Water Usage vs. Tree Growth: The Ecological Trade-Off of Urban Trees in Southern California

By Peter Ibsen, Ph.D. Candidate; Department of Botany and Plant Science, University of California Riverside

For the past several years, I have taught ecology classes to hundreds of undergraduate students at University of California Riverside and have found that the urban tree is an ideal common ground to discuss the ecological concept of “trade-offs.” You see, every single student has experiences with urban trees, from backyard lemons, to tire swings, to taking a nap in the shade of a sycamore on a hot day. Urban trees provide all these services and so many more, yet, the future of urban trees in dry regions is in doubt, and we, as urban residents, have to make serious considerations about ecological trade-offs between water usage and tree growth.

With the assistance of hundreds of citizen scientists, I am conducting a study of urban tree function across the climate gradient of Southern California. My early results find that the urban forest comprises a broad spectrum of ecological strategies regarding tree growth and water usage. It is possible to find species exhibiting all combinations of “fast/slow” growth and “liberal/conservative” water usage.

Figure 1. Ten species of urban trees oriented in an ecological trait space. Each quadrant represents a growth-to-water use strategy. Trees are represented using the US Forest Service IDs by species.

However, my research has discovered two very important trade-offs of water use and tree growth. When separating out the urban trees found in coastal southern California from those found in desert regions, what appears is an interesting difference between the two communities. The desert urban forest exhibits functions of a faster growth and liberal water usage. This goes against some conventional thinking that people plant more water-conserving trees in the desert. They key finding here is that trees and parks are heavily irrigated, and as long as a species can withstand the heat, they can take advantage of both the water and abundant sunlight for growth.

Figure 2: The effects of irrigation in Palm Springs. The difference between the irrigated Desert Highland Park and the natural Palm Springs environment on the right highlights this effect.

I also studied how individual species may change functional strategies when planted from the coast to the desert. By measuring the difference of plant water pressure before dawn (when plants have low water pressure) and in the middle of the day (when plant have higher water pressure), I am able to calculate the water status of a species at a certain location. A higher water status implies that the tree is losing more water to the environment. I discovered that all species studied, save one, increase their water usage when moving from a coastal environment to a desert one.

Figure 3: The differences in water status of California urban tree species. Most species exhibit an increase in water status when they are planted in the urban desert.

When taking all these results together, there is a clear management trade-off. In hotter and more arid environments, urban trees have the potential to experience faster growth at the expense of increased water usage. For urban stakeholders, this is serious consideration. Faster growth means quicker establishment of shade and greater cooling of air temperature. However, increased water usage has serious consequences for an area prone to extended droughts. As the future Southern California climate is predicted to become hotter and drier, our results highlight the uncertainty of our urban forest. My research will continue to add more species to the study, with a goal of both higher resolution of results, and ultimately an idea of which species might be “the right trees, for the right place, in the right time.”

And, thank you to all the community scientists who helped find the trees included in my study, including partners across the Greater Los Angeles region! Please let me know if you have any questions.

The Urban Resiliency Project in the Classroom: An Evolution

By Prof. Maura Palacios Mejia

I am a professor at California State University, Los Angeles teaching Fundamentals of Writing for Biologists. I am an advocate for students learning science through active learning. In the Spring of 2014, I partnered with the Urban Resiliency Program to bring citizen science into my classroom. My objectives were simple; get students outdoors and active in the community through research and generate a dataset for their research paper. With the help of the Urban Resiliency team, I incorporated several programs into my course.

My students are urban city dwellers that work part-time and commute to school. This weekly workload plus the horrible Los Angeles traffic leaves them very little time to pursue activities outside of school, let alone explore the outdoors. I come from a similar background, and I can recall a strong disconnect from nature, even with all the biodiversity that exists in urban areas. Connecting with nature can have a profound impact ranging from mental, physiological, and therapeutic benefits. I knew this partnership with the Urban Resiliency Program could be very beneficial for both parties. However, I faced a challenge in convincing my students, who range from microbiologists to pre-medical students, that this project was worthwhile. I placed the project in a context that appealed to their fields of interest by discussing topics such as the changes in microbe communities to the human health benefits of trees in urban areas.

