Extreme climate events are exposing coastal plant communities to stressful environmental conditions. Yet, the ecological and evolutionary mechanisms maintaining the resilience of native plant populations in response to these unpredictable events are unknown. Symbiotic associations with soil microbes have been demonstrated to increase plant fitness of populations living in extreme habitats. Here we assessed the ecological role of plant-microbe interactions for conferring resilience to extreme climate events, using baldcypress (Taxodium distichum) seedlings. We conducted a manipulative flooding experiment under controlled humidity and temperature conditions. Two-month-old seedlings were inoculated with sterile or non-sterile swamp soil collected from below adult baldcypress trees growing in seasonally flooded soil banks in Bayou Chevreuil, Louisiana. After a month of growing in inoculated soils, seedlings were exposed to three hydrological regimes that simulated extreme drought and flooding events, and typical hydrological conditions reported over the last decades at Bayou Chevreuil. Plant growth was tracked during the course of the experiment. Plant performance was estimated as the total dry biomass produced. At the end of the experiment, roots were harvested, fixed, and stained to evaluate the presence of arbuscular mycorrizhae (AMF) and dark septate endophytes (DSE). AMF and DSE were quantified using a modified magnified intersections method.
Results/Conclusions
Plants grown in sterile soil showed two times lower biomass in the extreme flooding treatment compared to the control and drought treatment groups, while seedlings that were inoculated with non-sterile swamp soil showed equal performance in each of the treatment groups. Abundances of AMF and DSE were significantly reduced in plants grown in sterile soil, indicating that the greater abundance of fungi in the non-sterile soils may have helped regulate plant response to flooded conditions. Contrary to our predictions, plants grown with sterile soil had six times higher growth and performance overall than plants grown with non-sterile soil. This may be evidence of negative plant-soil feedbacks, due to the provenance of the inoculation soil from below adult baldcypress trees. We concluded that fungi conferred resilience to changes in hydrological regimes for baldcypress seedlings, suggesting that the plant-fungal interactions may positively influence their overall establishment despite extreme climate events.