Date of Award

Spring 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Forestry and Environmental Studies

First Advisor

Brodersen, Craig

Abstract

Ferns are the second-most diverse major clade of green, vascular plants, and yet, they have been historically excluded from ecophysiology, ecology, and evolution studies. Ferns were once the dominant vegetation making up forested ecosystems, and while the fern lineage predates seed plants (gymnosperms and angiosperms), the majority of extant ferns diversified alongside and even after angiosperms, the flowering plants. The theory that ferns were literally overtopped by angiosperms includes the hypothesis that over evolutionary time ferns had to adapt to new ecological niches that are less desirable, such as shaded understories or bright and dry tree canopies growing on branches as epiphytes. Many extant fern species are still living in these marginal habitats, and the questions pertaining to the physiological function of ferns in these niches remain under-explored.Fern success in these marginal habitats is perhaps surprising, given that fern physiology is thought to be rather constrained; ferns do not produce secondary growth that would facilitate substantial height growth as it does in woody plants, and fern stomatal guard cells are thought to be relatively large and sluggish, unable to rapidly adjust to changes in environmental stimuli. The overarching question of my dissertation is how do ferns function physiologically in less desirable habitats despite their anatomical constraints, and specifically how do they cope with the environmental stressors associated with these marginal habitats that affect water transport and photosynthesis? I tackle this question from multiple angles in my dissertation work. My first chapter examines the physiological adaptations of the terrestrial, wintergreen fern Polystichum acrostichoides. This fern is an example of a species that has been relegated to the low-light, shaded understory beneath angiosperm-dominated canopies, but has adopted a wintergreen strategy for extending photosynthesis throughout times of the year when the leaves have access to high light conditions. However, this fern—common in northeastern forests—must survive the winter and experience freezing temperatures, which threatens the xylem function. My objectives were to identify the physiological and anatomical strategies that P. acrostichoides uses to survive the changing seasons and preserve the photosynthetic and hydraulic systems. Results indicated that P. acrostichoides relies on flexible xylem bundles that allow for leaf hinging to maintain a warmer leaf temperature in the winter months, and that while freezing temperatures did induce air bubbles to form in the xylem, the bending xylem bundles preserved the remaining hydraulic link to the leaf tissue that allowed photosynthesis to continue in the spring. My second chapter focuses on the epiphytic resurrection fern, Pleopeltis polypodioides. This fern is an example of a species that has been relegated to the canopy and branches of trees, but it has an uncommon ability to survive and recover from extreme desiccation. While this species is known to be able to take up water through the leaves, the physiological coordination between the photosynthetic and hydraulic systems that allows for resurrection remained unknown. Thus, the goal of this chapter was to examine the timing of photosynthetic and hydraulic decline and recovery and to determine if leaves can rely solely on foliar water uptake for their recovery. Results illustrated that the xylem is quick to fill with air during desiccation, and much slower to recover. While photosynthetic decline aligned with xylem decline during desiccation, photosynthetic recovery occurred prior to xylem recovery, due to foliar water uptake. Finally, my third chapter examines stomatal and vein anatomy, stomatal behavior, and water relations in multiple species of xeric (dry-adapted) and mesic (wet-adapted) ferns. The first part of this chapter created a broad dataset of stomatal and vein anatomy measurements in ferns to improve our understanding of the coordination between stomatal density and vein density in ferns. These leaf anatomy measurements show how leaves are built for gas exchange through stomatal pores and delivering water to the leaf tissue through xylem in the veins. The results showed that ferns had low vein densities compared to angiosperms, but xeric ferns had higher vein densities than mesic ferns. The second part of this chapter was to measure stomatal behavior and water relations in a subset of xeric and mesic species; as a whole, the xeric species had higher stomatal conductance, higher water content, and faster water turnover through the leaf than the mesic species. This dissertation provides insight into fern ecophysiology, particularly the adaptations and strategies that ferns employ to survive in marginal habitats. Furthermore, how ferns regulate transpiration and photosynthesis has important implications for our understanding of the physiological evolution of all land plants.

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