Organ Pipe Cactus National Monument

View from the visitors center trail at Organ Pipe Cactus National Monument.  In this photo jumping cholla, saguaro, and organ pipe cacti can all be seen with the Ajo mountains in the background.

Organ Pipe Cactus National Monument is located right on the boarder of Mexico as far south as you can go in Arizona. The monument is quite a ways off the beaten path and probably the most dangerous monument in the national park system. There are multiple boarder patrol checkpoints that are actually in or near the park and boarder patrols can be found all over the park. Tragically, a few years back a park ranger was killed in a boarder incident and things were quite dangerous within the park. Today however, a barrier fence has been put into place along the monuments boarder and dangerous sections of the park have been closed to visitors. The park is significantly safer today than it was several years ago. Unfortunately, Senita Basin, the only population of the columnar senita cactus is currently closed due to these issues. Fortunately, other sections of the park such as the Ajo Mountain Drive are open and offer spectacular views of the Sonoran Desert. For a desert, the monument is quite green and hope to a decent amount of vegetation. This unusually green desert is a result of this particular desert being one of the wettest deserts in the world. Organ Pipe receives about 10 inches of rain annually with significantly more falling at higher elevations. Both winter and summer rainfall seasons also contribute to the amount and diversity of vegetation here.

View along the Ajo drive in Organ Pipe Cactus National Monument

A total of 28 species of cacti can be found within the park. Several of these are at their northern most limits, and are prevented from migrating further north due increased number of days with freezing temperatures to the north. Organ pipe cacti are one of these species and can be found in abundance within the park. The amazing diversity of cacti can be found on the Ajo Mountain Drive, and it is spectacular for organ pipe viewing. Most commonly, the organ pipe is found on upper slopes facing south. The sun warms southern slopes just enough to prevent colder freezing temperatures that prevents the cacti from growing on colder northern slopes. Upper slopes also are slightly warmer due to warm air rising up these slopes. If the growing tissue of an organ pipe freezes for too long of a period of time or too many times in the winter it will kill the plant. So these slightly warmer areas give the cacti an added edge so they can become established. I also have a strong suspicion that organ pipes prefer soil types typically found on upper bajada slopes. Even with this added warmth however, conditions are not absolutely perfect. The organ pipe sill needs some help from what is called a nurse plant. Small shrubs, mesquites, ironwoods, and palo verdes all help protect young organ pipes from the intense summer heat and sun. The cooler temperatures and shady conditions also help hold the water in the soil for longer. On occasion, large rocks can even provide these added benefits of shadier, cooler, and slightly wetter conditions. Nurse plant associations can be found for several different species. The saguaro cactus has very similar nurse plant requirements. A different type of nurse plant association that can be found within the park is that of the jumping cholla and pincushion cactus. The jumping cholla is a rather large shrubby-tree like cactus that looses an abundance of spine dense joints. These joints naturally fall from the cholla and often will form a mat around the mother plant. Nothing really wants to go close to these piles of spiny cactus joints making it a perfect place for the small pincushion cactus to live.

Organ pipe cactus

White Tank Mountains Goat Camp Trail

Desert grassland with mainly tobosa grass located at the top of the White Tank Mountains west of Phoenix, Arizona.

Goat Camp Trail in the White Tank Mountains west of Phoenix is the longest trail in the park.  It can be done as a 13 mile loop along with the Mesquite Canyon and Bajada trails or a 12 mile hike from trail head to end and back.  The trail head is just past the park entrance on Black Canyon Road.  The first mile and a half of this trail is pretty easy starting at 1600 feet in elevation and hiking up a bajada.  This bajada is fairly interesting.  It has been highly disturbed by some major flood events along the dry washes that come out of the mountain canyons.  This disturbance is evident increasingly as you hike towards the mountains by the presence of large boulders laying on the surface.  Normally, these boulders would be sitting lower in the sediments of the bajada being at least partially buried in the dirt.  Large floods however coming from the canyons washed away a lot of these sediments but were not strong enough to wash away the larger and heavier rocks.  So the rocks remained in place while the finer textured sediments washed downslope.  These unburied boulders become increasingly common upslope where the flood, or floods, were more powerful.  These floods resulted in increasingly variable disturbed soil conditions higher up on the bajada and closer to the mountain.  As a result of the more variable soil conditions plant diversity also increases up slope.  Sections of undisturbed soils typically have triangle leaf bursage while the disturbed sections brittle brush.  Other species such as palo verde and jojoba also seem to like the flood disturbed soils.

