Functional ecology of cryptogams: scaling from bryophyte, lichen, and soil crust traits to ecosystem processes

‘Cyanobacterial colonization by the N-fixing genus Nostoc has been shown to increase the N status of bryophytes, but we are only just beginning to learn how changes in environmental factors may influence N-fixation rates in cryptogams.’ To date, efforts to develop analytical frameworks for functional traits, including large trait databases with standardized protocols (e.g. Cornelissen et al., 2003), have focused on vascular plants, primarily the angiosperms and gymnosperms; however, other photosynthetic organisms such as cyanobacteria, bryophytes, lichens, and ferns can be equally important to primary productivity and biogeochemistry in terrestrial ecosystems (Lindo & Gonzalez, 2010; Porada et al., 2014). In this context, an emphasis on terrestrial autotrophs other than the seed plants, a paraphyletic group defined by the seemingly antiquated term ‘cryptogam’, can be ecologically informative. Cryptogams possess unique suites of traits that enable them to be competitive in niches where seed plants struggle. In these habitats, cryptogams play important ecological roles relating to soil stability and fertility, biogeochemical cycling, and community succession. Bryophytes in particular can dominate in forest understories, dryland biocrusts, and peatlands, making cryptogams particularly important in our understanding of the functional trait relationships in these ecosystems. Some of these trait relationships may mirror those already described in seed plants, but others represent alternative ecological and evolutionary trade-offs. At the 101st meeting of the Ecological Society of America, 7–12 August 2016, in Fort Lauderdale, FL, USA, an organized oral session (OOS 17) was held on the functional ecology of cryptogams. The session included researchers from the United States, Canada, and Europe, and focused on a diversity of cryptogam taxa and terrestrial ecosystems, as well as the application of techniques from molecular to ecosystem scales. This report focuses on the main themes of this session, and describes efforts to unify trait-based analyses of cryptogams (and the community of scientists that study them) to provide an integrative framework for future research. As a result of their evolutionary and natural history, cryptogams possess traits that are distinct from vascular plants, many of which relate to reproduction (spores, asexual propagules), water relations (poikilohydry, desiccation tolerance), and morphology (shoot architecture, leaf structure). Such traits can simultaneously be used to explain responses to biotic and abiotic environmental factors, and their effects on community and ecosystem function (Fig. 1). In this context, it is often helpful to distinguish between response traits (those that allow an organism to grow and reproduce in a given community and environment) and effect traits (those that directly affect features of the community and ecosystem surrounding the organism). The importance of this distinction in the context of cryptogams was highlighted at the meeting by Hans Cornelissen (Vrije Universiteit Amsterdam), since different trait types may require different research perspectives and methodologies. Notably, there is the possibility of overlap between response traits and effect traits, with some traits linked to both organismal and ecosystem function (Cornelissen et al., 2007). Many features of cryptogam communities, including diversity metrics, sex ratios, and community assembly processes, are directly linked to the morphological and physiological traits of the cryptogams that comprise them. Jessica Coyle (University of North Carolina) described how reproductive mode (i.e. vegetative vs sexual propagules) related to species assembly patterns in epiphytic lichen communities, but many other qualitative morphological traits in lichens had a limited ability to predict community structure, highlighting the need for quantitative trait analyses. In a restoration context, Matthew Bowker (Northern Arizona University) discovered that competitive interactions among mosses in dryland biocrust communities can be overcome by the introduction of a ‘universal facilitator’ species such as the N-fixing lichen Collema tenax. In an experiment on the drivers of community structure through colonization, Nicholas McLetchie (University of Kentucky) showed that reproduction and the degree of dehydration tolerance (also called desiccation tolerance) trade off to influence the ability of liverworts to colonize novel habitats. At the global scale, John Shaw (Duke University) described how the Sphagnum Genome Project (Shaw et al., 2016) is allowing us to understand the underlying genetics and genomics responsible for adaptive trait evolution. Shaw's research revealed evidence of early Sphagnum diversification in boreal regions and subsequent range expansion into tropical climates. The same research also identified a strong phylogenetic signal for interspecific variation along the hummock–hollow gradient, showing how microhabitat preference can shape peatland physiognomy and community assembly. The physiognomy and composition of cryptogam-dominated communities strongly influence ecosystem processes. Kirsten