Evaluating the Use of Mycorestoration on Decommissioned Roads in Arizona

While restoration scientists have established protocols to guide the process of road ripping, little research has been done to assess the effectiveness of these procedures in restoring ecosys­tem processes (Switalski et al. 2004; Elseroad 2003). The Society for Eco­logical Restoration defines a restored ecosystem as self-sustaining in struc­ture and function, resilient to normal ranges of stress and disturbance, and able to interact with contiguous ecosys­tems (SER 2002). A road that is merely ripped and then left to recover on its own may not meet these criteria. Luce (1997) concluded that ripping leads to a temporary and marginal improvement in hydrologic and ecological function.

Fungi and Roads
Mycorrhizal fungi form important relationships with over 90% of the plants on Earth, whereby the fungus receives photosynthates from the plant while the plant benefits through improved water relations and nutrient uptake, and protection from pathogens (Allen 1991). Saprophytic fungi are primary decomposers of dead or­ganic material, and their underground vegetative structure, called mycelium, is important in nutrient transfer, soil stability, and ecosystem function (Hunt and Wall 2002; Setala and McLean 2004). Road construction and subsequent use removes organic matter and leaves a compacted mineral surface devoid of fungal propagules (Amaranthus and Trappe 1993; Harvey et al. 1979). Decommissioned roads, ripped or not, are also lacking in fungi that complete essential processes such as nutrient cycling and plant community support (Morman and Reeves 1979; Reeves et al. 1979). In theory, then, effective road restoration may utilize fungal inoculum to assist plant and microbial communi­ties to achieve pre-disturbance condi­tions.

There have been only limited explo­rations of the possibilities of using fun­gal inoculum in ecosystem restoration. Johnson (1998) examined the effects of mycorrhizal inoculation on a weedy non-mycorrhizal plant (Salsola kali) and a perennial mycorrhizal plant (Panicum virgatum) in an abandoned mine set­ting. Results indicated that plots with mycorrhizal inoculum resisted invasion by the exotic weed and enhanced the growth of the perennial native grass, but manipulating conditions to favor mycorrhiza formation (addition of soil organic material) may be more cost effective than the expensive process of inoculation. Stamets and Sumerlin (2003) have used fungal inoculum to restore roads in the Pacific Northwest. In a practice they call “mycofiltration,” saprophytic and mycorrhizal fungi are inoculated into mulch to accelerate de­composition and provide the improved conditions offered through mycorrhizal associations. Advantages of this tech­nique include sediment containment, moisture enhancement, habitat recov­ery, soil structural improvements, and aesthetic enhancement (Stamets and Sumerlin 2003).

There has never been a rigor­ous study done to evaluate the use of fungal inoculum in road restoration in a southwestern ponderosa pine forest. The purpose of this study was twofold: to evaluate the effect of mycorrhizal inoculum on plant establishment and abundance on a closed and ripped road in a northern Arizona ponderosa pine forest, and to evaluate the efficacy of saprophytic fungal inoculum in coloniz­ing ponderosa pine mulch.

Experimental Design
In August, 2004, three logging roads at approximately 7400’ elevation in a ponderosa pine/gambel oak forest 6 miles west of Flagstaff, Arizona were selected for the experiment. The roads were built to facilitate a 2003 timber sale and were closed and ripped by September 2003. The roads were flat (0-2% slope), with low canopy cover and clayey soils of basaltic origin.

Upon each road, five 11m long by 2m wide experimental blocks were installed end-to-end in succession. Blocks were divided into four 1.5m by 2m plots with 1m by 2m buffers between each plot, resulting in 2m long buffers between blocks. Four treat­ments were assigned to the plots: A) control, B) mycorrhizal inoculum C) mulch + mycorrhizal and saprophytic inoculum, and D) mulch. All plots were seeded with a native seed mix consist­ing of Elymus elymoides, Festuca arizo­nica, Muhlenbergia wrightii, and Lupinus argenteus. Ponderosa pine woodchips (mulch) were applied to a depth of ~4 inches after seeding. The mycorrhizal inoculum was “Mycogrow Micronized Endo-Ecto Seed Mix” and the sapro­phytic inoculum was the “dowel spawn” of the species Hypholoma capnoides.

Results

Vegetation Response
We collected data in mid-October, 2004, two months after application, and mid-October, 2005, 14 months after application, identifying each species and counting every plant. In 2004, Treatment B plots (those with only mycorrhizal inoculum) had the highest abundance of grass seedlings, but con­trols had the highest species richness. In both cases the differences between those two non-mulched treatments were insignificant.

In 2005, after a full year, control plots had the highest richness and abundance for all species, though not significantly more than Treatment B. Mulched plots (Treatments C and D) had significantly lower species rich­ness, grass seedling abundance, and total species abundance in both years (ANOVA P<0.001). While mulched plots (Treatments C and D) suppressed the establishment of most species, oak seedlings preferred them over non-mulched (Treatments A and B) (ANOVA P=.0157), and did especially well on one of the roads.

Fungal Response
In 2004, 70% of inspected dowels showed successful survival and colo­nization, but by 2005 only 34% showed signs of success. The inoculum on one of the roads was vigorously colonized by a resident soil fungus, and there was 0% survival by 2005. The two roads that did better were examined more critically for resident soil fungi. Sixty-eight percent of sampled subplots that had been inoculated were colonized by resident fungi, and 60% of mulch-only plots were colonized. There were no relationships between success of inocu­lation and presence of resident fungi.

Discussion
The results of this study provide little support for fungal inoculation of ripped roads in Arizona ponderosa pine forests at this time. The mycorrhizal inoculum appeared to have no effect on plant establishment in mulched or non-mulched plots. Johnson (1998) used soil from a nearby location as a source of fungal inoculum and had success, and Bagley (1999) suggested using local soil to prevent introducing exotic fungi. It is likely that the inoculum in the present study simply died as it was not adapted to local conditions. Also, St. John (1996, 1998) noted that inoculum placed on the soil surface will have less chance of working than inoculum that is buried. In this study inoculum was lightly raked into the soil and it prob­ably was not deep enough. Future trials with powdered inoculum would benefit from adding it at the time of ripping to increase mixing in deeper soils. As plant roots tend to be most dense near the surface, any deeper than 8 inches for the inoculum is not advised.

Mulch had a strong inhibitory effect on plant growth, except for favor­ing the sprouting of oak seedlings. The reduced understory plant establish­ment is unfortunate, but in terms of per­manently restoring roads, oaks may be a more effective plant. As they mature, they will become natural barriers to ve­hicle passage, contribute to organic soil creation with their annual leaf fall, and their roots will be effective at loosening the compacted soil of the road.

Since the majority of mulched plots became colonized by fungi that already existed in the soil or nearby, the inocu­lation of the saprophyte was unneces­sary. That the mulch was colonized by local fungi is a testament to the ubiq­uitous nature and effective life history of nature’s most important degrader of wood. Those interested in trying this idea in different regions should con­sider locating or developing inoculum that is adapted to the environmental conditions of their region. Until aggres­sive regional strains of mycorrhizal or saprophytic fungi can be cultured and produced for distribution, like in the Pacific Northwest, these methods will remain less effective at restoring roads than spreading local soils and mulch and then allowing local fungi to colo­nize naturally.

— Joseph Trudeau is a Research Assistant at the Ecological Restoration Institute and a student at Northern Arizona University in Flagstaff.

References

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