Vegetation Measurement and Monitoring

5.3: Ecological Sites

Video Presentation

Watch this video presentation for an overview and discussion of rangeland classification and assessment.

To add captions to this video click the CC icon on the bottom right side of the YouTube panel and select English: Corrected captions.


Learning Guide

Land Classification

One of the primary reasons for gathering information about rangeland vegetation is to assess the current condition or ecological status of the land. We use this information to determine whether our management objectives for the site are being met, whether the land is as productive as it could be, and whether fundamental ecological processes are functioning properly. Assessment of a site’s condition or status is always done relative to what the site is capable of producing, or what can be expected from the site.

The ability of a site to function at a certain level is related primarily to abiotic characteristics such as soils, topography, and climate. We cannot assess the status of a site without first identifying the type of land that is being considered, and knowing what that type of land is capable of producing. Therefore, we categorize lands based on a suite of characteristics, and use a land classification system to organize these lands into meaningful hierarchies. These systems of classification provide a framework for determining the status of natural resources on rangelands.

Land classification and assessment allows appropriate and relevant decisions to be made at local, state, regional, and national scales. The criteria used to classify land generally depend on the intended use of the classification. For example, someone interested in real estate might classify land based on monetary values. Land classification systems used by natural resource managers focus mainly on land use, and abiotic and biotic characteristics.


Land Classification Systems

There are two main systems used by federal land management agencies to classify land: The NRCS Land Resource System, and the USFS Terrestrial Ecological Unit Inventory (TEUI). Although there are subtle differences between them, both systems share common elements in their approaches to classifying land. However, the TEUI is almost exclusively applied to forest ecosystems, where the NRCS system is applied to a large diversity of ecosystems, including rangelands. For this discussion of land classification systems, we will focus exclusively on the NRCS Land Resource System, and refer interested readers to the USFS TEUI Technical Guide for a detailed presentation of that system.


NRCS Land Resource System

The Land Resource System developed by the United States Department of Agriculture (USDA)-NRCS is a planning tool and framework for organizing resource information at multiple spatial scales. It is broadly applied to rangeland systems, as well as forest and agricultural cropping systems.

Information used to develop this system is derived from the:

  • National Soil Survey Information System (NASIS)
  • Ecological Site Information System (ESIS)

The emphasis of the NRCS Land Resource System is on land as a resource for uses. The dominant characteristics of the system and the basis for its classification include:

  • Land use (type [crop, range, urban] and ownership [public v. private])
  • Elevation and topography
  • Climate (precipitation and temperature)
  • Water (ground water, surface stream flow, etc)
  • Soils (dominant types)
  • Potential natural vegetation
  • Disturbance regime (fire, herbivory, drought, flooding, erosion)

 

NRCS Land Resource System Levels of Organization

There are five major levels of the Land Resource System (Figure 1):

  • Level 5:  Land Resource Regions
  • Level 4: Major Land Resource Areas
  • Level 3: Land Resource Units (formerly Subresource Areas)
  • Level 2: Soil Interpretations (includes Ecological Sites)
  • Level 1: Soil Map Units (includes Soil Series)

Figure 1. A hierarchy of land classification levels.

These levels of classification provide a framework of information that is relevant to farming, ranching, conservation, forestry, engineering, recreation, and various other land uses.  Each level addresses a different degree of generalization and scale, where level 5 is the largest scale and level 1 is the smallest:

Level 5: Land Resource Regions (LRRs) are geographical regions similar in climate and productivity with associated Major Land Resource Areas (MLRAs). In the United States, including Alaska, Hawaii, the Caribbean, and the Pacific Basin, there are 28 LRRs (Figure 2). Each LRR approximates broad agricultural markets.

Figure 2. The Natural Resource Conservation Service (NRCS) map showing the distribution of Land Resource Regions (LRRs) in the coterminous United States.

Level 4: Major Land Resource Areas (MLRAs) are geographically associated land resource units (LRUs) that are similar in climate, geology, soils and biology. There are 278 MLRAs in all. Each MLRA is associated with only one LRR, and MLRA’s can extend across state boundaries (Figure 3). The USDA Handbook 296 provides detailed descriptions of each MLRA, including the following elements: the extent of each MLRA in the states where it occurs, total area, major cities, highways and culturally significant lands, physiography, geology, climate, water, soils, biological resources, and land use. These classifications are used extensively to inform state, interstate, regional and national planning.

Figure 3. The Natural Resource Conservation Service (NRCS) map showing the distribution of Major Land Resource Areas (MLRAs) in the coterminous United States.

