A within-lake occupancy model for starry stonewort, Nitellopsis obtusa, to support early detection and monitoring.

To efficiently detect aquatic invasive species early in an invasion when control may still be possible, predictions about which locations are likeliest to be occupied are needed at fine scales but are rarely available. Occupancy modeling could provide such predictions given data of sufficient quality and quantity. We assembled a data set for the macroalga starry stonewort (Nitellopsis obtusa) across Minnesota and Wisconsin, USA, where it is a new and high-priority invader. We used these data to construct a multi-season, single-species spatial occupancy model that included biotic, abiotic, and movement-related predictors. Distance to the nearest access was an important occurrence predictor, highlighting the likely role boats play in spreading starry stonewort. Fetch and water depth also predicted occupancy. We estimated an average detection probability of 63% at sites with mean non-N. obtusa plant cover, declining to ~ 38% at sites with abundant plant cover, especially that of other Characeae. We recommend that surveyors preferentially search for starry stonewort in areas of shallow depth and high fetch close to boat accesses. We also recommend searching during late summer/early fall when detection is likelier. This study illustrates the utility of fine-scale occupancy modeling for predicting the locations of nascent populations of difficult-to-detect species.

Novel determination of effective freeze-thaw cycles as drivers of ecosystem change.

Soil freeze-thaw cycles (FTCs) profoundly influence biophysical conditions and modify biogeochemical processes across many northern-hemisphere and alpine ecosystems. How FTCs will contribute to global processes in seasonally snow-covered ecosystems in the future is of particular importance as climate change progresses and winter snowpacks decline. Our understanding of these contributions is limited because there has been little consideration of inter- and intrayear variability in the characteristics of FTCs, in part due to a limited appreciation for which of these characteristics matters most with respect to a given biogeochemical process. Here, we introduce the concept of effective FTCs: those that are most likely linked to changes in key soil processes. We also propose a set of parameters to quantify and characterize effective FTCs using standard field soil temperature data. To put these proposed parameters into effective practice, we present FTCQuant, an R package of functions that quantifies FTCs based on a set of user-defined parameter criteria and, importantly, summarizes the individual characteristics of each FTC counted. To demonstrate the utility of these new concepts and tools, we applied the FTCQuant package to re-analyze data from two published studies to help explain over-winter changes to N2 O emissions and wet-aggregate stability. We found that effective FTCs would be defined differently for each of these response variables and that effective FTCs provided a 76 and 33% increase in model fit for wet-aggregate stability and cumulative N2 O emission, respectively, relative to conventional FTC quantification methods focusing on fluctuations around 0 °C. These results demonstrate the importance of identifying effective FTCs when scaling soil processes to regional or global levels. We hope our contributions will inform future deductions, hypothesis generation, and experimentation with respect to expected changes in freeze-thaw cycling globally.

Effects of childhood setting and interaction with nature on academic performance in introductory college-level courses in the environmental sciences.

This study explored the relationships between student background and academic performance in college introductory environmental science (ES) courses at a large U.S. research university with the premise that this analysis may inform teaching practices, curricula, and efforts to increase retention. We surveyed over 700 students across eleven introductory ES courses and used multiple linear mixed-effects regressions to model the data. We found that students who grew up in rural settings or who had frequent childhood interactions with natural environments earned higher grades, on average, than students from urban settings or with fewer childhood interactions with natural environments. Our results indicate that students reporting frequent childhood interactions with forests, for example, were projected to earn grades up to 1.5 letter grades higher in these courses than students with no such interactions. In addition, students with frequent childhood interactions with nature were likelier to report that such interactions helped them in their ES course, suggesting that these students may recognize the value of these experiences. Greater interest in the subject matter also correlated with higher ES course grades, whereas amount of prior ES coursework did not. We discuss the possible implications of these correlations for ES academic performance and educational practice.

Flower power: Floral and resource manipulations reveal how and why reproductive trade-offs occur for lowbush blueberry (Vaccinium angustifolium).

Plant reproductive trade-offs are thought to be caused by resource limitations or other constraints, but more empirical support for these hypotheses would be welcome. Additionally, quantitative characterization of these trade-offs, as well as consideration of whether they are linear, could yield additional insights. We expanded our flower removal research on lowbush blueberry (Vaccinium angustifolium) to explore the nature of and causes of its reproductive trade-offs. We used fertilization, defoliation, positionally biased flower removal, and multiple flower removal levels to discern why reproductive trade-offs occur in this taxon and to plot these trade-offs along two continuous axes. We found evidence through defoliation that vegetative mass per stem may trade off with reproductive effort in lowbush blueberry because the two traits compete for limited carbon. Also, several traits including ripe fruit production per reproductive node and fruit titratable acidity may be "sink-limited"-they decline with increasing reproductive effort because average reproductive structure quality declines. We found no evidence that reproductive trade-offs were caused by nitrogen limitation. Use of reproductive nodes remaining per stem as a measure of reproductive effort indicated steeper trade-offs than use of the proportion of nodes remaining. For five of six traits, we found evidence that the trade-off could be concave down or up instead of strictly linear. Synthesis. To date, studies have aimed primarily at identifying plant reproductive trade-offs. However, understanding how and why these trade-offs occur represent the exciting and necessary next steps for this line of inquiry.

Bearing fruit: flower removal reveals the trade-offs associated with high reproductive effort for lowbush blueberry.

Past studies have shown that taxa from disparate groups often respond similarly to reduced reproductive effort. These common responses imply that high reproductive effort trades off with a consistent set of other life functions for most angiosperms, albeit modulated by their growth form and life history. However, many questions remain about reproductive trade-offs in plants, including just how many other life functions they involve, how diverse these functions may be, and how the severity of these trade-offs may vary through time. To address these questions in a long-lived, iteroparous shrub, we performed flower removal on plots of lowbush blueberry, Vaccinium angustifolium (Ericaceae), over 3 years. We found significant physiological differences between removal and control plots for ten diverse traits. Vegetative phenology was shifted earlier by about 20% in removal plots, and removal plots had about 15% more vegetative biomass by mid-season as well. Removal plots produced about 10% more ripe fruit per reproductive node by harvest than control plots, and reproductive nodes in removal plots produced at least one fruit by harvest about 6% more often. While fruit water content and titratable acidity were increased by removal, other fruit traits, such as sugar content and fresh mass, were not. The strength of the removal effect varied significantly by year for seven traits; for many, such as vegetative mass/stem and ripe fruit production/node, the effect was stronger in years with more stressful abiotic conditions. Our results demonstrate that there are tangible but variable costs to high reproductive effort for flowering plants.