After a mile and a half or so, the trail begins to head up the mountain.  This trail has an elevation gain of about 1700 feet, topping out around 3300 feet.  With the elevational gain, average annual temperature decreases several degrees and rainfall increases several inches.  At the mountain base, rainfall averages about 8 inches annually.  At the higher elevations rainfall probably averages around 14 inches annually.  There is a rain gage at the top of the White Tanks but apparently gusty winds around the peak prevent it from collecting rain properly, so it is difficult to get an exact measurement of rainfall at the peak.  Regardless, the vegetation tells us that rainfall is significantly higher and temperatures slightly lower.  In the past decade or so I have seen snowfall above 3000 feet in the White Tanks only one time and never below that level.  The lower temperatures, specifically lower freezing temperatures in winter, make conditions less ideal for cacti such as the saguaro.  Saguaros are not able to survive freezing tempertures for longer than 24 hours.  While the saguaro does grow near the peak it is quite rare in comparison to lower elevations.  This is at least partially due to the increase in the amount of time freezing temperatures occur at the higher elevations.

I think there may be another, possibly better, explanation for the decrease in cacti towards the top of the White Tanks though.  Around 3000 feet the vegetation strongly shifts towards a desert grassland dense with tobosa grass.  Tobosa increases because of the increased rainfall and because it can survive the freezing temperatures quite easily.  Dense tobosa grass is possible out competing the cacti at these higher elevations.  Another indicator of a problem for cacti at these higher elevations is the presence of charcoal.  Obviously charcoal indicates fire has been present in the area at sometime in the past and cacti in general do not survive fire very well.  Grass, such as tobosa, however, are very flammable and actually encourage fire to some extent.  Grass, unlike cacti, are very adapted to fire through.  While I have never seen or heard of a grass fire at the top of the White Tanks, the charcoal is evidence that it has happened at some time in the past.  Even if fire happens only once every few decades, that is enough to severely limit the population of cacti in the area.

A few other plants that are relatively common at the higher elevations include desert agave and crucifixion thorn.  Most of the desert agaves are pretty small and almost all appear to be clones that have grown from root sprouts of older plants.

Crucifixion thorn.

Desert Agave

December Ephemeral Drainage Flow

A dry wash the morning after a flash flood came though.

The mid-December rain is the most reliable rainfall we receive here in the Sonoran Desert.  This rainstorm is almost like clockwork.  Every December, usually around the 15th or so, a strong Pacific frontal storm system brings rainfall in from the northwest.  One-half to one inch of rain pretty much falls across the entire desert with higher totals in the mountains.  Of the past ten Decembers, only one failed to produce any rainfall and that was during one of the driest winters on record in Arizona.  This year was picture perfect with one-half to one inch of rain falling in the Sonoran Desert between December 13th and 15th.  With this rain being almost like clockwork, the flow of the normally dry washes also flow during this rain almost like clockwork.  This year was a little odd in that the rain was spread out over a three day period making flows a little weaker than normal.  Typically, dry washes require a significant amount of rain over a short period of time in order to generate enough runoff to supply a flow.  A lot of drainages did flow at least a little though. 

Of course, a lot of rain over a short period of time helps these washes to flow in the desert, but there are other factors involved also.  Geology, or geomorphology, are probably the most important factors in determining flow.  Geomorphology is simply a scientific term that describes how landforms came about and how they function.  One of the functions of geology and geomorphology in the landscape is to determine how and where water flows.  For example, shallow unbroken bedrock is going to prevent water from seeping down into the soil and therefore will result in greater amounts of runoff.  Type of soil also matters in the amount of runoff produced.  Rain seeps very slowly into clay soils so a lot of runoff can be generated.  Sandy soils however can quickly absorb a lot of rain so not much will runoff.  Number of rocks also makes a difference.  Soils with fewer rocks have more runoff than soils with more rocks.  Rocks on a soil surface slow the speed of runoff and with slow speeds of runoff the water has more time to be absorbed into the soil.  Size of the dry wash also makes a difference with smaller washes flowing more frequently than larger washes.  However, larger washes tend to run longer than small washes when they do flow.  Larger washes simply need a lot more water to flow. Depending on the combination of these factors some washes will flow a few times annually while others will only flow a few times a decade.

