Long-Term Vegetation Transects (2012)

Long-Term Vegetation Transects (2012)


Background: Abiotic and biotic conditions across the entire region have been characterized by rapid change over the past decade. These changes include shifts in land cover, land use, and climate. Several large wildland fires have removed sagebrush from a large portion of the Upper Snake River Plain over the past ten to twenty years. INL Site specifically, nearly 60,000 ha have burned in the past five years alone. Soil disturbance associated with fighting wildland fires and soil disturbance associated with general increases in the use of remote backcountry areas are notable at the INL Site and throughout the Intermountain West. Finally, some of the hottest and driest years during the 60-year weather record occurred during the past decade. All of these factors contribute to increasing stress on native plant communities and potentially set the stage for a period of dramatic change in vegetation composition across the region.

The Long-Term Vegetation (LTV) Transects and associated permanent vegetation plots were established on what is now the INL Site in 1950. Vegetation abundance data, including density and cover, have been collected periodically once every two to ten years from plots located along two macro-transects, which are perpendicular to one another and intersect near the center of the INL Site (Figure 10-1). In 2011, 89 plots were sampled, which represents the twelfth LTV sample period. Eleven plots were sampled again in 2012 because they burned just a few weeks subsequent to sampling in 2011. Fourteen ancillary plots were also established to better address mechanisms affecting trends in sagebrush cover. Results from those analyses are included with the current LTV effort as well. LTV data are generally used to monitor vegetation condition and change in sagebrush steppe communities across the INL Site, while specific uses range from support for NEPA to conservation management planning. They are also uniquely suited to characterize native vegetation response to climate, land cover, and land use change.


GSS-ESER-163 The Idaho National Laboratory Site Long-term Vegetation Transects: Understanding Change in Sagebrush Steppe - Amy D. Forman,Jackie R. Hafla,Roger D. Blew - January 2013

The current LTV technical report, which integrates data from the most recent sampling effort, is organized into four chapters, each with specific objectives. Chapter 1 provides a brief overview of the LTV project and summarizes changes since the previous LTV sample period. Recent land cover change from wildland fire and changes in the amount and timing of precipitation are discussed, along with the potential for those factors to affect native vegetation. A summary of vegetation monitoring and research outside the scope of the LTV, but which inform our interpretation of the LTV data, were included in Chapter 1 as well.

The second chapter is an update of long-term trend analyses for major vegetation functional groups across the INL Site. It includes information about native shrub, grass, and perennial forb abundance, as well as distribution and abundance patterns of non-native species. The results of analyses presented in this chapter provide an indication of general vegetation condition on the INL Site.

The T-17 fire burned 11 LTV plots just a few weeks after data collection had been completed in 2011, providing a unique opportunity to monitor post-fire recovery on well-characterized sites. We resampled the 11 burned plots in 2012, and Chapter 3 compares vegetation abundance and composition immediately pre- and post-fire. Eventually, these data will be used to develop a better understanding of how pre-fire condition affects post-fire recovery and to help identify indicators of potential post-fire recovery issues in the first few years after a wildland fire.

Chapter 4 includes INL-specific data and discussion about big sagebrush (Artemisia tridentata) population biology and its effects on sagebrush steppe plant communities. In the late 1990s and early 2000s it became evident in the LTV data that big sagebrush was undergoing a dramatic and prolonged period of decline. The nature of big sagebrush decline on the INL Site coupled with increasing conservation pressures for sagebrush-obligate species, made obvious the need for an investigation into big sagebrush ecology. Fourteen plots were established in the center of the INL Site (Figure 10-1) for the purposes of characterizing big sagebrush population dynamics. Results of the big sagebrush population study, as well as the implications of these results on conservation measures and land management strategies at the INL Site are also included in this chapter.

LTV sites
Figure 10-1. Map of the INL Site with plot locations for the LTV permanent plots and the ancillary Sagebrush Demography study plots.

Accomplishments through 2012:

During the 2012 growing season, a full suite of abundance and distribution data were collected on the 11 plots that burned in the T-17 Fire in late August of 2011. Those data, along with data from the 2011 project database were integrated into the primary LTV database after undergoing final QA/QC and data verification/validation processes. The final, comprehensive project database was archived on the ESER server. Data analyses were finished and a draft of the LTV technical report, as described above, was completed by the end of 2012. The report was finalized shortly thereafter.

