Ferns have been exposed to herbivorous insects since the latter evolved in the Devonian. Currently, ferns suffer similar percentages of leaf herbivory as angiosperms. Therefore, they often use a combination of chemical defenses as protection against herbivores. In this review, we summarize the distribution of five groups of biomolecules that may act as chemical defenses of ferns: phytoecdysteroids, flavonoids, thiaminase, cyanogenic glycosides, and alkaloids. For each of these biomolecules, we briefly discuss their biosynthesis, mode of action, and currently known taxonomic distribution in ferns, and include examples to illustrate their observed concentrations in different fern tissues. We conclude with a discussion of ferns that accumulate heavy metals, which may also serve in their defense against herbivores. Finally, we discuss research gaps to encourage future research in this widely understudied and ecologically important field of investigation.

Two long-term studies were conducted in a rainforest in Puerto Rico that included measurements of leaf and plant functional traits of the common fern species Steiropteris deltoidea. A Fern Demography study (1993-2009) compared annual variation and effects of a category 3 hurricane (Georges, 1998) on fertile and sterile leaf traits. A second long-term study (2003-2019), the Canopy Trimming Experiment, evaluated annual variation in growth and reproduction of S. deltoidea in response to two experimentally simulated and one category 4 hurricane (Maria, 2017). In the Fern Demography study, differences between fertile and sterile leaf production rates and plant leaf count of S. deltoidea were significant while leaf lengths and lifespans did not differ between leaf types. Fertile (but not sterile) leaf production increased three-fold after Hurricane Georges but declined 10-fold by the end of the study. Leaf lifespans of cohorts emerging before and in the three years after Hurricane Georges were significantly shortened by tree and debris fall. Elevated production of fertile leaves and increased plant leaf counts followed the two simulated hurricanes of the Canopy Trimming experiment and two natural hurricanes. Steiropteris deltoidea exhibits a level of interannual flexibility in some growth and reproductive traits in response to a changed understory environment that suggests it may be a good indicator species for evaluating microhabitat hurricane effects. Although S. deltoidea exhibited resilience, predicted increases in frequency and magnitude of hurricanes in response to climate change may test the limits of life history strategies of rainforest understory ferns.

Phenological leaf characters of tropical ferns are often correlated with rainfall and temperature, especially in regions with pronounced dry and wet seasons. In this study, we describe the community- and species-level phenological patterns of leaf phenophases (young, non-fertile, fertile, senescent) of terrestrial ferns and their relationship with environmental variables in a Tropical Dry Forest over 15 months. At the community level, each phenophase was related to a different variable. The young phenophase was positively associated with precipitation, while non-fertile was positively associated with humidity, fertile was negatively associated with canopy cover, and senescent phenophase was not associated with any variable. At the species level, all fern species showed the peak leaf production of each phenophase in the rainy season. However, each phenophase was present during different periods of the rainy season; at the beginning, it was the young phenophase, in the middle, the non-fertile and fertile phenophase, and at the end, the senescent. Such phenological studies will help us understand how fern species change over time and modify their strategies, especially at the community level, in the face of imminent global climate change.

Mountains are the global centers of fern diversity and at the same time strongly affected by climate change, raising the question of how fern species and communities will respond to these changes. In the present review, which also includes our own unpublished data, we first outline the challenges of identifying distributional boundaries in ferns. We suggest that the elevational ranges of many fern species are determined by geographical constraints such as low mountain tops and sea level, as well as habitat availability rather than by climate. We then show that climate-range limits of ferns are driven by numerous physiological processes, not only involving the effects of cold and drought stress at high elevations, but also of drought stress coupled with high temperatures at low elevations, and possibly even of such poorly considered factors as low frost tolerance at low elevations in the absence of snow cover. Finally, there is also some evidence for biotic limitations, such as interspecific competition and the negative influence of leaf litter, especially in species-rich assemblages without extreme climatic factors. Overall, we find that elevational distributions of ferns are determined by a broad suite of factors, many of which do not involve physiological tolerance to climate or only indirectly so, and therefore that reactions of ferns to climate change will likely be species- and context-specific. We also emphasize the paucity of studies focusing both on the physiological limitations for fern growth and reproduction, and on biotic interactions affecting fern distributions. To overcome these knowledge gaps, we advocate a range of further studies, including resampling of old vegetation plots, lab experiments, and transplantation experiments, on both gametophytes and sporophytes.

