A Grand Experiment for Palm Trees

Measuring Sinolao hesper palms in Sonora, Mexico

Christa is part of a research team focusing on a threatened palm and how various management practices (i.e. cattle grazing) affect its population in the Sierra de Alamos-Rio Cuchujaqui Protected Area in the state of Sonora, Mexico. Read her previous post, Conserving Threatened Palms.

Looking across the arroyo, where our experiment is set up, at the hundreds of Sinaloa hesper palms Brahea aculeate dominating an area the size of a football field, I have to remind myself that we are working with a palm listed as “vulnerable” on the IUCN (International Union for Conservation of Nature) Red List and “threatened” by the Mexican government. Such large populations are rare across the palm’s range, which includes parts of just three Mexican states. Even in the small sea of palms before me, I see signs that the population is stressed: the number of seedlings is conspicuously low, few adult palms produced fruit or even flowering racemes in the past year, and a number of dead trunks dot the population.

Hesper palms in a plot in the arroyo

None of this is terribly surprising, given the stressors we know the population has been subjected to, namely years of cattle grazing and intensive leaf harvesting. The cattle munch on the shorter palms during the driest time of the year, as the palms are one of the few green things on the landscape. Local people harvest palm leaves to make roofs that are quiet in the summer rains and keep a home relatively fresh and cool during the dry season’s heat. Such roofs are also popular for restaurants and other tourism facilities along Sonora’s west coast. The loss of leaves from cows or people doesn’t usually immediately kill the palm, but with the need to put on many new leaves, the palms may not have energy left to produce fruit. Some may not even be able to recover fully from the leaf loss and slowly die. Though both result in leaf loss, the impacts of harvesting and grazing may be different: they affect slightly different size classes of palms, vary in the completeness of defoliation, and have different secondary impacts (i.e., trampling hooves). And we don’t know exactly how either is affecting the population.

A palm shows the sad results of a high harvest.

In the experiment set up in the arroyo, Postdoctoral Fellow Leonel Lopez-Toledo is attempting to tease out the effects of the two stressors. We have rented two plots from a local rancher and built a fence around one to keep the cattle out. Each plot is divided into three parts: no leaf harvesting, low-harvest intensity, and high-harvest intensity. The low- harvest intensity spares the youngest leaves and follows other rules of thumb used by harvesters in the local communities. The high-harvest takes everything useful and is what is typically done by people who come from cities and coastal communities. We will then track every palm—all 1,500 or so—for the next few years, measuring growth, changes in leaf length and shape, number of leaves put on, number of fruits produced, and more. Each year we hope to see how the palms are responding to the different treatments and what these responses may mean for the palm population as a whole.

Our field assistants come from the local communities and were clearly uncomfortable mimicking the outsiders’ high-harvest approach. I’m sure the sight of the completely leafless palms was just as disconcerting to them as it was to me. On the other hand, their traditional low-harvest rules may be sustainable.

A rare hesper palm seedling

If Leonel’s experiment can demonstrate that it is, the next step would be to work to make the low-intensity harvest an enforceable rule—for outsiders as well as locals. The grazing part of the experiment may lead to recommendations in range management or restoration for the ranch owners. Getting actual changes from the recommendations would be challenging, to be sure, but limiting the stress these threatened palms are under will be key to their survival, especially in the face of other stressors such as climate change.

However, step number one is to flesh out the problem. So, I turn back to the small sea of palms, ready to start measuring.

Christa Horn is a senior research technician in the Applied Plant Ecology Division of the San Diego Zoo Institute for Conservation Research.

One Response to A Grand Experiment for Palm Trees

  1. I am glad that the San Diego Zoo is also involved with saving endangered species of plants. Effort is being made to preserve the Sinaloa hesper palm, but there are other critically endangered palms. The argun palm of southern Egypt and northern Sudan, for conservation of this species can very easily be tied in to the fact that the palm is mentioned 22 times in the Quran. So it is not only a natural heritage of this region, but has religious significance as well. The most critically endangered palm in the world is Hyophorbe amaricaulis on the island of Mauritius, the same island where the extinct dodo lived. It would be nice if the zoo can be involved with the preservation of this species.