Recently published science applies remote-sensing tools to BLM-managed rangelands and provides an unprecedented record of how the vegetation across this enormous area has changed over the past 30 years
The research shows that both annual grass invasion and tree encroachment are ongoing and present clear threats to the health of public rangelands and the wildlife that live there.
Andrew Kleinhesselink, a WLFW-affiliated researcher with the University of Montana, utilized the Rangelands Analysis Platform to conduct an unprecedented assessment of trends in vegetation cover and production for all BLM rangelands (except Alaska) from 1991 – 2020.
Monitoring vegetation changes on the vast rangelands of the American West has long presented substantial challenges for land managers. The expansive geography and a patchwork of private, federal, tribal, and state management combine to make traditional vegetation monitoring complicated, time-consuming, and cost-intensive.
Measuring rangeland vegetation from the field is a laborious process of travelling to carefully selected monitoring plots, identifying the plants within them, and measuring their abundance. To analyze trends or changes in vegetation, researchers must return to the same plots, during the same season, year after year, repeat the collection process, and then compare results to previous data sets.
This has long been the only way for the Bureau of Land Management to monitor the 233 million acres of public and neighboring private rangelands it is responsible for managing in the American West. Currently, the BLM collects these critically important vegetation data on more than 50,000 field plots through its Assessment Inventory and Monitoring Program. This is a critical source of vegetation data but given the scale of these public lands, there are inevitably many areas that are rarely or never monitored.
Scientists have long hoped that satellite imagery, which has been available since the late 1980’s, could eventually be used alongside field data to monitor vegetation on BLM rangelands. Like a code waiting to be decrypted, the challenge has been translating satellite data into vegetation quantity and quality measurements comparable to data collected from the field. In the past several years, new advances in the field of remote sensing have made this dream a reality.
Learn more about this research and the findings from the study’s lead author in this Ask an Expert.
The BLM manages roughly 245 million acres of land in the U.S. These landscapes are remarkably diverse and include southwestern deserts, sagebrush-steppe rangelands, grasslands, higher elevation forests, and riparian zones along rivers, lakes, and creeks. BLM lands are managed for multiple uses including recreation, grazing, timber production, mining and oil and gas development, and habitat for lots of wildlife. It’s a huge amount of land with a lot of different uses.
Land management is divided among ten state-level offices and more than 100 field offices nested within those state offices. Because grazing has been one of the primary uses of BLM lands for well over a century, the agency further divides much of its land into a system of 21,000 “grazing allotments”, though the public land within these allotments is used for more uses than just grazing.
Rangelands are big and diverse, and they change through time. Our goal with this research was to look at how vegetation on BLM-managed land has changed and to present that data in a way that can help the BLM address large-scale threats facing the rangelands it manages.
Productivity and vegetation cover can vary a lot, even over small areas. For example, one BLM field office may have sagebrush-steppe rangeland, mature forests, and riparian areas. In the arid West, rangelands can also vary during the year or from year to year. They can be dusty and dry during drought and then transform during wetter years when they burst with lush grasses and wildflowers.
This dynamism makes it hard to tell what’s happening over time, especially with vegetation. Humans aren’t always able to notice small changes on landscapes that happen over the course of decades, so if trees start to encroach onto rangeland or cheatgrass becomes more abundant, it can be challenging to quantify these changes over large areas.
To track changes over time, we use data from the past that are consistent and accurate. Remote sensing technology provides this type of data because it covers the entire country and it goes all the way back to 1980’s. By analyzing 30 years of data, we focused on the long-term changes in vegetation rather than shorter-term fluctuations that could be caused by weather or other disturbances.
This is why remote sensing is such a breakthrough. It allows us to look at vegetation production and cover across the entire West from 1991 to the present, and it allows us to analyze a specific location, or locations, over that time period.
Our goal for this science, and WLFW’s model of co-produced science in general, is to make it useful and helpful for on-the-ground management. From the beginning, the BLM played a key role in this research and several of the co-authors on this paper are BLM scientists and land managers.