All of these things factors also determine what lives where along a dry wash.  Flow is normally very short in duration in a wash.  This is simply because flowing water quickly is lost as it is absorbed into the sediments of the stream bed.  Though flowing water is lost, the water is not entirely lost.  Water is stored in these sediments for long periods of time after surface flow ends.  Depending on the depth of this moisture and the depth of the sediments differing plants will occupy the area.  Typically, deep sediments with relatively frequent flows will be occupied by blue palo verde and desert willow.  Areas of fewer or shorter flows typically have yellow palo verde.  Other plants such as acacia's, ironwood, wolfberry, and mesquites can be somewhere in-between. 

Fall Leaves in a Sonoran Desert Riparian Zone

A Sonoran Desert riparian area in fall along Cottonwood Creek.

The desert is most definitely not known for spectacular fall colors.  Fall colors do however, find their place along some of the wetter desert water courses.  If perennial water sources are available, even if it is hidden below ground a short distance, the roots of large deciduous trees will find there way to it.  Sycamores, cottonwoods, and willows are all relatively common along streams and washes with perennial sources of water.  Even Arizona walnut and ash trees can be found in some of the more stable riparian zones.  These trees do not display the brilliant hues of red and orange common to eastern forests but do show off bright yellows that are in stark contrast to the dried out browns and greens of the desert.    Desert fall leaves are quite a rarity and are quite unique.  Typically, perennial water sources are considered perched water tables.  A perched water table simply is water that accumulated above the surrounding water sources, most often a result of bedrock that prevents water from penetrating deeper into the soil and out of reach of plant roots.

A recent hike I took demonstrated this concept extremely well.  The hike was along Cottonwood Creek near Lake Pleasant north west of Phoenix.  The majority of this hike is along Cottonwood Creek, which really isn't much of a creek considering water only flows in this creek a few hours every year.  The rest of the year the wash remains mostly dry, except for a few locations.  Nearly all washes in the desert are called dry washes, and for good reason: they are completely bone dry the majority of the year.  A few washes, such as Cottonwood Creek are fortunate enough to have areas that always remain wet.  Cottonwood Creek owes this moisture to its underlying geology.  First off, the creek bed lays at the base of two small bajadas between two small mountain ranges.  One bajada lays to the north of the creek bed and one to the south.  These bajadas and bedrock of the mountains are relatively steep and provide ample runoff to Cottonwood creek so it will run during periods of heavy rainfall.  Moisture is quickly lost into the deep sediments of the bajada and placed out of reach of deciduous tree roots.  In areas where bedrock are shallow though, moisture cannot penetrate deeply and remains closer to the surface within reach of plant roots.  Bedrock can also push water flowing underground towards the surface.  At these locations large deciduous trees take advantage of the shallow moisture and can in a few places form small but beautiful wooded areas.

Wildlife may not be obvious in these small wooded areas, but if you look at the ground you are sure to see evidence of animals.  Javelina and mule deer heavily utilize these small areas and their hoof prints are normally abundant.  In some area, such as along Cottonwood Creek, wild donkey's are also abundant and heavily utilize these areas.  The abundance of shade, food, water, and cooler conditions during hot dry summers gives great value to these areas for every creature.

