Tuesday, October 13, 2015

Analysis of Soil and Vegetation Maps: Accuracy and Utility for Describing Actual Habitats

There are four sources of landscape information from maps at the project level a few miles on a side. Topogaphic maps, satellite maps, soil service maps, and vegetation maps.

Comparing soil and vegetation maps at this scale is complicated by inaccuracies of both map sources and the strange ambiguity of aerial photography. Soil was mapped by NRCS into 6 major soils. However, two of the soils are described as compound soils, regions of undefined patches possibly intergrading continuously into one another. For example, Pyote-Maljamar soils (PU on soil map) have a layer of fine sand everywhere, but there are unmapped bits and pieces of caliche at around 50 inches (Maljamar soils) in a matrix of deep sand (Pyote soils).

Soil map created using the NRCS Web Soil Survey showing major soil types. PT and PU are deep sands, BH and KO are shallower silty soils, and TF is intermediate. (PA and BA are extensions of PT and TF, respectively, in Eddy county.)

Soil Profiles:

	PT	PU		TF	BH		KO
Soil Name	Pyote	Pyote	Maljamar	Tunuco	Berino	Cacique	Kimbrough
0-10	A: Loamy fine sand	A: fine sand	A: fine sand	A: loamy fine sand	A: fine sand	A: fine sand	A: gravelly loam
10-20				AC: loamy fine sand	Btk: sandy clay loam	Bt: sandy clay loam	Bkm: cemented material
20-30				Bkm: cemented material		Bt: sandy clay loam
30-40	Bt: Fine sandy loam	Bt: fine sandy loam	Bt: sandy clay loam			Bkm: cemented material
40-50			Bt: sandy clay loam
50-60			Bkm: cemented material
Type	Sandy eolian deposits	Sandy eolian deposits	Sandy eolian deposits	Sandy eolian deposits	Sandy eolian deposits over sandy calcaereous alluvium	Calcaerous eolian deposits	Calcaerous alluvium and/or eolian deposits

Selected Soil Properties

	PT	PU	TF	BH	KO
Depth to restrictive layer	>200cm	127cm	43cm	>200cm	15cm
Calcium Carbonate (CaCO3)%	2%	2%	0%	17%	15%
% sand	75.8%	81.9%	78.6%	62.6%	43.0%
Ksat (inches/hour)	7.8	8.5	12.6	1.7	0.5

In this part of NM, depth to a restrictive soil layer indicates the presence of caliche near the surface. These petrocalcic horizons are denoted Bkm on the soil profile. KO has the shallowest effective soil, followed by TF. Some parts of BH appear quite shallow, but in the table the depth to a restrictive layer is listed as greater than 200cm, possibly because some of the soil (i.e. the Berino component) lacks a caliche layer. Caliche is composed of calcium carbonate, so BH and KO are listed with the most calcium, and the least sand in their profile.

Saturated hydraulic conductivity (Ksat) refers to the ease with which pores in a saturated soil transmit water. The estimates are expressed in inches/hour for ease of comparison to possible rainfall rates. They are based on soil characteristics observed in the field, particularly structure, porosity, and texture.

Restrictive soil layers and overall soil texture contribute to the ability of a soil to drain water. PT, PU, and TF are listed as very well drained soils because they can all drain more than 7 inches of rain an hour, whereas BH and KO are significantly less porous, draining only 1.7 and 0.5 inches of rain an hour, respectively. Most of the water from heavy rains probably runs off of these soil types, limiting the amount available to grow plants.

Saturated hydraulic conductivity is considered in the design of soil drainage systems and septic tank absorption fields. It probably has the greatest impact on plant production of any soil parameter in SE NM.

Hydraulic conductivity is the rate at which a soil can absorb water. Red areas have the least ability to absorb rainfall, while blue areas have the greatest ability to absorb rainfall. Map created using NRCS Web Soil Survey.

Vegetation Map

Vegetation map from USGS GAP Vegetation Mapper uses NatureServe Ecological System Classification.

