Geoscience Reference
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Noy-Meir ( 1973a ; 1973b ; 1974 ) described what he termed the 'inverse soil texture
effect' on plant production in semi-arid grasslands and formulated the 'pulse-reserve-
response' concept for deserts in North America and elsewhere to describe the influence
of pulses of rainfall on soil water and associated plant responses. The inverse soil tex-
ture effect simply means that in wet environments, fine-textured soils such as clays
and clay loams will support a denser plant biomass than plants growing under a sim-
ilar wet climate on coarse textured soils such as sands and loamy sands, but as the
rainfall diminishes a threshold is reached in which sandy soils support a greater plant
biomass than fine-textured clay soils. Later workers have confirmed, refined or modi-
fied these two cardinal principles of desert plant ecology (Ogle and Reynolds, 2004 ).
The 'Westoby-Bridges pulse-response hypothesis', as it was termed by Noy-Meier
( 1973b ) because it was based on their unpublished data, was identified as a key tenet
of desert ecology. In essence, plants (and animals) adapt to erratic rainfall by only
responding to certain high rainfall events, which stimulate growth and reproduction
and allow the organisms to establish reserves to tide them over in times of reduced
rainfall (Ogle and Reynolds, 2004 ).
Liang et al. ( 1999 ) analysed the biomass dynamics and water use efficiencies of five
plant communities in the short grass steppe of Colorado, finding more efficient use of
water in plants growing on coarse-textured soils, thereby confirming the importance
of the inverse texture effect. However, the rainfall threshold, or crossover point, is not
everywhere the same and varies with elevation and mean temperature during the grow-
ing season, as Epstein et al. ( 1997 ) discovered in their studies of the influence of tem-
perature and soil texture on above-ground primary productivity in grassland ecosys-
tems of the semi-arid to arid Great Plains region of the United States. They noted that
the crossover point to the inverse texture effect only began when the mean annual rain-
fall amounted to 800mm, or far higher than expected fromobservations elsewhere, and
they pointed out that because grassland ecosystems coincided with mean annual pre-
cipitation amounts between 250 and 1,000 mm/year, their results showed the need to
factor in ambient temperature (itself controlled in part by elevation), as well as rainfall.
Belsky ( 1990 ) had earlier investigated the proportions of trees to grassland in East
African savannas in relation to the relative influence of variations in rainfall, temper-
ature and soil texture on the availability of soil moisture and soil nutrients during the
growing season, concluding that many of the grasslands were 'edaphic grasslands', or
grasslands determined by soil type. One may note here that the volcanic soils of East
Africa differ greatly from those developed on sedimentary or metamorphic rocks in
other desert regions, and they display considerable geochemical variation, with some
more prone to set hard and impede seedling germination, creating what can be called
'edaphic drought', so that soil structure or soil aggregate stability is another important
variable influencing plant growth in dry areas.
Reynolds et al. ( 2004 ) studied the interactions between plant responses to rain-
fall variability in the Mojave, Sonoran and Chihuahan deserts of North America,
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