Browsing by Author "Havrilla, Caroline, committee member"

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Open Access
A comparative analysis of wetland and riparian vegetation on Bureau of Land Management land in the western US
(Colorado State University. Libraries, 2023) Binck, Elin, author; Sueltenfuss, Jeremy, advisor; Reynolds, Lindsay, committee member; Smith, Melinda, committee member; Havrilla, Caroline, committee member
In 2011, the BLM deployed its first of three Assessment, Inventory, and Monitoring (AIM) programs as a large-scale, standardized ecological monitoring effort across the agency's land. The first two programs, known as Terrestrial AIM and Lotic AIM, were designed to sample all terrestrial and river ecosystems throughout the landscape. In 2019, the agency piloted its third AIM program, specifically targeting riparian areas and wetlands. This study addressed two main questions: 1) How do wetland and riparian areas sampled with the Terrestrial AIM program compare to those sampled with the Riparian and Wetland (R&W) AIM program, and 2) What are the drivers of plant community composition of the wetlands and riparian areas sampled on BLM land? I developed a set of criteria to identify sites sampled with Terrestrial AIM that had characteristics of wetlands or riparian areas. I then compared vegetation cover, floristic quality metrics, and species composition using nonmetric multidimensional scaling (NMDS) to those sites sampled with R&W AIM. R&W AIM sites had much greater foliar cover, hydrophytic species cover, and perennial cover, but Terrestrial sites had slightly higher floristic metric values. I similarly analyzed the R&W sites on their own, incorporating wetland-specific data that is collected with the new program. I found that sites that met the criteria to be classified as wetlands in the Terrestrial data were a distinct population from the sites sampled with R&W AIM. The main drivers of plant community composition among sites sampled with R&W AIM were elevation and the distribution of surface water, but impacts of grazing were also apparent. All sites assessed by both AIM programs had floristic quality metrics characteristic of highly impacted wetland systems. This study indicates the value of the new R&W AIM program for its ability to perform wetland-specific ecological monitoring, provide valuable data on the health of wetlands, and provide baseline condition that can help guide land management practices into the future.
Open Access
CONSERVATION MANAGEMENT PRACTICE IMPACTS ON RANGELANDS IN CALIFORNIA
(Colorado State University. Libraries, 2025) Banuelos, Ashley, author; Paustian, Keith, advisor; Cotrufo, Francesca, committee member; Havrilla, Caroline, committee member
Rangelands hold potential for mitigating climate change through soil organic carbon (SOC) storage. SOC plays a critical role in plant growth, soil structure and water retention, yet significant degradation of the world's soils poses major risks to forage production and water quality. To address this, California has promoted the adoption of conservation practices to restore SOC storage. Given California’s diverse climatic zones, climate-specific conservation strategies are necessary, as climate influences the effectiveness of different practices. These practices not only affect overall SOC stocks but also influence how SOC is stabilized in the soil, particularly through the formation of SOC fractions - particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). POC generally contributes to the short-term carbon pool due to its rapid turnover however, when microbial activity is limited, its decomposition slows, allowing it to persist in the long-term carbon pool. In contrast, MAOC is more inherently stable and primarily associated with long-term carbon storage. This thesis investigated the effects of three conservation management practices - riparian restoration, tree plantings, and perennial seeding - on SOC storage in California rangelands. We used a retrospective paired-site analysis, comparing ‘restored’ (i.e., locations where a conservation practice was adopted) and ‘unrestored’ sites (i.e., a nearby similar location but lacking adoption of a conservation practice). Restored sites varied by the time since conservation practices were adopted, providing a chronosequence approach to estimate SOC and SOC fractions (POC and MAOC) change over time. While overall SOC differences between restored and unrestored sites were inconclusive, clear trends between practice types emerged within the restored sites. In drier regions, perennial seeding had higher POC stock compared to riparian restoration and tree plantings. Climate significantly influenced apparent SOC accrual in tree plantings, with a rate of 3.1 Mg C ha-1 yr-1 observed in moist climates, while in drier climates, SOC stocks were lower in tree planting sites compared to the unrestored sites. However, soil under tree canopies had 9% higher SOC content compared with soil sampled between trees, outside the tree canopy. Canopy cover appeared to promote proportional contributions to both POC and MAOC, highlighting the potential of tree plantings to increase SOC stocks, in relatively cooler, wetter regions. These findings underscore the importance of climate-specific conservation strategies for maximizing carbon storage in rangelands, particularly given the challenges inherent in managing these dynamic ecosystems. The variability in the