Adam S. Wymore

Genes to ecosystems: exploring the frontiers of ecology with one of the smallest biological units

Genes and their expression levels in individual species can structure whole communities and affect ecosystem processes. Although much has been written about community and ecosystem phenotypes with a few model systems, such as poplar and goldenrod, here we explore the potential application of a community genetics approach with systems involving invasive species, climate change and pollution. We argue that community genetics can reveal patterns and processes that otherwise might remain undetected. To further facilitate the community genetics or genes-to-ecosystem concept, we propose four community genetics postulates that allow for the conclusion of a causal relationship between the gene and its effect on the ecosystem. Although most current studies do not satisfy these criteria completely, several come close and, in so doing, begin to provide a genetic-based understanding of communities and ecosystems, as well as a sound basis for conservation and management practices.

Contrasting rRNA gene abundance patterns for aquatic fungi and bacteria in response to leaf-litter chemistry

Abstract.  Few investigators have examined simultaneous bacterial and fungal responses to leaf-litter chemistry in fresh water. We tested the hypothesis that bacteria would be more abundant on labile litter with lower concentrations of defensive compounds, whereas fungi would be more abundant on recalcitrant litter. We used quantitative-polymerase chain reaction (qPCR) to measure the abundance of bacterial 16S and fungal 18S ribosomal ribonucleic acid (rRNA) genes and found that these groups responded differently to leaf chemistry. Bacterial 16S rRNA genes were 4× more abundant on labile than on recalcitrant litter. In contrast, fungal 18S rRNA genes were 8× more abundant on recalcitrant than on labile litter. Peak bacterial gene abundances on day 6 were related to leaf-litter % bound condensed tannin (r2  =  0.38), and peak fungal gene abundances on day 14 were related to % soluble condensed tannin (r2  =  0.49), % bound condensed tannin (r2  =  0.34), and % lignin (r2  =  0.33). Leaf-litter C∶N ratios were not associated with microbial gene abundance. The ratio of fungal 18S:bacterial 16S genes also increased along the 1st axis in a principal components analysis of phytochemical variables. The early peak in bacterial rRNA gene abundance may indicate the role of bacteria in the early decomposition of leaf litter. rRNA gene abundance patterns demonstrate that bacteria and fungi have different patterns of growth and productivity in response to leaf-litter chemistry.

Nutrient uptake along a fire gradient in boreal streams of Central Siberia

Fire can transform the boreal forest landscape, thereby leading to potential changes in the loading of organic matter and nutrients to receiving streams and in the retention or transformation of these inputs within the drainage network. We used the Tracer Additions for Spiraling Curve Characterization (TASCC) method to conduct 17 nutrient-addition experiments (9 single additions of NO3– and 8 combined additions of NH4+ and PO43–) in 5 boreal headwater streams underlain by continuous permafrost and draining watersheds with a range of burn histories (4–>100 y since last burn) in the Nizhnyaya Tunguska River watershed in Central Siberia. Hydrology, ambient nutrient concentration, and the ratio of dissolved organic C (DOC) to nutrients drove rates of nutrient uptake in the streams. Nutrients were taken up with greater efficiency and magnitude under conditions with high flow and reduced diffusive boundary layer (DBL), regardless of watershed burn history. Ambient molar ratio of DOC∶PO43– explained some variation in ambient uptake velocity (υf) for NH4+ and PO43–. We also observed tight coupling between ambient rates of NH4+ and PO43– uptake across the watershed burn-history gradient. These data suggest that fire-driven changes in stream chemistry may alter N and P retention and subsequent export of materials to downstream receiving waters. Climate change is likely to enhance the frequency and intensity of boreal forest fires and alter the extent of permafrost. Therefore, understanding the interactions among C, N, and P in these Arctic systems has important implications for global biogeochemical cycling.

Direct response of dissolved organic nitrogen to nitrate availability in headwater streams

Despite decades of research documenting the quantitative significance of dissolved organic nitrogen (DON) across ecosystems, the drivers controlling its production and consumption are not well understood. As an organic nutrient DON may serve as either an energy or nitrogen source. One hypothesized control on DON concentration in streams is nitrate (NO3−) availability. Synoptic surveys of DON and NO3−, however, have yielded inconsistent spatial and temporal patterns. Using a nutrient pulse method we experimentally manipulated stream NO3− and measured the response of both the manipulated solute and ambient concentrations of DON in three New Hampshire headwater streams. This direct experimental addition of NO3− often altered ambient DON concentrations in situ, with both increases and decreases observed. The overall relationship between NO3− and DON suggests that DON is primarily used as a nutrient source in these streams, as evidenced by net DON accumulation with added NO3−. However, strong underlying seasonal patterns in the response to NO3− addition are also discernable, indicating that the role of DON can switch between serving as a nutrient source to an energy source (as evidenced by net DON reduction with added NO3−). We also observed differences in the NO3−—DON relationship (net DON accumulation vs. net DON reduction) in two streams less than five miles apart when experiments were conducted within the same month. Based on these results, we expect the role of DON within ecosystems to vary among watersheds and throughout the growing season, alternating between serving as a nutrient and energy source depending on environmental conditions. With the incorporation of a new field-based method we demonstrate that the ambient DON pool can be manipulated in situ. This approach has the potential for furthering our understanding of DON across ecosystems.

