Temperate tree seedlings have similar drought vulnerability despite having different hydraulic drought responses in adults

Climate change is projected result in higher frequencies of drought events across the world and lead to reduced performance in many temperate tree species. However, many studies in this area focus specifically on adult tree drought responses and overlook how trees in other age classes might differ in their vulnerability. Evidence shows that seedling drought response can differ from that of adults and furthermore that demographic performance in the seedling age class will have disproportionately strong effects on the assembly dynamics of future forests, together suggesting that understanding seedling drought responses will be critical to our ability to predict how forests will respond to climate change. In this study, we measured four indices of hydraulic response to drought (leaf water potential, photosynthetic capacity, non-structural carbohydrate concentration, and hydraulic conductivity), as well as interaction effects with shade treatments, for seedlings of two temperate tree species that differ in their adult drought response: isohydric Acer saccharum and anisohydric Quercus rubra. We found a strong isohydric response in A. saccharum seedlings that included conservation of leaf water potentials (>-1.8 MPa) and reductions in non-structural carbohydrate concentrations consistent with reduction of stomatal conductance. Quercus rubra seedlings were able to survive to more negative water potentials, but only rarely, and they showed a similar reduction in photosynthetic capacity as was found for A. saccharum. Our results suggest that, although Q. rubra seedlings display some anisohydric responses to drought, they are more isohydric than adults. Both species seem to be relatively similar in their vulnerability to drought despite the differences predicted from adult drought response, and our results suggest that seedlings of both species will be similarly vulnerable to future drought events.