Barrel Cactus Part 1

Compass Barrel cactus

The barrel cactus is one of the most common cacti in the Southwest.  There are four different species common to this area of the country, the most common of which are the compass barrel and the California barrel.  It can be extremely difficult to distinguish between these two common species of barrels.  In southern Arizona, such as around Tucson, the compass barrel is the most common of the two cacti.  In central Arizona such as around Phoenix, southern California, and even into the depths of the Grand Canyon the California barrel cactus is the most common.  Their ranges overlap in central Arizona and their similarities are pretty extensive.  Without closer investigation you may not be able to determine what specific species a particular barrel is, there are however a few differences that may help in identification.  First off is shape.  Of course, barrel cacti are all sort of barrel shaped.  The compass barrel is a little more wide and plump than the California barrel.  The California barrel  is a little skinnier.  The second way to distinguish between the two is by looking at the spines.  Both have very interesting spines which are often red colored.  This red coloration gives a sharp contrast to the dark green of the cacti's body, especially after a rare rainfall.  Both cacti also have flattened central spines that have a ribbing pattern on them.  The central spines are also hooked, giving both cacti another common name of fishhook barrels.  Compass barrel cacti spines are however considerably more hooked than California barrels.  Compass barrel central spines are a full "fishhook" shape and were in-fact used as fishhooks by some Native Americans.  California barrel central spines are closer to a 90 degree curve than an actual fishhook.  These are the best ways, though not necessarily foolproof ways of distinguishing the two while out in the desert.

In our next post we will talk about the leaning habit of barrel cacti.

California barrel cactus front left of picture.

Creosote Bush

The Creosote Bush (Larrea Tridentata) is a relentless desert plant growing in the deserts in both North and South America.  In North America it is found in the hot Mojave, Sonoran, and Chihuahuan Deserts where it is possibly out numbers all other perennial plants.  The only North American desert where it is not found is the Great Basin, simply because it is too cold.  The Creosote is so relentless it can occupy the poorest soils in flat basin areas between the mountain ranges of these deserts.  Driving through the flat lands of these deserts you can drive mile after mile past near mono-cultures of this plant.  Its roots are so effective at extracting moisture from the soil that it is often very difficult for other plants to become established near Creosote.  Creosote roots can extract water from soil that is seemingly dry, surviving up to two years without rainfall.  They can also extract nearly all water from the soil, thus preventing any from ever reaching the water table. Creosotes are so good at all this that they can in-fact survive for over 11,000 years!

Monsoon Season: Breaking the Desert Drought

About two months ago I posted on how a 4th of July storm broke a severe drought we had been experiencing throughout all of 2012 (Monsoon Season and the Drought).  As always with desert rain patterns though, you never know if the rain is going to keep coming or if a single rainfall event was just a fluke.  Fortunately, we have had a pretty good monsoon season that began with an earlier than normal large rainfall event and still seems to be going.  As of now, most of the desert surrounding Phoenix has received about three inches of rain in the past two months, which is slightly above average.  As a result of the rain and additional humidity, temperatures have actually been cooler than normal.  We of course have had our 110 degree plus days, but nothing like what we have had the past several years. 

The effects of rainfall on the desert over the past two months has been quite dramatic.  The drought had been so severe that mesquites and acacias had gone leafless which is fairly rare.  Ironwoods also were loosing many leaves and yellowing, which is extremely rare.  Other plants such as wolfberry and palo verdes were also leafless.  Creosotes were loosing leaves quickly and leaves that did remain were often brown or yellowish.  Nearly all triangle leaf bursage looked as if it were completely dead and often brittle bush was just a bush of white crispy sticks.  As you looked out across the desert in late June it appeared to be a crispy brown landscape without much life.  The rain however changed all this very quickly.  Within days of the first rain, new bright green leaves began to sprout.  The sustained rain allowed for these leaves to keep growing and for new stems to begin growing also.  Creosotes show this dramatic change quite well.  Creosotes still retain some of the old more brownish leaves from the drought period.  Directly above these brown leaves though bright green leaves are growing like crazy.  Ironwoods, palo verdes, wolfberries, mesquites, and acacias are also all full of leaves.  One thing I love about the desert after rain is all the different shades of green that color the landscape.  Each one of these plants has a slightly different shade, from the dark thick green of the ironwood, to the yellowish light green of the palo verde.  There also has been enough rain for the wolfberries and creosotes to flower.  Many wolfberries are in-fact loaded with fruit right now as a result of the rain.

All of this has had very positive effects on the wildlife.  I have noticed good populations of gambles quail as well as some healthy rabbit populations.  I am also sure many of the song birds are benefiting by the increase in berries and bugs.  The additional water and grass growth should also be having a positive effect on mule deer, hopefully increasing fawn survival.

So for now, the drought has been broken and with continued rainfall we can hopefully keep from returning back to drought conditions.  As of now, the National Weather Service is predicting the return of El Nino this fall and winter, which often means more rainfall.  A lot of weather scientists hold La Nina responsible for the drought in the Southwest over the last few years. 