Operation Tree Canopy & Resilient Trees
For the first year, the hypotheses tested centered around the goal of the study, which involved comparing a measured variable, such as tree diameter, to drought. James Pineda comments “The outdoors broadened our horizons about the biological field, giving us a different angle compared to just the conventional laboratory specimens and compounds.” This easily implemented non-traditional classroom approach impacted my students in a way I was not expecting, shaping the dynamic, attitude, and interests of the entire class.

The following year students were given the freedom to address their hypothesis given the variables collected, and the research papers became very interesting. For example, one student focused their research paper on social justice issues by conducting green space analysis across Los Angeles County Parks in relation to race, education, and income. The main findings of the paper suggest that most parks are composed of concrete rather than green space and, despite most studies correlating smaller sized parks to race and income, this particular report was not in agreement. Another student explored the potential of using these variables to predict the effects of mitigating urban heat island effects across Southern California. They found that although the measured variables are important at a smaller scale, park size and structure played a stronger role in parks promoting cooling effects.

Operation Healthy Air: Habitat Mapping
This past summer, with the switch to Operation Healthy Air, students were asked to participate in Habitat mapping of locations where sensors had been placed to measure temperature, humidity, and ozone. They mapped a total of 33 locations spanning the Long Beach Area. The students also set up four sensors on campus, two in parking lots and two in lawns that were 200 feet apart. The students were not asked to write a report on this activity, but we did have a discussion session on the importance of air quality. I was surprised to learn how much my students did not know about the health effects of poor air quality. However, I was pleasantly surprised to learn about the issue of poor air quality and the joint efforts of San Pedro and Long Beach through the Clean Air Action Plan to reduce air pollution produced from the ships, trains, trucks, terminal equipment and harbor craft that operate in and around the ports. I was also fortunate to contribute habitat mapping of eight locations through the California Naturalist Program this past fall, which aims to foster a diverse community of naturalists and promote stewardship of California’s natural resources through education and service.

Some recommended improvements to the Habitat Mapping software are correction for glitches, such as not being able to access locations that are too close to one another, freezing of the screen with complete elimination of the work already inputted, and making the activity more fun by adding a reward for productivity. The class experienced a lot of technical difficulties, and frustrations in completing projects of various sites.


Students enjoyed getting the experience of being true scientists, carrying out a study from beginning to end for the Urban Resiliency Program. They independently developed a hypothesis, collected and analyzed data, and produced a final written product. For most students, this was their first experience outside of the classroom, directly applying what they had recently learned to an ongoing project with real-world implications. They started to view urban space differently, taking into account natural areas.

Working outdoors as a group also made students form stronger bonds and give them a sense of community that transitioned into the classroom and into long-lasting friendships. For the Operation Healthy Air project, student Madeline Currey states, “I was surprised at how many different ways green-spaces/plants can be categorized! Habitat Mapping taught me the importance of low-impact land use on wildlife conservation in urban areas.” The overall response by the class on the topic of air pollution was positive and the class wanted to contribute to collecting and analyzing of data.

I enjoy being a teacher because of the benefit of continually improving and growing through working with students. Collaborating with the Urban Resiliency Program for this project was my first exposure to “citizen science.” This experience taught me how combined efforts at a local scale can create a real impact in advancing research and solutions to current problems. This experience also generated an unforeseen outcome of an article published in the Applied Biodiversity Science Perspective Series (http://hdl.handle.net/1969.1/166246). This article discusses the use of citizen science to engage underrepresented communities, particularly co-creative equal partnership in an effort to address community-specific environmental issues.Additionally, this article also explored training programs as a mechanism to provide environmental job skills and empowerment through policy training for disenfranchised communities to advocate for environmental justice. My class will team up with the Urban Resiliency Program again this summer to continue on the Operation Healthy Air project, but in the meantime we have partnered with the Audubon’s center at Deb’s Park to carry out a study on natural alternative methods for pest control management of gopher populations.

I highly encourage educators at all levels to immerse their students, and even themselves, in citizen science projects such as the Urban Resiliency Program. I found a lot of support by staff in materials for the classroom, such as PowerPoint’s and video trainings, as well as materials for the field. My students were surprised at how fun learning, carrying out, and contributing to science could be. These type of collaborations are also an excellent mechanism to promote diversity in science, key to innovations and advancements in the field.