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This post contains a "Phase 1 Survey", which will help finalize a state-and-transition model. If you have not read the background information on this type of model before, please review the post "state-and-transition background". If the link doesn't work, the post is in the archive located to the right of the blog frontpage (it will be found under July 2010).
Please study the following state-and-transition model for Clayey Fans in Petrified Forest National Park and surrounding areas, then respond to the survey at the end. We suggest that respondents spend less than 1.5 hours reviewing the information and responding to the survey; additional time is optional but not expected. Respondents may refer to any information source they already know of, but are not expected to conduct an extensive literature review (that is our job). Rather they are expected to answer based upon general principles, past experience, and knowledge of this and similar ecosystems.
Ecological Site Background: See NRCS ecological site description for details. Clayey Fans are fluvio-alluvial features found below exposures of the Chinle Formation in the painted desert region, including Petrified Forest National Park (Fig 1). Soils are loamy ranging from fine sandy loams to clay loams, to sandy clay loams. The proportion of clay represents the influence of the Chinle formation relative to other coarser parent materials. The soils are of shallow to moderate depth. Precipitation ranges from 15 -25 cm (6- 10 inches) per year, and falls in the form of cool season rain & snow, and high intensity summer monsoons. Elevations are around 1800m (5500 ft). Currently vegetation is primarily grasslands variously dominated by Sporobolus airoides, or Pleuraphis jamesii, with a shrub component most often composed of Atriplex obovata or Atriplex canescens. There is considerable data on the vegetation community composition in the form of percent cover data at 20 points, and categorical vegetation type for101 accuracy assessment points in a recent USGS vegetation mapping effort. The detailed data contain only one case, where grazing- the most likely stressor- is active, otherwise grazing has been retired within the park for over 40 years. Data on ecosystem functional attributes, ground cover, or spatial attributes of vegetation are confined to only 10 datapoints in the National Park Service Inventory and Monitoring Program. Thus data on the range of community structure are fairly rich, but data on ecosystem function are sparse.
Fig. 1. Extent of Clayey Fans (beige) in the Survey of Apache County, Central Part (Miller 1975). The boundary of Petrified Forest National Park is shown. Ecological sites are not databased for the western, Navajo County portions of Petrified Forest National Park, and are left blank intentionally. (click image for larger version)
Figure 2. State & Transition Diagram for Clayey Fans. Boxes represent states, arrows represent possible transitions and dashed boxes represent phases within states (Click to enlarge image).
STATE 1. REFERENCE GRASSLANDS AND PALATABLE SHRUBLANDS. Miller (1975), suggested that a climax community is most likely to consist of Sporobolus airoides, Pleuraphis jamesii, and Ericameria nauseosa. Based upon typical response of community members to grazing, it was proposed that Sporobolus would be expected to decrease, whereas Bouteloua gracilis, Pleuraphis jamesii, and Ericameria would be expected to increase. Also, the possibility of Juniperus invasion was mentioned if grazing were to reduce fuels. The current ecological site description acknowledges a shrub component in the potential vegetation state, primarily Atriplex canescens, and does not consider Ericameria an important "climax" community member. It goes on to suggest that the most likely increasers are Gutierrezia, Ericameria, Cacti (presumably Opuntia), and annuals. We conducted an analysis of two datasets which failed to document these increaser-decreaser behaviors. Rather, putative grazing impacts seem to reduce all species, and the dominance structure is fairly similar in more and less recently grazed sites.
We acknowledge an important spatial mosaic dictated by two abiotic factors which are partially independent: salinity and distance from a wash. Clayey fans are alluvial features and thus contain washes. Near washes, local dominance of either Sarcobatus vermiculatus or Ericameria nauseosum, with various codominants is possible but does not cover a large area (Thomas et al. 2009). More distant from the wash we find a spatial mosaic of Atriplex obovatum, Pleuraphis jamesii, and Sporobolus airoides communities, and Atriplex canescens, Pleuraphis jamesii communities. Only occasionally are true Sporobolus grasslands observed in small patches despite > 40 years of rest from grazing (Thomas et al. 2009). Of course, intermediates between these four scenarios are likely to occur as well. DeCoster & Swan (2008) list Sporobolus airoides, Atriplex obovata, and Pleuraphis jamesii as the three most abundant species, but the high standard deviation in cover supports the assertion that there is a large degree of spatial heterogeneity.
S1Phase1. TERRACE COMMUNITIES. This spatial phase occurs on infrequently active, stable terraces near washes. They are shrublands, either dominated by Sarcobatus vermiculatus when saline, or Ericameria nauseosum when less saline. Based upon their palatability Sarcobatus communities are susceptible to degradation whereas Ericameria communities are likely to be resistant.
