In arid and semi-arid ecosystems, dominant tree species create dramatic mosaics of plant islands of fertility and relatively barren plant interspaces that exert immense pressure on ecosystem processes and offers an ideal opportunity to explore the impact of bacterial communities. We evaluated potential links between soil respiration and N mineralization, and community co-occurrence networks and predicted gene function across three tree island microsites (i.e., beneath tree canopies, at the canopy edge, and in interspaces) in a replicated field experiment in thirty-eight woodlands sites in the Great Basin Desert in UT, USA. Additionally, we potentially intensified the effects of tree islands by creating a treatment where whole trees were shredded and the resulting fine woody debris (FWD) was deposited onto the soil surface and measured a suite of characteristics relating to the metabolic functional state of communities (i.e., microbial efficiency as the microbial quotient, C substrate quality, biomass, and dissolved organic C) to improve our interpretation of potential links between function and structure. We found that tree islands were the predominant driver, creating highly complex and connected assemblies of bacterial populations and easily discernable differences in abundance and composition of predicted functional genes. Specifically, communities directly beneath Juniperus and Pinus canopies were comprised of at least 5.2-times more connections between bacterial taxa than present in networks from interspace and edge. Using Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) to predict the gene expression, differences in the functional potential mirrored shifts in network complexity. Tree island communities expressed 236 genes with many related to the degradation of polyaromatic or polycyclic compounds, while interspace communities expressed only 66 genes associated with the decomposition of more labile C substrates. We observed a robust tree island microsite effect on all ecosystem processes, with soil respiration rates increasing 12% and N mineralization decreasing 29% in canopy than interspace soils demonstrating that a more recalcitrant substrate from a sole C source provided high amounts of low quality of DOC and lead to a decrease in metabolic efficiency, but ultimately selected for a specific community assembly. Alternatively, communities at the edge of canopies, experiencing both tree island and interspace soil conditions generated similar levels of soil respiration as canopy soils regardless of not selecting for a highly connected community and/or specific genes suggesting that a diverse composition of labile and recalcitrant C substrates from multiple sources (e.g., trees, perennial grasses, annual grasses, and forbs) potentially elevates function by promoting the activity of a wide range of taxa. Our results identify that tree islands exert enough pressure to create distinct interactions between bacteria and alter gene expression resulting in changes in ecosystem function, but the link between structure and function is mediated through the diversity and quality of C substrates.

In boreal forest streams, woody debris is an important habitat component. Stream invertebrates and salmonids such as brown trout benefit from in-stream wood. The studies presented in this thesis explore how drifting stream invertebrates respond to addition of fine woody debris, and how young-of-the-year (0+) brown trout behave in habitats with and without fine woody debris. The first paper reports results from a field experiment where fine woody debris was added to streams, and invertebrate drift was measured in order to detect impacts of the fine woody debris on drift density, biomass and taxon diversity. In the end of the season, the fine woody debris-affected drift samples showed higher density, biomass and taxon diversity than the control samples. In the second paper, I describe effects of fine woody debris on 0+ brown trout, studied in laboratory stream channels. Trout were tested in habitats without fine woody debris, with an intermediate fine woody debris density, and with a high fine woody debris density. Swimming activity and foraging time were significantly lower when fine woody debris was present than when it was absent. More time was spent sheltering at the high fine woody debris density than at the intermediate one. The increasing exploitation of fine woody debris for biofuel purposes should be considered in relation to the effects on brown trout and stream invertebrate habitat. / <p>Artikel 1 ingick i avhandlingen som manuskript med titeln "Effects of fine wood addition on invertebrate drift in boreal forest streams". Nu publicerad.</p>

Recent and anticipated global change has focused concern on the role of forests in ecosystem functioning and carbon sequestration. Of key importance is identifying relevant factors that drive carbon and nutrient dynamics and the consequences of changes in these processes. Systems undergoing invasion by invasive woody species are particularly prone to changes. This study examined the branch and crown biomass and decay dynamics for the invasive Chinese tallow tree in Mississippi, USA as well as the influences of stand and site conditions on biomass modeling and arthropod contributions to fine woody debris decay. Coupling biomass and decay models presents a method for modeling carbon sequestration and nutrient turnover rates at the stand level. These predictions will aid our understanding of the consequences of ecosystem change, especially those driven by invasive species.