Every time I step outside, I notice something unusual: a plant doing something unexpected, a tree with an odd growth, or a patch of leaves that refuses to disappear. I catch myself wondering, What is that? And then, just as quickly, I move on without ever looking it up. What’s that weird lump on a tree? How does leaf decomposition work? If you’re anything like me, you’ve probably had the same questions about the small, overlooked relationships and processes shaping the plant life around us. Here are some of my considered questions, finally answered. 

Burls

Fig 1. A tulip tree in the Smokies has numerous flattened outgrowths known as burls. Photograph by Alice Sheffield, [Post and Courier].

Fig 2. Photograph by Kiyyah, [Gardening Know How].

Have you ever seen a tree with a weird lump or bulge and thought, “What on earth is that?” Don’t worry, I’ve wondered the same thing.

That outgrowth or woody lump outside of a tree is called a burl. They are abnormal, rounded growths on tree trunks, branches, or roots from disorganized cell growth caused by stress. This stress can include physical injury, insect damage, fungal or bacterial infections, viruses, drought, or other environmental challenges (Kuhns; Guida). Subsequently, you will see more burls in cities, parks, and the open countryside than in forests. Those trees are subject to a greater risk of injury and stress and have a higher rate of infection (Richter).

Since burls take years or decades to form, it’s often impossible to determine what first caused them (Guida). Burls are not harmful to trees and, in fact, can be seen as a sign of compartmentalized damage. While they are typically a stress response, they can also be considered a sign of a tree’s resilience to recover from damage. Since they don’t pose a direct threat to tree health, it’s best to leave the burls intact. Cutting off a burl can cause more harm than good and may lead to an infection and weaken the tree’s structural stability. Yet, people do harvest burls. They are a sought-after material, prized for their unique grain for furniture and woodworking.

Galls

Fig 3. Jumping oak gall caused by cynipid gall wasps. [UMN].

Fig 4. Goldenrod gall formed on the stem of a goldenrod plant, created by the larva of the goldenrod gall fly. Photograph by Leslie Mertz, [Entomology Today].

As a kid (or adult) playing in leaves, you may have noticed discolored bumps along the back of them. Or, the odd tumors that grow on the stems of some plants. Those growths are called galls.

Galls are abnormal lumps, growths, or swellings on plants that can appear on leaves, stems, branches, buds, flowers, fruits, or roots. They can look like bumps, spiky or fuzzy growths, or swollen stems. They are commonly caused by insects and mites like flies, midges, and wasps, but can also be from fungi, bacteria, or viruses, often serving as both habitat and food for the gall-inducing agent (Guida).

Galls form when an outside organism manipulates the plant’s growth hormones. An organism, like an insect, will release chemical or hormonal signals when feeding or laying eggs that cause the plant’s normal growth to be distributed, mimicking plant growth hormones, causing cells to divide and enlarge (“Insect and Mite Galls”). These chemicals redirect nutrients to the gall, creating a structure around the developing insect. 

The plant produces the gall to protect against the attack of the parasite. The objective is to neutralize the toxins by producing a gall, localizing the parasite, and forcing it to extreme specialization. Typically, it neither helps nor harms the plant; galls are typically only harmful with heavy infestations. But the organism within the gall greatly benefits from the gall. It acts as a food source and protection from predators and the weather.

Galls are a prime example of evolutionary specialization. They have evolved to find teeny niches within the ecosystem that are perfectly evolved to take full advantage of the microbiome to thrive in their environment. This hyper-specialization means that most gall-inducing organisms are dependent on a particular host and cannot be found on any other plant species (Guida). Because of this, it is easy to identify the organism that caused the gall once you have identified the plant, because it is “virtually unheard of” for another species to make the same kind of gall on the same species in the same location (Guida).

An example of this in the Natural Lands is the Canadian and tall goldenrod galls. The goldenrod gall fly produces ping-pong ball-sized galls along the stems in response to the larvae secreted into the stems. Goldenrod galls provide an example of dual selective pressures. In forested areas, smaller galls are favored because birds such as woodpeckers and chickadees preferentially locate and consume larvae in larger galls. In contrast, in open fields, larger galls are favored because they are more difficult for parasitic wasps to penetrate and lay eggs in. Thus, gall size reflects a trade-off between predation and parasitism that varies by habitat.

Cankers

Fig 5. Bacterial Canker. Photograph by Akchamczuk, [Gardening Know How].

Fig 6. Old canker on trunk, [The Morton Arboretum].

Now, some trees have growths that look more like scars or bulging open wounds. Cankers are localized diseases that result in areas of dead tissue on the bark, branches, or trunk. Caused by various fungal or bacterial pathogens that enter through natural wounds in the tree or through pruning and kill a patch of tissue (“Canker”). Mechanical damage (from cars, weed whippers, and lawn mowers), environmental injury (frost cracks or sunscald), pruning, and borer insects can promote infection by canker pathogens (“Identify and Manage Canker”). When the tree can’t fully heal that spot, it becomes sunken, discolored, and sometimes oozes sap or resin. Over time, the edges may try to grow over the damage, creating a target-like or calloused appearance. They can range in size from less than an inch to covering the entire trunk of a tree. Unlike most galls and burls, they are harmful to the tree. If left untreated, cankers will girdle the tree, wrapping all the way around and blocking water and nutrient flow, eventually killing it. 

Leaf Decomposition

Fig. 7. Abby Sikora, Oct. 2024.

Have you ever been walking around the forest in the spring or summer and seen abundant leaves on the ground? Shouldn’t they have disintegrated by now? But for real, what is going on with the leaves? 

Leaf decomposition is a process that keeps ecosystems running. It explains why forests don’t have endless piles of leaves and why the soil stays fertile. Leaves decompose through a combination of physical fragmentation, microbial activity, and chemical breakdown, returning nutrients to the soil and supporting ecosystem health.