Some of the more important vegetation composition patterns resulting from incorporation of the 2011 LTV data into the long-term trend analyses were related to the abundance and distribution of non-native species. Crested wheatgrass (Agropyron cristatum) abundance has continued to increase linearly since about 1990 (Figure 10-2) and is of particular concern because it has invaded the plots were it is found, it continues to increase in the plots it occupies, and where present, it occurs with much greater mean abundance than comparable native, perennial bunchgrass species. Cheatgrass (Bromus tectorum) distribution increased between the 2006 and 2011 sample periods, and cheatgrass abundance, which historically fluctuates, increased significantly over the past five years (Figure 10-3). Introduced annual forbs, primarily desert alyssum (Alyssum desertorum), continued along a trajectory of exponential increase which began in the mid-1990s. It doesn’t appear as though increases in non-native species were at the expense of the native herbaceous understory, but these trends do mark a departure from ranges of historical herbaceous composition.
Figure 10-2. Trends in total perennial grass cover, native perennial grass cover, and introduced perennial grass (crested wheatgrass) cover from 1950 to 2011 for the core subset of plots on the Long-Term Vegetation Transects at the Idaho National Laboratory Site. Data were collected using line-interception methods and are represented here as means ± 1 SE. Numbers in parentheses at the top of the frame indicate the number of plots for which data were available in each sample year.
Figure 10-3. Density and frequency trends for Bromus tectorum on the Long-Term Vegetation Transect permanent plots at the Idaho National Laboratory Site from 1950 to 2011. Data are means ± 1 SE. *Frequency data are missing from the 1995 data archives.

The 2011 LTV surveys provided an opportunity to assess the relationship between pre-and post-fire vegetation condition when the T-17 Fire burned 11 of the LTV plots only a few weeks after they were sampled. Despite the extremely dry conditions during the first growing season following the T-17 Fire, recovery of native perennial grasses, was notable and was consistent with results reported from earlier studies at the INL Site (Table 10-1). Results from this limited data set, indicate a striking post-fire decline in introduced annual species such as cheatgrass. These results suggest a different post-fire response of introduced annuals than may be otherwise expected.

Table 10-1. Mean absolute cover by functional group and one-way repeated measures ANOVAa results comparing pre- and post-fire vegetation on 11 Long-Term Vegetation Transect plots at the Idaho National Laboratory Site. Table10-1

Previous reports from the LTV data have demonstrated a decline in big sagebrush cover that is not associated with loss due to fire. Between 1975 and 2006 average big sagebrush cover on the unburned, core LTV plots declined from more than 20 percent to less than 10 percent. In order to better understand the losses of big sagebrush cover and the declines in stand condition at the INL Site, we completed the sagebrush demography study. Big sagebrush was generally much younger than we had anticipated (Figure 10-4). Based on our results, mechanisms controlling big sagebrush stand replacement appear to be related to a combination of general precipitation patterns and fine-scale microsite conditions. Across the study site, annual recruitment patterns are cyclic and patterns in annual age class size reflect patterns in annual precipitation, but recruitment in some stands appears to be more affected by annual precipitation than in others. Our results suggest that disturbance is not required for stand replacement. In fact, all stands sampled for this project had a mean age of living individuals of less than 25 years and a mean age of individuals at death of less than 50 years, indicating that natural rates of turnover at the INL Site are much higher than expected. Figure10-4
Figure 10-4. Annual age class distributions for 636 big sagebrush plants sampled on the INL Site. Data are pooled from 14 sample locations.

Plans for Continuation: The 2011 sample effort, along with the database updates and technical report are complete. The LTV plots are scheduled to be sampled again in their entirety in 2016.

Publications, Theses, Reports: GSS-ESER-163 The Idaho National Laboratory Site Long-term Vegetation Transects: Understanding Change in Sagebrush Steppe - Amy D. Forman,Jackie R. Hafla,Roger D. Blew - January 2013