Thrip composite
by Kate Wing

Western sword ferns (Polystichum munitum) usually form a lush evergreen understory in redwood and mixed evergreen forests. They are well-adapted to periodic droughts, and like redwood trees, have leaves that can absorb dew and the fog drip which falls from trees year-round. [Dawson 1998] This winter though, many of the sword ferns looked silvery, brownish, or in severe cases, appeared to be entirely dead. What is going on?

In dry years, sword ferns photosynthesize less, grow fewer leaves, and produce less of the chemicals that they use to protect themselves from pests. Over the past two years, Muir Woods has had half its normal rainfall, and there has been a 33% reduction in the number of foggy days compared to 70 years ago. [Johnstone and Dawson 2010] It seems likely that drought stress has made the ferns more vulnerable to damage by thrips, very tiny sucking insects that live on the underside of the leaves. If you were to turn over a frond, (and put your reading glasses on if necessary!), you could just barely see the thrips as long thin black spots on injured fronds.

With the assistance of Christopher Cook, Inspector with the County of Marin Dept. of Agriculture, thrips were collected in Muir Woods. Michael Forthman, an entomologist with the California Dept. of Food and Agriculture, recently identified them as greenhouse thrips (Heliothrips haemorrhoidalis), an invasive species native to South America and introduced to the U.S. prior to 1870. As a non-native species, greenhouse thrips have the potential to cause greater damage to ferns than native thrips because there are fewer natural predators present to keep their populations under control and the ferns have not had time to develop specific defenses against them.

According to Mia Monroe of the National Park Service, there have been previous episodes of fern dieback in Muir Woods, most recently in 2012-2014. However, sword ferns are known to be remarkably resilient [Baer 2016] in part because they store carbohydrates in their crowns and rhizomes. In the prior cases, within a year or two after the damage occurred, the ferns sent out new croziers (unfurling fronds) from their surviving crowns and recovered.

In Muir Woods, we were happy to see fresh fronds emerge this April. If we get adequate rainfall next winter, the ferns should be out of the woods, figuratively speaking. If the drought continues, it is possible that further reduction in the crowns’ energy stores could cause permanent dieback of some of the weaker plants. Longer term, climate change might expose the ferns to continued drought stress. Beyond hoping for a rainy winter, sword fern survival may depend on our efforts to reduce climate change.


Alex Baer, James K. Wheeler, and Jarmilla Pittermann. Not dead yet: the seasonal water relations of two perennial ferns during California’s exceptional drought. New Phytologist Vol. 210, Issue 1, pp 122-132 April 2016.

T.E. Dawson. Fog in the California redwood forest: Ecosystem inputs and use by plants. Oecologia 117: 476– 485. 1998.

James A. Johnstone and Todd E. Dawson. Climatic context and ecological implications of summer fog decline in the coast redwood region. PNAS March 9, 2010 107 (10) 4533-4538;

Emily B. Limm and Todd E. Dawson. Polystichum munitum (Dryopteridaceae) varies geographically in its capacity to absorb fog water by foliar uptake within the redwood forest ecosystem. American Journal of Botany Volume 97, Issue 7 July 2010.

US Forest Service. Fire Effects Information System, Polystichum munitum.