apparent response to conservation practice adoption from the retrospective paired-site analysis raised questions about potential confounding factors. While this approach offers an alternative to long-term experiments by leveraging existing conservation practices, it introduces inherent uncertainties, particularly concerning prior disturbances that may influence SOC storage. A key assumption of the paired analysis is that vegetation and soils were approximately the same on both sites within a pair before the adoption of conservation practice. However, even when controlling for factors such as soil type, topography, and current vegetation, differences in past land use - such as disturbance events occurring at one site but not the other - could have led to SOC stock differences prior to when conservation practices were implemented. These historical land-use differences may obscure or exaggerate the measurement inferred impacts of conservation practices, highlighting the need to account for site history when interpreting SOC dynamics in retrospective studies. To address this, we analyzed remote sensing imagery to evaluate site conditions, prior to conservation practice adoption, identifying disturbance events and assessing vegetation cover and soil exposure from historical observations dating back to 1984. Our analysis revealed that 12 out of 36 paired sites experienced a disturbance event, on only one of the sites within a pair, including mastication, tillage, and burn events, potentially confounding the assumption of similar SOC stocks prior to the time of conservation practice adoption. Additionally, restored sites with significant pre-treatment differences in vegetation cover and bare soil exposure often originated from more degraded conditions compared to the unrestored site in the pair. This suggests a potential selection bias toward implementing conservation practices on more degraded lands, emphasizing the need to account for pre-existing site conditions in retrospective studies. Integrating remote sensing into paired-site analyses enhances the accuracy of assessments of conservation practice effectiveness assessments on SOC dynamics. This study underscores the importance of both climate considerations in conservation management and the value of remote sensing tools for improving SOC research methodologies.
Open Access
Evaluating Bouteloua gracilis cultivars' performance after drought; The role of the soil microbiome
(Colorado State University. Libraries, 2024) Donne, Carina, author; Smith, Melinda, advisor; Havrilla, Caroline, committee member; Trivedi, Pankaj, committee member; Metcalf, Jessica, committee member
Drought has affected the Great Plains throughout history, most notably during the Dust Bowl of the 1930's. While most drought events are not as severe as the Dust Bowl, they still cause significant agricultural losses every year. As research has begun to uncover the mechanisms and responses of drought, there are still unanswered questions. For instance, the mechanisms of ecosystem recovery after drought ends remain relatively unexplored. It is possible that intervention methods such as reseeding will need to be done to help restore ecosystem structure and function after drought. After the Dust Bowl, it was a common practice to reseed native grasses, such as Blue Grama (Bouteloua gracilis), in sites severely impacted by the drought. Given forecasts of droughts on par or even more severe than the Dust Bowl, reseeding may need to be employed more frequently in the future to enhance post-drought recovery. However, with reseeding efforts, it is imperative to understand the adaptability of cultivars to the environmental conditions in which they are planted. One aspect of environmental conditions that has rarely been examined the soil microbiome. Here, I used a common garden experiment that included two cultivars of B. gracilis that were planted with soil microbial inocula extracted from either previously droughted or non-droughted soils. These soils were collected from a recently ended four-year drought experiment in the shortgrass steppe of northeastern Colorado, which caused the widespread loss of B. gracilis. The goal of the greenhouse experiment I conducted was to examine whether the post-drought legacy of altered soil microbial communities affected the growth and performance of two common cultivars of B. gracilis. I assessed plant performance by measuring weekly height to estimate relative growth rate and at the end of the experiment, I measured plant above- and belowground biomass. I found no significant differences in relative growth rate or plant biomass, and minimal differences in the bacterial community composition between the two cultivars. These results suggest that the post-drought legacy of altered soil bacterial communities did not differentially affect growth and performance of the two common B. gracilis cultivars evaluated in this study, and that the growth of these cultivars did not differ in their effects on the soil bacterial communities found under ambient vs. previously droughted conditions. Overall, both cultivars may be suitable for reseeding in the shortgrass steppe grassland after extreme drought, yet further studies are needed to examine a broader range of B. gracilis cultivars and whether soil bacterial communities previously exposed to extreme drought would allow for improved growth and performance of different cultivars to future drought conditions.