Life of a Cactus Part 10: Nurse Plants

The pulp within a cactus fruit contains hundreds to thousands of seeds.  Unfortunately, very few of these seeds will ever germinate, and probably less than 0.1 percent of these seed will ever grow to mature plants.   Life is just too dangerous and for the seed and young plants.  The first problem a cactus seed faces is also a blessing.  The succulent flesh of a cactus fruit is like a magnet during the bitterly dry and hot summer.  Birds and animals gorge themselves on these fruits as a rich source of moisture and nutrients.  As they eat the fruit though, many, if not most of the seeds are also eaten.  This isn't such a bad thing, as long as the seeds are not crushed by chewing.  The seeds are especially adapted to remaining intact and passing directly through the digestive tract unharmed.  With most cactus fruit being brightly colored and located high on the plant, these fruits are especially enticing to birds, which may be the only organisms capable of reaching the fruits on taller cacti.  For example, the saguaro cactus holds its fruits tens of feet off of the ground, only allowing birds to access it.  Being birds do not chew food, instead swallowing it whole by the beak-full, most of the seeds can pass through the digestive tract unharmed and be deposited in their fecal matter.  Kind of a disgusting start to life but true never-the-less. 

Young Saguaro Cactus growing under the canopy of a Palo Verde.

Being birds of course frequently perch and sleep in shrubs and trees, most of their fecal matter, and therefore cactus seeds, will be deposited below.  This is the ideal environment for a cactus seed to germinate and grow in.  The shrub or tree provides shade which creates a slightly cooler and moister environment for the seed to germinate and grow in, something much needed in the desert.  The shrub or tree also provides cover and protection from predators which might eat recently germinated seeds.  Soil also is slightly more rich in these locations also.  This environment under the shrub or tree is termed a microenvironment and the plant that makes it is called a nurse plant.  Of course, the nurse plant is termed such because it helps, or nurses, young plants such as cacti to maturity in the microenvironment they create.  A microenvironment is a small area, such as under a tree canopy, that has slightly different conditions than the surrounding environment.  Within the Sonoran Desert Triangle-Leaf Bursage is the most important nurse plant.  Very few plants are able to grow without the nursing aid of a Bursage microenvironment.  This is sort of odd considering Bursage is such a small desert shrub, usually only reaching 20 or so inches in height.  Bursage is however one of the few plants capable of establishing itself without a nurse plant and is extremely abundant across the desert.  Other plants such as Palo Verde trees are well known nurse plants but not as important as Bursage.  This probably is because Palo Verde can't become established without a nurse plant and are not as common as Bursage.  Interestingly, the most common nurse plant for Palo Verde is also Triangle-Leaf Bursage. 

Montezuma Castle National Monument: Montezuma's Well

Montezuma's Well, part of Montezuma Castle National Monument.  This is the well viewed from the cliffs surrounding the pond.

The Montazuma's Well portion of Montezuma Castle National Monument is the lesser known of the two sections of the park.  It has far fewer visitors every year, but I personally find this section far more interesting.  First of all, the "well" itself is pretty interesting.  This "well" is simply a hole in the ground pond, quite an unusual landscape feature and I know of nothing quite like it.  This pond is located on a hill top that caved in as water dissolved away the limestone bedrock forming the hill.  Nearly vertical cliffs line all sides of the pond and there are well over 100 stairs down into this hole along the trail.  Ancient Native American ruins are built into many of the cliff walls around the pond.  If you think about it, this would have been quite a nice place to live 1000 years ago or so.  At least compared to the surrounding desert.  The surrounding Upper Sonoran Desert is similar to what is found at the Montezuma Castle portion of the National Monument, however, it is slightly higher elevation.  This means it is slightly cooler and receives a little more rain, just enough to support Juniper trees in addition to all the other plants found in the desert we discussed in our previous post.  While Junipers do add another food source, a little more shade, and a source of materials, it still doesn't make living out in the open desert possible.  Within the "well" area, the pond moderates extreme temperatures, supports trees that provide shade, and provides water.  All this making for a nice place to live during ancient times.  Every time I have visited the well in the summer though, I still think it would be an unbearably hot place to live.   I have noticed the cliff walls where the ruins are located are considerably cooler though.  Hanging out in the cliff walls during the heat of the day was probably a major way ancient inhabitants avoided the heat. 