Level 3: Land Resource Units (LRUs) are conceptual subdivisions of MLRAs based on climate, water resources and land use. These units were historically named subresource areas, and are synonymous with LRUs. At the present time, Level 3 designations are under revision by the NRCS, and maps showing Level 3 delineations are not available. For this reason, it is especially important to use Level 1 Soil Map Unit information to confirm the ecological site before assessing resources on a specific site.

Level 2: Soil Interpretations are the first aggregation of soils into land units that respond similarly to management and disturbance. They include ecological sites, forage suitability groups, wildlife habitat suitability groups, cropland suitability groups, forest suitability groups, and recreation suitability groups. In rangeland management, the ecological site is the primary level of classification at which resources are assessed. The second lesson in this module focuses on ecological sites in the context of rangeland ecology and resource assessment.

Level 1: Soil Map Units are the foundation for the Land Resource System hierarchy and the fundamental basis of all classification levels. Guidelines for soil series and map unit classifications can be found on the NRCS Web Soil Survey webpage.

 

Using the Land Resource System

How you use the land resource system depends on your purpose for seeking information. If you need to assess vegetation resources at a specific location, you will need to identify the ecological site(s) at that location. At the other extreme, if you are interested in gathering information at a regional scale, the USDA Handbook 296 is a valuable resource.

Let’s consider an example using a rangeland site on Otero Mesa in southern New Mexico. By examining the maps in USDA Handbook 296, we determine that Otero Mesa is located MLRA 42 in the Western Range and Irrigated Region, Land Resource Region “D” (Figure 4).

Figure 4. A grassland site located on Otero Mesa (approximately where the number “42” is located on the map) in MLRA 42: Southern Desertic Basins, Plains and Mountains.

Determining the ecological sites on the area of interest (AOI) is a multi-step process:

  1. Using the USDA Web Soil Survey (WSS), delineate an AOI polygon over the site.
  2. Access the soil map for the AOI in the WSS (Figure 5).
  3. Open the map unit description for each map unit.
  4. Record the name and Site ID number for each ecological site listed in the description.
  5. Access the list of approved Ecological Site Descriptions (ESDs) for the designated MLRA (in this case, MLRA 42) from the Ecological Site Information System (ESIS).
  6. Review the list of ESDs for MLRA 42, and open or download the ESDs associated with the soil map units.

Figure 5. An AOI polygon (blue lines) with coordinating soil map units (red lines) on the USDA Web Soil Survey webpage.

Note: Soil maps are developed at various scales, and often soils are mapped as soil associations or soil complexes. Therefore, you cannot rely on identification of an ecological site solely from web-based information. The identification of each ecological site in the study area MUST BE CONFIRMED DURING A VISIT TO THE SITE. This requires examining the soils (dig a small soil pit!) and physiographic features such as position in the landscape and slope, in order to ensure that the ecological site has been properly identified.

Once the ecological site has been determined, vegetation, soil, and other data collected from the site can be used to assess the status of the site.


Self-Check Activity

The following questions are designed to test your knowledge and understanding of concepts and principles of rangeland classification and assessment.

 

1. Match the following.
Major Land Resource Areas
Geographical regions that approximate broad agricultural market regions.

Unselect

Land units that respond similarly to management and disturbances in managed rangelands.

Unselect

Basis for Land Resource System hierarchy and the foundation of all classification levels.

Unselect

Geographically associated areas similar in climate, geology, soils, and biology.

Unselect

Ecological Sites
Geographical regions that approximate broad agricultural market regions.

Unselect

Land units that respond similarly to management and disturbances in managed rangelands.

Unselect

Basis for Land Resource System hierarchy and the foundation of all classification levels.

Unselect

Geographically associated areas similar in climate, geology, soils, and biology.

Unselect

Soil Map Units
Geographical regions that approximate broad agricultural market regions.

Unselect

Land units that respond similarly to management and disturbances in managed rangelands.

Unselect

Basis for Land Resource System hierarchy and the foundation of all classification levels.

Unselect

Geographically associated areas similar in climate, geology, soils, and biology.

Unselect

Land Resource Region
Geographical regions that approximate broad agricultural market regions.

Unselect

Land units that respond similarly to management and disturbances in managed rangelands.

Unselect

Basis for Land Resource System hierarchy and the foundation of all classification levels.

Unselect

Geographically associated areas similar in climate, geology, soils, and biology.

Unselect

2. We categorize lands based on a suite of characteristics, and use a ____________ to organize these lands. Select the appropriate statement to fill in the blank.

3. There are many ways that land may be classified. Which of the following are factors used in classifying land in the NRCS Land Resource System? Select all correct answers.

