Chihuahuan Desert Ferns

Cheilanthes fern, Fort Davis National Historic Site, TexasNormally, I’m outside every day in September enjoying the cooler temperatures, brilliant sunshine, and abundant life inspired by the afternoon thunderstorms of the monsoon season.

But not last week.

Rain (torrential at times), heavy cloud cover, and temperatures in the low 50s kept me huddled in the house muttering about the imminent arrival of the next Ice Age. As I stared morosely out the window, watching the rain drip from the roof, I began to wonder: What happens to the butterflies, bees, and bugs when the temperatures go from warm and dry to cool and wet in a single day?

Thinking this was the perfect subject for a blog (I’m sure you’ve all been dying to know), I donned my rain jacket, grabbed my camera gear, and wandered through the mist to the Fort Davis National Historic Site. Although chiefly known for the 19th century fort buildings, they also have a wonderful nature trail that loops from the fort site, up the ridge that separates the Fort from Davis Mountains State Park, and back down again. Here, I was sure I would find my sleeping bugs.

I probably would have, too, if I hadn’t gotten distracted. Every place I peered and poked for insects, I was confronted by ferns. Beautiful, lush, green ferns. The magic of desert rain.

Dancing Bommeria, a Chihuahuan Desert fern, growing near Fort Davis, Texas.Most people don’t hot-foot it to the desert to look for ferns. But perhaps they should. According to botanists Sharon Yarborough and Mike Powell, there are 64 different types of ferns that grow in the Trans-Pecos region of Texas.

Ferns can grow in the desert because they’re experts at finding little microhabitats that are just a tinier bit wetter than the surrounding landscape. Look for them growing from cracks in the rock, under the shelter of a tree, or peeking out from beneath a boulder. Depending on when you look, though, you may not be overly impressed. In fact, during most of the year, desert ferns are brown, crispy, and look—well—dead.

Chihuahuan Desert ferns dehydrate during the warm, dry months and wait for rain.But fear not! Playing dead is just one of the many adaptations that ferns have for surviving in the desert. And what an adaptation it is. Your average plant can drop to about 75% moisture content before it begins to wilt and beg for water.  Creosote, one of our common desert shrubs, can lose about half its moisture before it begins to look a bit peaked.

But the ferns. Oh, the ferns. They are the masters of dehydration. During the dry season, some species can desiccate to less than 6%—or as dry as many seeds—but begin to green up, unfurl, and grow within an hour after a refreshing rainfall. When it dries up again, so do the ferns.

But ferns don’t give up their moisture easily. They may be able to survive as a brittle frond, but life is better when you’re green and photosynthesizing. So to stay green as long as possible, the ferns (and many other desert plants) have ways to reduce moisture loss.

Small Leaves

Small Leaves

Waxy Coating

Waxy Coating

Protective Hairs

Protective Hairs

Thickened Leaf Margins

Thickened Leaf Margins

Small is better. Most desert ferns are much smaller then their rainforest cousins. Remember that during the process of photosynthesis the plant takes in carbon dioxide and releases oxygen and water as a waste product. The smaller the leaf, the less water is released.

Be waxy. Most water is lost (transpired) from special cells on the leaf called stomata. These open and close to absorb and release gases (carbon dioxide, oxygen, and water). But other parts of the leaf surface and stem can lose water through simple evaporation. To help protect from this “incidental” water loss, many of the ferns have a cuticle of waxy deposits on the leave surface that make them thicker and seal in the water.

Get hairy. Have you noticed how many ferns are grey and fuzzy? If you look closely, you’ll notice that the color is due to a dense coating of hair. These light-colored hairs serve a couple of purposes. First they reflect light making the leaf surface temperature cooler and they efficiently trap rain, dew, and other forms of moisture to raise the humidity around the leaf surface. Keeping the air surface immediately around the leaf cooler and more humid reduces the evaporative demand from the atmosphere.

Thick leaf margins. The last defense against moisture loss is the ability of the leaflets to roll, thus protecting the surface and the spores. Many fern leaflets have thickened margins that enhance the ability of the leaf to roll.

Not all ferns found in the desert have these adaptations for dry conditions, though. Others simply seek places where there is enough moisture. Delicate maidenhair ferns, for example, can be found on many shaded rock faces with perennial seep springs. You can find these beautiful ferns at the bottom of Modesta Canyon at the Chihuahuan Desert Nature Center or in the grotto at Ojito Adentro, Big Bend Ranch State Park.

