Poor
competitors find refuge amongst specialists
Until
recently I knew gypsum, CaSO4·2H2O, mainly as a component
of some paints and as an insulating material in the building industry. It
certainly does not appear to be a very promising substrate for plant growth.
Thus, Palacio et al. (Xaragoza and Móstoles, Spain and
St Paul, MN, USA, pp. 333–343) state that gypsum soils often form a hard
surface crust that makes germination difficult; they tend to be mechanically
unstable with unpredictable water relations, while their basic chemistry
includes not only excesses of Ca and S, but also of Mg. Although these features
may compromise plant survival, there are plants that grow and even thrive on
gypsum soils. The authors suggest that these fall into two categories:
gypsovags, gypsum-tolerant plants that find refuge in this stressful
environment as a strategy for avoiding competition; and true gypsum specialists
or gypsophiles that are more or less confined to gypsum soils. Based on their
work in Spain,
the authors subdivide the latter into regionally dominant species and narrowly
distributed endemics. They have carried out a thorough chemical analysis, in
relation to soil composition, of plants representing all three groups and have
subjected the data to detailed statistical analyses. It is clear that in terms
of chemical composition, regionally dominant gypsophiles differ from gypsovags.
In particular, the former are able to accumulate nutrients that are scarce in
gypsum soils, in addition to being tolerant of higher than normal
concentrations of Ca and especially of S and Mg. Gypsovags, despite their
ability to grow in these soils, do not show either of these features (implying
incidentally that they possess mechanisms for excluding the more toxic
elements). Intriguingly, the narrowly distributed gypsophiles are similar to
the gypsovags. This work has thus provided significant information on a
somewhat neglected aspect of plant biodiversity. This is important both for
conservation of specialized floristic assemblages and for plant breeding, the
latter in relation to growing crops on more marginal land.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