Deane-Coe (St Mary's College of Maryland) showed that three co-occurring Sphagnum species differed greatly in both C and N fixation rates, according to their micro-topographic preferences (hummock to hollow). Linking cryptogam physiological traits to ecosystem C cycling, Tobi Oke (University of Guelph) used intraspecific trait analyses to show that bryophyte tissue decomposability influenced ecosystem CO2 production, and was correlated with shoot water content. Similarly, Cornelissen described how bryophyte shoot C : N and decomposability directly relate to C turnover in high latitude cryptogam-dominated systems. Environmental water availability and tissue water content can drive C fixation and growth in bryophytes in a diversity of ecosystems and taxa (e.g. Williams & Flanagan, 1996; Coe et al., 2012), and several contributors showed how bryophyte shoot morphology and cellular anatomical features influence water retention and storage. Steve Rice (Union College) used a three-dimensional (3D) thermal imaging system to determine that vertical temperature gradients exist in the shoot systems of feather mosses, relating directly to evaporative water losses and C-fixation potential. Oke found that other shoot traits such as capitulum mass and branch density in Sphagnum related to plant water status, and at the cellular level, the density of hyaline cells (specialized cells used in water storage) was correlated with water holding capacity. In a test of the diversity vs productivity hypothesis exemplified by Tilman et al. (1997), Bowker showed that primary production in dryland biocrust communities consisting of bryophytes, lichens, and cyanobacteria was linked to key facilitator species (e.g. the lichen Collema) rather than overall biocrust diversity, suggesting that C cycling in biocrust communities may depend more heavily on the functional traits of particular taxa. The ability to predict C and N cycling is crucial to our understanding of the effects of changing environmental conditions in terrestrial ecosystems. Primary productivity in peatland systems, which store nearly one-third of terrestrial C, is controlled by environmental factors that limit photosynthesis in the dominant plant, Sphagnum moss. Deane-Coe described how water table depth influenced the rates of C fixation in three species of Sphagnum from a kettle bog in New York. In particular, the absolute depth of the water table drove C fixation to a greater degree than intra-season water table variability. At the ecosystem scale, Catherine Dieleman (University of Western Ontario) used intact peat monoliths to show that increased temperatures are likely to increase the amount of dissolved organic C in soil as a result of Sphagnum replacement by less recalcitrant graminoid species, with implications for CO2 release to the atmosphere. Nitrogen cycling in cryptogam-dominated systems is partly mediated by cryptogam microbiomes, where traits related to the presence and activity of bacterial symbionts can influence N-fixation rates at shoot and ecosystem scales. Cyanobacterial colonization by the N-fixing genus Nostoc has been shown to increase the N status of bryophytes (Deane-Coe & Sparks, 2016), but we are only just beginning to learn how changes in environmental factors may influence N-fixation rates in cryptogams. David Weston (Oak Ridge National Laboratory) described several ongoing experiments associated with the Spruce and Peatland Responses Under Climatic and Environmental Change (SPRUCE) project (http://mnspruce.ornl.gov/), where CO2 and temperature are manipulated in a long-term field experiment in an intact Sphagnum-dominated boreal forest. In particular, Weston has found that the diversity of N-fixing bacteria associated with bryophytes decreases with increased temperatures, while the relative abundance of Nostoc increases. Deane-Coe also noted that N-fixation and cyanobacterial colonization in Sphagnum declined in taxa colonizing the middle and top of peat hummocks in response to water table manipulation. Collectively, this research suggests that N-fixation in cryptogams may be particularly sensitive to environmental change, perhaps more so than C-fixation. Cryptogams possess unique suites of traits distinct from vascular plants, many of which directly relate to ecosystem processes. The work presented at this organized session illustrated the diversity of current trait-based research in cryptogams, as well as the need to continue lines of questioning about response traits and effect traits in cryptogams, particularly in the face of global change. A common theme among all of the talks in this session was the call to develop an integrative trait framework for cryptogams. In particular, a trait database that includes correlation structures among response traits, effect traits, and traits that fall into both categories, would make a strong contribution to our understanding of unifying relationships between plant and ecosystem processes. The authors wish to thank all of the speakers in the organized oral session (OOS 17) as well as the Ecological Society of America for hosting the session at their annual meeting. The authors also thank David Hanson for assisting with revisions of this manuscript.

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