Montezuma's Well at the outlet, where the pond flows into one of the surrounding cliffs.

If you aren't at least sort of amazed just by the landscape structure of the well, than hopefully learning about the aquatic biology of the pond itself will amaze you.  The biology of the pond is truly weird, almost sci-fyish.  First of all, because the spring that feeds the pond come out of limestone there is a huge amount of carbon dioxide dissolved in the water.  This does not mean the water is poisonous, but does mean that fish cannot live in the pond.  Which would have been a bummer to me if I was an Native American living there 1000 years ago.  Algae, however, thrive on the abundance of carbon dioxide.  This algae supplies food to loads of amphipods, which are sort of like tiny freshwater shrimp.  At night time, huge numbers of non-blood sucking leaches migrate towards the surface to feed on the amphipods.  It is rather creepy imagining a mass migration of leaches swimming towards the surface. 

Vegetation along the canal the empties into Beaver Creek.

Once outside of the well, the trail also takes you down a cliff opposite the pond cliffs and along Beaver Creek.  This is actually my favorite section of the entire monument.  As mentioned before, water flowing out of the well flows into one of the cliffs surrounding the pond.  On the other side of the cliff, the water flows out of another cliff along the creek.  This water enters Beaver Creek in a truly beautiful desert oasis.  This area is very wet and shady.  Temperatures are far cooler here in the shade and thick vegetation.  Huge Sycamores and Ash trees fill the area.  The area has enough moisture to support beautiful Columbine flowers and even some poison ivy, so watch out.  This verdant green area is quite a refuge from the surrounding harsh desert landscape.  

Montezuma Castle National Monument

Montezuma's Castle

I recently made a trip to Montezuma Castle National Monument in central Arizona.  The National Monument is just a short trip off of the I-17 north of Phoenix and has two parts, the Montezuma Castle which is the main part, and Montezuma's Well which is a lesser visited section of the park.  Today, we will be discussing the castle portion of the monument.  We will discuss the well at a later time.  Both of these sections have some pretty interesting biology.  I will save some blog space by letting you read-up on the monument at the National Park website if you are interested:  http://www.nps.gov/moca/index.htm

Instead of giving the basic information on the park, I'll try to give you a little different perspective.

The tiny Beaver Creek that runs past Montezuma's Castle makes big changes in the desert landscape.  Much more water runs underground in this stream than above ground, feeding the trees of this desert oasis.

During the summer, Montezuma's Castle can give us modern humans a good perspective of what a desert oasis is like.  Us moderners are used to air conditioning, running water, and produce filled super markets everywhere we go.  Obviously this wasn't the case in the desert less than a century ago.  Hiking around the monument in the sweltering heat and scorching sun of summer should get one major point across to everyone about living in the desert.  That is: water is life.  The rolling limestone hills and mountains surrounding the monument are covered with relatively sparse Upper Sonoran Desert vegetation.  Water for drinking is completely absent and shade is minuscule.  Yuccas, prickly pear cacti, Creosote Bush, and Grey Thorn are the dominate plants, none of which cast any significant amount of shade.  Finding or not finding shade in the summer can mean life or death.  Think about how much hotter it is in the sun than it is in the shade.  Temperatures the weatherman gives us everyday are always taken in the shade.  If you were to take the temperature in the desert sun it might be thirty or more degrees hotter.  130 plus degrees is not easy for the body to handle and can quickly lead to life threatening heat stroke.  This is exactly why ancient Native Americans settled along Beaver Creek, where the water from the creek mean life.

Gazing up into an Arizona Sycamore tree along Beaver Creek in Montezuma Castle National Monument.  This tree casts life giving shade that decreases the temperature by tens of degrees.

