Recipient Organization
FOREST SERVICE
201 14TH ST SW YATES BLDG
WASHINGTON,DC 20002-6405
Performing Department
(N/A)
Non Technical Summary
The increasingly common occurrence of flash droughts in grasslands of the Northern Great Plains ("northern grasslands") challenges ranchers and other land managers who constantly decide when and where their livestock will graze. To sustain long-term forage production, rangeland managers are cautioned not to resume grazing on drought-stressed areas too quickly after the end of a drought. Translating this general guidance into concrete decision points for a specific grassland requires (1) understanding the processes behind dominant plant species' and plant communities' recovery from drought, (2) information on how grazing after a drought impacts these processes, and (3) a practical, robust indicator of grassland recovery following drought. We will address these needs for northern grasslands through a combination of field and greenhouse experiments that measure plant sensitivity to, and recovery from, drought occurring in different parts of the growing season and under different grazing conditions. We aim to provide land managers information that will help them make decisions that result in long-term sustainability of their operations and the natural resources on which they rely. Such decisions are made more challenging by growing climate variability and the flash droughts this produces. Our research will provide mechanistic, species-specific information needed to predict how northern mixed-grass prairie will respond to new climate circumstances, and therefore, how rangeland management practices may shift to adapt to these circumstances to maintain ecosystem health and productivity.
Animal Health Component
30%
Research Effort Categories
Basic
70%
Applied
30%
Developmental
0%
Goals / Objectives
Overarching goal: Grassland managers are cautioned to not resume grazing on drought-stressed areas too quickly after the end of a drought in order to allow time for plants to recover. Our goal is to provide information on how the recovery of the individual plant (physiology) and plant populations (demography) link to overall community recovery (productivity) following seasonal flash drought in the northern Great Plains. This will assist in determining when a grassland is recovered enough to resume grazing after drought and how to determine when a grassland has reached that point of recovery.Objective #1: Experimentally test how two current grazing practices during and after flash drought influence forage production recovery in northern grasslandObjective #2: Investigate the interactive effects of drought timing and grazing on the links between plant physiological and population processes and community-level productivity during and after flash drought.Objective #3: Propose a practical, robust indicator of northern grassland recovery to inform managers' post-drought decision making.
Project Methods
The field experiment will be conducted at two sites in southwestern South Dakota, on Buffalo Gap National Grassland (BGNG) representing the two dominant soil types (Clayey and Loamy). The sites differ in dominant plant species, with the C3, mid-height, rhizomatous grass western wheatgrass at the clayey site and the C4, short, sod-forming grass blue grama at the loamy site. Each site was fenced to exclude cattle in 2022 and will remain so for the duration of the experiment. Experimental plots will be 4 m x 3 m in size and separated by a minimum of 4 m. Drought-grazing combination treatments will include grazing management responsive to early or late drought, grazing management not responsive to early or late drought, and their appropriate controls. Early-season drought will be conducted in two different years; late-season in one year. Drought-grazing combination treatments will be assigned to plots in a randomized block design within each site. Drought treatments will be achieved by reducing ambient precipitation by 50% in either the first or second half of the growing season using a widely used rainout shelter design. Grazing treatments will be achieved by mowing to a specified stubble height followed by timed, controlled trampling by horses. Drought-responsive grazing will target a stubble height representing moderate grazing (residual standing herbage of 60% of mean annual peak standing herbage), whereas grazing unresponsive to drought will be represented as a stubble height of 3 cm.In the field experiment, we will measure plant population/demography and community properties every year of the experiment. These include aerial cover, plant density, and stem density by species before implementing any grazing treatment. Physiological and additional demographic measurements will be conducted in the year of treatment and recovery on the three dominant species at each site. Stretched culm height, stretched length of the longest leaf, leaf-stage development, light-adapted leaf chlorophyll fluorescence, and stomatal conductance will be measured on the same 3 tillers per plot once per month in May-September. In March, May, early July, September, and early November of