Patterns in biological soil crust recovery in Conservation Reserve Program fields, Washington State

Background and Research Objectives

Historically, shrub-steppe ecosystems dominated over 645,000 km2 of the western North American landscape, extending from northern California into southern Canada (Rickard et al. 1988). This ecosystem is characterized by shrubs, most often species of sagebrush, Artemisia spp., perennial bunchgrasses, a diverse forb component, and, in the spaces between the vascular plants, a biological soil crust, also known as a biotic, microbiotic, or cryptogamic crust. Preceding settlement, shrub-steppe was the prevalent habitat type throughout much of eastern Washington (Daubenmire 1970, Vander Haegen et al. 2004). Since then, however, mainly through conversion to crop production, over 60% of the shrub-steppe in Washington has been lost (Vander Haegen et al. 2004). The remaining shrub-steppe is highly fragmented and usually severely degraded mainly by livestock, invasion by exotic plants, and changes in fire frequency (Vander Haegen et al. 2004). Concurrent with these impacts to the shrub-steppe habitats has been the loss and degradation of biological soil crusts in eastern Washington. Biological crusts are fragile and readily disturbed, often degrading before vascular plant assemblages (Belnap et al. 2001).

Living soil crusts are important components of healthy shrub-steppe ecosystems. They are comprised of complex associations of organisms that includes lichens, bryophytes (including mosses and a few species of liverworts), single-celled algae, cyanobacteria, and fungal hyphae intermixed with plant roots, litter, and soil (Belnap et al. 2001, Belnap 2003). Soil crusts perform a number of ecological functions that contribute to the ecosystem integrity and health of shrub-steppe (Belnap et al. 2001, Evans and Johansen 1999, Hilty et al. 2004, Jones and Rosentreter 2006, McIntosh 2003, Ponzetti and McCune 2001). They bind soil surfaces increasing soil stability by reducing or eliminating soil displacement (Belnap 2003, Schulten 1985). The complex surficial microtopography of a biological crust creates a boundary of still air and further protects the soil from wind erosion (Eldridge and Kinnell 1997, Lehrsch et al. 1988, Leys and Eldridge 1998, Neuman and Maxwell 1999). Crusts have been shown to increase water infiltration rates (Eldridge 1993). The presence of an intact biological crust appears to inhibit the establishment of cheatgrass (Bromus tectorum L.) and other invasive species (Belnap et al. 2001, Kaltenecker et al. 1999, Wicklow-Howard et al. 2003). Lichens and cyanobacteria fix atmospheric carbon and nitrogen, contributing to the overall productivity of a plant community (Belnap 2001, Belnap et al. 2001, Evans and Belnap 1999). In some cases, vascular plants in areas of well developed crusts have higher accumulations of essential mineral nutrients than in sites that lack crusts (Belnap et al. 2001, Ridenour and Callaway 1997). Crusts may enhance the establishment and survival of vascular plant seedlings by increasing the availability of essential mineral nutrients (Harper and Belnap 2001, Pendleton et al. 2004). Recent research (Li et al. 2006, Zack et al. 2003) has shown that the presence of biological soil crusts significantly increases the diversity and abundance of insect species in arid areas.

Because of its ecological importance and the constant threats to both extent and quality, shrub-steppe warrants special management consideration. One initiative is the Conservation Reserve Program (CRP) which is currently the only large-scale effort designed to manage the restoration of shrub-steppe for use by native wildlife in the Columbia Basin. Administered by the US Department of Agriculture (USDA), this voluntary program pays farmers to take agricultural lands out of production in order to achieve conservation objectives that include enhancing wildlife habitat and reducing soil erosion. Through the CRP in Washington, over 400,000 ha of former croplands have been managed, through planting and restricting use, by the CRP. Although CRP fields were planted with a variety of non-native grasses early in the program, an increasing number of fields have been planted with native grasses, forbs, and arid-land shrubs as the program has developed. In some cases, native shrubs, particularly big sage, and some native herbaceous species have seeded into former cultivated areas from adjacent shrubsteppe habitats. Research, in particular by the Washington Department of Fish and Wildlife (WDFW), has been developed to evaluate the potential role of the CRP in longterm shrub-steppe conservation in the Columbia Basin. To date, this research has mainly focused on wildlife, in particular birds, mammals, and reptiles (Vander Haegen et al. 2004).

