Picea chihuahuana Martínez

Pinaceae

Endemic to México where it is severely fragmented due to deforestation and increasingly threatened by poor regeneration as well as warming and drying associated with climate change.

Distribution

México in SW Chihuahua and W Durango. The species spans a north–south range of 687km, but stands are found in three clusters, each separated by about 300km (Ledig et al., 2000). Recent research indicates that the scattered stands of spruces (Picea spp.) in the Sierra Madre Occidental and Sierra Madre Oriental are relicts of the last glacial period and that Holocene warming resulted in the extinction of spruce in the Valley of Mexico and contraction of the range northward. In addition, the lower elevational range of Chihuahua spruce in the Sierra Madre Occidental contracted upward at least 510 m in elevation between 13000 yr BP and the present (Ortega-Rosas et al., 2008).

The population may not exceed 2500 to 3000 mature trees with a total population including seedlings and saplings estimated to be about 43000 in 38 stands (Jaramillo et al., 2006). Subpopulations are usually smaller than 350 trees, show significant levels of inbreeding and a lack of geneflow (Ledig et al., 1997)

Habitat and Ecology

Picea chihuahuana occurs in scattered relict populations on north-facing high mountain sides, often in canyons, at elevations between 2150 to 3200(-3400) metres above sea-level. It grows in poor, barren, but always moist mountain soils of alluvial origin, usually near permanent streams, but in the Sierra Madre Oriental also on calcareous lithosols. The climate is cool and moist, with annual precipitation between 800mm and 1300mm, mostly as summer showers, but in the western part of the range also in winter; snow only at the highest elevations. It is mainly associated with Pinus strobiformis, P. pseudostrobus and P. ayacahuite. Other pines species, Pseudotsuga menziesii var. glauca, Abies durangensis, A. vejarii (in Sierra Madre Oriental), Cupressus lindleyi (= C. lusitanica Mill.), and C. arizonica may also occur. Broad-leaved trees include Quercus castanea, Q. rugosa and Prunus serotina.

Conservation Status

Global status

Endangered B2ab(ii,iii,v)

Global rationale

The area of occupancy is within the threshold for Endangered, subpopulations are severely fragmented and there is an ongoing decline due to fires, logging and grazing. On this basis alone Picea chihuahuana meets the B2 critieria for Endangered. The number of mature individuals is uncertain but could be less than 2500.

Global threats

The subpopulations of P. chihuahuana are widely scattered and very small, with a total of fewer than 100 to about 350 mature trees in each of the ca. 25 localities known. It is possible that other relict populations are hidden among the pine forests of the Sierra Madres of northern Mexico, yet its total area of occupancy (AOO) is unlikely to exceed 500km² and is by all accounts much less than this (here calculated to be 275km² with a 5x5 km grid); in addition it is severely fragmented. Unaware of its botanical significance, loggers have exploited this species where they have encountered it, reducing the number of mature individuals. In many stands natural regeneration has been observed to be poor or at best slow. Given 21st century projections of warming and drying in this region, the habitat in the majority of currently occupied localities habitat will disappear: persistence of this species will require human intervention to assist its migration to and successful colonization of suitable recipient ecosystems (Ledig et al. 2010)

National Status

According to the Secretaría de Medio Ambiente y Recursos Naturales (2010), this species is in danger of extinction.

Conservation Actions

In the short term, more subpopulations or localities with this species are in need of being put into protected areas, as only a few are at present within such areas. In the longer term, assisted colonisation would be necessary as, under current climate change impact predictions (Ledig et al. 2010), the habitat in the majority of currently occupied habitat will disappear. To facilitate this, a range wide programme of seed collection and ex-situ conservation is urgently required before declining seed production makes such work impossible (Ledig et al. 2010; Mendoz-Maya et al. 2015).