be critical for the survival of subalpine Engelmann spruce saplings that are exposed above the snowpack in later winter to early spring. Transpiration is greatly reduced in small saplings while engulfed in snowpack. For exposed trees, the availability of soil water may be critical in late winter, when transpirational demands increase. Increased rates of transpiration in response to loss of snowpack, coupled with low sapwood water reserves and an extended period of soil frost in windswept areas, may prevent Engelmann spruce from regenerating in open areas both above and below the tree line. Cuticular damage by windblown ice is probably more important at the tree line, but damage caused by desiccation is likely to be more important at lower elevations. Despite wind damage, the species tends to grow taller than others at the tree line. It is shade tolerant, but not so much as subalpine fir. Thus, it is somewhat dependent on fires to outgrow competitors, although its thin bark and shallow roots make it vulnerable to fire as well. Spruce bark beetles attack the tree, being particularly deadly to groups which have stood for centuries. It is also susceptible to avalanches. Although older spruce forests are not very useful to animals for forage, they are so in the aftermath of fires, which allow many other plants to rise. Engelmann spruce-shaded streams are exploited by trout. Additionally, aphids produce galls which hang from the tree and look similar to cones when they dry out. Subspecies and hybrids Two geographical subspecies (treated as varieties by some authors, and as distinct species by others) occur: Picea engelmannii subsp. engelmannii (Engelmann spruce). All of the range except as below. Picea engelmannii subsp. mexicana (Mexican spruce). Two isolated populations on high mountains in northern Mexico, on the Sierra del Carmen in Coahuila (Sierra Madre Oriental) and on Cerro Mohinora in Chihuahua (Sierra Madre Occidental). Engelmann spruces of the Madrean sky islands mountains in the extreme southeast of Arizona and southwest of New Mexico also probably belong to this subspecies, though this is disputed. The Engelmann spruce hybridises and intergrades extensively with the closely related white spruce (Picea glauca), found further north and east in the Rockies, and to a lesser extent with the closely related Sitka spruce where they meet on the western fringes of the Cascades. Uses Native Americans made various medicines from the resin and foliage. Engelmann spruce is of economic importance