Water Clover (Marsileaceae) is a Chihuahuan Desert fern that grows in puddles during the rainy season.But my favorite fern is the water clover. This aquatic fern survives the dry season as a packet of spores (sporocarp) in the mud of ephemeral pools. When the rainy season arrives and the pools fill with water, the gelatinous interior of the sporocarp swells, rises to the water surface and splits, releasing the male and female spores. Once they do what males and females do, the newly fertilized embryo sinks to the bottom of the pool, takes root and begins to grow, eventually spreading its four leaflets across the surface of the pool to begin the cycle of life again.

It’s starting to rain. Time to go look for ferns!

Posted in Chihuahuan Desert, Plants Tagged , , , , |

Puddle Parties: Just Hanging Out With The Guys

Puddle PartyI should know better.

In hot pursuit of a butterfly, I came around a bush and WHOOSH! I was immediately enveloped in bright yellow and pumpkin orange confetti. I’d stepped into the middle of a puddle party.

Puddle parties are social gatherings for butterflies. Dozens of butterflies—almost exclusively male and often young—will congregate in a single spot to sip from mud, a pile of excrement, the blood of fresh roadkill (I’ll spare you the visuals for that one), or even turtle tears (No, seriously. Check it out!).

The butterflies are searching for sodium, an ingredient that’s typically pretty scarce in a nectar-feeders diet, but critical for egg production.

The sodium-sipping males pass on their salty nuptial gift to the females during mating. Studies have shown that female butterflies that receive sodium from their male partner have larger eggs which produce more fit offspring.

But why is sodium-sipping a social event? Why not just go find your own puddle? Or bloody rodent. Or whatever?

Sleepy Oranges

As passing on sodium to your mate gives you a distinct evolutionary advantage, so does hanging out with the guys. There are benefits to participating in a puddle party.

Benefit 1: It’s easier to find the “good stuff” if you just follow the crowd. Male butterflies use visual cues to recognize brethren of the same (or similar) species. Once they see a gathering (a sure indication that someone’s found a good puddle) they’ll join in, swelling the ranks of the party.

Benefit 2: You’re less of a target. Puddle parties typically take place on the ground where the butterflies are more vulnerable to predators such as lizards. If you’re by yourself, you’re easy pickings. In a crowd though, there are a lot more eyes to watch for movement and you become “one of many” when your puddle party explodes into flight to avoid a predator (or an oblivious photographer).

Benefit 3: If a little advertising is good, then a whole lot of advertising is even better. Many butterflies have bright colors that warn “I don’t taste good, so save us both a lot of hassle, and don’t even try to eat me.” Gathered together in a puddle party, the message becomes a bit stronger: “WE DON’T TASTE GOOD, SO SAVE US BOTH A LOT OF HASSLE, AND DON’T EVEN TRY TO EAT US.” And if you try, we’re all going to fly away in a huge WHOOSH and you won’t be able to focus on a single one of us anyway. Ha!

Two-tailed swallowtails

IMG_5457

Not all butterfly species participate in puddle parties. In my experience the most social of the butterflies appear to be swallowtails, blues, and sulphurs. Blues and sulphurs have grand mixed-species parties with marine blues hanging out with Reakirt’s blues or Sleepy Oranges mixing freely with Cloudless Sulphurs.

But swallowtails seem to be more aloof. Two-tailed swallowtails, those yellow and black beauties that soar high overhead, prefer to gather with others of their own species. Pipevine swallowtails may not be quite as picky and have been known to crash another swallowtails party.

It’s easy to host a puddle party in your yard if you have a butterfly-friendly garden (see my story on Butterfly Gardening for more information on that subject!). Choose a bare patch of dirt or sand, pour some salty water over it, and wait for the party to begin. It helps if your puddle stays damp for awhile, so you might want to put the sand or dirt into a shallow dish (such as a terracotta plant saucer) and keep adding salty fluids as it dries out.

Butterflies are fascinating creatures that do a lot more than just flit from flower to flower. While we know a lot about butterfly taxonomy, there’s still much to learn about butterfly behavior. So grab your binoculars, settle in, and just watch. Who knows what you’ll discover!

 

Posted in Chihuahuan Desert, Insects Tagged |

White on White

Sunset at White Sands National Monument

Imagine this.