Leaf-litter leachate is distinct in optical properties and bioavailability to stream heterotrophs

Dissolved organic C (DOC) leached from leaf litter contributes to the C pool of stream ecosystems and affects C cycling in streams. We studied how differences in leaf-litter chemistry affect the optical properties and decomposition of DOC. We used 2 species of cottonwoods (Populus) and their naturally occurring hybrids that differ in leaf-litter phytochemistry and decomposition rate. We measured DOC and nutrient concentration in leaf leachates and determined the effect of DOC quality on heterotrophic respiration in 24-h incubations with stream sediments. Differences in DOC composition and quality were characterized with fluorescence spectroscopy. Rapidly decomposing leaves with lower tannin and lignin concentrations leached ∼40 to 50% more DOC and total dissolved N than did slowly decomposing leaves. Rates of heterotrophic respiration were 25 to 50% higher on leachate from rapidly decomposing leaf types. Rates of heterotrophic respiration were related to metrics of aromaticity. Specifically, rates of respiration were correlated negatively with the Fluorescence Index and positively with Specific Ultraviolet Absorbance (SUVA254) and T280 tryptophan-like fluorescence peak. These results reveal that leaf-litter DOC is distinctly different from ambient streamwater DOC. The relationships between optical characteristics of leaf leachate and bioavailability are opposite those found in streamwater DOC. Differences in phytochemistry among leaf types can influence stream ecosystems with respect to DOC quantity, composition, and rates of stream respiration. These patterns suggest that the relationship between the chemical structure of DOC and its biogeochemistry is more complex than previously recognized. These unique properties of leaf-litter DOC will be important when assessing the effects of terrestrial C on aquatic ecosystems, especially during leaf fall.

Critical zone structure controls concentration-discharge relationships and solute generation in forested tropical montane watersheds

Concentration-discharge (C-Q) relationships are poorly known for tropical watersheds, even though the tropics contribute a disproportionate amount of solutes to the global ocean. The Luquillo Mountains in Puerto Rico offer an ideal environment to examine C-Q relationships across a heterogeneous tropical landscape. We use 10-30 years of weekly stream chemistry data across ten watersheds to examine C-Q relationships for weathering products (SiO2(aq), Ca2+, Mg2+, Na+) and biologically-controlled solutes (dissolved organic carbon [DOC], dissolved organic nitrogen [DON], NH4+, NO3-, PO43-, K+, SO42-). We analyze C-Q relationships using power-law equations and a solute production model, and use Principal Component Analysis to test hypotheses regarding how the structure of the Critical Zone controls solute generation. Volcaniclastic watersheds had higher concentrations of weathering solutes and smaller tributaries were approximately 3-fold more efficient at generating these solutes than larger rivers. Lithology and vegetation explained a significant amount of variation in the theoretical maximum concentrations of weathering solutes (r2 = 0.43 - 0.48) and in the C-Q relationships of PO43- (r2 = 0.63) and SiO2(aq) (r2 = 0.47). However, the direction and magnitude of these relationships varied. Across watersheds various forms of N and P displayed variable C-Q relationships, while DOC was consistently enriched with increasing discharge. Results suggest that PO43- may be a useful indicator of watershed function. Relationships between C-Q and landscape characteristics indicate the extent to which the structure and function of the critical zone controls watershed solute fluxes.