Open Access
Implications of facilitation and heterogeneity for plant community restoration in the Rocky Mountain region
(Colorado State University. Libraries, 2023) Harris, Rebecca F., author; Paschke, Mark, advisor; Havrilla, Caroline, committee member; Rhoades, Chuck, committee member; Smith, Melinda, committee member
Seeding is the most widespread and viable method to revegetate degraded dryland ecosystems. Despite its prevalence, efforts to establish plants through seeding frequently fail. Variable weather conditions, competition with dominant and invasive species and herbivory pose barriers to the establishment of seeded plants. As a result, there is a great need and demand for innovative restoration practices that can aid revegetation efforts. Pit, mound and slash pile treatments create spatial and resource heterogeneity and thus can provide niches for different plants to find suitable habitat. I completed a four-year study that tested the impacts of seeding in conjunction with heterogeneity treatments on measures of plant community structure and cover compared to untreated but seeded plots and unseeded controls. Research sites were located in California Park, Colorado USA, a 11,000-ha Artemisia cana (silver sagebrush) meadow in the Medicine Bow Routt National Forest. In 2018, replicated test plots containing four treatments (unseeded control, seeded only, seeded plus soil pits and mounds, and seeded plus slash) were established at degraded sites in California Park. Seeded plots received a high diversity native seed mix (39 species) at a high rate (1496 Pure Live Seed [PLS] / m2). I monitored seeded species density and total plant species cover in the summers of 2019 through 2022. I analyzed treatment effects on plant Shannon Wiener diversity (H), richness, seeded abundance (plants / m2), Pielou's evenness (J) and percent cover with linear mixed-effects models. Plots that received the seeded only and seeded plus soil pits and mounds treatments contained higher seeded species diversity than control plots in 2019 and 2022. While seeded plus slash pile treatment plots increased seeded species diversity in the first year of monitoring, this effect diminished after 2019. It is important to note, no seeded diversity differences emerged between heterogeneity treatment plots and untreated but seeded plots in any year. Findings suggest that seed application can increase the diversity of desirable seeded species and may be a useful tool to shift plant community composition. Shrub subcanopies accumulate moisture and nutrients in a manner that can facilitate understory plants. Consequently, seeding beneath shrubs may improve seedling establishment and survival in dryland ecosystems where harsh environmental conditions are present. I set up a study to evaluate whether seeded shrub islands had higher seeded species abundance and richness compared to seeded interspaces and unseeded controls. Study sites were located outside of Grand Junction, Colorado in Dominguez Escalante and McInnis Canyons National Conservation Areas (NCA) (Figure 2.1). Six sites were established in fall of 2020, three within sagebrush and three within salt desert shrublands. Five seeded shrub islands, five seeded interspaces, five unseeded shrub island controls and five unseeded interspace controls were established at each site. In addition, I evaluated if the impacts of these treatments were different between sagebrush and salt desert shrublands. Furthermore, I assessed whether plant functional groups and soil chemical properties were impacted by treatment type. I analyzed treatment effects on seeded species richness, seeded species abundance, cover by plant functional group, surface soil moisture, temperature and chemical properties with separate linear mixed-effects models. While sub canopy microsites were on average nutrient enriched and cooler than interspace plots, they did not facilitate seeded species establishment. In contrast to expectations, seeded interspaces contained higher seeded species richness than all other plot types. Lower richness in seeded shrub islands may be due to sodic, saline and or saline-sodic soil conditions found beneath shrubs within the salt desert. In addition, high cover of introduced perennial grasses beneath shrub islands may prevent seedling establishment at sagebrush sites. Results suggest that interspaces provide more favorable conditions for seeded plant establishment than shrub islands in both systems. Land managers are interested in levering positive plant-plant interactions (i.e., facilitation) and heterogeneity to improve seeded plant establishment and plant community restoration outcomes. I investigated whether applying a native seed mix either beneath shrub islands, or in tandem with heterogeneity treatments, increased seeded plant establishment and diversity respectively. I hope this research can contribute to improving plant community restoration outcomes in degraded dryland ecosystems.
