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.