A networking resource devoted to biological soil crusts and the researchers who study them. We will provide a means for international scientists to communicate, share their research, share important news and announcements, ask questions and find collaborators. We will also provide a space for informal writing on research, opinion, and ideas (now seeking posters!).

Monday, December 30, 2013

Behold! the Bryotron!

Its been a while since I last posted. November and early December kept me plenty busy…and that means no posts (that's why I need additional bloggers here). But anyway, was has excited me lately is our success culturing biocrust organisms, particularly mosses. I had envisioned and found some small funds to create an automated moss growing system, and put Kyle and Anita in charge of final design and implementation. They completely upgraded the design, and it has been a great success. We have 2 experiments running now for different projects, but are planning many more.

Kyle Doherty, moss farmer.
We started growing Syntrichia caninervis and Syntrichia ruralis in a growth chamber environment, but have largely abandoned these efforts and made the jump to the greenhouse late last summer.


We are Arizona's largest consumer of urine sample cups!
One experiment subjects Syntrichia ruralis sourced from different populations to different environmental conditions. The other is seeking optimal growing conditions for both Syntrichia species sourced from northern Utah.


Nostoc "volunteers" among Syntrichia shoots.

This fibrous mat in between shoots is actually a Scytonema colony.
When we inoculate with field collected material, we add several hitchhikers. Thus far, they're primarily desirable species such as N-fixing cyanobacteria, so we have no problem with this.

Sunday, November 3, 2013

Maestre Lab: News related to our Nature article

One for the drylands enthusiasts & desert addicts....See below a link to press coverage of our recent Nature paper on global nutrient cycling patterns in changing drylands, led by Manu Delgado-Baquerizo. Nice work, Manu!


Maestre Lab: News related to our Nature article: In the 31th October issue of Nature we published the article “ Decoupling of soil nutrient cycles as a function of aridity in globa...

Friday, October 25, 2013

Biocrusts of Northern Arizona National Monuments Post 5: Sunset Crater National Monument

Typical conditions in Sunset Crater National Monument. a) Landscape view, showing open ponderosa pine forest and cinder-covered interspaces. b) A patch of the moss Ceratodon purpureus associated with organic matter. c) A typical cinder-covered interspace with little soil development.n



Sunset Crater-Soil development at Sunset Crater is rather minimal due to the recent geological origin of the parent materials. Areas with particle size distributions less than 2mm can be found, but are quite rare. Therefore, we did not extend our models to Sunset Crater due to the paucity of biocrust habitat. Survey crews did occasionally observe patches of moss cover, mostly Ceratodon purpureus, often associated with organic matter enrichment (Figure 9b). There was no observation of any cyanobacterial development on these sites, and only very minimal cover of the soil lichen Cladonia (note: rock lichens are quite abundant however). Interestingly, extensive moss cover was observed adjacent to the road possibly due to a N-sloping roadcut.


This is the latest in a series, see here for a lichen key, here for a moss key, here for a description of Walnut Canyon biocrust, here for a description of Wupatki biocrust.


Tuesday, October 22, 2013

Ecological thresholds report

Not exactly biocrust-focused, but biocrusts turn up repeatedly in this report.


Bowker, M.A., Miller, M.E., Belote, R.T., and Garman, S.L., 2013, Ecological thresholds as a basis for defining management triggers for National Park Service vital signs—Case studies for dryland ecosystems: U.S. Geological Survey Open-File Report 2013–1244, 94 p., 

