Performing Department
(N/A)
Non Technical Summary
Grassland to shrubland ecosystem transitions with increasing woody plant encroachment (WPE) alter key ecosystem services in rangelands, notably production of herbaceous plants used for livestock forage and soil retention. Over 70% of global rangelands exist in arid and semi-arid drylands, which commonly experience WPE and are vulnerable to degradation with increased temperatures and precipitation variability due to climate change. Despite application of various methods to remove woody plants and restore essential ecosystem services, WPE management frequently yields disappointing and variable results, risking poor return on expensive restoration treatments. This is likely to a critical threshold or "tipping point" in the transition from the grassland to shrubland state in which the characteristics of the ecosystem change so substantially that reversing the transition may never occur, even with active intervention.Understanding where these thresholds lie in the continuum between grassland to shrubland ecosystem state change, and how WPE management could be applied strategically given these thresholds, is critical to effectively restoring rangelands affected by WPE and the ecological benefits they provide into the future. The overarching goal of this project is to quantify cross-scale factors impacting the plant community response following WPE management to improve predictability in dryland restoration outcomes. This will be accomplished by modeling plant community data in response to factors that can drive ecosystem change from the relatively small scale (e.g., increasing soil erosion with loss of grass species) to the large scale (e.g., land management, weather) across a dryland grassland to shrubland gradient in the northern Chihuahuan Desert. Identifying the relative influence of these factors on the plant community following WPE management will refine our understanding of where thresholds lie along the grassland to shrubland transition, and at what point(s) along the transition land owners and managers can apply management to reliably meet restoration goals.
Animal Health Component
0%
Research Effort Categories
Basic
50%
Applied
50%
Developmental
0%
Goals / Objectives
The overarching goal of this project is to quantify cross-scale factors impacting the plant community response following woody plant encroachment (WPE) management to improve predictability in dryland WPE restoration outcomes. This will be accomplished by modeling plant community data in response to drivers of ecosystem change from the patch scale (i.e., between-plant soil erosion with increasing bare ground) to the landscape scale (i.e., land management, weather) across a grassland to shrubland gradient in the northern Chihuahuan Desert.Two objectives will be addressed to reach this goal:Objective 1: Evaluate the plant community response to WPE management and climatic factors across a shrub encroachment gradient. Plant functional group abundances will be modeled in response to herbicide treatment (applied for WPE management), seasonal precipitation and temperature data, and pre-treatment encroachment severity to quantify the importance of these variables in predicting post-treatment outcomes across the encroachment gradient.Objective 2: Test the efficacy of Connectivity Modifiers (ConMods) to aid herbaceous species recovery within shrub interspaces alongside herbicide treatment. Accelerated erosion with shrub encroachment can reduce the likelihood of herbaceous species re-establishment even after WPE is managed, which will be examined by modeling plant functional group abundance in shrub interspaces with and without ConMods in combination with plots and treated and untreated with herbicide.
Project Methods
Experimental Design: The project study site was established on the Jornada Experimental Range (JER), which is situated within the northern extent of the Chihuahuan Desert ecoregion in southern New Mexico, USA (32.5 N, 106.45 W) in a semi-arid to arid climate. Precipitation averages 25 cm per year, with approximately 60% of annual precipitation falling during the monsoon season between July 1 and September 30 in short, intense rainfall events from convective storms.Twenty sets of paired plots (n=40 plots total) were established along a Honey mesquite (Prosopis glandulosa Torr.) encroachment gradient in a Before-After-Control-Impact Paired Series design (BACIPs). Each set of paired plots spans a 384m x 300m area and contains two 5-ha plots (167 x 300m each) separated by a 50m buffer area to allow for potential herbicide drift and spatial separation between treatments. Paired plots were placed in pastures that have experienced similar grazing and management history and oriented to cover relatively congruent plant communities over the span of the paired plot area (i.e., comparable ecological state across the site). Paired plots were also restricted to Sandy and Shallow Sandy ecological sites to control for properties that could impact vegetation outcomes following herbicide treatment. One plot within each pair was treated with an aerial application of Sendero (684 g/ha, active ingredient aminopyralid + clopyralid) and Remedy Ultra (280 g/ha active ingredient triclopyr) mixed with a surfactant (Aero Dyne-Amic, 210 g/ha) and drift control agent (Justified, 105 g/ha) in late August and early September of 2021 following recommendations for P. glandulosa