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Research

The Walnut Creek Center for Education and Research provides access to many, as yet under-utilized, research resources in the Central Arizona Highlands. The Center’s leasehold is ideal for intensive research that needs protection from vandalism, frequent maintenance, water, or energy. This level of protection is a rarity for field research conducted in northern Arizona. In the surrounding region, the wide variation in habitats, the existence of relatively unaltered and protected public lands and the willingness of the Prescott National Forest to allow manipulative treatments present tremendous potential for biological research. Ultimately, we hope that research conducted at WCCER will help land managers in the region prioritize restoration activities and to develop specific restoration plans. This is a topic of great interest across multiple ecosystems in the area. Several specific restoration issues include 1) how best to return ponderosa pine forests to structures seen before fire suppression/grazing, 2) understanding the implications of pinyon-juniper woodland encroachment on grasslands, and 3) the need to balance riparian protection and restoration with ongoing economic uses of the land such as grazing.

Recent research at Walnut Creek

Abbott’s Riparian Area Survey

Abbott (2000) preformed research on the riparian area at Walnut Creek (see riparian area map below) to gain a baseline understanding of the vegetation occurring in this area. During this study 10 transects were established, surveyed and permanently marked for future re-surveying (see Abbott 2000 for transect design and location). Their study measured understory plant vegetation cover as well as foliar high density along these riparian transects. In 2008, 8 transects were re-surveyed and 2 new transects were established in the meadows adjacent to the riparian areas.

See examples of data collected and maps by clicking the image below.

Walnut Creek Biological Inventory Final Report

Download the report.

Download ESRI shapefile of the foliar height density transect locations

Land Use and Habitat Change

Vegetation change in response to variations in human land use is a major theme for the WCCER research community and a number of characteristics of the Walnut Creek region suggest that it could yield significant insights in this line of research. There are long-term plots in the Prescott National Forest on which forest structure and understory vegetation have been monitored. Researchers from the School of Forestry and the Ecological Restoration Institute at NAU are working with Prescott National Forest to relocate and resample these plots. The Prescott National Forest has detailed vegetation maps of the area from a survey done in 1912. We plan to follow the model of researchers at NAU who have digitized similar data for the Coconino National Forest and compared the vegetation to current data. This will then provide a valuable resource for many researchers and classes.

Resources for examining pre-historical vegetation change are rich in the region. There are archaeological resources, which will aid in the understanding of pre-historical human impacts and there are records of pre-historical vegetation preserved in packrat middens and peat deposits. Tree rings and fire scars also represent an important source of information about processes controlling vegetation in the area. Initial surveys indicate longer fire return times than found in other areas of the Prescott National Forest.

Restoring Endangered Riparian Ecosystems

The Center’s namesake, Walnut Creek, is itself an important resource. Walnut Creek and its tributary, Apache Creek, are rare perennial creeks in this semi-arid region. They are home to lowland leopard frogs (Rana yavapaiensis), canyon tree frogs (Hyla arenicolor), and speckled dace (Rhinichthys osculus) and native riparian trees such as willows, cottonwoods, and walnuts occur along the creek and in its floodplain. This riparian forest habitat is endangered in Arizona (Noss et al. 2000). The section of the creek that runs through the Center’s leasehold have been fenced to exclude cattle and horses since 1999. A detailed biological inventory documented the condition of the creek and riparian area before the fencing so that its effects can be studied (Abbott and Glomski, 2000). This lack of grazing makes this section of Walnut Creek an even more important resource. WCCER hopes to be a model site for riparian restoration in the area.

In addition to research on the restoration of this riparian zone, some unusual species occur there that warrant further study. For example, asexually reproducing stands of the hybrid species Hinckley’s cottonwood (Populus x hinckleyana) are thriving near the active channel of Walnut Creek, where it appears that exposure to air has stimulated root sprouting. Research has shown that hybrid cottonwoods may be important for maintaining biodiversity of dependant species (McIntyre and Whitham, 2003). The variety of habitats present in and around the riparian corridor supports an impressive diversity of species. The biological inventory of the field station described 279 species of vascular plants, 17 species of reptiles, 4 species of amphibians, 131 species of birds and 37 species of mammals (of which 9 were bats). It also described some unusual vegetation associations, including a terrace ecotone where flood dependent trees such as Arizona walnut (Juglans major) and Fremont cottonwood (Populus fremontii) are found alongside typically upland species such as Utah juniper (Juniperus osteosperma). This association may be due to the evolving morphology of the stream channel and consequent reduction of flood frequency at the terrace level (Abbott and Glomski, 2000).

