Park not suitable for Joshua trees

There has been a lot of negative talk lately.

I’m speaking of the decline of Joshua trees in Joshua Tree National Park. Based upon a lot of research, the trees are not doing well in what is one of our most visited local attractions. Researchers agree it is our changing climate that is taking a serious toll on America’s largest yucca.

But are Joshua trees doing poorly everywhere? Both the long and short answers are no. There are places where Yucca brevifolia, the technical name of the giant yucca, is thriving. Global warming and recurring droughts don’t seem to be hindering Joshua trees at Lee Flat in Death Valley National Park. The population there is clearly expanding.

I know this because in 1995, twenty years ago, I established a study site and began monitoring the trees every year. Each time I arrive for the annual update I discover new Joshua trees. Almost none of the young trees that appear have died. Rather they slowly grow taller and wider in spite of occasional droughts. At about six feet in height immature trees flower and then new branches appear the following year. Only one old tree died during the two-decade-long study period but it was replaced with more than a dozen young trees. Across the whole of Lee Flat, juvenile Joshua trees are appearing, immature trees are flowering for the first time and most adult trees appear healthy, extraordinarily healthy. In short, the population is thriving.

Why are Joshua trees doing so well at Lee Flat and doing so poorly in Joshua Tree National Park? Why isn’t a changing climate negatively impacting the tree in Death Valley National Park?

Based upon the available evidence, there are three reasons why Lee Flat is likely to be the best place to view Joshua trees in the years and decades ahead. First, Lee Flat is near the northernmost limit of Joshua tree distribution in the Southwest. In contrast, Joshua Tree National Park lies along the southern edge of the tree’s historical range. Because of its northerly location, Lee Flat receives less solar energy and, as a result, is not as hot. Lower temperatures mean there is less evaporation of rainfall and more water seeps into the soil. As a result, equal amounts of precipitation will be of more benefit to Joshua trees growing at Lee Flat than in Joshua Tree National Park. It pays to live in the north.

Secondly, Lee Flat is higher in elevation than any of my three study sites in Joshua Tree National Park (and in most areas where Joshua trees occur in the Park). Temperatures are generally lower at higher elevations and lower temperatures reduce evaporation rates. Again, equal amounts of rainfall will be of greater benefit to Joshua trees at Lee Flat than in Joshua Tree National Park if for no other reason than the elevations are different. Being high is advantageous if one is talking about Joshua trees.

Finally, and as its name indicates, Lee Flat is very flat. Most rain does stays where it falls and does not flow somewhere else, away from the Joshua trees. Areas in Joshua Tree National Park that are of comparable elevation to Lee Flat (about 5,200 feet above sea level) are typically on steep slopes where rain can quickly run off down ravines and washes.

In these times of a changing climate, Joshua Tree National Park may be too far south, too low and too steep to maintain a healthy population of Joshua trees.

Cornett is author of Population Dynamics of the Joshua Tree in the Proceedings of the First Natural History Conference, Death Valley National Park.