Virtual Mt. Lemmon Field Trip

Along the lower slopes in Sonoran Desert scrub we will also see many shrubs and small trees. Brittlebush is a drought-deciduous shrub that is prominent here, especially in drainages. In most years it produces broad leaves with fewer hairs in the winter and spring, and drops these to produce smaller much hairier leaves in the summer. Smaller leaves plus hairs that reflect light and slow air movement around the leaf will all reduce water loss from transpiration, which is important for survival through the warmer period of the year. The hairs make summer leaves appear a much lighter green--almost white sometimes. Desiccated winter leaves that have not yet dropped are visible along with the light-colored and expanding summer leaves (upper left). Broad and less hairy (and therefore greener) winter leaves are in the upper right.

The lower images were taken in April when brittlebush was in flower. Brittlebush will produce a fairly vibrant display of color in the springtime in most years, even when precipitation in last few months has been well below average.

Saguaros are quite common as we begin the climb. Suitable habitat for saguaro is determined primarily by summer-fall precipitation and temperatures, and perhaps more importantly, by winter temperatures. Saguaro seedlings less than one year old are especially vulnerable to freezing temperatures. It’s often observed that saguaros will not survive on sites where on average there is at least one day per year when the temperature does not go above freezing (0° C). As we proceed up the highway, turning northeast above Molino Canyon, fewer mature saguaros occur, even on the warmer south-facing slopes.

This map shows 1-km cells where at least one mature saguaro occurs. As the mean January temperature decreases below about 10˚C saguaros do not occur.

The vegetation of Molino Basin is also dominated by perennial grasses. However, sufficient precipitation occurs here to support more trees, specifically oaks. Oaks, which are in the genus Quercus and the family Fagaceae, are most easily identified by their production of acorns. This genus is taxonomically complicated as hybridization between what are considered different species is common. Thus, intermediate types (interspecific hybrids) are also common. In the Santa Catalina range there are seven species of oaks, many of which occur in Molino Basin. This community is referred to as an oak woodland, which means one in which trees are present, but at lower density (number of individuals per some unit of area) than we would see in a forest.

In this course we focus a lot on sight identification of plants to families. The same knowledge and set of procedures can be applied to the sight identification of individual species—something that you actually have some experience with already. If you are an Arizonan, you could probably sight identify a saguaro long before you took field botany! For natural historians, identifying individual species is often very important. We’ll learn later in this course that when confronted by an unknown plant species--one we can't sight identify--being able to sight identify the family will greatly simplify species identification in most cases.

Like all good natural history, sight identification begins with an understanding of the environment you find yourself in at any point in time. We start by answering a series of questions: What’s the plant community type and elevation? What is the slope and aspect? What soils are present, how deep are they, and is water nearby or will it accumulate? Are fires, droughts, floods, or windstorms common in this environment? What animals (including humans) and other plant species might be affecting the species you are interested in? Knowing these geographical and ecological details, and then considering them together, will greatly reduce the number of possible species we might expect a a given site.

With this narrowed list of species, we then look closely at the plants to further limit the possibilities and hopefully come to a conclusion. (We have produced an  extended checklist  for sight identification.) As we do this, realize that everyone will do sight identification slightly differently. Accumulating botanical knowledge and experience with plants will likely change how you do sight identification. We will go through some examples of sight identification at stops along our field trip, beginning with a prominent oak woodland species.

This is the first of our sight identification examples: A tree with broad, stiff, and toothless leaves that are blue green above and paler below. Fruits are egg-shaped acorns with 1/3 in the cap-like cup. The bark is gray or whitish and fissured in small plates. Secondary and lower veins are not prominent (ridged) in the leaves.

The observation that acorns are present is critical as this is a defining trait for the genus Quercus. Knowing the appearances, preferred habitats, and elevation ranges of southern Arizona oaks would lead us to conclude that this is Mexican blue oak. It’s found at the upper edge of desert grasslands and in mid-elevation woodlands and canyons between about 1300 and 1650 m. This species is often browsed by deer and many small mammals consume acorns. Examining images and descriptions of this species from a reliable source such as  SEINet  is a good way to confirm your identification, although this requires an Internet connection.

