Thursday, December 13, 2018

Norway Maple - Invasive Species

Norway maple as an urban and suburban tree is so well established that it feels counter-productive to proclaim that it is an invasive species. It is. The Commonwealth of Massachusetts prohibits the import, sale, trade or propagation of Norway maple trees. The ban dates to 2005, when the Massachusetts Invasive Plants Advisory Group proposed an initial list, last updated February 2017. From “The Massachusetts Prohibited Plant List prohibits the importation, sale, and trade of plants determined to be invasive in Massachusetts. This ban also covers the purchase and distribution of these plants and related activities, and includes all cultivars, varieties and hybrids of the species listed.” The rules do not affect existing Norway maples already within the landscape, only bringing in or creating (from seed) new trees.

Norway maple cultivars offered by garden supply businesses
include varieties that have 'copper' or purple leaves. In this
photograph, the dark leaf came from a sun-exposed part of
the tree while the green leaf was from deep shade.
Worldwide, there are more than 100 species of maple trees (genus: Acer). Most are native to Asia. Here in New England the native species we see are red silver and sugar maple. In leaf, there is a nice mnemonic to remember which is which: each leaf has three lobes if r-e-d or five lobes if s-u-g-a-r. Leaves on silver maples – also known as swamp or water maples - have five lobes, but this species thrive best near or in wetlands, and so differentiates from sugar maple. Norway maple leaves also have five lobes, but differences in the leaf stem and bark help us tell the difference. For sugar maple, a snapped stem seeps clear, whereas for Norway maple, white. Sugar maples also have a more shaggy bark. In the fall, the native species color up in the orange to red spectrum, while Norway maple leaves lean toward yellow/orange.   

Three differences make Norway maple a yard bane compared to red or sugar maple. Seed production is more prolific, meaning that all summer one will be pulling seedlings out of garden beds. Branches are more likely to break in storms – due to weaker wood – and so more time playing pick-up-sticks. Roots are very close to the surface, to the point of stunting or stopping any grass, weed or groundcover plants underneath. As roots extend sideways roughly as wide as the crown of the tree above ground, this can create a large area of bare earth under the tree.     

The winged seeds are called ‘samaras’. Why? I don’t know.  The term describes all tree seeds that are incorporated into a flattened, papery casing so that they are easily windblown. Much like oaks and beech trees, heavy maple seed production occurs every two to three years. Interestingly, for sugar maples good sap yield in early spring presages a strong seed year, but then sap production is reduced for the spring following that strong seed year, suggesting that the trees have only so much carbohydrate reserves to either create seeds or promote growth. Samaras tend to detach from their stems on windy days, which promotes better dispersal.   

Norway maples have a long history in North America. Credible reports date the introduction to the mid-1700s for New England, perhaps a century later for the west coast.  During the mid-twentieth century urban and suburban plantings of Norway maple trees were common, especially as a replacement for the loss of American elms from Dutch elm disease. With adequate sunlight, adult trees can be 40-50 feet tall and equally wide. The species is tolerant of poor soil and a range of water conditions from drought to wet soil, but fares poorly as a sidewalk installation tree as it prefers to establish a wide, shallow root bed. Lifespan is short compared to native species. Yard, park and cemetery plantings done 50-70 years ago show their age in increased loss of large branches from storms followed by slow replacement growth.

As to why designated invasive in Massachusetts, seeds from suburban plantings are wind-blown into bordering forests. There, due to its shade-tolerant nature as a seedling and sapling, Norway maples out-compete native species. The dense canopy it creates combined with its shallow root system means that forest diversity declines. The loss of understory plants cascades into a less hospitable environment for insects and the animal species that prey on them.

Conservation agents for Massachusetts cities and towns may consider establishing an anti-invasive plants program, but the reality is that killing mature Norway maple trees – the most effective way to stop seed production – is lower on the list than addressing faster-growing/spreading invasive species such as Oriental bittersweet and Japanese knotweed.

Previous plant winners of this column's "Invasive Species of the Year" are in a September 2012 item at Future candidates include Japanese barberry, purple loosestrife…

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