Beaver Creek might not look like much, but its effect on the landscape is dramatic.  The water-less shade-less landscape surrounding Beaver Creek quickly is transformed into a more moist and shady habitat the nearer you get to the creek.  Real trees become abundant near the creek, replacing the diminutive pathetic excuses for trees further away.  Smaller desert trees such as Desert Willows, Mesquites, and Acacias, become common along the outer edges of the riparian area.  Riparian areas are simply the vegetation adjacent to water.  As you move closer huge Arizona Sycamores, Arizona Walnuts, Velvet Ash and Cottonwoods become abundant and cast a dense shade on the ground.  Hackberry, Mesquites, and Acacias are also common in the undergrowth of these large trees.  This shady more moist environment is far more hospitable and inviting than the surrounding desert.  In-fact, you can get the feeling this shady desert oasis might have even had a paradise like sense to it to ancient desert dwellers.  While the actual creek might not look like much, remember, a much large amount of water is flowing slowly underground.  This underground water feeds the deep rooted riparian trees tens of yards away, creating an abundance of life in the desert.

Beyond all this, the creek of course also supplied plenty of water for agriculture for ancient inhabitants.  The riparian vegetation also supplied the ancients with plenty of wild foods to eat such as mesquite bean pods.  Furthermore, the riparian area was not only attractive to humans but also to wildlife, which were hunted.  On our trip we saw an abundance of wildlife including two snakes, squirrels, wrens of various species and a rather tame Summer Tanager.  

A rather tame Summer Tanager found at the monument.

Life of a Cactus Part 7: The Cactus and Freezing Temperatures

All that water stored inside the cactus can be quite a problem certain times of the year.  Freezing temperatures can create ice crystals that burst cells causing tissue damage.  Depending on the amount of tissue that freezes and how well a cactus is adapted to freezing temperatures, freezing can actually kill a cactus.  Most species of cacti are not well adapted to freezing and for that reason most are tropical and subtropical.  Tropical climates never freeze while subtropical climates occasionally freeze.  The extreme southern United States, such as southern Arizona, Florida, and California are all subtropical.  The subtropical Sonoran Desert of Southern Arizona and Sonora, Mexico typically average freezing temperatures a few nights every year or fewer.  Freezing temperatures rarely last more than a few hours and results in the abundance of cacti found in the Sonoran Desert.   Saguaro cacti are the largest cacti that can tolerate freezing, but only as long as freezing temperatures last less than 24 hours.  Areas that receive freezes lasting longer than 24 hours have no saguaro cacti.  Organ Pipe cacti are much more frost sensitive and only survives along the U.S.-Mexico border in Arizona and further south.  Some species such as the Cordon can survive no freezing whatsoever and therefore only survive further south in Mexico.  A handful of prickly pears are able to survive very deep freezes for long durations of time.  Several species, such as Plains Prickly Pear, grow in grasslands of the central United States up to the Canadian border.  One species, Brittle Prickly Pear, grows nearly to the Arctic Circle surviving temperatures as cold as -40 degrees Fahrenheit.  Species able to endure freezing temperatures actually remove water from their cells, essentially dehydrating themselves, so ice crystals will not burst and kill cells.

Brittle Prickly Pear, able to survive temperatures of -40 degrees F by pumping water out of its cells so ice crystals do not burst cells.

Life of a Cactus Part 6: Callus

Woodpecker entering a Saguaro "boot" used for nesting.

The last water conserving feature of cacti we will discuss is the abundance of mucilage all cacti produce internally.  If you were to cut open a cactus and touch the internal tissues you would notice the cut surface as being wet.  It would not be a watery wet though, it would be a slightly sticky and slimy wet.  This sliminess, known as mucilage, is a result of polysaccharides, or carbohydrates, produced by the cactus.  This mucilage helps hold on to water, slowing evaporation.  It also functions to form a callus or scab once it is exposed to the air.  This callus functions similar to how scabs function for us.  When we bleed a scab forms to prevent further loss of blood and to prevent infections from entering the body.  When a cactus is damaged it also forms a scab to prevent bleeding of water as well as to prevent infections from entering the cactus.  The scab also hinders predation of the cactus by being a barrier between the cactus flesh and potential predators.  Often the callus can become several millimeters thick.  If you are around cacti at anytime, look for damaged sections of the plant.  The callus will be a tan to black coloration on the damaged portion.

Saguaro boots like this one will often survive long after the cactus decays away.