Marsileaceae is a unique family of semi-aquatic ferns mainly growing in seasonal wetlands worldwide. These habitats present several challenges, since plants go from being wholly submerged to being exposed to aerial conditions, increasing drought stress. Although heterophylly has been studied as an adaptation to these environmental changes, there are still many unanswered questions concerning the mechanisms underlying the ecology of Marsileaceae. We studied the presence of circadian regulation in stomatal conductance, carbon assimilation rate, intrinsic water-use efficiency (iWUE), and leaf movement of four species from all three genera of Marsileaceae, and related our findings to possible water stress adaptations. No circadian regulation was detected in Pilularia globulifera, whereas Regnellidium diphyllum and two Marsilea species had an apparent rhythm in their stomatal conductance and iWUE, with species-specific patterns. Moreover, light-independent leaf movement was only found in Marsilea species. Taken together, the rhythm in iWUE and leaf movement, along with other anatomical traits for conserving water, infers different strategies to either increase carbon gain or reduce water use in Marsileaceae. Our study represents the first steps towards understanding the underlying drivers and adaptive value of circadian regulation in this family.

The Marsileaceae is a small family of semi-aquatic ferns displaying numerous traits commonly observed in angiosperms, including heterospory, sophisticated hydraulic architecture, and high rates of atmospheric gas exchange. Despite these similar traits, Marsileaceae is comparatively ecologically limited. Most species are found in Marsilea which is sister to Regnellidium and Pilularia, together these two genera include only seven species. Here we studied the anatomy and physiology of Marsileaceae to better understand the potential constraints on ecological and species diversity in this family. We focused on epidermal anatomy and stomatal responses to changes in light and water availability, which are unique amongst ferns. We found two evolutionary strategies in Marsileaceae, one of morphological simplification, physiological inflexibility, and aquatic specialization in Pilularia; which contrasts with a strategy of maximizing photosynthetic carbon gain at the expense of high rates of water loss in Marsilea and Regnellidium. We conclude that aquatic environments provide evolutionary opportunities for physiological innovation with regard to stomatal function, as well as selective pressures that drive the canalized evolution of highly specialized aquatic forms.

Climate change is expected to increase temperature and temporal precipitation variability leading to higher evapotranspiration and more frequent and severe droughts. While advancements are being made in our understanding of how plants will respond to these changes, gaps remain in our knowledge of species-specific drought response. This is especially true among herbaceous plant communities, including ferns and other seed-free vascular plants. Previous hydraulic work on ferns has almost exclusively concentrated on the leaves, with very little information on the rhizome, which is surprising given that the rhizome is the long-lived perennial organ (making it more costly and important in species survival). Only recently have rhizome hydraulics been explored in the context of drought stress. Similar to observations in many woody trees, fern leaves tend to desiccate and hydraulically disconnect before the perennial stem experiences significant levels of drought-induced embolism, suggesting strong vulnerability segmentation. These findings have significant implications for fern survival during drought. In this review we expand on these observations, integrating information from previous work on plant hydraulics and ecophysiology, to understand the implications of vulnerability segmentation on the response of ferns to future climate change.

The coming decades are predicated to bring widespread shifts in local, regional, and global climatic patterns. Currently there is limited understanding of how ferns will respond to these changes and few studies have attempted to model shifts in fern distribution in response to climate change. In this paper, we present a series of these models using the country of New Zealand as our study system. Ferns are notably abundant in New Zealand and play important ecological roles in early succession, canopy biology, and understory dynamics. Here we describe how fern distributions have changed since the Last Glacial Maximum to the present and predict how they will change with anthropogenic climate change – assuming no measures are taken to reduce carbon emissions. To do this, we used MaxEnt species distribution modelling with publicly available data from gbif.org and worldclim.org to predict the past, present, and future distributions of 107 New Zealand fern species. The present study demonstrates that ferns in New Zealand have and will continue to expand their ranges and migrate southward and upslope. Despite the predicted general increased range size as a result of climate change, our models predict that the majority (52%) of many species' current suitable habitats may be climatically unsuitable in 50 years, including the ecologically important group: tree ferns. Additionally, fern communities are predicted to undergo drastic shifts in composition, which may be detrimental to overall ecosystem functioning in New Zealand.