Select criteria for stratifying the study area example

Step 3: Select criteria for stratifying the study area (if necessary)Step1_AIM

The terrestrial landscape of interest is variable, ranging from Wyoming big sagebrush plains to 10,000-foot mountain peaks.  Terrestrial monitoring will utilize LANDFIRE Biophysical Settings (BpS) as strata to distribute sampling effort across the landscape, but not for reporting purposes.  BpS represent potential natural vegetation based on biophysical environment and historic disturbance regimes.  They also represent a consistent strata layer that can be used across the western US.  

Sample points per BpS will be proportional to BpS area on the landscape except where monitoring objectives and sample sufficiency analyses suggest alteration (Figure 1; Table 2; see steps 4 and 5).  Within the broader RMP area, additional stratification of sampling by sage grouse habitat areas (PHMA and PGMA) is not necessary to report on sage grouse habitat conditions within the study area because a majority (70%) of the study area is inside PHMA.   


The geospatial data layers used to define strata were derived from the BLM’s AIM Master Sample for terrestrial and aquatic systems and included:

  • Terrestrial strata: LANDFIRE Biophysical Settings (BpS)
  • Stream/river strata: Strahler stream order categories from the NHD+, medium resolution version 2.0

Figure 1.  Terrestrial and aquatic statistical sample design addressing resource management plan effectiveness. The field office contains greater than 70% PHMA and thus additional strata were not required to meet minimal sample sizes for that reporting unit (layers not shown). Points were drawn randomly such that every point or stream segment within the area has a known chance of being sampled, enabling inferences across the entire landscape.  When additional information is needed for smaller areas (e.g., grazing permit renewals) sampling can be intensified in those areas.


Table 1. Management objectives, example AIM indicators, and the methods to be used evaluating the attainment of management objectives. Assessing the attainment of management objectives requires a statement of the desired condition per indicator (i.e., condition threshold), as well as the amount or proportion of the resource that can deviate from from a condition threshold before management actions are needed.
Management objectives (summarized from above) Example indicators Condition determination method Condition thresholds Proportion of allowable departure (e.g., acres or stream km)1
Soils (LHS#1; RMP) Soil Aggregate Stability;

Bare Ground


Professional judgment





Healthy productive plant and animal communities (LHS#3; RMP) Plant Cover

Species Richness

Professional Judgment >=50%

>=15 sp per plot



Special status, threatened and endangered species (RMP; LHS#4; Sage grouse plan amendment) Sagebrush Cover

Perennial Grass + Forb Cover

Sagebrush Height

Predominant Sagebrush Shape2

Proximity of trees/other tall structures to sage-grouse leks2

Proximity of sagebrush to sage-grouse leks

Research (see citations in Northwest CO Greater Sage Grouse RMP Amendment and EIS, Table 2.3, Page 2-29) 10-25% *

>15% *

12 to 32 in *

Not quantitatively specified *

>50% in spreading
Trees/other tall structures are none/ uncommon within 1.86 mi of lek

Protective sagebrush cover occurs within 328 feet of lek

<20% of each habitat type can deviate from conditions specific to that type *
Riparian areas (LHS#2) Bank stability Professional judgement Minimal departure: > 80%; moderate departure: 60 – 79% < 20%
Riparian areas (LHS#2) Canopy cover Deviation from regional reference values Minimal departure: > 29%; moderate departure: 28 – 7% < 20%
Water quality (LHS#5) Macroinvertebrate biological integrity CDPHE3 multi-metric index (MMI) Mountains: < 42

Transition: < 42

Plans & Xeric: < 22

< 15%
Water quality (LHS#5) Temperature CDPHE numeric criteria Tier I warm water: 29oC daily maximum < 15%
1Different management objectives/areas (e.g., grazing allotment, PHMA, wilderness study area) are likely to have different management objectives and thus condition thresholds and/or the amount or proportion of allowable departure of a indicator from a condition threshold.
2Supplemental indicator
3Colorado Department of Public Health and the Environment (CDPHE)


Table 2. Summary of biophysical settings (strata) and associated plot sample sizes used in the terrestrial GRTS sample draw.  The number of plots per strata is proportionate to the available acreage, with a minimum sample size of three per strata.  Plot weights are the scaling factors used to extrapolate plot level data to the entire sample area; in this example, individual sample plots represent between 10,000 and 12,500 acres.  

Strata – Landfire Biophysical Setting Group Stratum acres Proportional area Plots per strata Plot weights
Wyoming Big Sage-Wheatgrass-3 370,000 0.31 31 11935.5
Mountain Sagebrush-Bluebunch Wheatgrass-Idaho Fescue-4 200,000 0.17 17 11764.7
Low Sage-Idaho Fescue-3 50,000 0.04 4 12500.0
Western Juniper-Utah Juniper-3 300,000 0.25 25 12000.0
Bluebunch Wheatgrass-Sandberg Bluegrass-1 150,000 0.13 13 11538.5
Quaking Aspen-4 100,000 0.08 8 12500.0
Black Cottonwood-Narrowleaf Willow-3 30,000 0.03 3 10000.0
Total 1,200,000 1.00 100 NA


Table 3. Summary of Strahler stream order categories (strata) and associated sample sizes used in the aquatic GRTS sample draw.  The number of sites per strata is proportionate to the available stream kilometers, with a minimum sample size of three per strata.  Site weights are the scaling factors used to extrapolate site level data to the entire sample area; in this example, individual sample sites represent between 20 and 53 stream kilometers.

Strata – Strahler Stream order categories Stream km Proportional length Sites per strata Site weights
Small streams (1st and 2nd order) 847.3 0.73 16 53.0
Large streams (3rd and 4th order) 250.8 0.22 5 46.3
Rivers (5th+) 59.2 0.05 3 19.7
Total 1157.3 1.00 24 NA

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