Desert Ecosystems: Their Resources in Space and Time

Abstract

The dynamics of desert ecosystems control levels of resources that are essential to the survival of desert biotas. Because precipitation is both low and relatively unpredictable in arid regions, the climates, topographies, and soils, of these areas present formidable constraints to resource availability in space and time. And for the same reason, the processes of production, consumption, decomposition, and nutrient-cycling in deserts are also highly irregular and difficult to predict with accuracy. For example, global models relating actual evapotranspiration to primary production and decomposition apply poorly in arid regions.Surprisingly great amounts of carbon are stored in desert soils, particularly in caliche deposits which represent a major ‘sink’ of carbon from the atmosphere. In Arizona desert soils, inorganic carbon exceeds organic carbon by a factor of > 10. Direct use of organic carbon is made principally by organisms that break down desert litter and simultaneously cause relatively high rates of nitrogen mineralization. While nitrogen is traditionally considered deficient in arid environments, its flux is considerable because of high rates of gain by fixation and loss by denitrification and volatilization. Nitrogen accumulates in ‘islands of fertility’ beneath desert shrubs where it becomes relatively available because of (i) its high concentration in plant litter, and (ii) reduced activity of any aromatic modifiers that retard decomposition.It is misleading in deserts to relate nutrient availability to yearly averages, as nutrients may become highly available following pulses of ‘effective’ precipitation. Moreover, mineralization and subsequent availability to plants of phosphorous, the ‘master element’ in nutrient cycling, are moderately independent of nitrogen mineralization and can proceed rapidly. Clearly, the case for nutrient deficiency in deserts may be overstated.Consumption of primary production has varying effects on rates of resource availability in desert ecosystems. Generally weak regulation of primary production is predicted for consumers of green vegetation, except occasionally during early drought. Carnivores should exert variable controls over their prey, while pollinators, seed-eaters, and detritivores—most of which are strongly soil-associated—should have the greatest impacts on primary production and nutrient cycling.

Soil resource heterogeneity in the Mojave Desert

“…The biologically limiting nutrients, once acquired from the soil solution through root uptake, remain under the plant through recycling via litterfall, microbial litter decomposition, and microbial immobilization (Crawford & Gosz, 1982;Caldwell & Richards, 1989;West, 1991).…”

Decomposition of Avena sterilis litter under arid conditions

“…Therefore, an increase in nitrogen was expected during the high rates of organic matter decomposition (Wallace et al, 1978;Crawford Above-ground below-ground Above-ground below-ground Above-ground below-ground 10/92 51)9 5 1 )9 5 1 )9 5 1 )9 5 1 )9 5 1 )9 12/92 60 78)6 7 3 )4 5 1 )4 7 0 8 2 )7 02/93 58 54)2 6 9 4 9 )4 5 6 )3 7 0 )1 04/93 38)1 2 2 )8 4 6 )9 3 5 )2 9 1 )9 4 5 )4 06/93 49)5 2 8 )6 5 3 )1 3 3 )5 3 6 )4 N . D .…”

Relationships between empirical and nominal indices of landscape function in the arid shrubland of Western Australia

“…Resilience of landscapes depends on the success of colonising species to rapidly revegetate bare-soil areas that result from loss of vegetated patches (Vitousek & Reiners, 1975;Mazzarino & Bertiller, 1999). Provided nutrients and soil are not transported out of the system following destruction of vegetated patches, low soil C:N ratios ((10 : 1) ensure rapid mineralisation of organic N (Crawford & Gosz, 1982;Wedin, 1999) and thus an available pool of N, at least in the short-term (Kelly & Burke, 1997), for rapid plant growth with favourable seasons. In both low-shrubland and low-woodland, C:N was usually less than 15 and more often less than 10 suggesting availability of N is unlikely, in the short-term, to reduce plant growth of colonizing species in these landscapes (Mazzarino & Bertiller, 1999).…”