the year of drought treatment and the year of recovery, we will take one core centered on a randomly selected individual of each of the three dominant species in each plot. Each core will used to count current-year bud and rhizome production per tiller and tillers produced from previous-year buds. In early July and early November, an extra core for each species will be taken for non-structural carbohydrate measurements.In all greenhouse experiments (GEs), treatments (including controls) will be applied to four perennial C3 grasses, four perennial C4 grasses, and a common invasive biennial forb (Melilotus officinalis). For each experiment, 8-10 reps will be used of each species.In GE #1, each species' drought sensitivity will be assessed for each month of a typical growing season (April-September). At the beginning of each month, water will be withheld from 8 randomly selected pots of each species and the top of each pot will be covered with parafilm to prevent evaporation from the soil. As the soil dries over time ('dry-down period'), we will measure photosynthetic rate, stomatal conductance, and chlorophyll fluorescence at ~0.2-0.3 MPa Ysoil intervals. We will also measure leaf water potential (Yleaf) twice during the dry-down period on each plant, with at least 2 days between these measurements. On the day all leaves in a plant species have wilted, we will make all measurements previously described and then water all plants of that species and maintain them at -0.1 to -0.3 MPa Ysoil. At 2, 4, and 8 weeks after we initiate rewatering we will measure photosynthesis, stomatal conductance, and chlorophyll fluorescence of the plants to assess recovery. Finally, after the 8-week recovery period following the final dry-down experiment (September), each whole plant (from all dry-down periods) will be harvested to assess root, tiller, and leaf biomass and non-structural carbohydrate storage, as well as bud production.In GE #2, we will assess the impact of two 'grazing' intensities on drought sensitivity and recovery of the same species in GE #1. The two intensities will be 40% and 90% biomass removal, applied at the beginning of a drought occurring at two different time periods. We will implement the droughts during the most and least sensitive time periods for each species. We will make the same leaf-level physiological measurements and monitor water use of the entire plant immediately following the drought treatment and 2, 4, and 8 weeks after rewatering begins in order to assess plant recovery. We will continue to water the plants through September and then harvest all individuals to quantify biomass, bud production, and non-structural carbohydrate storage.Greenhouse experiment #3 will start similar to GE #2. Grazing will occur at the beginning of a dry-down. After watering resumes we will simulate grazing at 3 different stages of recovery: 'pre-recovery, 'current management', and 'full recovery'. We will use the physiological recovery data from GE#2 to select the time periods following drought to initiate grazing. For 'pre-recovery', we will select a time period when physiology has not yet recovered to maximum post-drought levels. Simulated grazing in the 'current management' treatment will occur when plants have reached their late June height based on field experiment data, and simulated grazing in the 'full recovery' treatment will occur when physiological rates have recovered to their maximum level based on GE#1 and #2. Controls in this experiment will experience dry-down and the first grazing treatment, but not the second grazing treatment. Prior to the second grazing treatment, chlorophyll fluorescence and stomatal conductance of plants will be measured. After the second grazing treatment, plants will continue to be watered and, once new leaves are fully formed, the full suite of physiological measurements will be made on individuals. Finally, all plants will be harvested at the end of the growing season to assess biomass, bud production, and non-structural carbohydrate stores.We define drought sensitivity as the difference in physiological, population, and community-level metrics between drought treatments and their respective controls during the drought period. We define drought recovery as the degree to which metrics regain pre-treatment or untreated condition after experimental drought conditions are removed. Generalized linear mixed models with a repeated-measures design and planned contrasts specific to Objective 3 will assess sensitivity and recovery for most metrics. To assess sensitivity and recovery of community structure in the field, we will use non-metric multidimensional scaling analyses of species or functional group aerial cover, plant/tiller density, and biomass. We will select a subset of the measured metrics to include in structural equation models representing pathways of response and recovery from the physiological level to ANPP. These models will directly address Objectives 1 and 2 and will inform selection of 1-3 easily measured metrics (such as leaf-development stage, plant height or leaf length) to develop into the recovery metric of Objective 3.