Consistent responses to moisture stress within diverse mountain fynbos communities revealed by multi-year in situ physiological measurements

Résumé fourni par la source

Abstract ● Understanding climate change impacts on the Cape Floristic Region requires improved knowledge of plant physiological responses to the environment. Prior studies examining physiological responses of fynbos species have consisted of campaign-based measurements, capturing snapshots in time of plant water relations and photosynthesis. We examine conclusions drawn from these studies by tracking in situ physiological responses of three species, representing three dominant growth forms (proteoid, ericoid, restioid), over two years using miniature continuous sap flow technology, in combination with long-term observations of xylem water potentials, gas exchange and assessments of xylem vulnerability to embolism. ● We observed considerable inter-specific variation in the timing and extent of dehydration-induced declines in productivity. Erica monsoniana (a shallow-rooted ericoid) exhibited steep within-season declines in sap flow and water potentials, and pronounced inter-annual variability in total daily sap flux (Js). Protea repens showed steady reductions in Js across both years, despite maintaining less negative water potentials. Cannomois congesta - a shallow-rooted restioid - was least negatively impacted. Following rehydrating rain at the end of summer we compared gas exchange recovery in the representative dry year with the normal year, finding significant seasonal, but not species, effects. Loss of function in the drier year was partially accounted for by loss of stem xylem transport capacity, but only in E. monsoniana and C. congesta. ● Hitherto unseen water-use patterns, including inter-annual variability of gas exchange that were driven by contrasting water uptake properties, reveal that mountain fynbos species use different mechanisms to cope with summer dry periods.

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