Is My Tree Dying? Warning Signs of Decline in Southern Arizona Trees, and When It Becomes Dangerous

Written by Justin Tornatore, ISA Certified Arborist | TIMBERx

Abstract

"Is my tree dead or just dormant?" and "Is my tree dying?" come up constantly, and in the desert they're harder to answer than they sound. Many native trees drop their leaves in response to drought or cold [1], and drought and heat damage may not show up until the following year [2][3]. This article organizes the warning signs that University of Arizona Cooperative Extension and the International Society of Arboriculture (ISA) identify into a top-to-bottom checklist: canopy, bark, trunk, root zone, and structure [1][2][4][5]. It also separates two questions that often get blended together: is the tree healthy? and is the tree safe?

Background: Decline Is Slow, and It Hides

Trees rarely die overnight. More often they decline over seasons or years, drawing down stored energy until they can't keep up with repairs. UA notes that in trees, reduced growth or death from drought and heat stress "may also occur in subsequent years," and that recovery depends on remaining carbohydrate reserves, water, and temperatures [2]. Tucson saw this firsthand after the record-setting heat of 2024. The Tucson Botanical Gardens' director of horticulture told KGUN 9 that some plant damage might not be visible until the next year [3].

Think of a tree's stored energy like a savings account. A stressful summer is a big withdrawal. The tree may look fine for a while, but if deposits don't catch up, the account runs dry, and that's when symptoms finally show.

What the Research Shows: A Top-to-Bottom Checklist

1. The Canopy

UA lists the typical signs of drought and heat stress as wilting, drying and browning leaves, leaf drop, branch dieback, sunscald on branches and trunks, and reduced or no new growth [2]. ISA's risk checklist adds leaves that develop an unusual yellow color or seem smaller than normal [4].

Watering problems show up here, and both too little and too much water can look alike from a distance. UA's Watering Trees and Shrubs separates them [1]:

• Under-watering: dry soil; older leaves turn yellow or brown and may drop; leaves wilted or curled.
• Over-watering: constantly damp soil; young leaves turn light green or yellow; young shoots wilt; leaves green yet brittle; algae and mushrooms growing.

2. Dead, or Just Dormant?

Before you assume the worst, know your species. UA notes that mesquites are deciduous and shed foliage in response to cold temperatures and prolonged drought. Some, like Argentine mesquite, are typically dormant for six to eight weeks in winter [5]. Palo verdes may stand leafless during drought and cold, relying on their green bark for photosynthesis [5].

A simple field check comes from UA's guidance on bark injury. Make a small cut into the suspect area with a sharp, clean knife. If the inner bark is soft and green, it's still alive. If it's brittle and falls away to expose the wood, that tissue is damaged [6]. Test a few branches, not just one, and keep the cuts small.

3. Bark and Trunk

• Sunscald: reddish or brownish discolored bark, progressing to sunken or split bark that peels back, usually on the south or west side [6].
• Cracks and splits: longitudinal cracks in the trunk, major branches, or branch unions are a risk warning sign [4].
• Cavities, cankers, and rotten wood: possible signs of internal decay [4].
• Slime flux on mesquite: dark, foul-smelling liquid oozing from cracks and wounds, caused by bacterial infection of the sapwood. UA notes infected trees live many years and slime flux is "more of a nuisance," though seriously infected branches may need removal if they become hazardous [5].

4. The Base and Root Zone

• Mushrooms or shelf-like conks at the base. ISA lists mushrooms at or under the tree as a warning sign [4]. On mesquite, UA describes Ganoderma root rot, a soilborne fungus whose fruiting bodies appear at the base during the summer rainy season. They start as white structures and develop into light brown, shelf-like conks. It causes slow decline and eventually death of mature trees, and there's no recommended treatment [5].
• Soil cracking or heaving. Cracks or separations in the soil can mean the roots are moving excessively. That's a warning sign for failure, especially if the tree is leaning [4].
• Recent changes. ISA asks whether the area has recently been changed by construction, grade changes, or new lawns or pavement [4]. Root loss from these changes may not show in the canopy for years.
• Buried trunk base. UA notes that foothill palo verde is extremely susceptible to deep planting or soil piled against the trunk, which predisposes it to rot and failure [5].