We analyzed a lot of data for the project — 30 years of vegetation change on roughly 233 million acres of diverse landscapes. We would have lost a lot of useful and important data if we had simply averaged out those changes across the entire study area.
On the other hand, remote sensing gives us vegetation and production data all the way down to a 30×30 meters plot of land – about the size of a baseball diamond. But there are more than a billion of these “pixels” covering BLM rangelands, so it would be impractical to have reported trends for each one!
In the end, we summarized vegetation trends at three spatial scales relevant for management: ecoregions, field offices, and grazing allotments. Ecoregions are the largest and each contains a particular climate, vegetation, and ecosystems. For example, one ecoregion covers southwestern deserts while another includes forested mountains. Field offices are the next largest and they provided a good way for us to organize the data at a regional level nested within the ecoregions. Finally, the BLM field offices often divide much of their land into grazing allotments, so we used allotments as our smallest unit.
This organization allowed us to highlight changes at broad ecosystem-level scales, at smaller field office scales, and for each of the 21,000 grazing allotments we analyzed. Additionally, these units are well understood by the BLM, and therefore helpful in informing on-the-ground management.
AIM started about 10 years ago with a goal of standardizing data collection on BLM lands. Standardizing data collection has been an enormous step forward for understanding the status of our public rangelands. However, even with 50,000 AIM plots we are only getting data from a fraction of the total land area managed by the BLM. Moreover, it would be enormously expensive to visit each of these plots year after year to determine how vegetation is changing annually. Remote sensing provides less detail about the vegetation than the AIM plots, but it covers a lot more land and it collects data much more frequently.
Nevertheless, the remote sensing data owes much of its value to the field data collected by AIM and by the NRCS’ Natural Resources Inventory program which collects similar data on 80,000 plots that are on private lands. This huge database of field data was required to “train” the Rangeland Analysis Platform, which is the remote sensing product we used in our research. In short, we use the field data to ensure that the remote sensing data is providing accurate estimates of vegetation production and cover data.
Further, we can compare our findings with AIM-collected data to get a more detailed understanding of what’s happening on the ground. For example, our research showed an increase in annual herbaceous plants in the Great Basin. RAP doesn’t know if that annual herbaceous plant is an invasive grass like cheatgrass or a native plant. The AIM data, however, does provide that level of detail. So, we can use the AIM data to get a much clearer idea of which species are driving the vegetation changes we’ve observed.
This research is meant to be used alongside AIM data for informing local management decisions about rangeland health.
Our analysis provides the first comprehensive look at vegetation trends on BLM-managed lands in the West, and our key findings reflect that broad geographic scope.
We found an increase in annual herbaceous cover in every ecoregion across the study area. We also found a decrease in perennial cover in most ecoregions. So, annuals are going up and bare ground and perennials are going down. Cover and production of annual plants now exceeds that of perennials on nearly 52 million acres of BLM rangeland. This is a big shift in the functional ecology of these public lands. Perennial plants, with deep roots, and more consistent vegetation cover are the foundation of healthy rangelands; they provide forage, store carbon, capture runoff and stabilize soil. Patches of bare ground are an important component of rangeland vegetation because they can limit the spread of rangeland wildfires. Annual plants, which dry out in the summer are great fuel for wildfire, and when these plants replace bare ground, it leads to wildfires spreading more rapidly and burning hotter.
We also found tree cover increased in half of the ecoregions we analyzed – forested mountains, the northern Great Plains, and the cold deserts. This is in line with other research that shows tree cover going up over the past 30 years throughout many North American rangelands. Tree cover increased on more than 108 million acres of BLM-managed lands, underscoring the threat woodland expansion poses to western rangelands.
In the northern Great Plains, which include parts of Wyoming, eastern Montana and the Dakotas, tree cover has almost doubled in the past 30 years. That’s a big deal with real impacts for wildlife like sage grouse.
