1. George Johnson, a ranger at Petrified Forest, initiated a range recovery study in 1972, on two transects representing Clayey Fans. The transects were very close to one another and represented a former corral and watering area for cattle (Johnson 1972). Johnson sampled vegetation in 1972, 1974, 1976, 1978, 1982, and 1984. Peter Rowlands relocated and sampled the transects in 1992. Regrettably no one has resampled the transects after the drought years of 1996 and 2002-2003.
Rowlands (1992) identified several interpretational problems, chief among them: 1) The transects are in atypical low-lying areas which have higher than typical potential for vegetative cover, 2) Johnson sampled in early spring in 1972 & 1974, but shifted to the summer monsoon season afterward, 3) the initial years of the study also coincided with increasing precipitation, making it difficult to separate dynamics driven by succession, and those driven solely by precipitation, and 4) Johnson used a small number of points for a point intercept method. Nevertheless, these data represents the only quantitative document of temporal change in vegetation on Clayey Fans.
Figure 1. Increase in vegetation over time after the cessation of grazing on two transects on Clayey Fans (Click for larger version).
Total cover increases impressively (almost an order of magnitude on transect B1), and nearly linearly over this time span (Figure 1). Rowlands cautioned that the increase in Johnson's data may be due to annually increasing precipitation during this time. However, the vegetation continued increasing at a similar rate through the late 80's and early 90's, when this trend did not hold, in my opinion suggesting that a simple "time since fencing" model is the most parsimonious predictor of this change.
Figure 2. Vegetation change along Johnson (1972) transects. Rows are points on the transects, columns represent years. Basal cover is shown, except when there was no basal cover, canopy cover is shown. Rowlands (1992) did not document spatial information, therefore the 1992 data is omitted from this figure (click for larger image).
Vegetation change did not conform to the model expressed in the ecological site description. Under intense grazing pressure, we would have expected a major decline in Sporobolus airoides, and replacement with annuals, cacti, Ericameria and Guttierrezia. The latter three were never observed. The 1972 communities were Sporobolus-dominated, and remained so throughout the monitoring period. There was a clear spatial dependency from one year to the next, where new colonization events strongly tended to be in a previously occupied position on the transect or adjacent (Figure 2). Most of the occupied portions on the transects can be traced backward to a position previously occupied by Sporobolus, or one adjacent. This paints a picture of Sporobolus as grazing-resilient facilitator of community recovery. Transect B1 seemed to add species over time, with multiple shrubs becoming more important, especially Atriplex spp. In contrast, transect B2 had most of its species present from very early on.
2. Petrified Forest National Park is composed of north and south blocks connected by a narrow strip of park lands. Fencing and phasing out of grazing was conducted gradually, starting in the south in the mid 1930's. The north boundary fence was not completed until 1963. The distribution of clay fans within the park is also bimodal with a northern cluster and a southern cluster. Therefore, differences in the vegetation monitoring dataset (Thomas 2009) between the north and south clusters in terms of vegetation composition and cover might reflect different lengths of time since fencing. Of course we cannot rule out that any differences are due to other factors that differ among the north and south clusters.
The total vegetative cover is clearly different between these two clusters: the longer rested south cluster has 50% greater vegetative cover than the north. Species richness is almost identical among the two clusters and the variance is large. An NMDS ordination of species composition indicates a modest degree of separation among the clusters. We can account for which species account for this separation in 2 ways: 1) rotating the ordination so that axis 1 maximizes the difference between north and south clusters, and then obtaining the correlations of each species with axis 1, and 2) conducting and indicator species analysis. The most important species were identified as having an indicator value > 25. Their correlation with the "northerliness" axis is presented (Kendall's Tau) along with the probability value generated in the indicator species analysis. The major correlates with the southern group -- those fenced longer -- were B. gracilis (tau = -0.38, P = 0.001), and P. jamesii (tau = -0.32, P = 0.01), and Salsola tragus (tau = -0.49, P = 0.001). The major correlates with the north group were Isocoma drummondii (tau = 0.40, P<0.0001), style="font-style: italic;">Parryella filifolia (P = 0.03). B. gracilis and P. jamesii, though often thought of as grazing increasers, are 2-3 times more abundant in the longer rested southern cluster. Surprisingly, Salsola is also more strongly associated with longer rest in this data, although it tends to be associated with less vegetated, erosion prone landscapes. It is difficult to to interpret the association of I. drummondii and P. filifolia with the more recently fenced north cluster. Both are known from more sandy habitats, and the germination of I. drummondii is suppressed by salt (Baskin & Baskin 1998) suggesting that this may have more to do with gradients of sand deposition than grazing; a hypothesis that we cannot rule out for any of these patterns.
Figure 3. Vegetational differences among north(fenced later) and south (fenced earlier) clusters of Clayey Fans sampling sites (Thomas et al. 2009). a. Cover is ~ 50% greater in the southern cluster. b. Richness does not differ. c. An NMDS ordination showing a moderate degree of separation between north and south clusters in unrelativized vegetation composition. (click image for larger version)
Conclusions: Despite that both of these analyses have their flaws, we can draw some inferences based upon the agreement between these two datasets. There is much uncertainty in drawing these conclusions, but they are based on the best evidence available:
1. The data do not strongly support that Sporobolus airioides is a grazing decreaser any more than other community members. Like the majority of species its abundance seems to be lower when time since grazing is shorter, however it may act as a pioneer facilitation the establishment of vegetated patches. There is no landscape-wide difference in abundance for this species among north and south clusters of sites.
2. Likewise the data do not strongly support that Bouteloua gracilis or Pleuraphis jamesii are grazing increasers. They were largely absent from Johnson's transects in 1972, and they are distinctly more abundant in the south cluster of the sites from Thomas et al. (2009) which have been fenced longer.
3. The most obvious putative effect of grazing is that cover can increase dramatically when grazing ceases. This demonstrates a large degree of resilience.
4. There is no evidence that Ericameria of Guttierezia increase in the presences of grazing, or decrease in its absence, in this ecosite. Ericameria may be common near streams, but otherwise both of these species are minor players.
References
Baskin, CC, Baskin, JM. (1998) Seeds: Ecology, biogeography, and evolution of dormancy and germination. Academic Press, San Diego
Johnson, GE (1972) (Appendices 1974, 1976, 1978, 1982, 1984). A report on a vegetative recovery study made on two study plots in Petrified Forest National Park. National Park Service, Unpublished Report.
Rowlands, PG (1992) Vegetation monitoring at Petrified Forest National Park: I. Preliminary report and summary of vegetational change observed on long-term monitoring plots. Cooperative Park Studies Unit, Northern Arizona University, Unpublished Report.
Thomas, KA, McTeague, ML, Cully, A, Schulz,K, Hutchinson, JMS (2009) Vegetation classification and distribution mapping report: Petrified Forest National Park. National Resource Technical Report NPS/SCPN/NRTR—2009/273. National Park Service, Fort Collins, Colorado.
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