Our coastal forest showed little effect from the first 10-hour exposure to salt water in June 2022 and grew normally for the rest of the year. We increased the exposure to 8 p.m. in June 2023, and the forest still looked mostly intact, although the tulip poplars were drawing water from the ground more slowly, which could be an early warning signal.
Things changed after a 30-hour exposure in June 2024. The leaves of forest tulip poplars began to turn brown in mid-August, several weeks earlier than normal. By mid-September, the forest cover was bare, as if winter had set in. These changes did not occur in a neighboring plot that we treated in the same way, but with fresh water rather than sea water.
The initial resilience of our forest can be explained in part by the relatively low amount of salt in the water of this estuary, where water from freshwater rivers and a salty ocean mix. The rain that fell after the 2022 and 2023 experiments washed away the salts from the soil.
But a major drought followed the 2024 experiment, so the salts remained in the soil. The trees’ longer exposure to salty soils after our 2024 experiment may have exceeded their ability to tolerate these conditions.
The seawater released during the Southern California fires is entirely salty seawater. And the conditions there were very dryespecially in relation to our forest patch on the East Coast.
Obvious changes on the ground
Our research group is still trying to understand all the factors that limit forest tolerance to salt water and how our findings apply to other ecosystems like those in the Los Angeles area.
The leaves on the trees turning from green to brown well before fall was a surprise, but there were other surprises hidden in the ground beneath our feet.
Rainwater that seeps into the ground is normally clear, but about a month after the first and only 10 hours of exposure to salt water in 2022, the water in the ground turned brown and stayed that way for two years. The brown color comes from carbon-based compounds leached from dead plant matter. It is a similar process to brewing tea.