control in New Mexico and western Texas.Herbicide treatment was randomly assigned within each plot pair, with the other plot serving as an untreated comparison.Objective 1: Three permanent, 50m transects were established within each plot in 2020 for data collection. Transects azimuths were randomized to avoid placing transects along any unknown, pre-existing gradients in vegetation or soil characteristics across the plots. Line-point intercept (LPI), plant height, canopy gap, and species richness data were collected on plot transects annually, beginning the year prior to herbicide treatment (2020), immediately following herbicide application (2021) and one-year post-treatment (2022) following the Monitoring Manual for Grassland, Shrubland, and Savanna Ecosystems.Data were collected over ten weeks beginning in late August or early September through early November each year in line with expected peak biomass production following monsoonal rainfall.To model plant community response to weather data in addition to P. glandulosa management, precipitation and temperature data will be extracted from five weather stations within 5km of the study plots to generate weather variables based on their expected importance in plant community dynamics for each year of transect vegetation data collection (2020 - 2022): temperature seasonality (standard deviation of max temperature*100), winter precipitation (total precipitation Oct to Mar), and summer precipitation (total precipitation between Apr to Sept), which includes monsoonal precipitation). Plant functional group abundances will be modeled in response to herbicide application, weather variables, and initial P. glandulosa encroachment severity using Structural Equation Modeling (SEM) to quantify direct and indirect relationships between predictors and plant community response variables using the piecewiseSEM package in the programming software R.Objective 2: Connectivity Modifier (ConMod) arrays were installed within P. glandulosa interspaces on a subset of paired plots (n=16; 8 herbicide-treated and untreated paired plots each) in the early spring of 2022. Paired plots were selected to represent the "middle" of the P. glandulosa encroachment gradient in which P. glandulosa is a significant component of the plant community (~15% cover) but 5-30% perennial grass cover remains.Each ConMod consisted of two 50 cm long by 20 cm high panels of galvanized hardware cloth fit together at the midpoint to create an "+" shape and wired to a rebar stake in the center. ConMods were installed in a 12-unit arrays approximately 1.5m apart in the center of P. glandulosa shrub interspaces, with distances slightly staggered to reduce open pathways through the array against the strongest spring wind events. Interspaces that did not receive ConMods (control interspaces) were arranged in the same manner with only the rebar stakes installed to serve as sampling reference points. Eight interspaces within each plot were randomly assigned to either the ConMod or control treatment (n=4 per treatment per plot). Accumulated soil deposition (soil depth), plant litter, and species-level abundance data (litter and plant cover estimated using a 5% cover frame) were collected within the footprint of six randomly selected ConMods or rebar stakes (0.3x03m) in each study interspace in the fall of 2023.Functional group abundance of plant species recruited in interspaces with and without ConMods will be modeled in response to herbicide application, plant litter and soil accumulation, and initial P. glandulosa encroachment severity using Structural Equation Modeling (SEM) to quantify direct and indirect relationships between predictors and plant community response variables using the piecewiseSEM package in the programming software R.Efforts: The results of this project will disseminated through several professional conference presentations and seminar presentations to the New Mexico State University (NMSU) academic community and the local chapter of the Native Plant Society of New Mexico. Two peer-reviewed journal articles will also be published along with a newsletter article to reach a broader non-academic audience in print, such as the SRM Good Grazing Makes Cent$ newsletter. Any appropriate management recommendations resulting from the this project will be contributed to management-accessible information tool(s), such as a NMSU Extension publication or applicable State-and-Transition model(s). More informal modes of sharing research findings, including sharing project updates through a professional website and social media outlets such as Twitter/X, will help extend the reach of the project to diverse audiences.Evaluation: A detailed Gantt chart has been developed by the PD alongside their primary mentor and co-mentor to track progress toward key project milestones. The PD will meet with both their primary mentor and co-mentor weekly to discuss progress towards these milestones, troubleshoot issues as they arise, and receive support on data analysis, writing tasks, and co-authored publications. The PD will also meet with their dissertation committee at least twice a year to assess progress towards the overarching project goal and preparedness for the final doctoral exam and dissertation defense. Beyond the individual progress of the PD, the success of the project will be evaluated by the production of the deliverables outlined in the "Efforts" section above.