You’re standing in the Tularosa Basin of New Mexico during the last ice age—let’s say about 30,000 years ago. A huge lake shimmers in the background. Herds of mammoth and camels squelch through the mud leaving long lines of tracks. Packs of saber-toothed cats and dire wolves follow, waiting for the opportunity to hunt.

The lake is not unusual for this time period, nor this geologic setting. The Tularosa Basin is completely enclosed, bound by mountains to the east, west, and south; and Chupadera Mesa to the north. Water that drains into the basin has nowhere to go, so—depending on the climate—it either accumulates or it evaporates.

During the last ice age, water accumulated. Temperatures were cooler, evaporation wasn’t as rapid, and there was simply more water. At least eight lakes dotted the basins of the northern Chihuahuan Desert in Arizona, Texas, and New Mexico.

But the giant lake of the Tularosa Basin (now known as Lake Otero) is different. Although the water sparkles and shimmers, it’s not “pure.” The mountains surrounding the basin are partially composed of gypsum deposits hundreds of feet thick. As rain or snowmelt flows across the surface, the water slowly but surely dissolves the minerals of the rocks, and carries them away in solution to eventually accumulate in the lake.

Now, jump forward about 20,000 years. The climate is changing. Temperatures are rising and there is less precipitation. The mammoths and camels no longer roam the lake shores. The dire wolves and saber tooth cats have disappeared. The lake is drying up.

As the free water evaporates, millions of tons of dissolved minerals become concentrated in less and less water. Knife-like crystals of selenite begin to grow in the supersaturated muds of the lake bed. Eventually, the water of the lake is gone and the crystals lie exposed. Battered by the wind, cracked during freeze/thaw cycles, broken as animals walk across them, tiny chips and flakes of gypsum from the crystals are ground down into pure white, sand.

The wind picks up the sand grains and bounces them across the landscape. They begin to accumulate around rocks, bushes, and anything stable enough to stop their movement. Sand dunes begin to form.

Take another giant step through time. Pay your admission and enter White Sands National Monument, an amazing system of gypsum sand dunes. These gypsum dunes are quite rare. They require a set of specific conditions to form: a source of gypsum, arid conditions (so the gypsum won’t just dissolve away), and the wind to pile everything up. The three largest gypsum dune systems in the world are in the Chihuahuan Desert (White Sands, Guadalupe Mountains National Park, and Cuatro Cienegas, Mexico).

Although the dunes seem barren, they’re actually teaming with life. But you need to be on your toes (or more likely, your knees) to see much. Many of the animals spend their days dug into cool burrows in the sand or tucked into the shade of a bush.

But the main reason that you may not see anything is because the animals have changed color to blend with their environment. Instead of the normal browns and tans of the desert, the animals of white sands are bleached blondes.

This color adaptation is exciting to researchers. After all, the dunes are only about 7,000 years old, so to find white versions of common animals shows speciation moving along at a fairly rapid clip.

The Bleached Earless Lizard (Holbrookia maculata ruthveni) shows the greatest adaptation to its environment of all the lizards at White Sands. On the dark, volcanic rock surrounding White Sands National Monument, the earless lizard is a dark brown. The populations at White Sands are nearly pure white.

Bleached Earless Lizard

A Bleached Earless Lizard blends into the white sand.

This adaptation was probably originally a matter of natural selection. A dark animal on white sand is an easy target for a predator. Since the Bleached Earless Lizard is active during the day and prefers open habitats, the darker lizards would have been removed from the environment, leaving lighter and lighter animals to breed.

Today, predators are probably still picking off darker lizards, but the lizards themselves have changed their way of looking for a suitable mate. Field and experimental studies have shown that White Sands males will display preferentially to the lighter White Sands females when given a choice, thus continuing the selection for lighter animals. Although the Bleached Earless Lizard is considered a subspecies of the general population of Holbrookia maculata, these behavioral changes may indicate that a new species is in the making.

Bleached Earless Lizard

Bleached Earless Lizard (Holbrookia maculata ruthveni)

While other reptiles and animals such as pocket mice and crickets all show adaptation to the white sand, perhaps the most astounding group (evolutionarily speaking) are the moths.