As an important watering hole for many species Walnut Creek has been the focus of several surveys of vertebrates and their movements across the landscape between the creek and upland habitats. However, much of the population-level research at Walnut Creek has focused on species interactions between plant and insects or between small mammals, their food sources, and disease causing organisms. Notable among these projects are the ongoing studies of rodents that carry hantavirus. The high degree of variability in precipitation from year to year make the area valuable for studies seeking to understand the connections between precipitation, plant growth and insect and disease outbreaks. The infrastructure at Walnut Creek also offers the opportunity to back up such correlation field studies with manipulative experiments.

Fire Behavior in Semi-Arid Southwest Ecosystems

Before widespread settlement of the West by Euro-Americans a high surface fire frequency characterized most ponderosa pine forests of the southwest, including at least portions of the PNF (Brown et al. 2000, Covington and Moore 1994b, Dietrich and Hibbert 1988, Sneed et al 2002). Since the late 19th century, natural fire has been excluded or reduced in many ecosystems by post-settlement land uses such as mining, logging, livestock grazing (Savage and Swetnam 1990), fire suppression, and changes in climatic patterns (Brown et al. 2001, Grissino-Mayer et al. in press, Swetnam and Betancourt 1990). Understanding and restoring fire is central to newly emerging paradigms for ecological restoration of forests and the ecosystem management approach that has been adopted by federal agencies, including the USFS (Covington and Moore 1994a, Fule et al. 1997). Fire frequency, intensity, and spatial extent can be reconstructed quite accurately in ponderosa pine forests using dendrochronological analysis of fire scarred trees if the forest historically experienced relatively low intensity fires (Brown et al. 2000). Recent research has demonstrated that fire regimes in ponderosa pine forests varied substantially through time and across space during the several centuries prior to Euro-American settlement in the mid to late 1800s (Baker and Ehle 2001). Most forests of pure ponderosa pine in Arizona and New Mexico were characterized by frequent, low-severity fires that maintained open forests with trees of many size and age classes (Fule et al. 1997). Despite a broad network of fire history sites in ponderosa pine forests throughout western North America, a large gap still exists in central Arizona, where forest managers are developing new management plans that will incorporate fire as a natural ecosystem process in the extensive ponderosa pine forests of the region. One pilot study (Sneed et al., 2002) has addressed fire history in ponderosa pine forests of the central Arizona highlands.

Ecological Responses to Climate Change

There is also reason to believe that ecosystems in the Walnut Creek region may be particularly sensitive to natural and anthropogenic changes in climate. Although temperature measurements at Walnut Creek only extend back into the mid 1950’s, patterns there are likely to be similar to patterns measured at Prescott, approximately 55 km to the southeast. A linear regression analysis of temperatures measured in Prescott from 1899 to 2002 indicates that average annual temperatures have increased by 0.16 + 0.03 °C per decade. The increase in the annual average of minimum temperatures is 0.32 + 0.03 °C per decade. There also appears to have been a distinct shift in the snowfall patterns at Walnut Creek during the 20th century. Annual snowfall measured at Walnut Creek is shown in Figure 3, along with a 5-year running average. From the beginning of the winter of 1916-1917 through the end of the winter of 1956-1957, the average winter season snowfall was 54 cm, with a standard deviation of 46 cm. From 1957 to 2002, the average snowfall was 5 cm with a standard deviation of 9 cm. Over this same period there was no clear trend in total precipitation.

The ponderosa pine type forest found in the Walnut Creek region exists at the lower end of the elevation range where it is typically found. Given that winter snowfall is the most important factor determining availability of soil moisture in the critical spring germination and growing season, the decrease in snowfall in the Walnut Creek region could have a dramatic impact on distribution of plant communities. During the last two years, the Prescott National Forest has Ecological Responses to Climate Change experienced a dramatic die-off of ponderosa pines (50% in many areas) that resulted from the combination of drought and an infestation of pine engraver beetles (Ips and Dendroctonus).