http://pubs.usgs.gov/of/2013/1244/

Introduction

Threshold concepts are used in research and management of ecological systems to describe and interpret abrupt and persistent reorganization of ecosystem properties (Walker and Meyers, 2004; Groffman and others, 2006). Abrupt change, referred to as a threshold crossing, and the progression of reorganization can be triggered by one or more interactive disturbances such as land-use activities and climatic events (Paine and others, 1998). Threshold crossings occur when feedback mechanisms that typically absorb forces of change are replaced with those that promote development of alternative equilibria or states (Suding and others, 2004; Walker and Meyers, 2004; Briske and others, 2008). The alternative states that emerge from a threshold crossing vary and often exhibit reduced ecological integrity and value in terms of management goals relative to the original or reference system. Alternative stable states with some limited residual properties of the original system may develop along the progression after a crossing; an eventual outcome may be the complete loss of pre-threshold properties of the original ecosystem. Reverting to the more desirable reference state through ecological restoration becomes increasingly difficult and expensive along the progression gradient and may eventually become impossible. Ecological threshold concepts have been applied as a heuristic framework and to aid in the management of rangelands (Bestelmeyer, 2006; Briske and others, 2006, 2008), aquatic (Scheffer and others, 1993; Rapport and Whitford 1999), riparian (Stringham and others, 2001; Scott and others, 2005), and forested ecosystems (Allen and others, 2002; Digiovinazzo and others, 2010). These concepts are also topical in ecological restoration (Hobbs and Norton 1996; Whisenant 1999; Suding and others, 2004; King and Hobbs, 2006) and ecosystem sustainability (Herrick, 2000; Chapin and others, 1996; Davenport and others, 1998).

Achieving conservation management goals requires the protection of resources within the range of desired conditions (Cook and others, 2010). The goal of conservation management for natural resources in the U.S. National Park System is to maintain native species and habitat unimpaired for the enjoyment of future generations. Achieving this goal requires, in part, early detection of system change and timely implementation of remediation. The recent National Park Service Inventory and Monitoring program (NPS I&M) was established to provide early warning of declining ecosystem conditions relative to a desired native or reference system (Fancy and others, 2009). To be an effective tool for resource protection, monitoring must be designed to alert managers of impending thresholds so that preventive actions can be taken. This requires an understanding of the ecosystem attributes and processes associated with threshold-type behavior; how these attributes and processes become degraded; and how risks of degradation vary among ecosystems and in relation to environmental factors such as soil properties, climatic conditions, and exposure to stressors. In general, the utility of the threshold concept for long-term monitoring depends on the ability of scientists and managers to detect, predict, and prevent the occurrence of threshold crossings associated with persistent, undesirable shifts among ecosystem states (Briske and others, 2006). Because of the scientific challenges associated with understanding these factors, the application of threshold concepts to monitoring designs has been very limited to date (Groffman and others, 2006). As a case in point, the monitoring efforts across the 32 NPS I&M networks were largely designed with the knowledge that they would not be used to their full potential until the development of a systematic method for understanding threshold dynamics and methods for estimating key attributes of threshold crossings.

This report describes and demonstrates a generalized approach that we implemented to formalize understanding and estimating of threshold dynamics for terrestrial dryland ecosystems in national parks of the Colorado Plateau. We provide a structured approach to identify and describe degradation processes associated with threshold behavior and to estimate indicator levels that characterize the point at which a threshold crossing has occurred or is imminent (tipping points) or points where investigative or preventive management action should be triggered (assessment points). We illustrate this method for several case studies in national parks included in the Northern and Southern Colorado Plateau NPS I&M networks, where historical livestock grazing, climatic change, and invasive species are key agents of change. The approaches developed in these case studies are intended to enhance the design, effectiveness, and management-relevance of monitoring efforts in support of conservation management in dryland systems. They specifically enhance National Park Service (NPS) capacity for protecting park resources on the Colorado Plateau but have applicability to monitoring and conservation management of dryland ecosystems worldwide.

Tuesday, October 15, 2013

Thanks for attending the Biennial Conference for Research on the Colorado Plateau

Posing by the giant french fries sculpture. L to R: Ferran Garcia Pichel, Trent Northen, Ana Giraldo, Blair Steven, Estelle Couradeaux, Jayne Belnap, Yev Marusenko, Aindrila Mukhopadhyay, Anny Chung, Sergio Velasco, Mandy Williams, Eva Robinson, Lindsay Chiquoine, Cheryl McIntyre, Deb Neher, Matt Bowker, Zach Aanderud, Kyle Doherty, Anita Antoninka
Many thanks to all the attendees of our Biocrust session at the Biennial Conference. This year we brought in several new faces and have cemented a strong tradition. I hope to see you all back next time around.

Sunday, October 13, 2013

The charismatic microflora of the desert: the incredible and important natural history of biological soil crusts



For those in the Flagstaff area, I'll be doing a talk about biocrusts for the general public, this Thursday, Oct 17 6:30pm. If you're not familiar with this event, imagine a department seminar with less jargon and more beer. It's fun (kinda wish department seminars were always like this!).

