This is a tale of two grass species introduced into two different ecosystems. Both are bromes, both are annuals; they are similar in many ways. Both proved to be successful invaders, but with vastly different effects.
Japanese brome was introduced into a Western wheatgrass- dominated system. Will a fire management plan to control Japanese brome work? Will Japanese brome plots burned in April decrease in density after the first year? That depends on the weather. The density of the seedlings is largely a function of precipitation in the fall and the amount of litter on the ground. In a dry year, litter amount is very important; in a wet year, it is less important.
Field experiments and a simulation model were used to determine whether fire control of Japanese brome would be possible. Results indicate that we can control but not eliminate Japanese brome with fire. Without fire western wheatgrass and Japanese brome made up 95% of the standing crop. There is more productivity in the system with the brome than without it. Other important elements of the system are not affected - it is a "successful" introduction - there is not much benefit but not much harm either. There is a reduction of some native species, but no major ecosystem effects.
In southern Idaho, Bromus tectorum or cheatgrass, has been introduced and now dominates the system. The prevailing management approach has been to seek more competitive species to plant. The biggest problem is that fires are more prevalent once the cheatgrass is established. Some 83% of fires occurred in cheatgrass dominated areas, and 90% of burned acreage occurred in cheatgrass dominated areas. It's not that the cheatgrass out-competes the natives - it's the alteration in the fire regime that's detrimental to the native plants. So we've often taken the wrong approach to the problem. The region is critical winter rangeland for mule deer and other animals, but cheatgrass has little value in the winter.
Thirty ungrazed study sites in the Snake River plains were selected. In one 700 acre Kipuka; there had been no grazing and no fires for 110 to 120 years. There were lots of perennials and sprouting shrubs, and cheatgrass was there waiting for a break. After the area burned, the sagebrush was dead. The soil under the shrubs has 4% organic matter compared to 0.5% between shrubs, so new shrubs tend to sprout under old ones. There was no sagebrush left at all after a couple of fires in a few years. Fires were typically small; sagebrush seeds are very short-lived (less than a year to less than two years). If a second fire occurs before replacement, that's the end of sagebrush there. One area has burned 16 times in 30 years. In that area, all the remaining species were introduced varieties and all were annuals.
Increasing fire frequency wipes out native, perennial species and encourages invading annuals. Cryptogams had almost 100% ground coverage without fires and perennial grasses dominated. These both drop off very fast with increasing fire frequency while exotic annuals increase proportionately. None of these areas were grazed, so vegetation could not be affected by grazing in these areas. Species richness declines based on fire probability in any given year. Small patchy fires were part of the system naturally, but what we have now, in cheatgrass-dominated areas, is a positive feedback loop, an accelerating spiral in which more cheatgrass yields more fires which yields more cheatgrass. The continuity of fine fuel in the cheatgrass dominated areas causes fire to move easily through these lands.
But in the first example of the Japanese brome, fire didn't effect the system much at all, in contrast to the experience with cheatgrass. Changes in nutrient cycling and water retention potential have profound implications. Shrubs trap snow; without shrubs, moisture flows out of the system. By killing native species and changing the fire regime, the invading species alters the hydrology and nutrient cycle of the system. Any useful functions of animals are lost because many animals don't come into cheatgrass environments.
So the lesson of this example is that we need to address the fire regime, rather than focusing on competing with the invaders, since competition is not the primary problem. There is not enough money to re-seed all these areas and besides, the most important factor is to reduce fire frequency. Programs that plant strips of crested wheatgrass ("green strips") have proven valuable in reducing fire frequency. Cheatgrass does not readily establish in these strips and they tend to stop fires. Although the green strips produce more fuel, its poor distribution stops many fires.
Grazing is an important part of the system and must be considered in any recovery plan. Recovery won't occur with poor grazing management. Although these crested wheatgrass strips do not develop into diverse systems; they are better than cheatgrass. When systems are degraded to such an extent that natives can't establish, how do we begin the recovery of lands in this poor a state?
We need to initiate processes that might lead to eventual return to native, healthy systems. Sometimes there is no possibility of returning a system to a native state so we should look at adding something of value to the system while recognizing that we can't restore it to its pre-disturbance state. It can help to just get a stable and worthwhile system established.
So there is a marked contrast between the effects of these similar grasses, introduced into different systems, but with very different results. There's a disturbance threshold, that once crossed, makes recovery difficult. The scale of this problem is depressing - it's huge. Was the process set in motion by grazing? These kinds of problems are usually associated with grazing but they can also occur without grazing. Grazing may accelerate the problem, but removing cattle in itself won't solve the problem once it is underway; we must also change the fire regime.
Some of these areas were critical winter range for pronghorns and mule deer. Now these animals are hanging around train tracks and potato fields because their old range is useless. Some areas can be grazed while others shouldn't be, and biologists aren't the ones who make these decisions. The American philosophy that "we can fix anything" is challenged by these kinds of problems. Irreversible changes in systems can and do happen.
If you take a pristine system of shrubs and grasses and throw heavy livestock grazing into it, the animals take out the fine fuel before the brush, maintaining the system at a middle state. We could not predict ahead of time whether Japanese brome or cheatgrass would become the problem invader. It's hard to predict which species will become problems in which habitats. Human impacts can also prevent fires - roads, drainage ditches, etc., but we usually find ourselves starting fires.
Strategies for Coping with Plant Invasions
Dr. Whisenant has used fire, chemicals, and mechanical strategies to kill invasive plants, but he seems to keep facing the same problems over and over. Why? Perhaps because we're treating the symptoms of these problems rather than their causes. Now he thinks carefully and asks lots of questions before attempting control strategies. He builds simulation models to design "management" programs (not "eradication"). Finding the weak link in a population offers the best chance of regulating that population in the long run.