San Diego Climate and Chaparral Fires

If you haven’t noticed, San Diego doesn’t get much rainfall. In fact, average rainfall in San Diego only averages 10.8 inches over the past 48 years (which is about half the amount of precipitation San Francisco gets)! This means that plants in San Diego County are somewhat drought-resistant. Fog, however, adds a great amount of moisture to the air which increases humidity and lowers plants’ evaporation rate. If you are new to UCSD, you will discover that fog has been known to reside over our campus for days or weeks at a time, especially in June and July.

Whether you have traveled around San Diego under its sunny or overcast skies, you may have noticed dense shrubs covering hillsides. This plant community is known as chaparral. Chaparral is the most prominent plant population that spreads throughout Southern California. These plants form dense thickets of woody shrubs on drier and/or shallow soils where woodlands cannot survive. Chaparral originally came from the Spanish word chaparro, which was first used to describe evergreen oak trees in Spain. Mixed chaparral areas can be found throughout La Jolla and the rest of San Diego, but you can view them locally at UCSD’s ecological reserve park and Torrey Pines State Park. The photograph below shows a mixed chaparral community north of Geisel Library.

A view of the chaparral

Photo taken from Google Earth

The dry soils and dense, woody shrubs of chaparral communities are unfortunately famous for catching fire. The fall season’s Santa Ana winds dry out vegetation, which spreads the nearly annual San Diego intense chaparral fires. These fires frequently reach temperatures of 1,200◦F, making them some of the hottest fires in natural environments in the world. Fire intensity is determined by many factors including temperature, humidity, wind, and foliage and stem moisture content. The fires of October 2007 burned 400,000 acres across San Diego County. The Witch Creek Fire, the Poomacha Fire, and the Rice Creek Fire ended up destroying 1259 structures and cost 7.6 million dollars to exterminate. Ashes fell from the sky for days and UCSD was subsequently closed for a week due to poor air quality.

Luckily chaparral shrubs have recovery strategies that have evolved in shrubs throughout the world. Most have woody root crowns below the soil surface that are protected from the heat of the fire. These root crowns are able to quickly sprout multiple new stems because they store carbohydrates, thus rain is not required for post-fire recovery.

UCSD Celebrates 50+ Years of Eucalyptus Trees

It's something that we learn to notice, speculate on, and eventually learn to ignore-- the hordes and hordes of eucalyptus trees found on the UCSD campus. They're everywhere and in every college. We pass by them every day, make wisecracks about their uniform dispersal, and bemuse ourselves with the Talking, Singing, and Silent individuals. These more artificially-preserved trees aside, the groves of trees on our campus have become living walls of greys and greens that we have come to chalk up as all one genus of plant. Of the thousands of trees currently growing on the UCSD campus, more than 220,000 are of the genus eucalyptus. However, how many distinct species of eucalyptus can be identified in their midst?

Photo courtesy of Dr. Heather Henter

No less than 18 different species of eucalyptus sprout around us. If you look closely, you might be able to spot the more obvious differences-- some have smooth bark while some are rough; some are white-grey while some are ashen and almost black. Which ever way you spin it, the eucalyptus are more diverse than we give them credit-- in Australia, where the trees are native, there are over 700 different species that have evolved over centuries of geographical isolation. But why in the world would our campus in sunny San Diego be covered by some smelly, invasive tree from across the ocean?

Back when UCSD was even conceived as an idea, the grounds where our campus currently stands was a eucalyptus tree farm. In the 19th century, eucalyptus was prized for its quick growth time and ability to thrive in San Diego's weather conditions. It was used both medicinally and for construction during a time of "timber famine" (however, the construction plans were later realized to be lacking in long-term strength). Consequently, this eucalyptus farm was also a place for alcoholics and county prison inmates to "sober up" and contribute back to society during their incarceration. After the interest and enthusiasm for eucalyptus died down, the trees remained in the area until the construction of UCSD in the mid 1900s. 95% of the original trees from the farm have been deforested or chopped at least once, save for a few exceptions-- the trees next to the Main Gym and Urey Hall have survived the last century without ever once being pruned or cropped.

Next time you see a eucalyptus tree on campus (which I can assure you will be quite soon), take a look around and see if the trees around it are similar or not. You'd be surprised as to how different things are once you start to pay attention. :)

Layers of a Tree Trunk and Tree Wounds

Many people believe there are two layers in a tree’s trunk: the outer bark they can see and the inner ‘wood’ that supports a tree. In fact, there are five layers in a tree trunk and the two layers most people know about are both actually dead.

The outer bark that some people like to carve into and write cliché messages like “Me+Y.L.=Love” is a tree’s protection layer from bugs and insulation from weather. Just inside the outer bark is the inner bark, also known as the phloem. This layer transports nutrients throughout the tree. The phloem soon dies and the older phloem becomes part of the outer bark. The cambium layer is the growing part of a tree’s trunk. You have probably heard that the age of a tree can be determined by a cross section of a tree trunk. This is correct, and the annual rings are formed by this layer. As long as a tree is receiving nutrients, hormones are formed, thus new bark and wood are produced. Sapwood is the new wood that the cambium layer forms. This layer transports water to the tree's leaves. Older layers of sapwood die and turn into the next layer, heartwood. Heartwood is the center of a tree trunk and will not decay or lose its extreme strength as long as the other layers of a tree are intact and functioning.

This is a tree located in the middle of the Student Services Center (down the stairs from Croutons) and shows a tree’s response to pruning. At one time a branch here was pruned. Notice that the clean cut is inside the outer bark. How did this happen? There are actually four layers that formed after the pruning occurred. The first three layers are known as compartmentalization. The first is something like a band aid, plugging cells directly around the wound (yes-pruning trees create an injury!). It protects the tree from further injury from infection and decay. The second layer consists of cells that form each year and act as guards against any infection entering past the first defense layer. The third layer resists lateral spread, using physical and chemical defenses. The final layer is known as a barrier zone. New wood that contains natural fungicides is formed. This wood is harder and stronger due to 1) an altered arrangement of cells, 2) an increase in axial parenchyma, which provides more energy to the injured tissues, and 3) the cells acquire toxins to kill off any invading organisms. This fourth layer is what you see directly in front of the pruned section of this tree.

Poke!

Mimulus aurantiacus, also known as the monkey flower, is one of the most interesting flowers on UCSD’s campus because of the steps in its pollination sequence. Unlike most plants that we come across daily, these plants are capable of very rapid movement! The monkey flower is a shrub native to California and can be found on campus north of Geisel Library in UCSD’s ecological reserve park. Although monkey flower colors vary, the campus' flowers are generally an orange-red. The stigma lobes, which look more or less like tiny white fish lips, are part of the female reproductive system in plants. What makes this flower unique is the closure of the lobes upon touch, an action that you can watch from beginning to completion in just a few seconds. If the lobes are touched without pollen, the lobes close but then will quickly reopen within the next day. If the lobes are touched with pollen, part of the male reproductive system in plants, the pollen will continue down the pistil to the ovule. If many pollen grains make it into the ovule, the lobes will never reopen. However, if only a few grains travel into the ovule, the lobes will most likely reopen. These plants are commonly pollinated by bees and hummingbirds, so be on the lookout for both if you if you come across these flowers, which bloom from March to July. Try touching the white stigma lobes and watching them close! We guarantee that afterward, you'll never be able to pass by a bush of monkey flowers without having an urge to prod them. Happy poking!