In 2004, the WDFW initiated a study designed to evaluate the re-establishment and recovery of biological soil crusts in recovering CRP fields. Although a great deal of research has been completed in other parts of North America and elsewhere on soil crusts, little research has been completed in Washington State. The few regional studies include Daubenmire (1970) who discussed crusts in his seminal work on shrub-steppe, Johansen et al. (1993) studying the effects of fire on the algal and cyanobacterial crust components on the Arid Lands Ecology Reserve, Ponzetti and McCune (2001) who studied soil crust community composition in relation to soil chemistry, climate, and livestock activity in the Horse Heaven Hills, and McIntosh (2003) who studied biological crust diversity and community assemblages at the Hanford Reach National Monument. Although some literature is available that discusses soil crust recovery following fire (e.g., Hilty et al. 2004, Johansen et al. 1986, 1993) or livestock trampling (e.g., Anderson and Rushforth 1982, Anderson et al. 1982, Evans and Belnap 1999, Johansen and St. Clair 1986, Kaltenecker et al. 1999), research is lacking that examines the reestablishment of a biological crust from a completely barren environment, in our case following long-term disturbance by plowing and subsequent crop management. The main aim of this paper is to initiate the assessment of patterns of recovery and succession of the biological soil crust in the CRP research program. This study is the first part of a larger study of biological soil crust recovery, succession, and classification in the Columbia Basin.

Research Constraints

An inherent problem with the study of biological soil crusts is the identification of their component lichen and bryophytes species. A concentrated period of training is often necessary before the identity of most of the taxa can be positively confirmed in the field or, if collected, in the laboratory. However, because most of the soil crust research has been undertaken by scientists who are well-trained in lichen taxonomy, a variety of references are available that help in lichen identification. The most comprehensive work is the recent soil lichen monograph by McCune and Rosentreter (2007), although Goward et al. (1994), McCune and Rosentreter (1995), and Brodo et al. (2001) are also useful. Lichen specimens often need chemical tests to confirm identification so laboratory work or sending specimens to experts is sometimes necessary. Bryophytes pose a more serious problem than lichens because arid land-specific guides are lacking, except for the regional treatments of Flowers (1973) and McIntosh (1986, 1989). Additional references include Lawton (1971) and Rossman (1977). However, the recently published Volume 27 for the Flora of North America (Buck et al. 2007) provides keys and some illustrative material for most of the dryland moss taxa likely to be encountered in the study area. In addition to taxonomic difficulties, sterile lichen thalli or juvenile mosses are often observed in plots, and identification is not possible in most of these cases, so some 'lumping' of observations is necessary. However, these taxa usually make up only a small portion of the soil crust.

Difficulties in identification have been noted Rosentreter et al. (2001) and Rosentreter and Eldridge (2002) who recommend using morphological groups (that is, grouping lichens or mosses of similar ecological function/response) and measuring the cover of these groups in plots versus individual species. Although this method does allow for a more rapid collection time, easier identification, and some quantification of large scale trends, it does not allow for fine-grained species by species analyses, an important step considering that there are probably 'response' overlaps within or between the various morphological groups. Further, mosses are often lumped as either tall or short mosses, and this is unsatisfactory with respect to mosses: in many instances, even if lichens dominate the crust cover, the diversity of mosses is relatively high, and, although we are still learning about their successional responses, many of the 'short' mosses are early successional and others are late successional, and some smaller mosses, if environmental conditions are favourable, may be scored as 'tall' mosses.

A further constraint on the present work is the general study design and the restrictions it places on subsequent data analyses. This soil crust study was broad-based and limited in time and scope, and it was necessary to gather within the pre-determined WDFW CRP sampling design (discussed below). Many of the sites that are used for comparison in the CRP project are kilometers apart, and are dissimilar with respect to a number of environmental variables, in particular soils (e.g., stoniness vs. sandiness), but also slope and aspect. Also, the past history (e.g., grazing, fire, seeding treatments) often varies considerably between sites. Thus, at this time and until more work is completed at a more local scale, it was necessary to provide only qualitative and semi-quantitative analyses of the data.