Killer in the salt marshes: What keeps plants from returning in a drought?

In 2011, China’s Yellow River was under an extreme drought, turning many salt marshes in the river delta into desolate wasteland. The government attempted replanting the marshes but was unsuccessful. A team of researchers worked throughout the drought period to investigate the causes behind the mass death of the salt marshes and find ways to manage droughts more effectively. They uncovered a hidden culprit neglected by the government — natural enemies.

Scientific models cannot accurately predict how long drought recovery will take. More plants die from drought and take longer to recover than climate and precipitation models assume. A team of researchers led by Qiang He at Beijing Normal University set up a six-year multi-faceted study in the Yellow River Delta to understand this delay in salt marsh recovery. The researchers suspect that a range of natural enemies — i.e., grazers and diseases — were the cause of the delay. They tested how plant enemies exacerbated drought effects and published the research results in Ecology Letters.   

The main plant that grows in the region is seepweed (Suaeda salsa), a plant whose stunningly neon red is so unusual that every year it draws many tourists to where the Yellow River meets the sea. Enemies to the seepweed include grazers, most notably Helice tientsinensis, a local crab species. To understand how the grazer was affecting seepweed, the scientists built multiple exclosures, areas enclosed by fencing which blocked the small leaf-eating crab from entering. Over the period of the drought and drought recovery, the researchers measured the amount of healthy vegetation in both the grazed and non-grazed areas.

By excluding crabs from the area only after the drought, He and his team saw the plants inside crab exclosures recovered quickly. Outside crab exclosures, in contrast, plants took years longer to recover. The researchers concluded that the crabs played significant role in disrupting the recovery process of seepweed in the Yellow River Delta.

In other words, these natural enemies were not just ancillary to the drought but “a likely powerfully counterpart” that not only act to kill ecosystems but suppress their ability to recover.

This finding has important implications. With climate change expected to bring stronger and prolonged drought periods, understanding such natural plant enemies might help us manage vegetated areas better and build resilience against the impact of climate change.