No. We used the BLM grazing allotment boundaries as a way of aggregating our results because they were a convenient, existing organizational structure in use by the BLM, not because we wanted to look at grazing impacts. Even with the advantages of remote sensing, it would be very difficult to tease out the impacts that grazing might be having from the effects of weather, soil, or fire. Understanding the effects of grazing in these systems is an important research goal, but our intent was first and foremost to provide a record of the vegetation changes we’re seeing on these lands.
Put another way, we researched how vegetation changed over time, but did not analyze the “why” of those changes.
From the get-go, this effort was intended to highlight vegetation changes happening on rangelands with the goal of helping to inform the BLM on where and how to address those changes. To that end, this research was done in collaboration with land managers and scientists in the BLM, including lead scientists in the AIM program. We also had an expert on rangeland monitoring from the USDA as part of the team. Coproducing this work was important in developing the analysis and in helping share it with the BLM.
Our research shows that both annual grass invasion and tree encroachment are ongoing and show no clear signs of slowing down. They present real challenges for the wildlife which rely on healthy, functioning rangelands. If the BLM wants to maintain these landscapes as functioning, natural habitats, our research helps pinpoint where it can implement targeted restoration in specific areas.
We’ve been collaborating with the BLM to present this research and to make our findings available to BLM and other range managers across the agency. Our hope is that, when combined with local data and local knowledge, our research will help the BLM address some of the big landscape-scale threats facing western rangelands in the most effective way.
I think our research helps highlight how remote sensing is transforming rangeland science. Remote sensing is still relatively new, and research like this further validates its effectiveness and utility.
Because of the scale of western rangelands, we need remote sensing to figure out where to target and prioritize areas where we’re going to get the most bang for our management buck. Idaho’s Cheatgrass Challenge uses remote sensing-based maps to identify where to work most effectively. And the Sagebrush Conservation Design that uses remote sensing to map intact rangelands also identifies woodland expansion and annual grassland invasions as the top two threats facing western rangelands.
When we know where those intact rangelands are, we can work to defend them from trees or invasive annual grasses. This “Defend the Core” strategy is also the foundation of WLFW’s Frameworks for Conservation Action in both the sagebrush and Great Plains biomes. Remote sensing can also teach us where landscapes have transitioned past the point where conservation can affect the change we would like to see. We can work with communities in those places to protect life and property from the impacts those changes are causing.
While our research focused on BLM-managed lands, these changes are happening on public and private lands with no regard to our human boundaries. Due to the matrix of ownership in the West, managers will need to work across boundaries. With remote sensing, we can see what’s happening consistently across all lands and make informed, cross-boundary conservation plans.
This project adds to a body of work that shows large, landscape-scale threats are degrading our rangelands. Moving forward, the big challenge for rangeland managers is how to protect rangelands from these big threats. We’re going to need new tools and new approaches because of the scale of these challenges. We can also easily track the effectiveness of conservation efforts across land ownership boundaries using remote sensing. I’m excited to use remote sensing to evaluate the effectiveness of large-scale conservation efforts.
Most conservation effectiveness research is done at a relatively small scale, but with remote sensing, we can cast a wider geographic net and learn from the few landscape-scale projects that are out there. I’ll be analyzing the on-the-ground impact of a large-scale conifer removal project in the Great Basin. That research will help inform other large conifer removal projects in the West.
I’ve worked in grasslands and rangelands since my undergraduate studies. I grew up in southern Oregon where there are oak savanna grasslands that have been heavily invaded by invasive annual grasses, so I’ve seen the impacts these threats have on the landscape.
Grasslands in North America are precious, and I’ve always been interested in conserving and restoring them.
I’ve always loved being outside. I’m way into iNaturalist, and I like to go out and record what I see and submit it to the iNaturalist app. I’m a birder and love observing natural history.
I’ve got the perfect suggestion! The Politics of Scale: A History of Rangeland Science by Nathan Sayre. It’s all about the various challenges BLM and USFS have faced trying to monitor and manage our public rangelands over the past 150 years. I highly recommend it for anyone interested in this topic.