All cacti form calluses, which may be helpful to the cactus but an annoyance to any animal that may want to feed on cactus flesh.  A lot of birds though find cactus calluses quite useful.  In large columnar cacti such as saguaros, woodpeckers will remove the spines and peck a hole.  Gila woodpeckers and flickers are the most common birds that do this in the Sonoran Desert.  These woodpeckers will excavate a hole large enough to nest in.  Of course, excavating a hole in a cactus will cause it to bleed but a callus will in short time line the hole.   Typically, this causes little harm to the plant itself in the long-run.  These callused holes are called “boots” and will remain for the entire lifespan of the cactus.  Once the boot is abandoned by the woodpecker, an assortment of other birds will also use it for nesting being it is warmer in the winter and cooler in the summer.  Birds using saguaro boots for nesting include elf owls, kestrels, ash-throated flycatchers, and purple martins.  In-fact several species of birds can be found nesting in the same large cactus if it has many different boots.  This is why saguaros have been called by some a “cactus hotel”.  Oddly, these boots are so durable that, years after a saguaro dies and most all remnants of the cactus have decayed, the boot still remains lying on the ground.

A callused hole in a Saguaro cactus.

Life of a Cactus Part 5: CAM Photosynthesis

This is a microscopic image of the surface of a cactus stem.  The oval shaped objects with dark areas in the middle are stomata, the pores through which cacti and other plants breath.

Once water is stored inside of the cactus water conservation does not stop.  Microscopic pores called stomata cover the green tissue of all plants, allowing them to “breathe” and carry out photosynthesis.  Through these stomata, all plants “exhale” water vapor and oxygen and “inhale” carbon dioxide.  The exhaled oxygen is a waste product of photosynthesis.  Inhaled carbon dioxide, along with water stored in the plant, are converted into sugar and starches by use the sun’s rays through photosynthesis.  Water vapor is passively lost through the stomata whenever they are open to inhale and exhale carbon dioxide and oxygen.  Even when the stomata are closed a small amount of water vapor is lost.  Nearly all plants open their stomata to carry out this breathing process during the day.  As a result, most plants are opening their stomata when it is hottest and are therefore releasing huge amounts of water through evaporation.  In areas where there is plenty of water this really isn’t much of a problem, if you have water to spare you have water to waste and can afford to use it quite liberally.

Crested Saguaro Cactus.

 In the desert, there is no water to spare and to waste.  The cactus therefore does the exact opposite of what almost all plants do; it opens its stomata to “breath” during the night and closes them during the day.  By closing stomata when it is hottest and opening them when it is cooler the cactus conserves huge amounts of water.  The problem with this is cacti still need a constant input of carbon dioxide in order to carry out photosynthesis during the day, and without the stomata open there is no direct supply.  To overcome this, cacti absorb huge amounts of carbon dioxide at night time when their stomata are open.  They do not absorb this carbon dioxide like a balloon would though.  Instead, they convert it to a chemical called malic acid, in which form the carbon dioxide can be stored until day light.  Once daylight appears the, malic acid is then converted back to carbon dioxide needed to carry out photosynthesis.  Plants only need to breathe when it is daylight and photosynthesis is being carried out.  So in the dark no photosynthesis is being carried out and there is no reason for the plant to breathe.  This is why most plants only open their stomata and breathe during the day, and because day temperatures are warmer, more water is lost through the stomata.  As said before, cacti open their stomata at night to breath, storing the carbon dioxide as malic acid for later use when daylight appears.  Once daylight appears, the cactus closes its stomata and sort of holds its breath, converting the malic acid back into carbon dioxide for use in photosynthesis.  By only opening their stomata in the cool of night, far less water is lost.

Cacti stomata not only open and close in an ideal fashion to conserve water, they are also specifically designed to conserve water.  Most plants have large amounts of relatively small stomata all over their green tissues.  Relatively small cells also open and close these stomata.  Cacti however have far fewer stomata but the stomata they do have are much larger.  Overall though, this decreases the amount of water that can evaporate through the stomata.  Cacti also locate their stomata is a shallow pit as opposed to directly on the surface of green tissue.  This protects the stomata from drying winds.  Lastly, the cells that open and close the stomata are huge in comparison to the typical plant.  This allows the cacti to firmly close their stomata so water vapor does not accidentally leak out.