Post-1935 Changes in Pinyon-Juniper Persistent Woodland on the South Rim of Grand Canyon National Park

Figure 1. Pinyon-juniper woodlands on the South Rim at Grand Canyon National Park.

NPS photo.

The discovery of datasheets from vegetation study plots established in 1935 in Grand Canyon National Park provided a unique opportunity to look at 76 years of change in pinyon-juniper woodlands across the South Rim of the park.

The importance of long-term ecological data

In the late 1990s, a Grand Canyon National Park (NP) employee discovered long forgotten files on a loading dock in the park. The files contained datasheets for 456 study plots that were sampled during a vegetation mapping project in 1935. As it was the oldest set of vegetation data known to exist for the park, as well as the entire Southwest, this was a valuable, exciting discovery. The datasheets contained a trove of information about the major plant communities on both the North Rim and the South Rim of the park—pinyon-juniper woodland and ponderosa pine, mixed conifer, spruce-fir, and quaking aspen forests. This brief focuses on the pinyon-juniper woodland community on the South Rim of Grand Canyon NP (Figure 1) and the changes that have occurred over three quarters of a century.

Long-term, ecological data sets can provide important information about ecosystems, communities and populations. This information helps public land managers make informed decisions about managing natural resources. The problem is that few long-term, landscape scale data sets exist. Although pinyon-juniper woodland is widespread in the western U.S., data covering large areas and changes across decades is scarce. The author of this study demonstrates the value of long-term datasets by examining the changes that have occurred since the original plot sampling in 1935, and making inferences about their relation to climate, fire regimes, and resource management.

Pinyon-juniper plant communities

Pinyon-juniper is largely made up of Pinus spp. and Juniperus spp., but has a wide range of stand structures. One common classification divides them into classes based on canopy structure, understory characteristics, and historical disturbance regimes. The class most common in the southwestern U.S. is pinyon-juniper persistent woodland. It occurs on uplands with shallow, often rocky soils and exhibits a multi-aged structure affected primarily by interactions with climate, insect outbreaks, and fire.

Humans have affected pinyon-juniper persistent woodland for millennia. Native Americans lived in pinyon-juniper persistent woodlands around Grand Canyon, where they hunted game, grew crops, and collected pinyon nuts for food. Euramericans brought widespread livestock grazing to the region in the 1860s, but that was gradually halted after the establishment of Grand Canyon NP in 1919. The practice of suppressing forest fires in the park began around 1905 and continued into the 1980s when fire was reintroduced into forests to mimic its natural role. Park management of pinyon-juniper persistent woodland, though, was largely limited to tree thinning treatments in areas near park facilities to reduce the likelihood of fires. With minimal human disturbance since 1935, the study plots in pinyon-juniper persistent woodland in Grand Canyon NP presented a unique opportunity to look at pinyon-juniper dynamics over an extended period of time.

Resampling the 1935 plots

In 2011, plant ecologist John Vankat used the rediscovered datasheets to locate and resample the original 1935 plots on the South Rim. Using the original map of plot locations, he determined the UTM coordinates for the plots that were in or near pinyon-juniper persistent woodland on the South Rim of Grand Canyon NP. He then went to the park to more precisely determine the location of the plots. Vankat was able to relocate 49 of them, and resampled them using the same methods that had been used in 1935.

For his analysis, Vankat further divided the plots into three subclassses based on the 1935 plot data:

• Core pinyon-juniper persistent woodland (35 plots dominated by conifers <10 m height)
• Transitional pinyon-juniper persistent woodland-ponderosa pine forest (7 plots, with both pinyon-juniper persistent woodland and ponderosa pine forest species)
• Seral pinyon-juniper persistent woodland (7 plots in post-fire succession in 1935)

Changes in tree species composition

Vankat looked at changes in tree density and basal area (trunk size) for the constituent species in each of the three subclasses of pinyon-juniper persistent woodland. He also looked at these changes for different tree diameter size classes of each species. Species that make up pinyon-juniper persistent woodland communities include one-seed juniper (Juniperus osteosperma) and pinyon pine (Pinus edulis).