Saturday, October 12, 2013

Tuesday, October 1, 2013

Thursday, September 26, 2013

Ancient soils reveal clues to early life on Earth

Ancient soils reveal clues to early life on Earth

New Nature paper out yesterday suggests using geochemistry that oxygen was in the atmosphere and cyanobacteria were in existence 3 billion years ago. No hard fossil evidence of the cyanobacteria exist...the line of thinking rests on the assumption that cyanos were the first oxygenic photosynthesizers. Anyways, cool paper.

Tuesday, September 24, 2013

Biocrust data repository

Preface: After a quick glance at the Maestre lab blog, I see they have added links to datasets deposited for public use on Dryad. Good idea. In fact I need to make some old data available myself, the only reason I haven't being that I'd have to sit down and document metadata (yuck!) and make sure the data was easy for someone else to use and I have so much other shit to do in every waking minute that it just hasn't been done. One day....be patient. In the meantime, thanks to Fernando & Co. for showing us how we should be operating.

It occurs to me that there ought to be a single place where someone could go an find links to datasets containing some form of biocrust data, and that this blog is the perfect launching pad. I'll have to think about a nice, more permanent way to do it, but in the meantime it occurs to me I can do it as a simple blog post that I will permalink on the top bar. It will be called Biocrust Data Repository just like this post (do you see it up there?), and I will periodically update it with your help. If you want a link to a dataset posted, leave a comment. You'll have to deposit the data somewhere such as Dryad or your own website, and I will link to the URL that you provide.

Dryad
This is the most widely used repository for data. Here's a search for the term "biological soil crust".

Figshare
This is a widely used repository for figures, presentations, and in some cases datasets. Here's a search for the term "biological soil crust".

Specific data resources (check back for updates)
Castillo-Monroy AP, Maestre FT, Delgado-Baquerizo M, Gallardo A (2010) Biological soil crusts modulate nitrogen availability in semi-arid ecosystems:insights from a Mediterranean grassland. Plant and Soil 333:21-34.




Escolar C, Martinez I, Bowker MA, Maestre FT (2012) Warming reduces the growth and diversity of biological soil crusts in a semi-arid environment:implications for ecosystem structure and functioning. Philosophical Transactions of the Royal Society B 367: 3087-3099.

Maestre FT, Puche MD (2009) Indices based on surface indicators predict soil functioning in Mediterranean semi-arid steppes. Applied Soil Ecology 41:342-350.

Weber B, Berkemeier T, Ruckteschler N, Caesar J, Heintz H, Ritter H, Brab H (2015) Development and calibration of a novel sensor to quantify the water content of surface soils and biological soil crusts. Methods in Ecology and Evolution http://dx.doi.org/10.1111/2041-210X.12459


To submit a link to an archived dataset, please leave a comment with the original paper citation (if applicable) and a link to where the data can be downloaded.



Sunday, September 22, 2013

Species of concern on the Colorado Plateau: Mosses & Lichens

Preface: nearly a decade ago I was asked to contribute to a book about species of concern in the Colorado Plateau ecoregion focusing on all taxa. I drafted up a section about mosses and lichens, focusing on gypsiferous species of biocrusts, which is a rare habitat. There are no federally listed mosses or lichens, not because they don't exist, but rather because the Endangered Species Act categorically excludes them. 

I asked Roger Rosentreter (lichenologist), and Lloyd Stark (bryologist) for tips on other non-gypsiferous species I ought to mention. 

Having heard nothing about the book in many years, I think I can assume the project is dead. In the meantime, I reckon its more useful here than on an old hard drive in an obsolete file format. I've supplemented it with links to images from the web, if they exist.

Regarding status, vulnerable means that it could conceivably be extirpated. For example rarity of habitat would render something vulnerable. At risk refers to a vulnerable species that is threatened by a stressor. 




MOSSES AND LICHENS OF GYPSIFEROUS SOILS

NODULE CRACKED LICHEN

Status in region: At risk

Status elsewhere: At risk in the Americas

DESCRIPTION ~ Nodule cracked lichen was first encountered in the Americas in the mid-1980's, and has been found to be locally common species confined to the gypsiferous soils of the Colorado Plateau. Although it is has a widespread distribution around the world, its preference for a very rare habitat type in North America makes it a species of concern. This lichen is composed of many scales of ~3-5 mm diameter with lobed margins. Generally the appearance of the lichen is white due to a covering of oxalate salts, but the thallus underneath is actually pale brown. It has black fruiting bodies immersed in the thallus that lack any kind of rim around them. Colonies are irregularly shaped and generally less than 5 cm in diameter.

