Functional traits across primary producer groups and their effects on tundra ecosystem processes

Vascular plants, lichens and bryophytes dominate the vegetation above the tree line. Individual species within these groups are characterized by a set of traits, such as nutrient content, height, growth rate, and leaf area. These traits can vary both within species and across species. For example, as temperature increase, the vegetation in general gets higher, either because the individual species grow bigger (within species variability) and/or because smaller species are replaced by larger species (across species variability). In this project we study if traits of vascular plants, lichens and bryophytes change in similar manners in response to the same climate, and if the relative importance of within vs across species variability is the same between the three organism groups. Studying traits are important because they may affect ecosystem processes such as nutrient and carbon fluxes. Therefore, we link plant traits to litter decomposition and nutrient release. Small animals, such as mites and springtails, consume and fragment litter as well as change the microbial communities through grazing and faecal production. To understand the link between plant traits and ecosystem processes, we will also study the direct effect of traits on these essential decomposers. We found that trait responses differed highly between functional traits and primary producer groups. Intraspecific variation was important for nutrient variables for vascular plants and lichens, but not for bryophytes. However, non-chemical traits and pH were mainly driven by changes in species composition (which we refer to as species turnover), for all three groups. For some traits, intraspecific variation was more important in lichens than in vascular plants and bryophytes, indicating that the lichens across our gradient show a great deal of intraspecific plasticity. Although we do not know if our results hold true across other elevational gradients, it suggests that the lichen community could be more resistant to climate change than the bryophyte and vascular plant community. This study is now published in Functional Ecology (doi:10.1111/1365-2435.13454). We have reviewed (Asplund & Wardle 2017 Biological Reviews 92: 1720-1738) the role of lichens in terrestrial ecosystems and draw attention to the important, but often overlooked role of lichens as determinants of ecological processes. We start by assessing characteristics that vary among lichens and that may be important in determining their ecological role. We then assess how these differences among lichens influence their impacts on ecosystem and community processes. As such, we consider the consequences of these differences for determining the impacts of lichens on ecosystem nutrient inputs and fluxes, on the loss of mass and nutrients during lichen thallus decomposition, and on the role of lichenivorous invertebrates in moderating decomposition. We then consider how differences among lichens impact on their interactions with consumer organisms that utilize lichen thalli. We then address how differences among lichens impact on plants, through for example increasing nutrient inputs and availability during primary succession, and serving as a filter for plant seedling establishment. Finally we identify areas in need of further work for better understanding the role of lichens in terrestrial ecosystems. The mat-forming lichen Cladonia stellaris, an important fodder for reindeer, produces usnic acid in the outermost layer and perlatolic acid in the medulla. In a study published in the lichenologist (Asplund et al 49:269-274) we studied effects of simulated global warming on these compounds in C. stellaris cultivated in climate chambers with. After two months of acclimation, +4 °C warming in one simulated growing season increased the concentration of usnic acid by 31% compared with ambient conditions, whereas the warming decreased the concentration of perlatolic acid by 14%. We have performed a decomposition study using tea-bags in plots that previously been exposed to four types treatments; warming with open top chambers (OTCs), fertilization (NPK), OTC + fertilization and a control. These treatments stopped in 2007, nine years before this experiment. We found that plots that had been warmed and fertilized had higher decomposition rates. We attribute to changes in species composition in these plots and not to differences in soil chemistry. This work is published as a master thesis and will be rewritten as a paper putting more emphasis on how traits of the plants and lichens influence decomposition. In a study published in Basic and Applied Ecology (doi: 10.1016/j.baae.2018.07.007) we show that nitrogen concentration in epiphytic lichens are twice as high on Norway maple compared with lime trees. This is due to bark pH causing a switch in lichen species composition between different tree species.

One outcome with this project is that we have shown the research community that it actually is possible to measure functional traits in lichens and mosses, which if often neglected by plant ecologists. Other studies prior to our (although not many) have measured functional traits on bryophytes or lichens. However, but studying these three organism groups simultaneously, we have managed to reach a broader audience for the often-neglected groups. As such, I think that the project has increased the understanding of the importance of including lichens and bryophytes in studies on alpine vegetation, also among plant ecologists. This knowledge allows us to better understand how important ecosystem processes, i.e. those regulating nutrient- and carbon cycling, as well as vegetation-climate feedbacks are driven by photoautotroph communities. In particular, our findings underline the importance of functional traits for understanding these relationships.

Functional traits are important drivers of ecosystem processes such as herbivory and decomposition and thus in regulating ecosystem nutrient and carbon fluxes. There has been much recent interest in understanding variation in vascular plant functional traits at the whole community level, and new methods have recently been developed to study the relative importance of within and across species trait variability in determining community level traits. As such, for vascular plants, community level traits are commonly found to be primarily driven by changes in species composition across environmental gradients, while changes in intraspecific traits are of lesser importance. Meanwhile, the relative importance of inter- and intraspecific trait variability for other functionally important groups of biota, such as lichens and bryophytes, are poorly understood. However, a recent study suggests that community-level responses of boreal forest epiphytic lichens are driven by intraspecific variability and that species turnover is unimportant. There has been growing recognition that for vascular plants, functional traits can drive the community composition of both herbivores and decomposers. Meanwhile, our knowledge of the role lichen and bryophyte traits play on consumer and decomposer communities are limited. Further, climate change is most likely to influence the relative abundance of lichens, bryophytes and vascular plants and their community level traits and thus the trait-driven ecosystem processes. In this light, we aim to study the link between primary producer traits, abundance of micro-arthropods and decomposition. Further, we are interested in how these links are influenced by climate change in alpine environments. The proposed project will be highly novel as bryophyte- and lichen-specific traits and their linkages to ecosystem processes such as decomposition have hardly been investigated.