Lesley R. Morris

Testing soil phytolith analysis as a tool to understand vegetation change in the sagebrush steppe and pinyon-juniper woodlands of the Great Basin Desert, USA

Better biological proxy methods are needed to understand changes in arid ecosystems over the recent past. Our objective in this study was to examine the utility of soil phytolith analysis to reflect vegetation changes over the past 200 years, from the end of the ‘Little Ice Age’ to present. The sensitivity of phytoliths to record vegetation changes for this time period was tested by sampling in locations where vegetation changes were known to have occurred based on human records. We used extraction weights to test for the trend in reduction of grasses and increase in woody vegetation over time. We used the relative abundance of soil phytolith morphotypes from native versus introduced grasses to look for the increase in non-native grasses. Then, we examined the phytolith assemblages from soils in a continuous core sampling method with 1 cm increments. Extraction weights decreased by half from the surface to ‘Little Ice Age’ layers and may reflect increasing pinyon-juniper woodland cover. The relative abundance of common introduced grass phytoliths increased by nearly half between the older segments and the surface, which could be a reflection of growing dominance of invasive grasses. The phytolith assemblages varied over time and with depth in continuous core sampling. The patterns of change in both detailed cores appeared to reflect known vegetation changes since the end of the ‘Little Ice Age’. Soil phytolith analysis has the potential to provide much needed biological proxy data for this time period in the Great Basin Desert.

Secondary succession in the sagebrush semidesert 66 years after fire in the Great Basin, USA.

ABSTRACT: The invasive annual grass Bromus tectorum (cheatgrass) creates multiple challenges as it spreads across the Great Basin, fueling repeated wildfires and dominating large expanses of land that were once sagebrush shrublands. The replacement of shrublands by annual grasslands has been widespread and much research has focused upon loss of wildlife habitat, altered fire regimes, and degraded ecosystem function. Monitoring of short-term plant community reassembly occurs in these systems, but considerably less is known about the long-term succession of native plant communities after fire. Using repeated measures in time over a 66-year period, we examined the species composition of two shrubland sites in the Great Basin. The sites burned completely in 1947 and density data on herbaceous species were reported one year and 41 years after the fire. At both sampling intervals, B. tectorum and other annual invasive species, dominated the sites. Our resampling 25 years later found B. tectorum no longer maintained dominance on the north-facing site and native grasses were common. The south-facing site still contained a high density of B. tectorum, but it was four times less abundant than in previous years. Our results are consistent with the few studies using historical data that show, in some instances, desert shrublands can transition out of an annual dominated state into a native perennial state over decadal time scales without intervention. This study highlights the importance of repeated long-term studies for improving development of restoration plans and state-and-transition models, as community trajectories may not be apparent for more than five decades following disturbance.

Can soil phytolith analysis and charcoal be used as indicators of historic fire in the pinyon-juniper and sagebrush steppe ecosystem types of the Great Basin Desert, USA?

Wildland fire intensity and area are increasing across the Intermountain West, USA, in a variety of ecosystem types including the pinyon-juniper woodlands and sagebrush steppe of the Great Basin Desert. Unfortunately, we do not know if there were historic analogues for these high-intensity stand-replacing fires because of the lack of fire scars that record evidence of them. Soil-charcoal and phytolith analyses have been successfully employed in other regions to garner information about fire regimes through the Holocene. We studied the utility of these methodologies and related taphonomic issues in soils in the Great Basin Desert. Our results showed that both microscopic charcoal and burned phytoliths can be found in soils with radiocarbon ages from modern to late Holocene. Microscopic charcoal abundance was more useful than size class as an indicator of recent local fire. Its abundance declined rapidly and remained low at 2.8 to 4.8 km from the edge of the fire. A Burned Phytolith Index shared a similar pattern starting at 0.4 km from the fire edge. Both proxies declined in abundance with depth in the soil at recent burn sites while remaining constant in unburned sites. We were unable to detect a signal for a known historic fire, however, this may have been a result of sample depth. Our results indicate that soil-charcoal and phytolith analysis can be used to examine questions about historical fires in these two ecosystems of the Great Basin Desert.

Assessing the Past: The Importance of Cultivation History in EBIPM Success

The introduction and spread of invasive plants has been consistently linked to the development of agricultural crops such as cereal grains, legumes, and forage grasses. This is because cultivating a crop involves both soil disturbance (e.g., plowing and har- rowing) and the introduction of plant species, both the in- tended crop and unintended contaminants that accompany crop seed.1 Cultivation is also consistently linked with the creation of land-use legacies, which continue to impact eco- system structure and function for long periods of time follow- ing human utilization of resources. These land-use legacies can alter soils, hydrology, and plant communities in ways that persist for decades, centuries, and even millennia.2,3 A com- mon legacy of cultivation is invasive plant species dominance, causing the site to remain “stuck” in a state of arrested succes- sion with early-seral weedy species dominating for decades. DOI: 10.2458/azu_rangelands_v34i6_morris