Research shows Joshua trees not doing well

(Editor's note: This is the last of three columns on Cornett's Joshua tree research at Upper Covington Flat in Joshua Tree National Park.)

What did our long-term research project on Joshua trees discover?

Joshua trees are not doing well at Upper Covington Flat in Joshua Tree National Park. At least they are not doing well within the boundaries of our one-hectare study site.

We started with 39 trees in 1988. Numbers could have increased, stayed the same or declined. They declined. By the spring of this year the number had dropped to 31 trees, a decline of 21%. This approaches a 1% decline each year. I consider this somewhat alarming as I believed the site was the best area in the park for Joshua trees.

And there was more bad news for Joshua trees. Recruitment, the germination and establishment of new trees, was insufficient to replace the old trees that had died. Of seven new trees that appeared in the 27 years of study, only 2 survived into 2015. Four, not two, recruits were needed to replace the four old trees that died.

Paralleling the absolute decline in tree numbers was a corresponding decline in the vigor of surviving trees. Granted this part of the evaluation was somewhat subjective, with the pitfalls described in my last column, but it clearly clarified the story.

In addition to a decline in total number of trees and shift towards an older population, the vigor of trees declined. In 1988, I considered 29 of the 39 tree present (74%) to be enlarging. Most of these trees were growing taller, wider and producing more branches. By 2015, the number of enlarging trees had declined to just 13 (42%). It seemed the vigor of surviving trees had declined along with a drop in numbers.

The remaining trees were considered either stable or declining. With plants, as with what some economists claim, if you're not growing your dying. So it goes with Joshua trees. On all of our study sites, trees downgraded to stable status almost never return to enlarging status. Most often they were downgraded a year or two later to "declining" and then they died.

One could argue that what was happening on our lone study site was not representative of the vitality of Joshua trees across all of Upper Covington Flat, let alone throughout Joshua Tree National Park. Perhaps something very strange was happening on our 2.5-acre sample?

But I found almost no evidence of thirsty animals chewing on leaves or removing bark to get at the moist tissue beneath. There was no evidence of disease and no evidence of serious insect attacks. There was, however, evidence of young trees dying at a rate greater than that of older, larger trees. Recall how 7 new, young trees died compared with four mature, big trees. Add another four young, small trees to the seven to reach 11. That's the total number of small, young trees that died within the study area during the study period because these additional four trees were present at the inception of the study.

That small, young trees were dying at nearly three times the rate of mature, established trees is significant. Young plants of any perennial species in a desert or semiarid environment most often die because of drought. They don't develop extensive enough root systems in time to overcome the inevitable drying out of the soil.

During the 27 years of the study, unusually severe and recurring drought plagued not just Joshua trees within the study site but throughout the Park, indeed the entire Mojave Desert. On top of the drought was an officially recorded increase in temperature of 3 degrees F. beginning in the 1970s. Warmer weather results in an increase in evaporation, robbing the trees of even more moisture.

In short, what was happening to Joshua trees on 2.5 acres at Upper Covington Flat was most likely happening to Joshua trees throughout the Park.

I am in the process of tabulating data on two other sites we established in the Park but preliminary indications are that Joshua trees are declining on these two additional sites as well.

Cornett is a desert ecologist and author of The Joshua Tree.