Water and life

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Most of the country is experiencing a snowy winter, but it is hard to imagine that cold here in the Sonoran desert. Our latitude and placement relative to the Pacific Ocean give us the climate of a typical midlatitude desert. Life thrives in the winter, especially in the canyons where the winter precipitation concentrates as runoff. Here are a few quick shots from the Aravaipa Canyon Wilderness.

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A javelina (Pecari tajacu) crossing the water.

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Cottonwood trees (Populus fremontii) caught in late afternoon light – leafless, but very much alive.

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Saguaros (Carnegiea gigantea) and sycamore (Platanus wrightii) in a canyon bottom.

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Insects (unknown species) mating in a late afternoon sunbeam.

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The last light of the day catches the canyon walls and is reflected on the water rushing through this wilderness. There is no snow, and that is just fine.

A paper canyon ghost

The Superstition Wilderness is rumored to hold the Lost Dutchman’s gold mine. But other treasures can be more readily found here…

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The landscape is a harsh volcanic one, dominated primarily by lava flows and ash deposition. The rocks are mostly basalts, tuffs, and rhyolites.

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Narrow canyons are incised into these planes. The canyons are where the mountains’ secrets are kept.

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Descending through these broken landscapes across boulders and woody debris, the bedrock of the canyon bottom can be hard to find.

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But occasionally bedrock sections do appear, and in these impermeable places water can be reliably found. At least, when there has been rain. When there has not been rain, the water slowly evaporates away. And the organisms that have purposed it as habitat and home begin to die. The landscape becomes alien.

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This was once a floating mat of green algae, a sort of floating plant. As the water began to disappear the algae began to fold itself over the rocks, and soon enough was bleached by the sun. The only remnant of earlier life is this ghost.

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The dead algae had a fibrous texture, and would rip and tear just like a rough paper. Green algae produce a range of polysaccharides to form their cell walls, including cellulose – a key component of modern papers. Interestingly, most paper is produced from wood, but wood comes only from vascular plants (which evolved from green algae) and contains lignin, a biopolymer that is typically removed chemically in high-quality paper to prevent the degradation that occurs for example in news-paper. No surprise, then, that this algal mat had such fine texture to it. The gold of the Superstitions will remain lost, but I was glad enough to find this ghostlike secret here.

Escape from a canyon

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In red rock country, the bottom of a canyon can be a difficult place for a plant to live. Why?

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Here you can see the depths of Boynton Canyon, where ponderosa pine (Pinus ponderosa) and blue spruce (Picea pungens) must grow tall and straight to survive. Growing deep in a canyon restricts the hours of direct sunlight to a scarce few – perhaps too few to support enough photosynthesis to offset respiration. On the other hand, canyons rarely endure major wildfires, preserving large populations of majestic trees that have grown up high enough to reach the sun.

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And here, in Oak Creek Canyon, you can see the constant risk of drought. Canyons are often wet places, serving as the lowest points of large watersheds, but the vagaries of the canyon’s history of downcutting mean that many portions are exposed and dry. Here you can see a bigtooth maple (Acer grandidentatum) blindly seeking for water, with blind roots pushing through hidden fissures in the canyon walls.

Canyon life has other downsides, as well. Flash floods can erase centuries of growth as they scour canyon bottoms, and the narrow confines of the walls can prevent fertilization or dispersal of seeds and so reduce fitness. They are beautiful but harsh places, and I am glad to not be a plant as I walk through their hidden worlds.

Guess the cactus

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My trip to México took me to the small town of Zapotitlán Salinas. The town and nearby hills were once home to Popoloca people, and served as an important trading center. Then the main resource was salt, but today the town exists primarily for the onyx and marble mining industries. It is also surrounded now by the Tehuacán-Cuicatlán Biosphere Reserve. I learned that the reserve provides some protection for the landscape, but that low wages for local workers and the dominance of mining interests mean that the landscape is far from being sustainably managed.

Even with these concerns, it is a beautiful place to see columnar cacti. Coming from Arizona, I am accustomed to landscapes dominated only by saguaros. Here the desert was shared by several species, which seemed very saguaro-like from far away, but not on a closer inspection.

