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.

Impact of Residential Prairie Gardens on the Physical Properties of Urban Soil in Madison, Wisconsin.

Prairie gardens have become a common addition to residential communities in the midwestern United States because prairie vegetation is native to the region, requires fewer resources to maintain than turfgrass, and has been promoted to help remediate urban soil. Although prairie systems typically have deeper and more diverse root systems than traditional turfgrass, no one has tested the effect of this vegetation type on the physical properties of urban soil. We hypothesized that residential prairie gardens would yield lower soil bulk density (BD), lower penetration resistance (PR), greater soil organic matter (SOM), and greater saturated hydraulic conductivity () compared with turfgrass lawns. To test this hypothesis, we examined 12 residential properties in Madison, WI, where homeowners had established a prairie garden within their turfgrass lawn. Despite a consistent trend in the difference between vegetation types, no significant main effects were found (i.e., a difference between vegetation types when averaged over depth) for any of the four soil properties measured in this study. Differences were found with depth and depended on a significant interaction with vegetation type. At the surface depth (0-0.15 m), soil beneath prairie gardens had 10% lower mean BD, 15% lower mean PR, 25% greater level of SOM, and 33% greater compared with soil beneath the adjacent lawns. These differences were not detected at deeper sampling intervals of 0.15 to 0.30 m and 0.30 to 0.45 m. Although not statistically significant, the consistent trend and direction among soil variables suggest that residential prairie gardens had changed the surface soil at a rate that marginally outpaced turfgrass and calls for controlled experiments to identify the mechanisms that might enhance these trends.

Coupling tree-ring delta13C and delta15N to test the effect of fertilization on mature Douglas-fir (Pseudotsuga menziesii var. glauca) stands across the Interior northwest, USA.

Nitrogen (N) fertilization causes long-term increases in biomass production in many N-limited forests around the world, but the mechanistic basis underlying the increase is often unclear. One possibility, especially in summer-dry climates, is that N fertilization increases the efficiency with which a finite water supply is consumed to support photosynthesis. This increase is achieved by a reduction in the canopy-integrated concentration of internal CO(2) and thus discrimination against (13)C. We used stable isotopes of carbon (delta(13)C) in tree rings to experimentally test the physiological impact of N fertilization on mature Douglas-fir (Pseudotsuga menziesii Franco var. glauca) stands across the geographic extent of the Intermountain West, USA. The concentration and the stable isotopes of N (delta(15)N) in tree rings were also used to assess the presence and activity of fertilizer N. We hypothesized that N fertilization would (i) increase delta(15)N and N concentration of stemwood relative to non-fertilized stands and (ii) increase stemwood delta(13)C as photosynthetic gas exchange responded to the additional N. This experiment included two rates of urea addition, 178 kg ha(-1) (low) and 357 kg ha(-1) (high), which were applied twice over a 6-year interval bracketed by the 18 years of wood production measured in this study. Foliar N concentrations measured the year after each fertilization treatment suggest that the fertilizer N had been assimilated by the trees (P < 0.001). The N fertilization significantly enriched stemwood delta(15)N by 1.3 per thousand at the low fertilization rate and by 2.4 per thousand at the high rate (P < 0.001) despite variation in soil N between sites. However, we found no significant effect of the N fertilizer on delta(13)C of the annual rings (P = 0.76). These data lead us to suggest that alternative mechanisms underlie the growth response to fertilizer, i.e., increase in canopy area and shifts in biomass allocation.

Carbon isotope discrimination in photosynthetic bark.

We developed and tested a theoretical model describing carbon isotope discrimination during photosynthesis in tree bark. Bark photosynthesis reduces losses of respired CO2 from the underlying stem. As a consequence, the isotopic composition of source CO2 and the CO2 concentration around the chloroplasts are quite different from those of photosynthesizing leaves. We found three lines of evidence that bark photosynthesis discriminates against 13C. First, in bark of Populus tremuloides, the δ13C of CO2 efflux increased from -24.2‰ in darkness to -15.8‰ in the light. In Pinus monticola, the δ13C of CO2 efflux increased from -27.7‰ in darkness to -10.2‰ in the light. Observed increases in δ13C were generally in good agreement with predictions from the theoretical model. Second, we found that δ13C of dark-respired CO2 decreased following 2-3 h of illumination (P<0.01 for Populus tremuloides, P<0.001 for Pinus monticola). These decreases suggest that refixed photosynthate rapidly mixes into the respiratory substrate pool. Third, a field experiment demonstrated that bark photosynthesis influenced whole-tissue δ13C. Long-term light exclusion caused a localized increase in the δ13C of whole bark and current-year wood in branches of P. monticola (P<0.001 and P<0.0001, respectively). Thus bark photosynthesis was shown to discriminate against 13C and create a pool of photosynthate isotopically lighter than the dark respiratory pool in all three experiments. Failure to account for discrimination during bark photosynthesis could interfere with interpretation of the δ13C in woody tissues or in woody-tissue respiration.

Decreased needle longevity of fertilized Douglas-fir and grand fir in the northern Rockies.

Changes in nutrient availability significantly affect canopy dynamics in conifers. To elucidate these effects, we experimentally fertilized mixed conifer stands at several sites across the northern Rocky Mountains. We measured needle longevity, total branch length and foliated length along the main branch axis, and determined mean retained cohort length on mid-canopy branches of shade-intolerant Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) and shade-tolerant grand fir (Abies grandis Lindl.). Needle longevity ranged from 6 to 8 years in Douglas-fir and from 7 to 8 years in grand fir on unfertilized plots. Fertilization significantly decreased needle longevity by 26 and 27% in Douglas-fir and grand fir, respectively. However, the foliated branch length remained unchanged following fertilization and was similar for both species, indicating a 33% increase in mean branch length per needle cohort in Douglas-fir and a 27% increase in grand fir. These data are consistent with the theory that foliated branch length and needle longevity are a result of the ecological light compensation point (ELCP), which links the inherent physiology of the leaf with the availability of resources in the leaf environment. Mid-canopy ELCP was approximately 74 and 71 cm from the branch terminus in Douglas-fir and grand fir, respectively, regardless of fertilization. We hypothesize that fertilization-enhanced needle production and annual shoot growth resulted in a higher rate of shading of older needles. The shaded needles were unable to maintain a positive carbon balance and abscised. The results demonstrate that foliated branch length of Douglas-fir and grand fir in the northern Rocky Mountains can be treated as a homeostatic response to fertilization, whereas foliar turnover is plastic.