Shade does not ameliorate drought effects on the tree fern species Dicksonia antarctica and Cyathea australis

We examined the responses of two tree fern species (Dicksonia antarctica and Cyathea australis) growing under moderate and high light regimes to short-term water deficit followed by rewatering. Under adequate water supply, morphological and photosynthetic characteristics differed between species. D. antarctica, although putatively the more shade and less drought adapted species, had greater chlorophyll a/b ratio, and greater water use efficiency and less negative delta C-13. Both species were susceptible to water deficit regardless of the light regime showing significant decreases in photosynthetic parameters (A (max), V (cmax), J (max)) and stomatal conductance (g (s) ) in conjunction with decreased relative frond water content (RWC) and predawn frond water potential (I-predawn). During the water deficit period, decreases in g (s) in both species started one day later, and were at lower soil water content, under moderate light compared with high light. D. antarctica under moderate light was more vulnerable to drought than all other plants as was indicated by greater decreases in I-predawn, lowest stomatal conductance, and photosynthetic rates. Both tree fern species were able to recover after a short but severe water stress.