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Bark Photosynthesis

It’s spring and all my favourite signs are here: snow is melting, birds are returning, and tree bark is photosynthesizing. Wait – what?! You might not know it, but many of PEI’s trees have stems and twigs that photosynthesize just like green leaves do.

Leaves are green due to chlorophyll, a pigment that converts sunlight, water and carbon dioxide into oxygen and carbohydrates (aka plant food). Tree bark has chlorophyll too, which is most easily seen in our Poplar (aka Trembling Aspen, Populus tremuloides, Photo 1). As much as 40% of Poplar’s chlorophyll can be in the bark. That’s especially important this time of year, as trees wake up for spring. With no leaves to generate food, bark photosynthesis gives the tree an extra boost of energy for buds to swell and leaf out.

Photo 1: That green hue on this Poplar stem is chlorophyll.

Bark photosynthesis is common in the mature trunks of thin-barked trees such as Poplar, Beech, and Birch but most of our deciduous trees can do this in their young twigs and new buds. Just remember that green isn’t the only colour that indicates bark photosynthesis: red xanthophylls and orange or yellow carotenoids can serve the same purpose. If you scrape away the lovely red outer bark of Red Maple twigs (Acer rubrum, Photo 2), or the red or yellow surfaces of our various Willows (Salix spp., Photo 3), you’ll find chlorophyll-rich green wood underneath. Those colourful outer compounds not only contribute to photosynthesis, they also protect the delicate new chlorophyll from too much sunlight.

Photo 2: Red Maple twig showing photosynthetic and photoprotective red xanthophylls covering green chlorophyll.

Bark and twig photosynthesis has advantages beyond spring growth. If a tree loses its leaves due to drought, insects, or fire, this auxiliary food supply can help it survive and leaf out again. That carbohydrate-rich bark and new growth can also be important food sources for mice, voles, and Snowshoe Hares during this lean time of year.

Photo 3: Two willows - one with red xanthophyll and the other with yellow caretenoids covering green chlorophyll.

Watch for green, yellow, orange, or red bark and twigs in early spring and know it’s helping the tree or shrub wake up and leaf out for the growing season ahead. Another part of PEI untamed!

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