Mountain Soils - Mountain Geography: Physical and Human Dimensions - page 318

come thinner, coarser and rocky, acidic, unstable, infertile, and immature. Human use
of soil for agriculture is constrained by characteristics of the soil at intermediate and
high altitude. Other human uses of mountain environments, including grazing, trekking,
and road construction, may be limited by mountain soils and can, in turn, alter soil char-
acteristics.
Soil productivity is affected by the totality of processes operating at any location. If
soils remain frozen or snow-covered for long periods, their operational level of physic-
al, chemical, and biological processes is greatly reduced. Low temperatures during the
growing season effectively limit the number and kinds of species that exist in a given
environment, and lack of moisture sets similar limits. Certain characteristics help to ex-
plain why many mountain soils are relatively unproductive. Shallow soils restrict the
penetration of roots, absorb less water, and do not insulate well against cold nighttime
or winter temperatures. Coarse-textured mountain soils that lack well-developed struc-
ture have low base-exchange capacities and low water-holding abilities. Low nutrient
levels may also limit plant growth in mountain soils. Luxuriant plant growth around an-
imal burrows and bird roosts in alpine areas reflects the high nitrogen and phosphorus
content of animal droppings at those sites, compared to the low values of those essen-
tial elements in the surrounding soil.
Recently, heightened interest in terrestrial carbon storage has brought global atten-
tion to the organic-matter dynamics and carbon storage of mountain soils. Concentra-
tions of soil organic carbon increase with elevation, provided that enough vegetation is
present to supply organic matter to the soil. This increase is attributed to the slower
rates of decomposition in cooler temperatures (Leifeld et al. 2009; Budge et al. 2011).
Although soil organic matter increases with altitude, this does not always connote an
increase in fertility. In fact, the reverse may be true. A study in Taiwan attributed high
humus (decomposed organic matter) contents of Andisols to aluminum toxicity and the
lack of phosphorus (Chen et al. 1999). Land-use practices that reduce soil organic mat-
ter or cause soil erosion are significant because they reduce the amount of carbon
stored in mountain soils.
Organic material is also important to mountain soils for its capacity to store and reg-
ulate the release of water. The importance of mountains in terms of their role in cap-
turing, storing, and releasing the freshwater used by over half of the world's population
is discussed further in Chapter 12. Mountains store water in snowbanks and glaciers,
lakes and bogs, and soils. Most human uses of mountain regions compact the soil, re-
ducing the available pore space and decreasing the ability of soil to absorb water. One
study in the Colombian Andes found soils to contain less moisture following long-term
grazing because soil compaction by grazing animals caused a higher proportion of rain-
fall to flow away as runoff (Hofstede 1995). Similarly, a 20-year record of sheep grazing
in high-elevation grasslands of the Ecuadorian Andes demonstrated multiple impacts:
an increase of bare land, a three- to fourfold reduction of water-retention capacity, and
an increased tendency toward water-repellent soil surfaces (Podwojewski et al. 2002).
The combination of these effects increases rainfall runoff and decreases the capacity of
the soils to regulate the storage and release of water.
Soil erosion can have dramatic consequences for thin mountain soils. The stability of
soils above timberline is largely due to the presence of a fairly complete cover of herbs
and grasses, which form a tight matrix of organic material and interwoven, fine roots. If