I went hiking in the Santa Catalina Mountains north of Tucson this weekend. It was a rare day, windy and snowing at some of the higher elevations – the first snow I’ve seen all this year. The cacti did not look particularly happy. In fact, the higher up the mountains, the fewer cacti there are – and eventually, there are no cacti at all. Why? It’s not limited water availability – there’s plenty of moisture higher up, and lots of other plants like cottonwoods and aspens that need lots of moisture. It’s actually temperature. If the weather remains cold for too long, water freezes inside of the plant, expanding into ice, and rupturing cell walls, causing tissue damage and potentially death. Cacti, having such a high water content, are particularly susceptible. And of course, they’re not adapted for life in the cold – they’re adapted for life in the heat, where dry conditions prevail and the occasional rains must be captured quickly for water storage. The cold mountains are not the niche for any cactus!

You might wonder, then, why other plants don’t succumb to the same fate. Why don’t oak trees, for example, freeze to death each winter? There are a few answers, but my impression is that we still have much to learn. First, plants often pull nutrients out of their extremities before winter, storing them in the roots. This way even if the cold is fierce in the air, nothing is sufficiently hydrated to cause ice fracture damage. Believe it or not, the ground (thanks to its high thermal inertia) is usually the warmest part of a landscape in the winter. A few feet down and it will be warmer than freezing in most parts of the world. Second, some plants avoid winter entirely, and die back, leaving only hardy seeds to begin growing once the chance of frost damage is lower. Third, some plants rely on snow for insulation, or special coloration to remain warm in the limited sunlight of winter (ever seen a green aspen trunk in the winter? It’s warm enough to photosynthesize!) And fourth, and the coolest – some plants make natural antifreeze molecules, that slow or prevent the formation of ice crystals at low temperatures. Do cacti do this? I suspect not – it would be an expensive investment for a desert-adapted plant. But I don’t know – maybe you do?

The other nifty thing about cacti is their internal plumbing. I’ve been writing a lot about leaves, but a cactus defies any common ideas of leafiness. Yet it still photosynthesizes, and still is green. How does it supply water and move resources around? I’ll leave it to you to imagine its insides, and perhaps go on your own desert adventure to look for skeletons…

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