Temperate ferns are often associated with low-light environments, such as the shaded understory of deciduous forests. While most of these fern species are deciduous, a few species are evergreen. Wintergreen ferns, such as Polystichum acrostichoides, retain their fronds throughout the year despite freezing temperatures and drastic seasonal changes in soil moisture and light intensity. Wintergreen understory leaves must tolerate the deep shade of the summer but also exposure to higher light intensities and low air temperatures when the canopy is open. Compounding the effects of temperature and light on photosynthetic capacity in the winter is the vulnerability of the vascular system to embolism resulting from freeze–thaw cycles. While there is a lack of understanding regarding winter embolism in angiosperms and gymnosperms, the knowledge gap is all the more significant for wintergreen ferns.
In their new study published in AoBP, Prats & Brodersen monitored the photosynthetic capacity and the functional status of the vascular system of P. acrostichoides for over a year in the Yale Myers Forest, Connecticut, USA. They found that the first night below 0 °C led to a 25 % loss of conductivity in stipes (the fern petiole), suggesting that winter-induced embolism occurred. Yet, they found that despite losing over 60% of their water transport capacity from freeze-thaw induced cavitation, photosynthesis recovered in the spring. The authors found that localized, frost-triggered frond flexibility allows the species to lay flat on the ground and maintain warmer leaf temperatures. This process is facilitated by highly flexible vascular bundles that bend without disrupting the water conducting pathway. The xylem of P. acrostichoides are flexible yet robust, tolerating winter embolism and hinging, all without damaging the photosynthetic machinery. These wintergreen strategies contribute to the success of P. acrostichoides in northeastern forests.
Kyra Prats is a PhD Candidate in Plant Ecophysiology in Craig Brodersen’s lab at the Yale School of the Environment, Connecticut, USA. She is also a Cullman Fellow and joint graduate student with the New York Botanical Garden. Her research focuses on ecophysiology of plants —particularly ferns — under environmental stressors, such as drought or freezing temperatures. She uses a variety of methods in the field and in the lab — from measuring photosynthesis and xylem water flow, to using X-rays and microscopes — to explore questions related to plant responses to the environment.