Excerpts from Jim Conrad's
from the February 23, 2007 Newsletter issued from Sierra
Gorda Biosphere Reserve Headquarters in Jalpan, QUERÉTARO, México
Not far west of high-elevation Pinal de Amoles the road crossed the mountain range and suddenly we were out of a green, highland landscape of pines and junipers into a gray-brown, barren-looking, bushy, semi-desert. Downslope a little just beyond the dusty-looking town of Camargo, at around 5500 feet in elevation (1700 m), I spotted a rutted, long abandoned track plunging into a valley, so that's where Rudolfo put me out.
Here and there in the cactus-rich scrub I found woody, cylindrical items abundantly ornamented with diamond-shaped holes that where so geometrically aligned that the objects almost looked manmade. You can see one at the top of this page.
It took me years of occasional desert visits to figure out that these are skeletons of cholla cacti. The common cholla below Camargo is the shoulder-high Tree Cholla, CYLINDROPUNTIA IMBRICATA. You can see from my picture what's tree-like about it: It has a trunk, as shown below:
This species may be even more abundant and far ranging than the previous species, extending north clear into Kansas and Colorado. It's also become an invasive weed in some countries.
You'll notice that both Tree Cholla and Christmas Cactus are members of the genus Cylindropuntia, and thus are closely related. One curious feature uniting them is that they both bear spines usually invested with a yellowish or whitish papery sheath. You can see such a sheath I've just pulled off a cholla spine below:
I can't figure out why the spines of these cacti bear sheathes. Many cacti produce sheathless spines and appear to do just fine. Sheathes on the Christmas Cactus spines that punctured my leg didn't remain stuck in my skin when I removed the spines.
Christmas Cactus's stem joints are so weakly attached to the main stem that when I tugged on its spine sheathes the stem joints came off before the sheathes did. However, when I tugged on Tree Cholla spines the sheathes came off first. Still, the Tree Cholla's outer stem joints also are fairly loosely attached. Loose joints latch onto the fur of any large mammal passing by, which serves the parent cactus's purposes, since broken-off joints can root and form new plants. Beneath Tree Chollas typically you see numerous broken-off joints lying around, their spines menacingly pointing in every direction.
Wild burros wandered the landscape below Camargo and I just wonder what kind of messes they get into with these cacti.