Our coastal forest showed little impact after its first 10-hour exposure to saltwater in June 2022 and grew normally for the rest of the year. In June 2023, we increased the exposure to 20 hours and the forest still seemed mostly undisturbed, although tulip poplars They took up water more slowly from the soil, which may be an early warning sign.
The situation changed after a 30-hour exposure in June 2024. Tulip poplar leaves in the forests began to turn brown in mid-August, a few weeks earlier than usual. In mid-September the forest canopy was bare, as if winter had come. These changes did not occur in a nearby plot, which we treated in the same way, but with freshwater rather than seawater.
The initial resilience of our forest can be partly explained by the relatively low salt content of the water in this estuary, where water from freshwater rivers and the salty ocean mix. The rain that fell after the experiments in 2022 and 2023 washed the salts from the soil.
However, after the 2024 experiment, there was a severe drought, so the salts remained in the soil at that time. The trees’ prolonged exposure to salty soils after our 2024 experiment may have exceeded their ability to tolerate these conditions.
The seawater released by the Southern California wildfires is full-strength salty ocean water. And there are the conditions they were very dryespecially compared to our forest plot on the east coast.
Changes perceptible in the ground
Our research group is still trying to understand all the factors that limit a forest’s saltwater tolerance and how our results apply to other ecosystems like the one in the Los Angeles area.
The leaves of the trees turning from green to brown well before fall were a surprise, but there were other surprises hidden in the ground beneath our feet.
Rainwater seeping through the soil is usually clear, but about a month after the first and just 10-hour exposure to saltwater in 2022, the soil water turned brown and stayed that way for two years. The brown color comes from carbon-based compounds leached from dead plant material. It is a process similar to brewing tea.