March of the Chollas

The cholla cactus (Cylindropuntia imbricata) is a fairly common sight in the Arkansas River basin of southeast Colorado, where it can occur as scattered individuals or in fairly dense stands. Eleven years after first monitoring cholla cactus and other shrubs at the Pueblo Chemical Depot (PCD), in Pueblo County, CHNP ecologist Renée Rondeau has documented a significant increase in cholla density in the greasewood shrubland at the site.

Anyone else hear that music from The Sorcerer's Apprentice?

In 1999 chollas averaged 550 individuals/hectare. By 2010, cholla density had increased to 720 individuals/hectare. Half of the plots were grazed by cattle up until 1998 while the other half have not been grazed since 1945; there was no difference in cholla densities between grazed/ungrazed plots.

Mean cholla densities for each year of monitoring. Bars show one standard error.

So why the increase in cholla density? Renée and others will be looking into this during the winter months, investigating climate changes as well as management changes. If anyone has any ideas about what might have caused this increase in chollas, feel free to leave a comment below, or email your thoughts to Renée (email can be found on our Contacts page).

Photograph of one of the transects at PCD in 1999. A few chollas are visible in the distance.

The same transect in 2010. The circles highlight "new" chollas. (Note: the buildings visible in 1998 were dismantled, but the water tower is still present.)