Article reviewed: Post-1935 changes in forest vegetation of Grand canyon National Park, Arizona, USA: Part 1 – ponderosa pine forest
By Vankat, J.L. 2011. Post-1935 changes in forest vegetation of Grand canyon National Prak, Arizona, USA: Part 1 – ponderosa pine forest. Published in Forest Ecology and Management, Vol 262 pp 309-235.
The plot line: In 1935 the National Park Service sampled hundreds of vegetation plots in Grand Canyon National Park (GCNP). In 2004, the author of this paper relocated and remeasured many of these plots. Ninety-nine of the plots were in ponderosa pine forest (PPF); these PPF plots were analyzed in this article. The author looked for changes in tree density and basal area between 1935 and 2004. Comparisons were made based on size class, species composition, and PPF subtype (dry, mesic, and moist).
Because dozens of comparisons were made, there was a battery of findings, with some species and forest types showing significant increases, some decreases, and others showing no change. Perhaps the most surprising finding was that when all PPF forest types were analyzed together, there was no change in tree density and basal area actually decreased. These results were accompanied by changes in species composition: The number of quaking aspen decreased by over 80% and the number of white fir increased by a shocking 631%.
When the three PPF subtypes were analyzed separately, a different set of changes were seen. Dry PPF forest, which is found at lower elevations and is transitional with pinyon-juniper vegetation, showed no statistically significant changes. Mesic plots, on the other hand, showed a large decrease in basal area, mostly due to loss of ponderosa pine in the medium size class. Despite this decrease in basal area, tree density did not decrease. This can be explained by a significant increase in the density of small trees, including a staggering 2351% increase in small white fir. Finally, moist PPF showed a large decrease in total tree density, mostly due to loss of quaking aspen. In these moist forests, quaking aspen went from being the most common tree in 1935 to being third most common after ponderosa pine and white fir in 2004.
In the second part of this article, the author attempted to place his findings in the context of other studies on historical forest structure in GCNP. Estimates of tree density and basal area at different dates were plotted and trend lines were fit to the graphs. The results in this section of the paper were highly variable. They suggested an increase since the 19th century in total density of saplings and trees. However, for just trees--excluding saplings--the graphs showed no clear trend.
Relevant quote: “Previous studies...in the Southwest indicated that historical dynamics...have involved increases in forest densities... However, this study -- the first to examine multi-decadal changes across a never-harvested Southwestern PPF landscape using resampled historical plots -- documents that changes...also have included decreases since 1935."
Relevance to landowners and stakeholders:
Historical forest information is crucial to our understanding of current forest conditions, management goals, and appropriate management techniques. However, previous studies in GCNP (and elsewhere) have relied on estimates from extrapolated data, reconstructions based on tree rings, and sampling of contemporary reference sites thought to represent historical conditions. These methods all have their own particular pitfalls that may introduce large uncertainties about historical conditions. For example, extrapolation of data depends on assumptions about rates of change. Thus it may be dangerously circular to use these studies to form conclusions about how forests have changed during the 20th century. Tree ring studies may have inaccuracies because of loss of evidence from older trees. Reference sites present problems because the current and recent impacts of climate, fire, and diseases can never exactly duplicate historical conditions. Because the paper discussed here relied on direct measurement of contemporary and historical plots, landowners may feel more confident about its findings.
The lack of increase in density in any of the PPF types or in overall PPF was surprising. One explanation is that forest densities had already increased by 1935. Another is that increases in small trees were offset by losses of larger trees. The larger trees may have suffered from competition with the smaller, vigorous trees. In either case, this paper highlights the insight that forest changes are not monotonic: even in the absence of fire or other disturbance, forests density may increase, decrease, or remain constant. Different forest types and subtypes may change at different rates and in different directions.
Relevance to managers:
This paper highlights some of the uncertainties about past PPF structure. Yet there are even greater uncertainties regarding the future. Climate, invasive species, fire, and societal pressure will have unpredictable, synergistic effects on forests. The author of this paper suggests that, in light of the uncertainty about both past and future, managers need to shift their focus away from emulating the "historical range of variation" (HRV) and instead think in terms of avoiding "thresholds of potential concern" (TPC). For example, future climate conditions may make it impossible to achieve the HRV in certain places. The TPC approach provides a much broader range of acceptable conditions.
Critique (I always have one, no matter how good the article is):
The first section of the paper describing the remeasured plots was well done and full of novel findings. The second section, in which the remeasurements were combined with other data on historical forest conditions, was weaker and detracted from the robust findings in the first section. The regressions in the second section were presented without any significance values. Presumably that is because each fit was based on only a few scattered points and they would not have been statistically significant. If that is the case, these graphs would have been better presented as eyeballed trends, without the R2 values and exact regression parameters.
Finally, the unique management history of GCNP suggests that this paper's findings might not translate well to areas outside of the park. Since the study site has never been harvested and has experienced some prescribed fire since the 1980s, its developmental trajectory might be quite different from forests in which timber has been cut and all fire has been suppressed.
Posted By - Assistant Professor of Cooperative Extension / Co-Director, Berkeley Forests / Adjunct Associate Professor of Forestry