Vulnerability Factors: Habitat specialist, restricted range

NEVADA GYPSUM MOSS

Status in Region: At risk

Status in other regions: At risk

DESCRIPTION~ This rare moss was recently described in 1995 as a result of morphologically well-developed collections made during California bearpoppy studies. Nevada gypsum moss appears to be a widely but sparsely distributed species of western North America, almost exclusively on gypsiferous soils. It is distinguished by other mosses of the same habitat by its lack of an awn (hair-like projection) on its leaf tips, and its dark green to black leaves that tend to spiral around the stem near the top. Its small stature (usually < 2mm tall) makes a handlens a must to observe these features. Many desert mosses have skewed sex ratios, but to date a male individual of this species has yet to be found making it one of the champions.

Vulnerability Factors: Habitat specialist, restricted range, possible low genetic diversity due to lack of sexual reproduction

DESERT CRATER LICHEN

Status in region: Vulnerable {EDITORIAL NOTE: THIS SPECIES IS QUITE ABUNDANT, EVEN DOMINANT IN A RARE HABITAT TYPE}

Status elsewhere: Widespread and stable


DESCRIPTION ~ Desert crater lichen is found on several continents but on the Colorado Plateau it is strongly restricted to the rare gypsiferous soils where large white populations can be visually impressive. This lichen forms rather large colonies often exceeding 5cm in diameter and is pure white. It tends to have a rugose, undulating surface and has large (~ 2mm) black fruiting bodies which are bowl shaped and sunken into the thallus surface like a crater. This species tends to have a very clumped distribution, so if you find some you are likely to find a lot.

Vulnerability factors: habitat specialist, restricted range

LARGELEAF GYPSUM LICHEN

Status in region: At risk

Status elsewhere: At risk globally

DESCRIPTION ~ Largeleaf gypsum lichen has been confusing lichenologists since the 1920's, and was not discovered in the US until 1990. Its unique fruiting body which grades into the vegetative thallus afforded it a designation as a new genus solely representing a new family, Gypsoplacaceae. It is a squamulose lichen with olive - tan squamules (scale-like mini thalli) usually about 0.5 - 1 cm in diameter. The fruiting bodies, when present, look like upraised brick red swellings on the squamules and may be irregular to dome shaped. Although fairly large in the soil lichen world, colonies are usually less than 5cm in diameter. This species is a rare one even within its specialized habitat which is also rare.

Vulnerability Factors: Low population density, habitat specialist, restricted range.

GYPSUM-LOVING RIM LICHEN

Status in region: At risk

Status elsewhere: At risk globally

DESCRIPTION ~ This lichen was unknown to science until collected in the San Rafael Swell in 1998, and has since been observed at scattered locations around the Colorado Plateau. Gypsum-loving rim lichen is chalky white to ashy gray and forms a tightly adhering crust on the soil surface. Its thallus (vegetative portion) is divided into small partitions called areoles. It bears black disk shaped fruiting bodies (1-2 mm dia.) with a white margin that are flush with or sitting slightly atop the thallus. A typical specimen is about 3 - 5 cm in diameter and irregularly shaped.

Vulnerability Factors: Habitat specialist, restricted range.



Habitat: These species are strongly restricted to arid and semi-arid sites with gypsiferous soils such as those derived from gypsum-bearing portions of the Carmel  Formation, the Paradox Formation, and the Moenkopi Formation (most of which occur at 5000 - 6500 ft). Such sites are found across southern Utah and in southwestern Colorado. They occur as components of the conspicuously well developed biological soil crusts generally found on these soils.

Threats and Concerns: On the Colorado Plateau, these species are endemic to an inherently rare habitat type, and are uncommon to rare within that habitat type even when undisturbed. Largeleaf gypsum lichen is a rare species even within this special habitat type. Most of the gypsum soils of the plateau are degraded to some extent by livestock activity and off road vehicle use, and truly undisturbed examples may be lacking. As population growth continues in the region, economical exploitation of the more pure gypsum deposits may occur to satisfy demand for products such as drywall. Gypsum areas are particularly popular with users of dirt bikes and all-terrain vehicles.

