Variability in Wind-eroded, Post-fire Sagebrush Steppe

Spatial and Temporal Variability in Soil, Vegetation and Aerodynamic Properties in Wind-eroded, Post-fire Sagebrush Steppe


Aeolian processes play a significant role in shaping arid and semi-arid environments. The semi-arid shrublands of southeastern Idaho are particularly prone to wind erosion following wildfire, and the INL has notably high levels of wind erosion. Vegetation has large influences on aeolian transport by providing soil cover and decreasing the erosive force of wind. Soil surfaces resulting from wind erosion influence vegetation recovery, so there are several strong feedbacks that exist in the soil-plant-aerodynamic condition of wind-eroded landscapes.


Heterogeneity in the spatial structure of plants and soils is particularly important for desert ecosystems, and pulses of post-fire wind erosion likely impact this heterogeneity. In deserts, enrichment of resources beneath perennial plants creates a matrix of islands of fertility and bare interspaces that can be exacerbated or reduced by wind erosion. Large amounts of heterogeneity exist in the soils and vegetation of the sagebrush steppe. This project focuses on heterogeneity in the relationship of soil and plants, specifically shrub “islands of fertility” and the relatively bare interspaces between them, in unburned areas and areas that burned and then experience high levels of wind erosion. We are also assessing aerodynamic properties of the vegetation community as it recovers from wildfire, and consequences of soil erosion for nutrient balances of sites.


Objectives
Our goal is to increase understanding of the relationships between vegetation and geomorphic and atmospheric processes. More specifically, we plan to address three main objectives.

  • To determine the relationship between post-fire heterogeneity of the soil surface morphology and vegetation, in replicate areas that are unburned or that have been burned and wind-eroded in the last several years.
  • To determine if there is temporal variability in the aerodynamic parameters friction velocity and roughness length at multiple scales and identify how it relates to vegetation recovery after fire.
  • To determine the nutrient exchanges occurring with wind erosion on the INL, in burned and unburned areas. Specifically, to determine horizontal sediment transport and associated nutrient redistribution occurring in the saltation zone in a sagebrush steppe ecosystem exhibiting an episodic period of aeolian transport following wildfire. We also examined how temporal trends in nutrient fluxes were affected by changes in particle sizes of eroded mass as well as nutrient concentrations associated with different particle size classes.


Accomplishments through 2009
Field studies were conducted during summer 2008 and 2009 at the sites of three wildfires on the INL Site. We characterized the soil surface morphological heterogeneity and the vegetation corresponding to the soil morphologies in the post-erosion environment. In addition, we collected data on temporal changes in aerodynamics (friction velocity and roughness length) and corresponding vegetation at the burn site. We analyzed sediment captured from air over an 18-month period for carbon, nitrogen, and particle sized. We also worked with University of Idaho colleagues to use ground-based LiDAR to evaluate surface microtopography in our sites.


Results
For Objective 1, we investigated the correspondence between physical, chemical, hydrological, and vegetation properties of shrub-island (coppice) and interspace soil surfaces (or microtopographies), in sites that were unburned or burned and wind-eroded (n = 3), in the Snake River Plain of Idaho. Soil chemical properties (total carbon, total nitrogen, organic carbon, carbonates, electrical conductivity, pH) were determined for samples collected from 0 to1 cm depth in the field in late summer. Soil physical and hydrological properties (mechanical strength, volumetric water content, water infiltration rate) were measured on each soil surface in the field in late summer. Plant abundance and diversity on each soil surface was recorded repeatedly throughout summer.


Coppices had (1) greater total carbon and nitrogen, electrical conductivity, organic carbon, and infiltration; and (2) less mechanical strength and water content than interspaces, and these differences were consistent in both burned and unburned areas. Only carbonates did not differ among the microsites, and only pH differed among the microtopographies in unburned but not burned conditions. Species richness and abundance of herbs were greater on coppices than on interspaces in unburned and burned landscapes. Assemblages of native plant species tended to vary more between coppices and interspaces and with burning than did assemblages of exotic species.


These data suggest that heterogeneity in microtopography is sustained following wildfire in the sagebrush steppe, even after large amounts of aeolian redistribution have occurred in the years following fire. Soil microtopography appears to influence plant diversity in this landscape by creating different micro-communities, and the relatively bare interspaces may increase water reserves for growth on coppices. Post-fire seeding done with the intent of soil stabilization or exotic plant invasions could reduce this important plant-soil heterogeneity.


For objective 3: In the burned site, C and N fluxes were as high as 235 g C m-1 d-1 and 19 g N m-1 d-1 during the first few months following wildfire, whereas C and N fluxes were negligible in the unburned site. The largest particle size class (> 500 µm) had the highest percent C and N, but sediment transport of this size class was small and therefore contributed relatively little to overall nutrient fluxes. Although sediment fluxes were similar for the suspension (< 106 µm) and saltation (106-500 µm) sized particles, percent C and N were considerably lower for the suspension-sized particles. Thus, temporal variation in C and N fluxes appeared to be largely attributable to the redistribution of saltation-sized particles, and to a lesser extent the redistribution of the suspension-sized particles. Aeolian sediment was also enriched in C and N relative to surface soils, suggesting that aeolian transport following a disturbance, such as wildfire, has the potential to impact nutrient redistribution and ecosystem function in a landscape that otherwise experiences little wind erosion.


Plans for Continuation
Further data on both objectives will be collected during spring and summer 2009.


Publications, Theses, and Reports

  • In May 2010 at the 16th Annual Wildland Shrubland Symposium, the INL wind erosion work will be showcased in 3 serial presentations in a session entitled “Wind erosion in warm and cold deserts: causes, consequences, and implications for land management” that M. Germino is convening.
  • We presented the findings at Ecological Society of America Annual Meeting in August 2009, and will again in 2010.
  • These data will comprise Amber Hoover’s M.S. thesis (expected finish is August 2010). We submitted a manuscript on the nutrient balance findings to Ecosystems.