Seedling drought stress susceptibility in two deciduous Nothofagus species of NW Patagonia

The physiological capacities of seedlings to cope with drought may be subject to strong selective pressure in the context of future climate scenarios, threatening the regeneration and sustainability of forests. Characterization of the responses and the variability between species is of interest to breeding and domestication programs. In this study, our main goal was to describe some of the physiological mechanisms involved in the drought response of Nothofagus nervosa and N. obliqua, two forest species of ecological and commercial importance (high wood quality) in NW Patagonia. We tested for differences in water status, gas exchange and survival in response to a gradually imposed severe drought. Based on cavitation vulnerability curves and hydraulic conductivity measurements, we can conclude that N. obliqua stems have higher specific hydraulic conductivity and somewhat lower vulnerability to cavitation than N. nervosa stems, leading it to sustain higher stomatal conductance under non-severe drought conditions. N. obliqua had higher photosynthetic capacity than N. nervosa, due both to characteristics of its hydraulic architecture and to its higher metabolic capacity. Our results indicate that both species present characteristics of plants susceptible to water stress. Also, both species showed behavior resembling an anisohydric response. This behavior results from a lack of stomatal control over transpiration while the soil dehydrates, probably accompanied by very high vulnerability to cavitation. In contrast, both species had similar high stomatal sensitivity to vapor pressure deficit when soil water was limiting.