Refining and Expanding Campus Tree Species Selections
Past efforts by University of Arizona faculty research yielded a rich collection of arid adapted tree species that both helped create Arizona’s economic base in the early 20th Century and produced innumerous species populating urban landscapes throughout the southwestern US. However, limitations on plant performance have and will continue to become increasingly challenging in the decades ahead. Though improved tree selection yield myriad community and environmental health benefits, those benefits will only be realized to the extent that we can accurately match new tree species to current and future climate variables. This project leverages availability of global data sets to establish potential for tree species to perform well in climates experienced in Tucson, Arizona. By extrapolation, observations of tree species in regions hotter, colder and drier than Tucson will be considered as "climate-ready" selections to introduce to campus for performance assessment.
Acacia pruinocarpa occurs naturally in the Northern Territory and in the Pilbara Region of Western Australia. It was brought to the UA through the Desert Legume Program as heat and drought tolerant choice for Tucson as it has been observed growing in regions where temperatures exceed 47C (118F) and annual precipitation is less than 300mm (11").
In addition to our own research relating to tree species and associated climate conditions, we wish to also collect information from practitioners who can refine our predictions relating to tree performance by sharing experiences that add insight regarding species performance. If you have experience observing or cultivating desert trees, please consider reviewing the list of some candidate species and sharing your insights for species familiar to you. To do so, complete a short survey. Surveys will close July 2025.
Share your knowledge here.
As the state of Arizona is situated in a desert biome, and the University of Arizona is a Land Grant Institution, early faculty members conducted research and education programs focused on searching for and testing performance of arid adapted woody plants from around the world to determine if any could be useful here. As a result of their work not only were many new commodity crops and landscape ornamental plants introduced into the trade. Their work helped establish the economic base for Arizona statehood in the early 20th Century and produced many of the species that populate urban landscapes throughout the southwestern US. Further, use of the campus as an experiment station culminated in a world class research collection, establishment of the campus as a living laboratory and prolonged use of campus open spaces for university programming and for the community good.
Early studies by UA faculty based tree selection choices on approximate climate similarity to other regions of the world. As such, they visited and then mined arid and semi-arid deserts around for potentially adapted species to bring home and test on campus. Clearly, they did their homework as many have stood the test of time! However, deserts are characterized by a variety of factors affecting performance. They can be hot and dry, semiarid, coastal, or cold. The Sonoran Desert is hot and dry with cold winters and as climate has shifted, the patterns and magnitude of environmental factors have made it increasingly challenging to find species adapted to all the conditions faced by landscape plants in Tucson. Furthermore, the built environment we inhabit is replete with microclimates and deviations from the “natural climate” experienced outside the city. Global climate changes coupled with the compounding effects of the built environment increase the need to refine plant selection processes and match plants thoughtfully to a planting site that will allow them to thrive in the decades ahead. Though myriad community and environmental health benefits may result from improved tree selection for current and future climate scenarios, those benefits will only be realized to the extent that we can accurately match new tree species to current and future climate variables.
Fortunately, the world is also flooded with new resources to support the process of searching for climate-ready tree species. Climate classification systems, large datasets specifying global observations of tree species, and improved methods for connecting with a global community of horticultural experts may all help to perpetuate the legacy of testing and trying new tree species for the American southwest. This project aims to leverage the history of tree selection work at the UA, with the resources afforded in the 21st century to curate a list of tree species with potential to perform well in both current and future climate scenarios.
The University of Arizona Campus Arboretum is located in the Sonoran Desert. This biome is characterized by an incredible diversity of vegetation, wildlife, as well as unique soil, climate conditions. The main University of Arizona campus in Tucson, AZ is also located in the middle of the Tucson Metropolitan area (MSA). In 2024, Tucson was home to 1,080,300 residents, experiencing an increase in population of 0.7% between 2022 and 2023, and ranking fourth fastest in population growth out of 12 western Metropolitan areas. Tucson is also among the country’s fastest warming cities. It has a hot, desert, climate (Köppen BWh) with two major seasons, a hot summer and mild winter where freezing temperatures are infrequently experienced.
The mean annual temperature of Tucson, AZ is 70.88°F (21°C). Based on models developed by the Intergovernmental Panel on Climate Change (IPCC), given the assumption of limited emissions (SSP2/RCP4.5), the mean annual temperature of Tucson will be 74.5°F (23.6°C) by 2050, and under the business as usual climate change scenario (SSP3/RCP7.0), the MAT of Tucson is predicted to be 79.7°F (26.5°C). Tucson data available from the National Weather Service as of 2024 indicated the mean temperature of the hottest month (MHM) was 30.05°C, the mean temperature of the coldest quarter (MCQ) was 12.37°C. Additionally, the mean annual precipitation (MAP) was 270mm/yr, while the mean precipitation of the driest quarter was 17.02mm. Based on data from 1991-2020, January and December are the coldest months in Tucson with the normal average cold temperature for January and December, being 66°F (18.9°C). The record low temperature (Tmin) of 6°F (-14°C) was recorded on January 7, 1913. Based on data from 1991-2020, June and July are the hottest months of the year with the normal warm temperature for June and July being 100.7°F (38.17°C). The record high temperature (Tmax) was recorded at 117°F (47.22°C) on June 30, 1990. Since Tucson is classified as a USDA 9a hardiness zone, it rarely experiences temperatures below -8°C (17°F) or above 44°C (112°F) (1). As such, for this study we set these as the target Tmin and Tmax, respectively, for our search criteria. Tucson averages 10.61 inches (269.5 mm) of precipitation per year, concentrated during the North American Monsoon of summer (~7.97” in June to September) and the Pacific storms of winter (~2.64” in December and January). However, the Campus Arboretum Collections Policy which informs succession plans for the Campus Arboretum, permits inclusion of species requiring less than 500 mm (20 inches) of annual precipitation. Given this we set the precipitation limit at 500mm but highlighted those fitting both the temperature criteria as well as the more conservative water requirements.
In this study, we compared observations of the collection taxa to observations of those species globally using the Global Tree Database and research grade iNaturalist observations. The geographic location of tree species observed globally was cross referenced with temperature and precipitation data available from the weather station nearest the observation site. Weather station data was obtained using weatherspark.com (2) and, for Australian observations, detailed and precise station data was obtained from the Bureau of Meteorology (3). In all instances the station name or number, and its elevation was noted along with the annual precipitation received at that location. Additionally, the maximum and minimum temperatures reported over the three years preceding the observation date (where known – otherwise the most recent years of data were used).
In addition to collecting global climate parameters for our existing species, we also wished to identify other tree species not previously considered for our collection. To this end, we identified regions of the world where climate was most similar to Tucson, where some of our top performing species naturally occur or where climate extremes were even greater than in Tucson. In those areas, we mined iNaturalist observations to record all observations of woody plant species that were then included for consideration.
To refine further the adaptation of these taxa to our climate we sought input from practitioners who have observed these plants growing in cultivation or in their native environment. To this end we invited those with expertise related to performance of potential trees in cultivated or natural settings (in urban forestry, conservation horticulture, botany or landscape architects) to provide input via an electronic survey (here). This input validated upper and lower temperature thresholds experienced in those settings where tree species are common and gleaned insights relating to plant health and performance.
These results will serve as the basis for guiding arboretum collection development by complementing the Campus Arboretum Collection Policy and university landscape succession plans.
References:
1. https://www.weather.gov/twc/TucsonDailyRecords
2. Weather spark.com
3. http://www.bom.gov.au/
Results will be posted after July 2025.