ABSTRACT:
Axis splitting is a widespread phenomenon in
desert shrubs, and has been reported for shrubs from several plant
families, both in old- and new-world deserts. It is so common in
dwarf shrubs of
arid environments as to be a defining characteristic of this growth
form. Although anatomists have described this phenomenon several
decades ago,
there has been only one ecological study of one species, Ambrosia
dumosa . The anatomical nature of the various splitting
mechanisms that have been found suggests axis splitting to be an
extreme form of hydraulic segmentation. The adaptive advantage of
clonal splitting in desert shrubs has yet to be determined, but it
appears to be largely a risk-spreading mechanism that enables
independent mortality of integrated hydraulic units (IHUs) or
ramets. This should be especially advantageous in heterogeneous,
water-limited environments, where soil water occurs in pockets too
small to support a large shrub-genet. Clonal splitting may cause
an increase in intraclonal competition among ramets, but there are also
indications that at least some species possess mechanisms to reduce
competition by minimizing root system overlap among ramets. Many
desert shrub species that undergo clonal splitting maintain a dense
clump
growth form, possibly because such a growth form has positive effects
on
water and nutrient status of the soil and long-term effects on other
soil properties.