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This first species is a tetetzo (Neobuxbaumia tetetzo), and was most common in lower areas. It grew tall and straight, and had a pale green color that harshly reflected the sunlight.

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This second species is a cardón blanco (Cephalocereus columna-trajani), and was more common on higher-elevation slopes. It seemed more curved and soft, and would sway softly in the wind. The heads were covered in long white spines that seemed almost like a soft mat of hair.

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This third species is a quiotilla (Escontria chiotilla), and was most common at lower elevation near dry riverbeds. It would grow into massive multi-armed structures, with a beautiful dark green skin.

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And this fourth species is a trick – it is a saguaro (Carnegiea gigantea) growing hundreds of miles north, in Tucson!

Cacti have converged many times on the stem-succulent, columnar growth form. It seems to be a very effective way to live in dry desert-like conditions. But not all cacti do it – some of the most basal species in the family (Pereskia spp.) are small leafy plants inhabiting very different environments. You can learn more in this Edwards & Donoghue (2006) paper. Or by simply walking among the beautiful and heavily-exploited landscapes of Tehuacán…

Volcanoes and sky islands

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The first thing I noticed on my visit to Puebla was the dust. The dust comes from many sources. The fallow agricultural fields. The desert. The active Popocatepétl volcano. The two million inhabitants of the city and their automobiles. The air is windy and dry in the winter, so the valley sits constantly under a yellow-gray layer of air.

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My friend Alicia, a Mexican ecologist, invited me on an adventure far above the dust. We were aiming to climb Volcán La Malinche (Matlalcuéyetl), which rises nearly 4,500 meters above sea level. (The mountain is named for a Nahua woman who became the interpreter of Hernán Cortés). The peak is an island in the sky, and an oasis of clean air and alpine biodiversity.

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One of the most marvelous things about mountains is how rapidly their ecology changes as you ascend. This mountain switched from an oak/alder (Quercus / Alnus) dominated forest rapidly to one dominated by the beautiful oyamel (Abies religiosa).

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As we reached 4,000 meters elevation, the firs dropped out and were replaced by pines (Pinus hartwegii) and the occasional juniper (Juniperus monticola). For someone recently-arrived from sea-level, this was a difficult climb, but one full of ecological magic.

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Climbing up higher still, the woody plants were gone entirely, replaced by a grassland growing on the volcanic ash, dominated by Festuca and a few forbs.

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Ice was common here, probably due to condensation from the clouds formed by orographic lift. Shadows trapped the ice where the sun did not reach.

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Higher up yet, no plants remained at all. We found nothing more than a barren landscape of ash, rock, and lichen.

At the top, we found ourselves high above the clouds on a beautiful clear morning. In places like this, the concept of a sky island is most evident. We talk about mountains as islands, in the sense that they trap species on their ridges that cannot pass through the valleys below. Here you can see the summit of La Malinche, with its collapsed caldera in the foreground, and the stratovolcano Pico de Orizaba (5636 m) some eighty kilometers away. As you can imagine, the animals and plants and microbes living on each peak show strong genetic differentiation because of their geographic isolation.

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I had never before seen such a clear illustration of sky islands, and would have stayed all day to enjoy the view.

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But the clouds began to rise, and safety lay below, so we descended.

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Back to the dust and the chaos of the city.

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Desert photo featured by BBC and BES

I’m traveling in Mexico this week, so a short post – check out the British Ecological Society’s annual photo contest! My image of Death Valley was the runner-up, and several other images were featured.

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You can see all the images at the BBC’s website.

Desert moons

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A full moon in the Sonoran desert is a beautiful thing, lighting up canyons and cacti in an unearthly shade of light that is unseen during the day.

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The transition from full to new has biological consequences, too. Many animals’ behavior changes depending on the levels of light available in the night. The foraging behavior (and competition) of many rodent species, for example, depends on the phase of the moon (e.g. Kotler 1984). And further from my desert home, dung beetles are able to navigate in darkness with the polarized light of the moon (Dacke et al. 2003).

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But in my home, the light of the moon is easily diminished by the lights of the city. This urban light pollution probably has major consequences for animal behavior. I wonder what an animal sees, looking out from the desert to the miles of glittering human development we have built in the last century.

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