5. Structure: The Safety Question

A tree can be healthy and still be hazardous. ISA's checklist asks: Are there large dead branches? Detached, hanging branches? Has the trunk developed unusually? Has the tree been topped or heavily pruned? [4] Dead branches in the canopy are "probably the most obvious potential hazards," and codominant stems (split trunks) can often be failure points [4].

Field Observations

The first thing I check on a struggling desert tree is the water. Irrigation problems are among the most common, and most fixable, causes of decline, and UA bluntly notes that "most people over water plants" [1]. Emitters that were never moved out as the tree grew, a controller schedule that hasn't changed since planting, or a broken line under the gravel are all easy to miss. Because over- and under-watering can look so similar from the street, I always want to see the soil, not just the leaves.

The second thing I look for is anything that happened to the roots in the last few years. A new wall, a pool, a trench, or a regrade often explains a canopy that's "suddenly" thinning.

Practical Recommendations

1. Check the soil before you change anything. Push a smooth metal rod or soil probe into the root zone. It slides through moist soil and stops at dry soil [1]. Mature tree roots generally need water to a depth of about two to three feet [2].
2. Do the scratch test on a few branches before writing off a leafless tree [6].
3. Don't fertilize a stressed tree. UA advises minimizing or skipping fertilizer during drought and heat, because it raises salinity in the root zone [2].
4. Don't prune heavily to "help" it. Removing canopy from a stressed tree reduces photosynthesis and exposes bark to sunscald [2].
5. Photograph the tree each season from the same spot. Slow decline is much easier to see side by side.
6. Call an ISA Certified Arborist if you see any of ISA's risk signs: large dead or hanging branches, cracks, cavities, mushrooms at the base, soil heaving, or a new lean [4].

Key Takeaways

• Leaf drop isn't always death. Many desert trees go leafless in drought or cold, so use the scratch test before deciding [5][6].
• Over- and under-watering cause many of the symptoms people notice, so always check the soil first [1].
• Mushrooms or conks at the base, soil heaving, cracks, and large dead limbs are safety issues, not just health issues [4][5].
• Drought and heat damage can lag a year behind the stress that caused it [2][3].

Bottom line from your local arborist: a declining tree is asking a question. The sooner someone reads the answer, the more options you'll have.

Talk to an Arborist

Worried about a tree on your property? Text Justin at (520) 451-8790 (texting is preferred over calling) or email Justin@TimberxTrees.com. TIMBERx is a veteran-owned tree care company led by Justin Tornatore, ISA Certified Arborist, serving Tucson, Oro Valley, Marana, Vail, Sahuarita, Green Valley, and Benson.

References

1. Call, R. E., & Daily, C. (2017). Watering Trees and Shrubs (AZ1298). University of Arizona Cooperative Extension. https://www.extension.arizona.edu/publication/watering-trees-and-shrubs
2. Schuch, U. (2021). Drought and Extreme Heat: Plant Responses and Landscape Maintenance Practices (AZ1876). University of Arizona Cooperative Extension. https://extension.arizona.edu/publication/drought-and-extreme-heat-plant-responses-and-landscape-maintenance-practices
3. Fish, R. (2024). Even desert plants have been feeling the record heat. KGUN 9. https://www.kgun9.com/news/community-inspired-journalism/midtown-news/even-desert-plants-have-been-feeling-the-record-heat
4. International Society of Arboriculture. (2021). Recognizing Tree Risk. TreesAreGood.org. https://www.treesaregood.org/Portals/0/TreesAreGood_Recognizing%20Tree%20Risk_0721.pdf
5. Schuch, U., & Kelly, J. (2012). Mesquite and Palo Verde Trees for the Urban Landscape (AZ1429). University of Arizona Cooperative Extension. https://extension.arizona.edu/publication/mesquite-and-palo-verde-trees-urban-landscape
6. Schalau, J. (adapted 2022). Sunscald (Backyard Gardener #25). University of Arizona Cooperative Extension, Yavapai County. https://extension.arizona.edu/sites/extension.arizona.edu/files/attachment/Sunscald.pdf