A Vegetation History

My Portal story begins in 1991 when Jim Brown offered me the opportunity to be the Portal postdoc. Among other things, this meant I organized the yearly ant census in which we spent about 2 weeks counting the abundance of ant colonies on the experimental plots. In one of my first summers, I hired Don Sias to help with the census. Don was a non-traditional student and had previously traveled extensively throughout the southwest. One day he mentioned to me that the vegetation in the San Simon valley looked pretty “beat up”. By that he meant the vegetation, dominated by shrubs, had the look of a grassland that had become desertified.

I was intrigued by Don’s comment for two reasons. First, Brown and Heske (1990) had recently described a significant increase in grass cover on Portal plots that removed kangaroo rats and mentioned that the site was “near the zone of natural transition from desert to grassland”. Second, while desertification is often associated with overgrazing, Heske and Campbell (1991) found no difference in vegetation across the Portal grazing fence despite 11 years of livestock removal – why hadn’t the grasses recovered with livestock removal if the site had once been a grassland?

When I returned to Albuquerque, I recall asking Jim about the vegetation history of the site. He said that he wasn’t sure and encouraged me to see what I could figure out. That led me to the U.S. General Land Office Surveyors notes. The surveyors described the dominant vegetation they traveled through as each mile-long section line was surveyed. Many, but not all, of the lines near our site were surveyed between 1875 and 1883 and the most common description was “good grass”. However, when the surveyors came back to complete their work in the early part of the 20th century, the descriptions were dominated by the words “scattered shrubs” while grass was not mentioned at all. This change in vegetation coincided with large introductions of livestock into the San Simon valley in the late 1880’s, and then a major drought in the early 1890’s that resulted in tremendous livestock mortality due to starvation.

Our understanding of the recent vegetation change at the site can help to explain the observed shifts in grass cover observed at Portal both over time and across the rodent treatments. It also prompted further investigations into desertification and the role of livestock in affecting soil compaction as a mechanism that helps to explain why reversals of desertification (a recovery of perennial grasses) can require several decades. Finally, it illustrates how careful observations by students and researchers that visit the site can lead to interesting questions and new discoveries.

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I’ve been very fortunate to have worked on the Portal system for almost 30 years. Change seems to be the constant at Portal – you never know what you’ll see and how things have changed since the last visit. This complex system continues to be a source of inspiration as we try to understand it (or, as Morgan says, “unravel her mysteries”).

Tags: history, plant ecology