Core pinyon-juniper persistent woodland exhibited major changes from 1935 to 2011, with its basal area having decreased by 47%. The density of mid-diameter one-seed juniper and pinyon-pine also decreased, and density of small-diameter one-seed juniper increased.

Transitional pinyon-juniper persistent woodland-ponderosa pine forest, which occurs at higher elevations where pinyon-juniper persistent woodland and ponderosa pine intermix, remained relatively stable over the 76 years. This subclass occurs on the most mesic of all the sites and therefore likely did not experience as much drought stress as core pinyon-juniper persistent woodland.

Seral pinyon-juniper persistent woodland represented the early, post-fire successional stage in 1935. Unsurprisingly, these plots changed the most of the three pinyon-juniper persistent woodland subclasses, with large increases in total tree density and basal area in the decades following fire. The seral plots appear to be shifting from a shrub-dominated successional stage to a tree-dominated stage.

Figure 2. Bert Fire 2016. This lightning-caused fire in pinyon-juniper woodlands south of Grand Canyon National Park was allowed to burn naturally to clean out accumulated fuels and improve forest health.

(https://www.fs.usda.gov/detail/kaibab/news-events/?cid=FSEPRD503989)

Effects of climate and fire on pinyon-juniper persistent woodlands

Researchers have found that changes in climate, principally precipitation and temperature, drive tree recruitment and mortality in pinyon-juniper persistent woodland. At Grand Canyon NP, between 1935 and 2011, periods of below average precipitation alternated with periods of above average precipitation. This likely resulted in alternating periods of higher drought mortality for pinyon-pine, which is more sensitive to drought than one-seed juniper, with intervening pulses of tree regeneration and establishment. Studies have also shown that periods of pinyon pine mortality are often accompanied by establishment of one-seed juniper, and such an increase in small diameter one-seed juniper occurred here.

Fire is another driver of pinyon-juniper persistent woodland dynamics, with low-severity surface fires being uncommon and generally having little effect (Figure 2). However, stand-replacing fires that occur every 400–600 years result in high tree mortality, followed by tree recruitment and establishment. Once burned, it takes 200-300 years for pinyon-juniper persistent woodland to return to the unburned state.

Implications for natural resource management

This study provides unique information on pinyon-juniper persistent woodland. It uses data obtained from resampling the oldest set of landscape-scale, quantitative study plots to document multi-decadal changes. The data obtained was essential to understanding current conditions and how pinyon-juniper persistent woodland changed post-1935. This, in turn, can help managers to develop ecologically-based management practices. Vankat focuses his management recommendations on Core pinyon-juniper persistent woodland. The data show this is a dynamic system in which alternating phases of tree recruitment and mortality are related to climate.

He recommends the following management strategies for pinyon-juniper persistent woodland in Grand Canyon NP:

• Management of pinyon-juniper persistent woodland should not focus on restoring historical stand structure and composition, but instead let naturally occurring processes prevail wherever possible. These processes include fire and insect outbreaks that historically followed droughts and led to varied spatial and temporal stand composition and structure.
• Active management may be needed when human activities may substantially alter natural processes. For example, action should be taken when invasive plants, like cheat grass, threaten to change the historical fire regime.
• Management of vegetation types adjacent to pinyon-juniper persistent woodland should consider the potential long and short term effects of these actions on pinyon-juniper persistent woodland.

Finally, as the climate continues to warm and droughts become more frequent and widespread, land managers are likely to face additional challenges in managing pinyon-juniper persistent woodland.

Information presented here was summarized from Vankat J. L. 2017. Post-1935 changes in Pinyon-Juniper persistent woodland on the South Rim of Grand Canyon National Park, Arizona, USA. Forest Ecology and Management. 394:73-85.

To read about other studies related to the 1935 vegetation data at Grand Canyon National Park, see:

Vankat, J.L. 2011. Post-1935 changes in forest vegetation of Grand Canyon National Park, Arizona, USA: Part 1 – ponderosa pine forest. Forest Ecology and Management 261(3): 309-325.

Vankat, J.L. 2011. Post-1935 changes in forest vegetation of Grand Canyon National Park, Arizona, USA: Part 2 – Mixed conifer, spruce-fir, and quaking aspen forests. Forest Ecology and Management 261(3): 326-341.