The decomposition process begins immediately after a leaf falls. Leaching occurs first, where rainwater washes away water-soluble compounds, like sugars, amino acids, and salts, into the soil, accounting for a significant portion of early mass loss, especially in wet environments (“What Happens When Leaves Decompose?”). 

Next is fragmentation, the physical breakdown, where wind, rain, freeze-thaw cycles, and the feeding activity of small invertebrates, like worms, break leaves into smaller pieces. Once physical barriers are removed, microorganisms like fungi and bacteria produce enzymes that chemically break down the leaf tissue (BCC). Bacteria break down simpler compounds while fungi break down harder materials. Detritivores, including insects like springtails, millipedes, and earthworms, further fragment leaves and improve soil aeration, facilitating microbial activity (Sharma). 

Finally, nutrients like nitrogen, phosphorus, and carbon are released into the soil for plants to reuse the nutrients in an ongoing cycle (‌“Do Leaves Disintegrate?”). During decomposition, carbon from leaves is partially released as carbon dioxide through microbial respiration, while some is incorporated into the soil as organic matter, enriching soil fertility and structure. Nutrients such as nitrogen, calcium, and other minerals are recycled, making them available for plants and sustaining soil ecosystems (‌“Do Leaves Disintegrate?”). 

This process is necessary for soil structure and function. As leaves break down, they improve soil structure, increasing its porosity, allowing roots to penetrate more easily, and enhancing air and water movement through the soil (Epiphan). It also improves the soil water-holding capacity. Leaves act as a natural mulch. Soil moisture retention reduces evaporation and decreases the need for irrigation that insults soil, keeping it cooler in the summer and warmer in the winter to protect plant roots from extreme temperatures (Epiphan). It suppresses weeds and acts as erosion control.

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Sources

‌BBC Bitesize. “What Is Decomposition and How Does the Carbon Cycle Work for GCSE Biology? – BBC Bitesize.” BBC Bitesize, 11 Nov. 2024, www.bbc.co.uk/bitesize/articles/z4jyjfr#z2737v4.

‌“Canker.” Gardenia, 2023, www.gardenia.net/disease/canker.

“Canker Diseases.” The Morton Arboretum, mortonarb.org/plant-and-protect/tree-plant-care/plant-care-resources/canker-diseases/.

‌“Do Leaves Disintegrate? The Science of Leaf Decomposition.” Biology Insights, 18 Aug. 2025, biologyinsights.com/do-leaves-disintegrate-the-science-of-leaf-decomposition/‌

Epiphan, Jean. “Why Leave the Leaves and How to Do It.” Agriculture & Natural Resources Agent, Morris County. Rutgers.edu, 2025, njaes.rutgers.edu/fs1369/.

“Goldenrod Gall – Entomology Today.” Entomology Today, 12 Sept. 2023, entomologytoday.org/2023/09/12/insects-plant-galls-pollution-detectors/olympus-digital-camera-39/.

Guida, Sam. “Galls, Burls, and Brooms of the North Woods.” Pinnule Publishing. 2025.

Gibbons, Whit. “ECOVIEWS: What Causes Burls on Trees?” Post and Courier, 2 Nov. 2024, www.postandcourier.com/aikenstandard/lifestyle/ecoviews-what-causes-burls-on-trees/article_dbd17766-97c8-11ef-808c-ef04591b94ef.html. Accessed 27 Apr. 2026.

Hahn, Jeffrey, and Ascerno, Mark. “Insect and Mite Galls.” Umn.edu, 2019, extension.umn.edu/yard-and-garden-insects/insect-and-mite-galls#aphids-and-midges-1346964.

“Identify and Manage Canker on Trees and Shrubs.” Purdueplantdoctor.com, 2025, purdueplantdoctor.com/factsheet/tree-15.

“Insect and Mite Galls – Agricultural Biology.” Agricultural Biology, Colorado State University. Mar. 2024, agsci.colostate.edu/agbio/ipm-pests/insect-and-mite-galls/.

Kuhns, Michael. “Can Trees Get Cancer?” Utah State University. 2020. Viewcontent.cgi. 

‌Mani, M.S. “Ecology of Plant Galls.” Springer, 2013, books.google.com/books?id=zvLqCAAAQBAJ&lpg=PP10&ots=fnuFOSLwA_&dq=how%20do%20galls%20form%20&lr&pg=PP1#v=onepage&q=how%20do%20galls%20form&f=false.

Richter, Christoph. “Biotically Induced Wood Characteristics”. In: Wood Characteristics. Springer, Cham. (2015) https://doi.org/10.1007/978-3-319-07422-1_6.

Sharma, Babita. “Decomposers- Definition, Types, Examples, Decomposition.” Microbenotes.com, 3 Aug. 2023, microbenotes.com/decomposers-definition-types-examples/.

Spengler, Teo. “Learn about Tree Burls and What Causes Them.” Gardening Know How, 5 Apr. 2022, www.gardeningknowhow.com/ornamental/trees/tgen/tree-burls.htm.

“What Happens When Leaves Decompose?” ScienceInsights, 10 Nov. 2025, scienceinsights.org/what-happens-when-leaves-decompose/. Accessed 24 Apr. 2026.

Waterworth, Kristi. “What Is Bacterial Canker: Bacterial Canker Symptoms and Treatment.” Gardening Know How, 5 Apr. 2018, www.gardeningknowhow.com/ornamental/trees/tgen/what-is-bacterial-canker.htm.

“What Is Bacterial Canker: Bacterial Canker Symptoms and Treatment.” Gardening Know How, 5 Apr. 2018, www.gardeningknowhow.com/ornamental/trees/tgen/what-is-bacterial-canker.htm.

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