S1Phase2. SALINE UPLAND COMMUNITIES. This spatial phase occurs close to or distant from washes and its soils may be more strongly influenced by the parent materials for the Chinle formation relative to other materials. This assertion is inferential and based upon the fact that Chinle parent materials are saline, and there is a salinity tolerance gradient in potential plant communities. This phase would be dominated by Atriplex obovatum (a salinity tolerant shrub; Hodgekinson 1987) either exclusively or accompanied by Pleuraphis jamesii, and Sporobolus airoides (a salinity tolerant grass; Ungar 1966). Pure stands of Sporobolus airoides are possible but much less common.
S1Phase3. MODERATELY SALINE UPLAND COMMUNITIES. This spatial phase occurs close to or distant from washes and its soils may be less strongly influenced by the parent materials from the Chinle formation due to mixture with other materials. These tend to support shrublands of Atriplex canescens with a herbaceous component dominated by Pleuraphis jamesii. These species are salt tolerant but less so than Atriplex obovatum and Sporobolus airoides (USDA NRCS 2002, Hodgekinson 1987).
S1Phase4. GRAZING COMPROMISED - Cover is sharply reduced in the majority of species. Differences in relative species composition attributable to grazing are not obvious (see analysis). Sporobolus and Atriplex spp., though reduced in abundance, may dominate the degraded community. Due to reduced buffering of erosive forces by vegetation, this state is likely subject to water erosion. This phase retains resilience and can recover high vegetative cover over 1-2 decades.
STATE 2. TAMARIX SHRUBLAND. This state is documented by Thomas et al. 2009 especially near the Puerco River (Thomas 2003), though it is not a major community type on the landscape. These invaded shrublands may arise due to replacement of Ericameria or Sarcobatus (Ladenberger et al. 2005), and because of the phreatophytic characteristics of Tamarix, they are unlikely to occur distant from stream channels. This state will be possible if the watershed is at least 10km2 and has some degree of persistent alluvial groundwater (Shaw & Cooper 2007). If high density is attained, these communities can burn, reinforcing dominance of the resprouting Tamarix. Also, Tamarix localizes salts at the soil surface in its leaf litter, making colonization by less tolerant species more difficult. For these reasons this state tends to reinforce itself.
STATE 3. ANNUALIZED. This state is dominated or co-dominated by the invasive exotic forb Salsola. Seasonally these can be productive, and dead tumbleweeds can afford some degree of protection against erosive forces. Sites such as these may be susceptible to water erosion seasonally because Salsola cover may be sparse.
STATE 4. SEVERELY ERODED. This state is largely theoretical. These fluvial-alluvial soils contain a substantial proportion of clays from the Chinle formation as source material. These bentonitic, shrink-swell clays have a very poor degree of stability when wet because aggregates expand, destroying the structure (Bowker et al. 2008). When such soils are lacking in biological crust development, as DeCoster & Swan (2008) indicate is the case in Petrified Forest, their aggregate stability tends to only be around 1.7 (Bowker et al. 2008) using the Herrick soil aggregate stability test (Herrick et al. 2000). DeCoster & Swan (2008) report a higher value for interspaces, ~ 3.7, suggesting either that some sites with less proportional abundance of bentonitic clay, and therefore more stability, exist, or that there is undetected and highly cryptic colonization by cyanobacteria conferring some stability. These soils are prone to forming physical crusts when dry however (DeCoster & Swan 2008), which resist wind erosion. A severely eroded state would be expected to have very poor plant cover, possible dominance by vegetation which is only seasonal such as Salsola, and clear evidence of active water erosion such as rills and gullies. This would be especially likely on sloping sites.
NOTES: Because Thomas et al. (2009) sampled extensively in Clayey Fans and never recorded the presence of Juniperus, the suggestion that Juniperus invasion could occur on Clayey Fans (Miller 1975) is not included in our model. This does not rule out that it happens outside of the National Park.
Explanation of Transitions
T1. If watershed is > 10km, and there is persistent alluvial groundwater AND a propagule source, Tamarix individuals may colonize. Due to vegetative reproduction, soil surface salinization, Tamarix dominance increases.
T2. Moderate but persistent grazing reduces preferred forage species allowing an increase of unpalatable plants.
T3. Relaxation of grazing allows recovery of palatable grasses and saltbushes.
T4. Moderate but persistent grazing reduces preferred forage species allowing an increase of unpalatable plants.
T5. Relaxation of grazing allows recovery of palatable grasses and saltbushes.
T6.Long-term low to moderate intensity grazing, especially in warm season when native C4 species are growing, OR increasing C02 in atmosphere, OR both, favors C3 cool season invasive annuals.
T7. Continued low to moderate intensity grazing combined with drought, or high intensity grazing alone, or extreme drought alone leads to devegetation and initiation of strong water erosion processes which present colonization barriers for plants.
T8. Continued low to moderate intensity grazing combined with drought, or high intensity grazing alone, or extreme drought alone- especially in the cool season when invasive annuals are growing - leads to devegetation and initiation of strong water erosion processes which present colonization barriers for plants.
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