4. What systems are used to develop the NRCS Land Resource System? Select all that are correct.

5. The emphasis of the NRCS Land Resource System in on land as a resource for ______. Select the appropriate response to fill in the blank.


 


Ecological Sites

Video Presentation

Watch this video presentation for an overview and discussion of ecological sites.

To add captions to the video, click the CC icon on the bottom right side of the YouTube panel and select English: Corrected captions.


Learning Guide

Introduction to Ecological Sites

The official NRCS definition of an ecological site is “A distinctive kind of land with specific physical characteristics that differs from other kinds of land in its ability to produce a distinctive kind and amount of vegetation and in its ability to respond similarly to management actions and natural disturbances.”

Simply put, an ecological site is a kind of land with similar potential to produce vegetation that responds similarly to management and disturbances. Ecological sites are distinguished by differences in soils, topographic position, and climate (Figure 1) – and these factors influence the potential to support the mix of species that characterize the plant community.

Bestelmeyer_SRMPilot_ESDintro_Page_13.jpg

Figure 1. A landscape image showing two ecological sites with their corresponding soil profiles and vegetation (image courtesy of Joel Brown).

The term ecological site is applied to range and forest lands that are not agronomically managed. Agronomically managed grazing lands, such as forage croplands and pasturelands, are defined as forage suitability groups. Forage suitability groups simplify soils information and provide relevant plant and soil science information for managers (National Range and Pasture Handbook chapter 4).


Site Potential

Site potential is the capacity of an area to support a specific type and amount of vegetation. The potential of a site is directly correlated with the type of soil, topography and climate that exists for a given area (Figure 2).

Figure 2. Two distinct ecological sites with very different potential; a) Deep Sand site (R042XA054NM) with typical black grama grassland composition, which is susceptible to shrub encroachment, and b) Clayey site (R042XA061NM) with tobosa grass composition and low vulnerability to shrub encroachment.

Therefore, site potential reflects the ability of a site to support a certain suite of plant species, and influences the production potential or total plant biomass that the site can support.

 

How is the Ecological Site Concept Used by Land Managers?

Ecological sites are distinctive areas in a landscape or management unit with similar ecological structure and function. For this reason, they are united by their response to disturbances and management.

Guide Management Decisions

Understanding how the vegetation on a site will respond to a disturbance or management action is fundamental to land managers. Understanding vegetation dynamics guides management decisions for a variety of uses:

  • Wildlife
  • Forestry
  • Hydrology
  • Grazing
  • Recreational Uses
  • Restoration Techniques

Describe Vegetation Dynamics

State and transition models (STMs) are conceptual models that describe vegetation dynamics, or how plant communities on a site can change in response to disturbances and management actions. STMs are presented as diagrams (Figure 3) that organize knowledge about the possible “states“, or stable plant communities, that a site can support, and the “transitions“, or process-induced shifts between states.

Figure 3. A conceptual state and transition model (STM) where two states are possible for a given ecological site and three community phases with their corresponding pathways exist in each state (adapted from Stringham et al 2001).

States are distinguished by relatively large differences in vegetation structure and in the functioning of ecological processes. Different plant community phases belonging to one state may exist on a site, and the pathways or shifts between communities are generally considered to be reversible. A conceptual threshold is crossed during the transition from one state to another, and this transition is considered to be “irreversible” without significant management actions such as shrub removal, reseeding, or other types of vegetation manipulation.

Let’s examine an actual STM from a Gravelly ecological site to illustrate this concept (Figure 4). There are three possible states: Mixed shrub-grassland, Shrub-dominated and Shrubland.

Figure 4. A state and transition model (STM) for the Gravelly ecological site (R042XB010NM) from MLRA 42.

Each state has multiple community phases (as shown by the small boxes inside the larger “state” boxes) and community phase pathways, or slight alterations in vegetation composition and production that are maintained within the general state. This model shows there are three transitions between the three states. Transitions 1 and 2 indicate potential transitions in two directions, and describe the mechanisms for both degradation and restoration. Transition 3 is unilateral, meaning there is no identifiable restoration transition to the previous state.

STMs provide essential information to assist land managers in several ways:

  • To identify the current state of a site
  • To predict how a site will respond to management actions and disturbances
  • To identify sites at risk of crossing a threshold and prompt timely management actions
  • To develop plans to address management needs

Ecological Site Descriptions

An ecological site description (ESD) is a document that provides information about the physical and ecological characteristics of an ecological site, and the relationships between those characteristics. ESDs are developed and maintained by the USDA-NRCS. The Ecological Site Information System (ESIS) is the national repository for ESDs.