Vegetation Map Key and Attributes

Table 3. Vegetation Map Key and Attributes

Color	ReGAP Community Name	Vegetation Type	Dominant Species	Accuracy
	Great Plains Shortgrass Prairie	Grassland	Biennial wormwood, Russian thistle	Low – should be mapped as disturbed area
	Mesquite Upland	Thornscrub	Mesquite, Catclaw Acacia, Mimosa, Yucca	High - mesquite dominant
	Sandhill Shrubland	Shrub	Shinnery oak, Catclaw acacia, Giant dropseed	Medium – not all dune
N/A	Sandy Plains Semi-Desert Grassland	Grassland	Purple three-awn, Sand dropseed, Sand muhly	Low – not mapped

The GAP national land cover data, based on the NatureServe Ecological Systems Classification, are the foundation of the most detailed, consistent map of vegetative associations available for the United States. The soil map is interpolated based on soil pits and vegetation patterns, so in a way it functions as a hand-drawn vegetation map. Vegetation patterns have changed from the time the soil survey was completed (1960’s?) to now. This GAP high-resolution vegetation map was produced via satellite mapping and computer algorithms.

The prairies of the southern Great Plains are also called the Llano Estacado, a region where vast flat to rolling uplands are covered with blue grama grass. However, this vegetation type is misclassified. GAP maps roads and disturbed areas with low grass as shortgrass prairie (brown on image) because these areas look similar to prairie in multispectral satellite imagery. It maps the rest of the project area as a fractal pattern of mesquite upland (mauve) patches and sandhill shrubland (green) patches.

Mesquite has spread throughout areas with deep sandy soils and now forms the default vegetation community across much of the area. Mesquite can also invade sandhills and desert washes and other coarse-textured soil areas. It is especially invasive in grasslands such as Sandy Plains Semi-Desert Grasslands, Great Plains Shortgrass Prairie, and Chihuahan Semi-Desert Grasslands.

Mesquite grows best when soils are deep, lacking the caliche or clay pan that would limit infiltration and storage of winter precipitation in deeper soils layers. Mesquite and other deep-rooted shrubs exploit the deep soil moisture that is unavailable to cacti or grasses.

The effects of major soil boundaries are evident: deep sand (PT and PU) soils support more sandhill shrubland, whereas soils with shallow restrictive horizons (BH and KO) tend to have more mesquite upland patches. The vegetation map fails to identify patches of Lehman lovegrass grasslands, or catclaw acacia shrublands, but it does correctly identify shinnery oak areas as sandhill shrubland.

However, the map misses out on an important intermediate community, sandy plains semi-desert grassland. Sandy plains grasslands are actually the dominant community throughout much of the project area. It is distinguishable on the ground by the greater proportion of grass than shrubs on sandy soils, often with Aristida purpurea, Muhlenbergia arenicola, and especially Sporobolus flexuousus. However, this community has been invaded by mesquite shrubs (some areas of which have been recently killed with herbicides) so these grassland patches can be difficult to distinguish from true shrublands.

Topo Map

A topo map shows that areas with accumulating sand are typically uplands, especially breaks in slope where winds drop eolian deposits. Eroding, exposed slopes reveal deeper, more-developed paleosoils, possibly Pleistocene clays (Steve Hall, 2006 Geomorphology of Mescalero Sand Dunes).

On top of soil and geomorphic landscape-determined vegetation patterns, local populations of invasive species have overlaid an unpredictable pattern of monocultures of Lehman Lovegrass, Artemisia biennis, and occasional plants of Salsola tragus around wellpads. Note that none of these invasive species are NM state-listed noxious weeds. There are also surprising areas of intact, diverse Chihuahuan grasslands with healthy stands of black grama , muhly arenicola, and sporobolus cryptandrus. All of the sand dunes here, despite presence of shinnery oak, and even some Artemisia filifolia, are coppice or hummock dunes that formed around shrubs during historical time (Hall, 2006).

Conclusion

Unfortunately, there are no map sources of reliable data on habitats and vegetation communities at field- or project area-scale. Each source provides valuable clues along with misleading simplifications, errors, and obfuscations of actual on-the-ground conditions.

Appendix: Soil Properties Maps

Depth to a restrictive soil layer: