At the turn of the 21st century the arid southwest experienced one of the worst droughts since the historic droughts of the 1930's and 1950's with accompanying high temperatures especially in the summer of 2002. Also accompanying the drought was panic over anthropogenic global warming that was bringing disaster to the forests and woodlands of the southwest. One such proclamation was made by numerous academics concerning rapid changes in the range of pinyon pine in northern Arizona. Changes that could eventually lead to the extirpation of the species over much of its range since the trees would not have time to extend there range upward and northward to cooler and wetter climates that were conducive to survival. Trees were dying by the millions all across Arizona and New Mexico. The drought had lowered natural defenses the trees had to survive or limit bark beetle infestations from the pinyon ips beetle. It was more than a little frightening to see. We saw it everytime we stepped outside and looked at the south facing slope of Cedar Mountain. Instead of green crowns of pinyon and juniper the dominant color was brown.
Before I try to answer these questions we have to look at what happened in light of the history of these woodlands and all the many factors that effect the survival or not of all these individual pinyon pines. In other words the results of this bug infestation cannot be boiled down to one single factor (climate change). This simple-minded approach to explaining change in forest communities was astounding to hear. Not least because the catastrophic pronouncements of the doom facing pinyon were coming from "experts" with PhD's in ecology, botany, and forestry.
History and Dynamics of Pinyon-Juniper Woodlands in Northern AZ
So where to begin. Let's go back to the 1870's and the introduction of grazing livestock to the region. Prior to the introduction of grazing the pinyon-juniper woodland was dominated by groups of large old pinyon pine and juniper. In the Cedar Mountain area the juniper was mostly one-seed with areas on more favorable sites of alligator and Utah junipers. The majority of the area within these stands was open area occupied by grasses, shrubs, and forbs. The species composition of grasses was likely very different than the current one which is dominated by blue grama and other hardy, grazing tolerant or increasing species. The original mixture was no doubt taller, thicker, and well-established and maintained by frequent fire which easily killed invading trees.
Cattle and sheep grazing had a huge effect on grass composition and abundance. The numbers of livestock put on these lands can hardly be imagined these days. Literally millions of head beat much of this land down to dirt and trees. Fire became non-existent and trees began to occupy most of the space formerly occupied by grasses. By the end of the 20th century tree density increased to levels never previously found on these sites.
So what? Trees are good. The more the merrier. However, without the check on tree growth from natural fire, trees became so thick that individual tree vigor (the health of each tree) was getting worse and worse. Warnings of impending problems were evident. For example, clumps of bark beetle mortality were becoming more frequent in pinyon pine. This was in spite of extremely wet conditions that were prevalent throughout most of the 1980's and 1990's. Something wasn't right. It was obvious that should we enter a dry, hot period similar to those that occurred in the 1930's and 1950's mortality from bark beetles would be epidemic. Between 1999 and 2003 we experience drought and heat on par with those of the 1930's. The result was predictable and in fact had been expected by those of us familiar with the changes that had been occurring throughout the region in pinyon-juniper woodlands as well as the ponderosa pine forest. I remember having numerous discussions at work expressing the expectation that these epidemics were imminent.
You should notice that in this discussion above the impact of climate change was not considered. And though the actual influence of climate change on the drought has yet to be determined, it is almost irrelevant to the discussion. Much more important was the fact that these systems were way out of there natural range of variability in terms of species composition, tree density, and fire frequency. It was and accident waiting to happen.
Results of the Bark Beetle Epidemic and Tree Mortality
So what was the overall effect of all this pinyon pine mortality? Well, as in most things dealing with complex natural systems the effects are a mixed bag of positive and negative. And we must view this reduction in overall tree density as an adjustment back towards a more sustainable condition. A crude, somewhat haphazard adjustment, but a generally positive one in terms of sustainability. So let's look at some of these effects. I will categorize them as "good" or "bad" based on a somewhat subjective analysis blending effects on plant and animal diversity, watershed condition, and tree health both as individuals and as tree communities.
Negative Effects
- Many large old trees that had existed for centuries succumbed to bark beetle attack because of their weakened condition due to competion with the many younger trees established post-grazing, as well as, the large buildup of bark beetle populations in these young trees that overwhelmed the natural defenses of the old trees. These trees had obviously survived many previous droughts but they had never entered one in such a poor state due to unnaturally high tree densities. Densities which had never existed prior to this.
- Due to all this dead material suddenly present on these sites the risk of a possibly catastrophic wildfire fueled by this material was increased temporarily.
Positive Effects
- Large numbers of pine snags have provided increased habitat for snag dependent birds and other wildlife. This is a temporary effect which is now decreasing as snags rot and fall over in great numbers (though they still provide benefits as downed logs)
- Watershed condition has improved greatly due to a huge increase in grasses, forbs, and shrubs which have much better soil holding and nutrient recycling characteristics than the previous tree-dominated landscape.
- This same increase in ground vegetation area over treed area has been beneficial to overall wildlife habitat diversity and no doubt has improved species richness and overall species diversity.
- Horizontal diversity of tree cover has been moved toward a more historical, pre-grazing structure that may favor the reintroduction of natural fire to these landscapes.
So now lets look again at the questions I posed concerning pinyon pine and what really happened here.
Is pinyon pine gone?
Hardly. Though some large old trees were lost, particularly from exposed south-facing slopes on cinder cones like Cedar Mountain, the species is certainly as abundant as ever and occurs still at higher numbers than it did historically. Most, if not all, of these trees had many established pine seedlings under them that were not effected by the beetle because of there small size. In the following photo you can there are at least 8 seedlings that were under these two that died that have been released and are already 3 to 5 feet tall.
Has the range of pinyon pine been reduced?
Not in any noticeable way. Pinyon pine remains abundant (maybe over-abundant) on the same areas it occupied before the beetle epidemic. The panic over rapid change in suitable conditions for species growth due to climate change is not evident. No doubt over the long term climate change (natural or man-influenced) will have an effect on the distribution of tree species in northern Arizona as it has since the last ice age.
What actually has been the result of all this tree mortality?
The effects have been largely positive on balance. Other than the loss of some old, large trees, this adjustment to the pinyon-juniper ecosystem brought on by the drought and resultant beetle epidemic has been beneficial to the system as a whole.
Here is a photo of the the approximate area covered in the 2002 photos of dead trees as it appears now.
Catastrophic and apocolyptic are not words that come to mind now when we see these landscapes. Change in natural systems is natural. When we have forced these systems through our actions into a condition that is outside the area of historic variability, nature will eventually make adjustments. Nature does not put a value on these as negative or positive but we have the responsibility as stewards of these lands to honestly evaluate the condition of them and manage them to improve their health. This may mean action or no action as long as we are consciously choosing and evaluating what the future holds. But we must understand all of the factors that are involved and consider them together. When we go off the deep end and/or focus on one factor without considering what we are missing we run the risk of not making the correct decisions about these lands that we are responsible for.