Open Access
Response and recovery of grassland plant communities exposed to multiyear drought differs across a precipitation gradient
(Colorado State University. Libraries, 2022) Ross, Maggie, author; Smith, Melinda D., advisor; Knapp, Alan, committee member; Havrilla, Caroline, committee member; Wilkins, Kate, committee member
Drought events are expected to increase in grassland ecosystems in many regions of globe due to climate change. Much is known about the effects of drought on grassland plant communities, yet it is difficult to compare responses across different grassland ecosystems because studies impose drought with varying characteristics. Further, few studies have documented plant community recovery, even though the impacts of drought can persist for multiple years. We experimentally imposed four years of extreme, growing season drought at four sites representing the major Central US grassland types (shortgrass steppe, mixed grass prairie, tall grass prairie) spanning a precipitation gradient. Growing season drought was imposed in two ways: 1) by reducing each rainfall event by 66% (chronic) or 2) by completely excluding rainfall until a similar reduction in precipitation as the chronic treatment was achieved (intense). Plant community responses to the two drought treatments were monitored for each year of the four-year drought treatments and four years following the drought to assess recovery. Overall, plant communities at the drier sites responded sooner to drought and took longer to recovery than the wetter sites. Plant composition was altered at all sites, which was largely driven by shifts in the dominant C3-C4 grasses and subsequent species reordering and to a lesser extent by changes in richness in evenness. There was a significant decrease in C4 graminoid abundance in response to drought at all sites with a corresponding increase of C3 annual grasses during the drought at the mixed grass sites but not until the recovery period at the shortgrass steppe. Cheatgrass (Bromus tectorum) invaded the shortgrass steppe during the drought and proliferated during the recovery period, which likely pushed the communities into an alternate state, and inhibited recovery after four years of ambient conditions. The northern mixed grass prairie also did not fully recovery after four years, which indicates that full plant community recovery can extend longer than the drought itself at these drier sites. While there is some indication that intense drought had a greater impact on communities than chronic drought, there is limited evidence to suggest that drought type significantly influenced plant community responses or recovery. These findings indicate that while the shortgrass steppe is water limited with drought adapted species, these xeric grassland plant communities are less resistant and resilient to multiyear drought than those in mesic grasslands.
Open Access
Semi-arid grassland ecosystem functional collapse after effects of five years of extreme drought
(Colorado State University. Libraries, 2023) Lenners, Alicia, author; Smith, Melinda D., advisor; Havrilla, Caroline, committee member; Ocheltree, Troy, committee member
A key outcome of climate change is an increase in the frequency and intensity of drought events in many regions of the globe. The largest impacts on ecosystem structure and function are likely to occur in water-limited ecosystems, such as semi-arid grasslands, potentially leading to a collapse of ecosystem function. While short-term studies have been conducted on various grassland ecosystems, the goal of this study is to fill in the gap of the effect multi-year extreme droughts have on the semi-arid shortgrass steppe of the Central US by characterizing the change in structure and function of these ecosystems. The drought was conducted between 2018-2022, and I had conducted various measurements over the summer of 2022 within the USDA-Central Plains Experimental Range (CPER) of Northeastern Colorado. The experimental drought was imposed using four rainfall exclusion shelters, two of which blocked 66% of precipitation from entering, and the other two remaining uncovered (control plots). Ten plots in each of the four shelters were measured weekly for soil moisture (%), soil temperature (°C) and soil respiration (CO2 efflux); twice per season for soil nutrient availability; and at the end of the growing season for aboveground (ANPP; stems and leaves) and belowground net primary production (BNPP; roots). The extreme drought resulted in an ~40% reduction in growing season soil moisture and an average 2°C increase in soil surface temperatures. Within the 13 weeks of study, drought led to an ~50% reduction in soil respiration (CO2 efflux). ANPP was drastically reduced (~99%) with extreme drought, while cactus surface area increased 3-fold. The extreme drought treatment also resulted in large reductions in BNPP measured from 0-30 cm (79%); however, root growth was reduced most in the shallowest soil depth (0-10cm) when compared to control plots. Lastly, there was an increase in nitrogen availability (both NH4+ and NO3-) with extreme drought by the end of the growing season. These results suggest that extreme, multi-year drought can cause an almost complete collapse in ANPP and significantly reduce BNPP particularly in the top 10 cm of the soil profile, which could have important implications for carbon sequestration. It remains unknown what impact the dramatic reduction in ecosystem productivity but accumulation of available nitrogen in the soil will have for recovery of the shortgrass steppe ecosystem post-drought, but it is likely that recovery will be prolonged despite the increase in soil resources.