Temperaturemoisture gradients below the timberline

Altitude, and the changes in environmental conditions that go with it, plays just as important a role below the timberline as at the top. For example, there is a well-marked temperature-moisture gradient from the warm, dry lowlands to the cold, wet mountain peaks in the north-western region of the USA shown in Fig. 3.18. The lower limit for these trees usually shows a gradual transition, apparently set by soil moisture levels. Ponderosa pine Pinus ponderosa, Douglas fir Pseudotsuga menziesii, Engelmann spruce Picea engelmannii (not shown in Fig. 3.18) and alpine fir Abies lasiocarpa are found at successively greater altitudes and their roots will usually grow in increasingly moist soils. This equates with their drought resistance, which is highest in ponderosa pine and least in alpine fir. In this situation temperature does not seem to be a major factor with these species, all of which can be grown at low altitude if watered. Also, dry atmospheres produce little effect as long as there is sufficient soil moisture.

Figure 3.19 demonstrates a more complex situation from an area further south. Here as many as ten species of pine can be encountered up a landscape gradient from the foothills to the crest of the mountain range (Richardson and Rundel, 1998). The vertical axis simply represents the altitudinal position of the communities involved, but the horizontal one compounds the influence of topography and soil moisture. Even if it received the same rainfall a steeply

Figure 3.18 Usual order in which coniferous trees occur with increasing altitude in the Rocky Mountains of north-western Montana. The relative altitudinal range of each species is shown by arrows. The dashed portion of each arrow is where the species is seral (early successional: one of the first trees to grow on bare ground) and the solid portion of the arrow is where the tree is the potential climax dominant (late successional and there to stay in successive generations). The temperature-moisture climatic gradient runs from the lowlands where the conditions are warm and dry to the timberline where they are cold and wet. There are several timberline types (h. ts): all have a timberline ecotone with a krummholz above them (see Section 3.5.1). (After Pfister et al., 1977. From Packham and Harding, 1982. Ecology of Woodland Processes. Edward Arnold.)

Figure 3.18 Usual order in which coniferous trees occur with increasing altitude in the Rocky Mountains of north-western Montana. The relative altitudinal range of each species is shown by arrows. The dashed portion of each arrow is where the species is seral (early successional: one of the first trees to grow on bare ground) and the solid portion of the arrow is where the tree is the potential climax dominant (late successional and there to stay in successive generations). The temperature-moisture climatic gradient runs from the lowlands where the conditions are warm and dry to the timberline where they are cold and wet. There are several timberline types (h. ts): all have a timberline ecotone with a krummholz above them (see Section 3.5.1). (After Pfister et al., 1977. From Packham and Harding, 1982. Ecology of Woodland Processes. Edward Arnold.)

sloping site on a hill is likely, assuming the soils to be similar, to experience lower soil moisture level than one which is level, and so would be shown more towards the right (xeric or dry) side.

The blue oak Quercus douglasii woodlands of the foothills frequently include the gray or digger pine Pinus sabiniana, while ponderosa pine dominates the forest from 1500 m to as much as 2200 m on drier slopes and south-facing exposures. At these altitudes singleleaf pinyon pine P. monophylla exists as small relict populations on steep canyon slopes. In mesic (moist) sites coniferous forests at these middle elevations are dominated by a white fir Abies concolor/ mixed coniferous forest with sugar pine P. lambertiana, giant sequoia Sequoiadendron giganteum and incense-cedar Calocedrus decurrens as associates. Western white pine P. monticola is a common associate on the drier slopes and exposed areas are strongly dominated by Jeffrey pine P. jeffreyi from 2200-2800 m. Above 2500-2800 m, sierra lodgepole pine P. contorta ssp.

Sierra Conifer Species Elevation Diagram
Figure 3.19 Diagrammatic representation of community positions along elevational and topographic-moisture gradients for the west slope of the southern Sierra Nevada: mesic = moist; xeric = dry. (From Richardson and Rundel, 1998. In Ecology and Biogeography of Pinus. Cambridge University Press.)

murrayana forms single-species forest. At the top, upper subalpine and timber-line habitats above the lodgepole pine zone have foxtail pine P. balfouriana as the most important species, with smaller numbers of whitebark pine P. albicaulis and limber pine P.flexilis.

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