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.

Air Quality & Health: Communicating New Research Findings

By Becca Fink


Many of us have seen Air Quality Index (AQI) maps informing us whether the air quality today is “Good” or “Hazardous.” These charts indicate the level of health concern for the day, as decided by the U.S. Environmental Protection Agency (EPA). What do we know about these standards? Do standards change when new research is published? What effects can air quality have on our health?


The AQI is a system that measures a set of pollutants, determined by the EPA, and the effect those pollutants can have on human health based on current levels. The AQI was developed as a user-friendly, easy to understand indicator of complicated information. However, new research indicates that even lower amounts of pollutants than those identified by the EPA can have negative consequences on our health and lives. How can researchers take this information and share it with the general public in an effort to improve people’s wellbeing?


A recent study suggests that environmental factors, such as a lack of green space and higher levels of particulate matter air pollution, can be a cause of chronic absenteeism (students who miss ten percent or more of school days for any reason). Particulate matter is a mixture of tiny particles in the air that, when inhaled, can cause serious health effects, specifically affecting the heart and lungs. The authors of the study note that “chronic absenteeism can affect success later in life, including impacts on education attainment, employment opportunities, incarceration risk, health status, and financial stability.” What this study makes clear is that it can take long-term studies to better understand the impact of air quality on our health and lives and that we must be willing to adjust our standards as new findings become available.


Health effects can be measured by examining the economic loss due to worse performance by employees. Harvard Business Review published a study in 2016 that found higher levels of air pollution can cause office workers to be less productive. The study “found a surprisingly robust relationship between daily air pollution levels and worker productivity…workers are 5%–6% more productive when air pollution levels are rated as good by the Environmental Protection Agency versus when they are rated as unhealthy.” It is likely this impact on economic productivity will make clear the impact of air quality on people’s health and be of interest to business owners. The authors note it could be compelling for policymakers as well, “All of this might really matter for the economy as a whole. For policy makers, the evidence changes the cost-benefit analysis of environmental regulation and suggests that prioritizing industrial expansion over environmental protection may actually undermine economic growth.”



These are just a select few of the many studies that have been published in the last five years that discuss air pollution. While there has been much new research linking air pollution to negative health consequences, it can still be difficult to communicate this to people who are impacted. This is of high interest to the Urban Resiliency Program, as we are always seeking ways to take complicated science and make it relevant and actionable. How can scientists effectively communicate the importance of setting higher air quality standards? How can they prove to individuals the effects air quality can have on their health, community, productivity, and future? The research is there, and the benefits of cleaner air are evident. Now, it is a matter of making these findings available and accessible to a larger audience.


A recent article published in Air Quality, Atmosphere, & Health focused on this issue. The article notes that “interventions will be implemented only if their relevance and impact can be conveyed clearly and convincingly to those empowered to set policy, the people responsible for implementation, and those likely to be affected by the intervention.”


While many countries have taken steps to introduce laws and bills that set higher standards for air quality, there is still much more that needs to be done. Local projects, like Operation Healthy Air, help to show the importance of involving the community in making large-scale change. When citizens get involved in the science, they see firsthand the value of making changes at home that can have larger impacts. These changes could be as simple as planting trees, replacing air filters, gardening, getting involved in local government, recycling, and much more.



As research is being conducted to learn more about air quality and its effects on human health, researchers need to take more time to communicate this clearly to larger audiences. Change to policy and practice occur when people understand and care about research findings. When individuals begin to see the relevance of research and understand that there are changes to be made, they may be more compelled to advocate for those changes. There are many steps in the chain from scientist to policymakers. We believe that an informed public can help at both ends, to create relevant research and to help communicate the results that shape our policies and make our communities more resilient and healthy.


The communications issue is not unique to this subject. Effectively communicating the importance of health and scientific research is a difficult task that requires reaching the right audience in the right way. At the Urban Resiliency Program, we are thankful for our team of dedicated citizen scientists who are open to learning more about scientific research, taking those learning outcomes, and applying them to their lives.