Conservation: When planning road and trail construction, gypsum areas should be avoided whenever possible. Enforcement of off-road vehicle regulations should be prioritized in these areas. Plant cover is poor on gypsum soils, therefore they offer relatively little forage value to livestock. Grazing of these fragile habitats could and should be phased out without creating major economic impacts. Because gypsiferous soils generally occur in relatively small patches, a network of small fenced reserves could potentially maintain the endemic biota.

Notes: Because several soil crust species and some vascular plant species are rare gypsum endemics, and gypsum soils cover very little area, it is practical and possible to conserve them all by conserving the habitat in small reserves. We thank Dr. Larry St. Clair of BYU for sharing his expertise on gypsiferous lichens, and Dr. Lloyd Stark of UNLV for information pertaining to Didymodon nevadensis.

OTHER MOSSES AND LICHENS OF CONCERN
LITTLE FRINGE MOSS
Crossidium seriatum

Status in region: Unknown

Status elsewhere: At risk globally

DESCRIPTION ~ This little fringe moss is an extremely rare western North American soil moss with a primarily hot desert distribution. It has never been collected in the Colorado Plateau but likely occurs in the more xeric portions, albeit very rarely. Although this species cannot be separated from lookalikes in the field, perhaps its best identifying characteristic is its extremely small size. A typical field specimen is frequently less than 0.5 mm tall and although it does generally have a white hairlike point on its leaf tips, it never appears as a white hairy cushion. With the naked eye, individuals look like little black dots.

Vulnerability Factors: Low population density, restricted range



Habitat: This moss occurs on sandy or gypsiferous soils of aridlands. Dr. Lloyd Stark suggests that this species is primarily centered around the gypsiferous soils near Lake Mead, thus its most likely habitat on the Colorado Plateau includes the more xeric gypsiferous substrates at the margins of the Colorado Plateau ecoregion such as those near St. George, Utah. Other possible localities include exposures of the Paradox formation in Cataract Canyon and adjacent side canyons.

Threats and concerns: Soil disturbances of various sorts are the most likely stressors: foot traffic, livestock grazing, and off road vehicles.

Conservation: Initially, the best strategy is simply to determine that the species does indeed occur on the Colorado Plateau so that the habitat characteristics can be better defined and stressors better identified. As a preemptive strategy, gypsiferous habit reserves should be developed as described previously. Fortunately, the most likely localities for this species happen to be in protected areas (Canyonlands National Park, and Glen Canyon National Recreation Area), but they could potentially be impacted by river users.

Entosthodon planoconvexus

Status in region: Vulnerable

Status elsewhere: Globally vulnerable

DESCRIPTION ~ This exceedingly rare moss is known on the Colorado Plateau from only one location in Canyonlands (deposited by the author at the National Park Service Southeast Utah Group's herbarium in Moab, Utah), and is known from only four other collections worldwide. Entosthodon planoconvexus is a short moss with rather large yellowish green leaves that are spreading when moist and shriveled when dry. Its sporophyte, when present, consists of an upside-down pear-shaped capsule borne on a reddish stalk.

Vulnerability factors: Low population density

Habitat: Because of its rarity, the habitat of this species is poorly defined although it tends to occur on dry soil at the base of rocks. The Canyonlands specimen grew in a dry sandy soil layer over rock and adjacent to a rock outcrop. It is an occasional associate of liverworts of the genus Targionia.

Threats and Concerns: The main concerns with Entosthodon planoconvexus are its naturally highly isolated small populations. Because it tends to grow in at least partially protected habitats, the population is likely stable, however stochastic events or disturbance could easily drive this species locally extinct. Livestock and recreation impacts are the most likely anthropogenic stressors of this species.

Conservation: It is difficult to recommend conservation strategies for this species without knowing what potential stressors are. Perhaps the best strategy is simply to determine where the species occurs so that the habitat characteristics can be better defined and stressors identified. Currently, bryophytes are not generally included in inventory and monitoring projects.

Notes: Dr. Lloyd Stark of UNLV provided helpful information on this species.