 

ESD information is presented in four major sections:
  • Site Characteristics – identifies the site and describes the physiographic, climate, soil, and water features associated with the site
  • Plant Communities – describes vegetation dynamics (including STMs), and the common plant communities associated with vegetation states
  • Site Interpretations – provides interpretive information relative to use and management of related resources
  • Supporting Information – provides relevant literature, sources of information and data used to develop the ESD, and information about the relationship of the site to other ecological sites

 

Site Characteristics

The site characteristics sections of ESDs typically include detailed information on features such as:

  • Physiography: elevation, slope, flooding frequency and duration, runoff class and aspect.
  • Climate: monthly precipitation and temperature, frost/freeze-free period, and vegetation growth/green up timing.
  • Soil: parent materials, texture, drainage class, permeability class, available water capacity, electrical conductivity, sodium absorption ratio.
  • Water: describes water system classification and characteristics, such as typical flooding regimes and drainage patterns

 

Plant Communities

The plant communities sections of ESDs typically include detailed information regarding:

  • Plant species
  • Vegetation states and
  • Vegetation dynamics as conveyed through state and transition models (STMs)

This section also includes information about annual plant growth curves, relative monthly plant production, and allowable plant production values used to calculate similarity indices.

 

Site Interpretations

The ecological site interpretations sections of ESDs provide alternative management options for each ecological site. They describe how changes in climate, disturbance processes and management can affect the biotic and abiotic components of an ecological site. Information provided in this section may include:

  • Wildlife considerations (species present, endangered species, preferred forage and habitat of the various species)
  • Livestock grazing systems commonly employed and optimal stocking rates
  • Monthly plant preferences of typical vertebrate herbivores (livestock and wildlife)
  • Hydrologic functions of the site
  • Recreational uses and opportunities (e.g. hunting, hiking, horseback riding, or camping)
  • Other potential products that might be obtained from the site (e.g. wood or native seed harvest).

 

Supporting Information

The supporting information sections of ESDs typically include links to associated similar ecological sites, details on the approvals and revisions of the ESDs, and relevant literature, information and data sources pertaining to the ecological site.


Calculating Similarity

Determining how similar a current plant community is to a reference, or desired, plant community is an important tool for land managers. Data on the production of species in given states during varying climatic periods (high and low precipitation years) are provided within each ESD.

Begin by identifying the ecological site:

  • Determine the MLRA in which the site occurs.
  • Determine and verify the ecological site in which the site occurs (Dig a hole and double check the soil classification).
  • Once the ecological site has been verified, navigate to the ESD for that ecological site from the NRCS-ESD reports webpage.
  • In the ESD, navigate to the Plant Communities section and find the Plant Species Composition table, which provides composition data for the major states (plant communities) on that ecological site (Figure 5).

Figure 5.  The Plant Species Composition table provides production data for both low and high precipitation years (shown in red dashed circle) in pounds per acre.

 

  • Determine which state is preferred, or desired, for management plans and objectives.

 

Calculation of similarity index from ESDs:

  • Randomly locate a sufficient number of plots to accurately estimate the biomass of the site (NRCS uses 10 quadrats, of either 0.96 ft2 or 1.92 ft2 frames). Make sure to use a plot size that is appropriate for the vegetation being sampled! (refer to the Biomass Module)
  • Record every species in each plot and measure its biomass (for example, NRCS uses double sampling).
  • Average the biomass of each species and convert the average to pounds per acre, or kilograms per hectare. The ESD Plant Species Composition table lists production as pounds per acre.
  • Sum the average biomass of each species into its respective species group (group number as listed in the ESD Plant Species Composition table).
  • List the resulting annual production values (lbs/ac) for each species group in the Determination of Similarity Index worksheet (columns D; Figure 6).
  • Determine the annual precipitation for the year that production measurements were taken.
  • List the annual production (lbs/acre) from the ESD for each species group in the Determination of Similarity Index worksheet (column C; Figure 6). Note: production values for low and high precipitation years are listed in the Plant Species Composition table.  Be careful that the correct values are selected!
  • Compare the production values of each species group on the site with the corresponding production values listed in the ESD, and select the lesser of the two production values (column E, Figure 6).
  • Sum the values for Columns C and E of the Determination of Similarity Index worksheet, and divide the sum of Column E by the sum of Column C.
  • Multiply the resulting number by 100. The result is the percent similarity of the site to the desired state.

Figure 6. The Determination of Similarity Index worksheet is used to compare a current plant community with a desired state.


Self-Check Activity

The questions and activity below are designed to test your knowledge and understanding of ecological site concepts, and your ability to calculate similarity indices based on ecological site description information.

1. Which are components of an Ecological Site Description (ESD)? Select all that are correct.

2. Which of the following are components of a state-and-transition model? Select all that are correct.