Climate Adapted Trees

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 many new commodity crops and landscape ornamental plants were 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, continuous use of the campus as an experiment station produced a world class research collection, and an established pattern of using the campus as a living laboratory and use of open spaces in support of both university programming and community benefit.
 

Early studies by UA faculty based tree selection choices on the approximate climate similarity of Tucson to other regions of the world. Their process was to visit and study trees from arid and semi-arid deserts for potential use in Arizona's low desert. They brought those species home to Tucson, planted them on campus and tested performance over time. Those proven to perform were introduced as major agricultural commodities or landscape ornamental trees used in cities throughout the southwest US. On campus, it is clear they did their homework well as many have stood the test of time into the 21st century! 

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 in which these trees are now planted are distinct and deviate considerably from the “natural climate” experienced outside the city.  This combination of global climate changes coupled with the compounding effects of the built environment highlight the need for equally dynamic processes in selecting plants for changing conditions. To adapt, plant selection processes must be continuously refined to ensure they are strategically and thoughtfully paired with conditions of urban planting sites so they will 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 refined search processes in identifying 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 build on the UA history of testing tree stress tolerance, assessing performance, and continuously reinvigorating the nursery trade by recommending proven performers. This project will accomplish this goal by leveraging the 21st century resources and data available that expand and refine capacity to curate the tree collection, and better predict 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 at latitude and longitude coordinates of: 32.248814, -110.987419, positioning it 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 current taxa in our collection 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, the annual precipitation received at that location. and where the information was available, elevation was also noted.  Additionally, the maximum and minimum temperatures reported over the three years preceding the observation date (where this information was available – 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 collated recommendations from the Campus Arboretum's new plants committee gathered over the years 2002 through 2024. We also 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 co-located there and added those to the species to be considered further. 

To further refine predictions regarding climate adaptation of these taxa we collected input from practitioners who had experience cultivating those species or who had observed these plants growing in their native environment. To accomplish this, 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, gleaned insights relating to plant health and performance and yielded additional tree and shrub species to consider. 

Ultimately, the woody plant species considered will be planted on campus for performance testing and evaluation, with the long-term goal of informing arboretum collection development and landscape succession plans. Temperature and precipitation data obtained was sorted and ranked to establish priority species for further consideration.  Data relating to upper and lower temperature thresholds and precipitation needs was sorted and ranked as follows: species commonly found in locations where winter temperature reaches 17°F, where summer temperatures reach 112°F and where annual precipitation is less than 500mm were ranked as priority for planting and performance testing on campus. Those species that met the precipitation requirement and summer temperature threshold were ranked second priority for planting and performance testing on campus as these would likely require special consideration for winter protection, a warm microclimate or siting on a south or west aspect of a building.  Those species that met the precipitation requirement and winter temperature threshold were ranked third priority for planting and performance testing as these would require special consideration for siting in a cooler microclimate and likely also require additional irrigation for cooling. 

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.