Nitrate uptake across biomes and the influence of elemental stoichiometry: A new look at LINX II

Considering recent increases in anthropogenic N loading, it is essential to identify the controls on N removal and retention in aquatic ecosystems because the fate of N has consequences for water quality in streams and downstream ecosystems. Biological uptake of nitrate (NO3−) is a major pathway by which N is removed from these ecosystems. Here we used data from the second Lotic Intersite Nitrogen eXperiment (LINX II) in a multivariate analysis to identify the primary drivers of variation in NO3− uptake velocity among biomes. Across 69 study watersheds in North America, dissolved organic carbon:NO3− ratios and photosynthetically active radiation were identified as the two most important predictor variables in explaining NO3− uptake velocity. However, within a specific biome the predictor variables of NO3− uptake velocity varied and included various physical, chemical, and biological attributes. Our analysis demonstrates the broad control of elemental stoichiometry on NO3− uptake velocity as well as the importance of biome-specific predictors. Understanding this spatial variation has important implications for biome-specific watershed management and the downstream export of NO3−, as well as for development of spatially explicit global models that describe N dynamics in streams and rivers.

Community Genetics Applications for Forest Biodiversity and Policy: Planning for the Future

In many ecosystems, the genetic variation within foundation tree species drive key ecological processes. Here we present four key findings from community genetics research that can be applied to the preservation of forest biodiversity and improvement of management policy. (1) Different tree genotypes support different communities and different ecosystem processes; (2) primary productivity is, in part, genetically-based and is linked to biodiversity; (3) with changing climate, gene by environment interactions will affect forests and their dependent communities; and (4) minimum viable interacting population theory and analyses of species interaction networks provide a framework for integrating genetics into each of the above topics. Inclusion of community genetics in forest management is important because species evolve, exist, and interact within the context of a community. This approach allows for the creation of policies that are less susceptible to pitfalls inherit to single species management. Specific policy suggestions include the incorporation of a community genetics perspective in federally-funded reforestation and restoration projects, the use of non-local genotypes where climate change is predicted, and the development of provenance trials in a community context to identify superior genotypes and mixes of genotypes that will perform best in a changing environment. Such amendments to policy would unite the efforts of research, production, and conservation to maximize tax-payer investment.

Indirect influences of a major drought on leaf litter quality and decomposition in a southwestern stream

Climate models predict that the southwestern United States will experience an increase in drought frequency and intensity with global climate change. We tested the hypothesis that leaf litter produced under natural drought conditions would have an altered litter chemistry profile and affect decomposition rates and macroinvertebrate colonization compared to non-drought conditions. To test this hypothesis we collected leaf litter from Populus fremontii, Alnus oblongifolia, and Platanus wrightii grown during an average precipitation year (2001) and a record drought year (2002) and performed an in-stream decomposition study using both litter types. Three major patterns emerged: 1) Drought conditions significantly altered litter chemistry for mature trees of three species; however, the direction and magnitude of change differed among species and litter chemicals; 2) Leaf litter mass loss was influenced by both differences among species and drought; yet, species effects were more pronounced over time than drought effects; and 3) After 69 days of decomposition, the structure of the macroinvertebrate community was uninfluenced by the drought effect on A. oblongifolia or P. wrightii litters, but there was a community-wide drought effect on macroinvertebrate communities colonizing P. fremontii litter. Many recent studies have explored the influence of drought on stream flow and water temperatures, but these results suggest that litter quality can change under different climatic conditions, but the overall decay of leaf material may not be dramatically altered by droughts. Understanding how forest-stream interactions may be altered by the various influences of climate change will allow for better predictions regarding how long-term disturbances may alter stream ecosystem functioning.

Integrated Interdisciplinary Science of the Critical Zone as a Foundational Curriculum for Addressing Issues of Environmental Sustainability

ABSTRACT Earths critical zone (CZ) is the uppermost layer of Earths continents, which supports ecosystems and humans alike. CZ science aims to understand how interactions among rock, soil, water, air, and terrestrial organisms influence Earth as a habitable system. Thus, CZ science provides the framework for a holistic-systems approach to teaching Earth surface and environmental science, especially related to environmental sustainability. Here, we describe efforts by an interdisciplinary team to create a full-semester, university curriculum that introduces upper-division students to CZ science. Course topics include a background in CZ science, key concepts and methods of CZ science, and units on land–atmosphere interactions, water budgets, landscape evolution, and biogeochemistry. The course culminates with a unit on human interactions within the CZ. Through interactive activities that use data sets from U.S. CZ observatories, the course emphasizes how a CZ framework is appropriate for teaching concepts across scientific disciplines, concepts of environmental sustainability, and the usefulness of CZ science for considering humanitys grand challenges. Materials can be integrated into existing courses or used as an independent course to maximize instructor flexibility; all materials were piloted in eight separate courses across a range of university settings. Although preassessments and postassessments of geoscience literacy did not show much change, students overwhelmingly agreed that they could use what they learned to help society overcome grand challenges. Thus, the holistic-systems approach advocated by CZ science and explored throughout this curriculum provides a unique opportunity to engage students in thinking about complex issues related to environmental sustainability.