Notice that doing sight identification requires very careful and through observation. Often at least two of your senses (sight and touch) are involved; sometimes smell and hearing are as well. Here, we're focusing on the most important of these senses, which is sight. Remember, that this field trip is giving you a chance to look at lots of images. With this, it's up to you to determine what you actually see the next time you are in the field. If you don't get the difference between these two, think about listening and how sometimes this is not the same as truly hearing.

Fire has played a significant role in the vegetation composition of much of the Santa Catalinas. Heading up Molino Basin, about 1 km north-northeast of the  Gordon Hirabayashi Campground  we can see evidence of the Aspen Fire, which burned here in 2003. Before that fire, this was an area with mostly scattered oaks (it is a woodland), a few alligator junipers, and a grass understory. This was what the site looked like in November 2003 about four months after the fire. Note that some basal re-sprouting is visible suggesting these trees are not dead. 

The image above shows what the site looked like in October 13 years later (2016). Many of the oaks that seemed dead in 2003 have recovered and new oak seedlings, as well as those of other perennial species have established. Vegetation recovery from wildfire may not involve conversion into a new community type, but it can occur. Many factors will determine what happens following a fire including the initial plant community type and intensity of the fire, the time since the last fire, and the environmental conditions following the fire, especially those affecting erosion and seedling establishment.

Here we can see the role of aspect in relation to species composition. The north-facing (northern aspect) slope of Bear Canyon is best categorized as a pine oak forest while the south-facing slope is oak woodland or other more drought-adapted community types.

Pine oak forests in this environment represent a very good example of the fluid nature of plant community boundaries. Oaks are generally adapted to drier and warmer sites while pines are found on those that are somewhat wetter and cooler. The transition areas are known as pine oak forests and there are many patches of this community type in the Catalinas.

Another sight identification example: We observe a tree in Bear Canyon with seed-producing cones > 8 cm long with sharp-tipped scales, needles 10-20 cm long, mostly 2-4 per fascicle, and bark yellow to red brown that is deeply furrowed and forms scaly plates.

There are four pines found in the Santa Catalinas. Knowing their appearances, preferred habitats, and elevation ranges would lead us to conclude that this is ponderosa pine. This is the most common pine species in Arizona and is found on open, generally drier slopes between 1500 and 2700 m. Many animals depend on ponderosa pine seeds including squirrels and chipmunks. Because it is so common and mature individuals are tolerant of low-intensity fire, this species is a critical component of natural resource management in many areas of Arizona.

The understory is also diverse, especially in the riparian zone. Golden columbine is one of the most attractive species here. This area was subjected to a variety of water and soil conservation activities during the 20th Century. This involved introducing plants—mostly grasses—not native to the Santa Catalinas and many remain common near the creek. 

Bear Wallow has numerous large trees, which could be defined as those with stem diameters at breast height (DBH) > 70 cm. Among these large trees, many have thick and corky bark that is deeply fissured and scaly with ridges. The needles, which are not in fascicles, are about 3 cm long. Seed-producing cones have three-pointed bracts between the scales. Given these observations and knowledge of preferred habitats and elevation ranges we would conclude that this is Douglas fir.

Seed cones of Douglas fir are very distinctive. The bracts under the cone's scales are often called “mouse tails” since they resemble the hind legs, tail, and back of a mouse. Bark on younger trees is much smoother and may be reddish brown. The species occurs at elevations between 2000 and 3200 m, but may be lower in some canyons. The tallest trees in Arizona forests are Douglas firs.

Another often tall (> 10 m) tree in Bear Wallow and higher in the Catalinas has smooth whitish/gray bark and nearly round leaves with a sharp point and finely toothed margins. The leaves are shiny green above and dull green beneath with the petiole longer than the blade and flattened.

These observations combined with knowledge of preferred habitats and elevation ranges would lead us to conclude that this is quaking aspen. This species occurs on wet or intermittently wet sites at elevations from 1900 to 3100 m. It is deciduous, often producing vibrant yellow fall color as leaves senesce. Quaking aspen may spread laterally by producing shoots from near-surface roots. Clonal groves develop via this spread after fire or other disturbances. Quaking aspen is the most widely distributed tree in North America, occurring in areas with cold and cool temperate climates from north of the Arctic Circle (near sea level) to near Mexico City (at high elevations) and from coast to coast.

The image at the right was taken in August 2020 after the Bighorn Fire, which burned vegetation on the steep slope in the background. The brown leaves on smaller trees under the aspens in the foreground were desiccated by flame heat, but did not burn.