MINNESOTA ROCK LICORICE
Lichinella minnesotensis

Status in Region: Unknown

Status elsewhere: Possibly vulnerable in western North America

DESCRIPTION~ This rock licorice lichen is apparently a North American endemic with a primarily eastern distribution. There are some rather disjunct collections from the midwest and west including a single collection from near Kanab, Utah. This black lichen is composed of clumps of ascending convoluted lobes. It is jelly like and semi-transparent when wet. Colonies are generally only about 1 cm in diameter.

Vulnerability Factors: Isolated populations

Habitat: Minnesota rock licorice is found in shallow fissures or crevices on rock outcrops. The sole Colorado Plateau collection was from a limestone substrate of the Timpoweap member of the Moenkopi formation in Grand Staircase-Escalante National Monument. It is difficult to characterize the habitat characteristics of this species on the Colorado Plateau because only one collection has been made.

Threats and Concerns: It is unknown whether this species is threatened by anthropogenic forces, buts its rarity in the region suggest it is vulnerable. Fortunately, its habitat type affords considerable protection.

Conservation: It is difficult to recommend conservation strategies for this species without knowing what potential stressors are. Perhaps the best strategy is simply to determine where the species occurs so that the habitat characteristics can be better defined and stressors identified if they exist. Currently, lichens are not generally included in inventory and monitoring projects.

Notes: Dr. Roger Rosentreter of the BLM provided information on his Kanab-area collection of this species.

HAIRY MOUTH MOSS
Trichostonum sweetii

Status in region: Vulnerable

Status elsewhere: Globally vulnerable

DESCRIPTION ~ Hairy mouth moss is known from only three collections on the Colorado Plateau (one a dubious identification) and only a handful more from western North America where it is endemic. This is a fairly large moss for arid regions (up to 2cm tall) and is an inhabitant of shady crevices. It has large, narrow, bright green leaves (2-3 mm long) which are spreading and widest just below the apex.  They may occur as small tufts or scattered individuals mixed with other species.

Friday, September 20, 2013

Biocrusts of Northern Arizona NAtional Monuments Post 4: Wupatki


This is the latest in a series, see here for a lichen key, here for a moss key, and here for a description of Walnut Canyon biocrusts.



Wupatki-Wupatki was the largest national monument examined. We found that in the case of potential biocrust abundance low to medium values were observed within the monument. All of our outputs suggest that the areas where biocrusts attain the greatest importance are the limestone flats above the Doney cliffs, including Antelope Prairie. This conclusion is deceptive because it does not take into account eolian reworked cinders. Surfaces covered by cinders are not available habitat for BSCs. We had no available spatial data on the extent of cinder deposits, therefore we generated a map of cinder cover based upon interpolation of our surface data (Figure 1). Amos et al. (1981) provide data on thicker cinder deposits, but do not address the thin eolian deposition of cinders that strongly influences western Wupatki. Because mapping cinder cover was outside the scope of our project, this data should be considered a rough approximation only. The cinder map reveals that a large proportion of Wupatki that otherwise could support BSCs likely does not because a large proportion of the available surface is covered with cinders (Figure 2a). There may however be less cinder deposition on the northern portions of the Doney Cliffs where our models predict high potential for BSC abundance, function and biodiversity.

Figure 1. Surface cinder cover in Wupatki National Monument, estimated by interpolation from non-systematic ground-based samples.

 
Overall, BSC cover is sparse in Wupatki, apparently due to several factors. The cinder deposits of the western portions virtually prohibit biocrust development because there is simply no soil at the surface, i.e. no available habitat. To the east, the Wupatki basin has less cinder deposition but is quite arid and hot, and is lacking in sandy soils which tend to support higher biocrust cover on the Colorado Plateau. This area consists mainly of highly eroded exposures of moenkopi shale, and alluvial terraces of various ages. Minor biocrust development was detected on some alluvial terraces, but was not clearly related to terrace age. Occasionally northern exposures (especially shrub mounds within such sites) supported some crust cover (Figure 2). It is possible, especially in the Wupatki Basin, that the landscape is not at its potential due to the legacy of disturbance both from livestock and the widespread prehistoric agriculture that occurred there. Calcareous sandy soils and non-bentonitic shale-derived soils in low disturbance conditions generally support greater biocrust cover than that observed in the Wupatki Basin.

Figure 2. Soil surfaces in Wupatki National Monument. a. A thin veneer of cinder deposition prohibits biocrust development due to little available habitat at the soil surface. b. A biocrust growing on the north side of a shrub mound on an alluvial terrace.