Seasonal xylem pressure potentials of two South African coastal fynbos species in three soil types

Abstract

The seasonal dawn and midday stem xylem pressure potentials of two South African coastal fynbos species, Thamnochortus punctatus and Leucospermum parile growing in three soil types were measured from July 1981 to May 1982. The results show that, the response of the two species was markedly different; the deeper rooted L. parile exhibited little or no moisture stress throughout the year while the decreasing spring-summer (September-March) potentials of the shallow-rooted T. punctatus suggest a degree of water stress in summer. The potentials of L. parile are high by comparison with those of other mediterranean shrubs, but both rainfall and shoot growth data suggest that the summer drought period studied was not extreme. L. parile appears to conform to Specht’s (1972) concept of a water-conservative, mediterranean, evergreen sclerophyll. P.C. Miller (pers. comm. 1981) who died unexpectedly in 1982 before much of his fynbos data was published, suggests that restioids, including T. punctatus, employ a ‘boom or bust’ water strategy, using a lot of water when it is available and conserving it when it is scarce; these data support this concept. In terms of soil type the observed differences in the xylem potentials of both species can be explained by differences in soil water-holding capacity. Die seisoenale dagbreek- en middag-stamxileem-drukpotensiaal van twee Suid-Afrikaanse kusfynbos-soorte, Thamnochortus punctatus en Leucospermum parile, wat in drie grondtipes voorkom is van Julie 1981 tot Mei 1982 gemeet. Die resultate toon dat die reaksie van die twee soorte merkbaar verskillend is; die dieper-gewortelde L. parile het min of geen vogtekort dwarsdeur die jaar getoon nie terwyl die verminderde lente-somer (September-Maart)-potensiale vir die vlak-gewortelde T. punetatus ‘n graad van vogtekort in die somer suggereer. Die potensiale van L. parile is hoog in vergelyking met dié vir ander mediterreense struike, maar beide reënval en lootgroeidata suggereer dat die somerdroogteperiode wat bestudeer is nie baie droog was nie. L. parile skyn aan Specht (1972) se konsep van ‘n waterbewarende, mediterreense, immergroen sklerofiel te voldoen. PC. Mliller (pers. mededeling, 1981) wie onverwags in 1982 gesterf het voor veel van sy fynbos-data publiseer kon word, stel voor dat restioïede, insluitende T. punctatus ‘n ‘maak of breek’-strategie aanwend deur baie water, wanneer dit besklkbaar is, te gebruik en dit te bewaar wanneer water skaars is; hierdie data ondersteun dié konsep. In terme van die grondsoorte kan die waargenome verskille verduidelik word in terme van die verskille in grond-waterhouvermoëns.