Contrasting survival strategies for seedlings of two northern conifer species to extreme droughts and floods.

Lowland northern white-cedar (Thuja occidentalis L.) forests are increasingly exposed to extreme droughts and floods that cause tree mortality. However, it is not clear the extent to which these events may differentially affect regeneration of cedar and its increasingly common associate, balsam fir (Abies balsamea (L.) Mill.). To test this, we measured how seedlings of cedar and fir were able to avoid, resist and recover from experimental drought and flood treatments of different lengths (8 to 66 days). Overall, we found that cedar exhibited a strategy of stress resistance and growth recovery (resilience) from moderate drought and flood stress. Fir, on the other hand, appears to be adapted to avoid drought and flood stress and exhibited overall lower growth resilience. In drought treatments, we found evidence of different stomatal behaviors. Cedar used available water quickly and therefore experienced more drought stress than fir, but cedar was able to survive at water potentials > 3 MPa below key hydraulic thresholds. On the other hand, fir employed a more conservative water-use strategy and therefore avoided extremely low water potential. In response to flood treatments, cedar survival was higher and only reached 50% if exposed to 23.1 days of flooding in contrast to only 7.4 days to reach 50% mortality for fir. In both droughts and floods, many stressed cedar were able to maintain partially brown canopies and often survived the stress, albeit with reduced growth, suggesting a strategy of resistance and resilience. In contrast, fir that experienced drought or flood stress had a threshold-type responses and they either had full live canopies with little effect on growth or they died suggesting reliance on a strategy of drought avoidance. Combined with increasingly variable precipitation regimes, seasonal flooding and complex microtopography that can provide safe sites in these forests, these results inform conservation and management of lowland cedar stands.

Linking physiological drought resistance traits to growth and mortality of three northeastern tree species.

Climate change is raising concerns about how forests will respond to extreme droughts, heat waves and their co-occurrence. In this greenhouse study, we tested how carbon and water relations relate to seedling growth and mortality of northeastern US trees during and after extreme drought, warming, and combined drought and warming. We compared the response of our focal species red spruce (Picea rubens Sarg.) with a common associate (paper birch, Betula papyrifera Marsh.) and a species expected to increase abundance in this region with climate change (northern red oak, Quercus rubra L.). We tracked growth and mortality, photosynthesis and water use of 216 seedlings of these species through a treatment and a recovery year. Each red spruce seedling was planted in containers either alone or with another seedling to simulate potential competition, and the seedlings were exposed to combinations of drought (irrigated, 15-d 'short' or 30-d 'long') and temperature (ambient or 16 days at +3.5 °C daily maximum) treatments. We found dominant effects of the drought reducing photosynthesis, midday water potential, and growth of spruce and birch, but that oak showed considerable resistance to drought stress. The effects of planting seedlings together were moderate and likely due to competition for limited water. Despite high temperatures reducing photosynthesis for all species, the warming imposed in this study minorly impacted growth only for oak in the recovery year. Overall, we found that the diverse water-use strategies employed by the species in our study related to their growth and recovery following drought stress. This study provides physiological evidence to support the prediction that native species to this region like red spruce and paper birch are susceptible to future climate extremes that may favor other species like northern red oak, leading to potential impacts on tree community dynamics under climate change.