The Santa Cruz Island ironwood tree is a modern survivor of an ancient disaster. Today, it only grows wild on the Channel Islands off the coast of California. What’s interesting is how it got there.The Santa Cruz Island Ironwood is descended from a long line of Ironwood trees that used to grow over a much wider area of the American Southwest (Raven and Axelrod, 1995). Fossils of their leaves have even been found out in the Mojave desert, near Death Valley. If you’re familiar with the climate in that area of the Mojave desert, you’re probably aware that it’s not an easy place for a tree to grow. That’s even more relevant for this tree in particular, because the Ironwood doesn’t handle weather fluctuations very well. However, Death Valley, along with much of the Southwest, used to be different.

 

            About 3 million years ago, Earth was just beginning to feel the effects of a climate shift. The temperature began dropping to its current level (Bartoli et al., 2005). Now you’d think that dropping temperatures would make Death Valley a better place for a tree, right? Well, there’s a problem. The problem with the dropping temperatures is that the plants weren’t adapted for the change. As the temperature dropped and rainfall patterns changed, the mainland ironwoods died out. Eventually, there were none left in the harsh deserts of the American Southwest. Nowadays, the Channel Islands are the only places that still maintain an environment like the one that early Ironwoods adapted to. Out there, the weather is pretty stable, especially in the protected valleys and canyons where the sun shines a little less harsh. That might change in the future, though.

 

            The Channel Islands sit off the coast off the coast of Southern California, which is all well and good. However, the climate of California is changing. One of the most important ways it’s changing is in the patterns of rainfall. By 2075, extreme summer dry seasons are expected to be at least twice as common as they have been in the 2010’s (Swain et al., 2018). The frequency of extreme rain events is also expected to increase greatly. Remember how I said the ironwoods do well in stable climates? Climate change hasn’t affected the modern ironwoods yet, but it killed most of the prehistoric ones. Now that humans have sped up the process, we need to watch the Ironwoods to make sure climate change doesn’t kill off the new ones too. And climate change isn’t even the only thing threatening the Santa Cruz Island Ironwood. To figure out their main problem, you need to ask one of the most complicated questions in plant biology.

 

            That question is “what is an individual?” With animals, it’s usually pretty easy to answer that question. Animals are self-contained bags of organs that functions independently of all the other individuals. In plants, it can be much more difficult to figure out what counts as an single plant. For example, if you go to Santa Cruz Island, you’ll find groves containing what appears to be hundreds of ironwood trees. However, their roots are all connected together into one giant individual per grove (Bushakra et al., 1999). They may have separate-looking trunks, but these trees are all genetically identical clones of each other, joined at the roots. Although there are about 32,000 trunks in total, there are only about 1125 separate individuals left. This isn’t good news.

 

            See, when you have a big colony of trees, pollinators like bees and hummingbirds probably aren’t going to bother flying to other colonies. That means that big groups of clones aren’t likely to reproduce with other populations of ironwoods because pollinating animals don’t have a reason to leave their home turf. There’s plenty of food in this grove, so why leave? Because the pollinators stay put, the species has yet another problem: the ironwoods don’t have much genetic diversity. I talk more about genetic diversity in the ginkgo tree stop, so make sure to check that one out too. The moral of the story is that losing genetic diversity is bad. Low genetic diversity means that a population is more vulnerable to problems like disease, so Ironwoods are in trouble.

            We really don’t want ironwoods to be in trouble, either, because they’re a big deal out in the Channel Islands. For example, those pollinating insects and birds I mentioned earlier need to drink nectar in order to survive. Even if these pollinators don’t always carry pollen far enough fertilize the eggs of another individual, they’re still a valuable part of the Santa Cruz Island ecosystem. They aren’t the only animals that rely on the ironwoods, either. Humans have also depended on the ironwood for thousands of years.

 

            The Chumash people of California have a long and distinguished history of traveling out to the Channel Islands, and they need the ironwood in order to do so. They get to the islands by paddling there in canoes. It’s a grueling voyage, with the closest of the islands being fourteen miles from the mainland. When canoeing fourteen miles over the ocean, it’s a good idea to pack a good paddle, which is why the shafts of Chumash canoe paddles are made with ironwood (Campbell, 2009). They’re actually called ironwoods because their wood is tough and durable. The Chumash people also use this wood for harpoon shafts and digging sticks (Santa Barbara Botanic Garden, 2015). Those tools are crucial for finding food out on the Channel Islands. The ironwood is a valuable part of Chumash culture, and deserves to be protected as such.

 

            Ultimately, humans do have the power to protect the Santa Cruz Island Ironwoods. We can grow them in captivity like we do here. We can help pollinate the groups of clones to improve their genetic diversity. We might even be able to slow the rate of climate change. We know how to keep this species alive. The question that remains is, “will we do it?”

 

 

 Works Cited

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Bartoli, G., Sarnthein, M., Weinelt, M., Erlenkeuser, H., Garbe-Schönberg, D., & Lea, D. W. (2005). Final closure of Panama and the onset of northern hemisphere glaciation. Earth and Planetary Science Letters, 237(1-2), 33-44.

 

Bushakra, J. M., Hodges, S. A., Cooper, J. B., & Kaska, D. D. (1999). The extent of clonality and genetic diversity in the Santa Cruz Island ironwood, Lyonothamnus floribundus. Molecular Ecology, 8(3), 471-475.

 

Campbell, P. D. (2009). Survival Skills of Native California. Salt Lake City: Gibbs Smith.

 

Outline of Topics for a General Tour [Pamphlet]. (2015). Santa Barbara, California: Education Department Santa Barbara Botanic Garden.

 

Raven, P. H., & Axelrod, D. I. (1995). Origin and Relationships of the California Flora. Sacramento: California Native Plant Society.

 

Swain, D. L., Langenbrunner, B., Neelin, J. D., & Hall, A. (2018). Increasing precipitation volatility in twenty-first-century California. Nature Climate Change, 8(5), 427.