5.3 Ecological Sites

Video Presentation

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.

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 sandy site with typical black grama grassland composition, which is susceptible to shrub encroachment, and b) clayey site 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

Demonstrate 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.

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:

• Physiographic: 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 ration.
• 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 ecological 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.

Ecological 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.

We calculate similarity indices from ESDs by:

• Randomly locate a sufficient number of plots to accurately estimate the biomass of the site (NRCS uses 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 ocular estimation with periodic calibrations).
• 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 biomasses of species by species 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 pounds per acre value of the desired state 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).
• Total the values for Columns C and E of the Determination of Similarity Index worksheet, and divide the total for Column C by the total for Column E.
• Multiply the resulting number by 100. The result is a percentage of similarity for 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 community, or state.