Overall, this dry forest is less pronounced and more seasonal than its southern cousin, particularly as one moves north to the margins of the Sonoran desert (Rzedowski 1994).
Common and characteristic plants include several acacias: boat-thorned acacia (Acacia cochliacantha), and tree catclaw, or tésota (Acacia occidentalis). The former, a shrub, or small tree, is the only local acacia with boat-shaped thorns. The latter acacia flowers prolifically in March, perfuming the air so heavily that it can often be smelled before it is seen. Another common species in the thorn forest is torote prieto (Bursea fragilis). The gum of this tree has been used for treating scorpion and other insect stings, as well as for mending dishes. Northern limits for many characteristic species of this region appear to be strongly influenced by low winter temperatures and decreasing summer precipitation (Turner et al. 1995).
In general, Mexican dry forests have the highest levels of endemism of all Neotropical dry forests (Ceballos and García 1995, Gentry 1995). 20% of Mexico’s vascular plant species (ca. 6000 spp.) occur in tropical dry forests, more than the contribution of Mexico’s tropical humid forests, or deserts (WWF and IUCN 1993). Although precise figures are not available, this region also supports a number of endemic and rare plants, including the arborescent morning glory or palo santo (Ipomea arborescens). This species flowers in the dry season, thus providing pollen to nectar-feeding long-tongued bats (Choeronycteris mexicana and Glossophaga soricina) – amongst the most important pollinators of the Sonoran region – at a time when few other plants are in flower. When the ping-pong ball like flowers fall to the ground they are browsed upon by white-tailed deer (Odocoileus virginianus). The botanist Howard Gentry reported 33 to 45 species in one-half acre of undisturbed forest (Bowden 1993), which is relatively rich for a tropical dry forest so close to the temperate zone. Overall plant richness rivals that of the Sonoran desert to the north (Turner et al. 1995). The ecoregion is also home to as many as 90 species of butterfly during the rainy season (Bowden 1993). This ecoregion is also notable as the northern limit for many Neotropical plant families (WWF and IUCN 1993).
Within the ecoregion, several areas have been identified as important areas for bird conservation, including the large Cuenca del Río Yaqui (Benitez et al. 1999). In addition, CONABIO has identified a number of terrestrial priority areas within the ecoregion, including Marismas Topolobampo-Caimanero and San Javier-Tepoca, both of which are about 4,000 km2 .
Although throughout extensive areas, such as along the upper Rio Cuchujaqui, over half of the original forest has been cleared, extensive blocks of intact dry forest may remain, particularly on the steep, uncultivatable margins of the mountains. There is evidence that these continue to support large carnivores and intact mammalian faunas (Arriaga et al. 2000). Habitat along the coastal plain has been heavily altered and degraded (INEGI 1998). The overall distribution of intact habitat in this ecoregion is unknown, due in part to the difficulty using remote sensing to accurately map intact dry forest. In general, however, dry forests have practically disappeared from Central America (Ceballos and García 1995). Of the roughly 2% that remains in the entire Central America region, this area supports some of the largest remaining tracts (Bowden 1993). There are no protected areas in the ecoregion.
Types and Severity of Threats
Agriculture, ranching, development for tourism, sport and subsistence hunting, are the primary threats to all Mexican dry forests (Ceballos and García 1995). The primary threat here is cattle ranching – particularly when it involves the complete removal of the original vegetation and the planting of buffel and other non-native grasses (Bowden 1993). In 20 years between the 1970’s and the 1990’s, vast tracts of forest between the cities of Navojoa and Alamos were cut down by hand or bulldozer, burned, and replaced with non-native buffel grass. There is hope for restoration, however. 12-20 years after clearing, the majority of original woody species are again evident (Bowden 1993). Low-level subsistence hunting and conversion for agriculture are lesser threats (Bowden 1993). Increasing temperatures due to human caused climate change pose a serious threat given the transitional nature of the ecoregion, and the narrow temperature and precipitation requirements of many of its native species. Comprehensive conservation strategies for this and other Mexican dry forests are urgently needed (Ceballos and García 1995).
Justification of Ecoregion Delineation
This xeric dry forest ecoregion represents the transition from desert (to the north) to dry forest (to the south). As mentioned above, the northern and southern limits of many species occur here. Initial linework follows the current landcover classifications of INEGI (1996) and reviews of linework were conducted at several ecoregion priority setting workshops (CONABIO 1996 & 1997). From this map we lumped the following classifications: "lowland spine forests", "lowland caducifolous forests", "sarcocaulous matorral", "pachycaulous matorral", "microphyll desert matorral", all subsequent agricultural areas, and "halophilic vegetation" along the coast. Later modification occurred from expert opinions, following suggestions from Rzedowski (pers. comm.) and others at a regional ecoregion delineation workshop.
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Prepared by: Tom Allnutt
Reviewed by: In process