Not much was known about the moths of gypsum dunes until 2006 when lepidopterist Eric Metzler was invited to conduct a long-term study at White Sands National Monument. He established a three km-long transect that cut across the four habitat types within the dune field: open dunes with no vegetation, interdunal spaces, the edge of the dunes, and open habitat outside the dune field.

Using light traps, Metzler and his colleagues collected moths at 11 sample sites along the transect. What they found was astounding. Among the thousands of moths collected were 24 undescribed species in 7 families. As you would expect, many of the new moth species are white or very pale in color.

As Metzler’s study shows, there is still much to be learned in the gypsum dune fields of the Chihuahuan Desert. So the next time you visit White Sands National Monument or venture out into the gypsum dunes at Guadalupe Mountains National Park, take a moment to wander. Watch for movement and holes in the ground. Imagine the life under your feet and admire the resourcefulness of the plants and animals that live here. Perhaps you, too, will discover a new species.

Posted in Animals, Chihuahuan Desert, Geology, Insects, Landscapes Tagged , , |

Dodder Discourse

Dodder

On a recent trip to Big Bend Ranch State Park, I was amazed by the amount of dodder growing by the side of the road. Tangled, yellow filaments—looking like a Silly String party gone wild—weighed down the branches of the shrubs.

Dodder is a stem parasite with a fascinating life history. It’s a plant that can see, smell, and even sweet-talk its host.

Dodder is typically an annual that starts life a little late, giving its unsuspecting host a head start. Once emerged, the seedling immediately begins to move, sweeping the area in a counterclockwise direction, looking for the perfect host. It can’t be just any plant. It has to be the right plant.

Dodders use phytochromes (pigments that plants use to detect light) and volatile cues to “see” and “smell” the plants around them. They avoid potential hosts that are too young or too sick leading one researcher to describe the search for a host as an “intelligent choice and intention.”

The dodder seedling must work quickly—its root organ will wither away within a few days leaving the dodder reliant on its host plant for almost all of its nutrients.

Once the dodder finds the perfect host (“welcome, come on in”), it wraps itself up the stem, producing haustoria that invade the host-plant cells. The hyphae or filaments of the haustoria hijack the host’s nutrient transport system, transferring food back to the dodder.

The host plant isn’t entirely passive during this process. They can, and often do, fight back with barrier tissues that block the dodder’s advancing hyphae or inhibitors that retard hyphae growth.

Research published in Science this week shows that dodder and its host plant “talk” to each through the exchange of messenger RNA molecules. So what are the plants talking about? Researchers aren’t in on the conversation yet, but one suggestion is that the dodder is engaged in some “sweet talk,” sending messages that instruct the host plant to lower its defenses and allow the dodder in.

A few species of dodder (who really are serious agricultural pests) have given the whole group a bad reputation. But Dr. Mihai Costea, a botanist at the Wilfrid Laurier University in Canada, says we shouldn’t be so quick to dismiss dodder.

According to Dr. Costea, dodder should be respected as a keystone species in some ecosystems.  Many host plants can be bullies. They grow fast and aggressively, crowding out the shy, retiring types. But dodder, through its ability to reduce the host plant’s biomass and alter the way it uses resources, can actually modify the structure of the plant community, keeping the bullies in check and allowing other plants to flourish.

I’m not sure which species of dodder I saw the other day (there are six that grow in the Trans-Pecos region of Texas), but I sure am glad I stopped to take a look at this fascinating little plant.

Sources

Costea, M. 2007 through present. Digital Atlas of Cuscuta (Convolulaceae). Wilfrid Laurier University Herbarium, Ontario, Canada.

Costea, M., and F.J. Tardif. 2006. The biology of Canadian weeds. 133. Cuscuta campestris Yuncker, C. gronovii Willd. ex Schult., C. umbrosa Beyr. Ex Hook., C. epithymum (L.) L. and C. epilinum Weihe. Canadian Journal of Plant Science 86:293-316.

Gunjune, K., M.L. LeBlanc, E.K. Wafula, C.W. dePamphilis, and J.H. Westwood. 2014. Genomic-scale exchange of mRNA between a parasitic plant and its hosts. Science 15 August 2014:345 (6198): 808-811.

Virginia Tech. 2014. Plants may use newly discovered molecular language to communicate. ScienceDaily 14 August 2014. www.sciencedaily.com/releases/2014/08/140814191939.htm.

Posted in Chihuahuan Desert, Plants Tagged , , |