Trees of the Preserve – July 13, 2019

Glen Stanosz holding a branch with Ash Yellow disease, called a witch's broom.

Ash tree afflicted with Ash Yellow disease.

​On a beautiful Saturday afternoon, Professor Glen Stanosz shared his knowledge of trees with five enthusiastic guests. As attendees strolled through the shaded paths near Picnic Point, Glen identified ash, oak, red pine, and white pine trees among other woody species. 
Each tree species was paired with a remarkable tale of conquest by fungal or insectivore invaders… occasionally intertwined with hopeful twists of perseverance as researchers discover some tree species are beginning to develop resistance against their pesky assailants:

• Ash trees face adversity from two fronts: emerald ash borer (EAB) insects and the Candidatus fraxinii bacteria which can harm ash trees’ ability to store and transfer water and nutrients, respectively. Glen noted that most ash trees in the Preserve have succumbed to the forces of Candidatus fraxinii, displaying the characteristics of Ash Yellow disease (a sparse crown, with leaves that exhibit a “cupped” shape as you look at higher branches of the tree, and a “witches broom” in newer branches) which reflect the consequences of a failing root system. However, the future of ash trees is not necessarily bleak according to Glen, as new research indicates that ash trees are developing resistance to both EAB and Candidatus fraxinii. 

 

• Oak trees commonly fall victim the coordinated acts of the Bretziella fagacearum fungus and their sap beetle accomplishes. The Bretziella fagacearum fungus can colonize in wounds of oak trees and create a mat of sweet-smelling spores under the oak bark to attract sap beetles. Fresh oak tree wounds also release a sweet smell and attract sap beetles— so sap beetles can effectively spread the Bretziella fagacearum fungus from oak tree to oak tree if as they alternate visits to alluring sweet-smelling fungal pads and equally appealing oak wounds. Researchers have discovered that sap beetles can detect oak wounds just 15 minutes after a wound is created— an impressive skill, but extremely problematic in the eyes of scientists who want to mitigate spread of oak wilt disease. Once established in the oak tree, Bretziella fagacearum spreads through the root system of the tree until it ultimately kills the oak.

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• Red pines have fungal predators of their own: Diplodia sapinea. Diplodia sapinea attacks the tips/new growths of red pine branches and leads to browning needles and branch die-back. The spores of the fungus can also rain down from larger red pines and find refuge on red pine saplings— usually killing these saplings. The Diplodia fungus affects red pines, but interestingly, the fungus does not harm white pines. This can consequently create an interesting sight: a healthy population of white pine saplings growing underneath the cover of an adult red pine tree. 

 

• Heterobasidion root disease (HRD)— caused by the Heterobasidion fungus— is the most damaging disease of coniferous forests. The fungus can land on pine stumps and spread through the root systems of these severed trees, eventually contacting and invading other pines with intertwining roots. The visual result is a “circle of death” or an enlarging circle of dying pine trees that expand outward from the original infected conifer. Glen says scientists are searching for ways to stop the HRD spread such as applying tar, urea, or even borate to recently formed pine stumps.

• White pine blister rust is a fungus that uses both gooseberry and currant bushes as well as white pines as hosts. This bright orange fungus spends the spring and summer seasons on the leaves of gooseberry and currant plants, using pollinating creatures as vectors to spread the fungus from plant to plant (the fungus tricks the pollinators into spreading spores by mimicking the color and texture of pollen). The fungus then overwinters on the needles of white pines. Over time, the fungus spreads towards the main stem of the white pine trees and induces a canker within this stem region which prevents effective water and nutrient transport throughout the tree— this process quickly kills smaller pines, where the distance between branches and the main stem is smaller, compared to larger pine trees. 

While trees face incredible feats against fungal and insect invaders alike, Glen described how this harassment is a result of trees’ tempting design. Trees are made of two main components: cellulose and lignin… in other words, trees are made of sugar and glue! So, if trees have the audacity to grow tall and advertise their sweet composition to the decomposers in their ecosystems, that must mean trees have some secret defenses of their own. Indeed, Glen skillfully pointed out trees using their defensive ability to compartmentalize their trunks; these trees created a seal that trapped an invading fungus on the interior of the trunk, allowing the living outer region of the trunk to service the tree unharmed. 
 
Finally, Glen referenced a non-fungal threat to forests: climate change. With observed warming trends in Wisconsin, Glen said it is possible that the composition of forests may change over time. Glen hypothesizes that Wisconsin may lose its maple and conifer trees, but retain hickory trees as hickory trees can be found in southern/warmer regions of the US. ​​Report and Photos by the Friends' host Olympia Mathiaparanam.