Winter Adaptation: Trees
We’ve been looking at how PEI wildlife adapts to our frigid winter conditions, but what about trees? Unlike animals that take shelter or herbaceous plants that retreat underground, trees must stand tall throughout winter storms, sub-zero temperatures, and brisk winds. Our conifer and broad-leaved trees take two very different approaches to winter survival.
Conifers such as Pine, Spruce and Hemlock keep their leaves all year, but that doesn’t mean they are active year-round; they go dormant just like broad-leaved trees do. Cold winter air is dry, and so those needle-like leaves are small and waxy to reduce water loss. The small size means less surface area to lose water or catch heavy snow, and a waxy surface further holds in moisture – just like a wax rind on cheese.
One problem living things have with freezing is that needle-like ice crystals can puncture cell walls, killing cells (and eventually, the entire plant or animal). Conifer needles contain proteins that bind to ice crystals – preventing them from growing and becoming sharp – thus providing some antifreeze protection. Additionally, conifers move water out of their cells in preparation for winter. This increases the concentration of sugar inside the cell, providing further antifreeze protection (sugar-water freezes at a lower temperature than does pure water).
Keeping your leaves all winter means you are ready to start photosynthesizing as soon as temperatures get above 0C (32F). But most adaptations in nature are trade-offs, and being an early riser can be a liability rather than an advantage depending on conditions. In late winter or early spring when the ground is still frozen, warm days or high winds can cause ‘winter burn’. Warm days cause pores in the leaves (called stomata) to open during photosynthesis; high winds wick moisture out of the leaves. In both cases, the tree can’t move water out of frozen soil to replace what’s lost through the leaves, and so needles may turn colour and die. Fortunately, this is rarely fatal. While the system isn’t perfect, most of our native conifers come through winter just fine.
Keeping or losing leaves is all about return on investment. It takes a lot of energy to make a leaf, and the tree must get more energy from that leaf than it took to make it. Broad-leaved trees such as Maple, Birch and Poplar have leaves with big surface areas to produce a lot of energy in a short time. But those same leaves would be lethal liabilities in winter. These trees get rid of those liabilities in fall, and so have only one growing season to recoup their investment.
Needles’ winter adaptations allow them to live for two to five years (or more), giving them a longer payback period. That means they can live in colder climates and places with shorter growing seasons, and explains why you can find conifers on mountaintops and in the far north where broad-leaved trees can’t survive. Winter adaptations are another cool part of PEI untamed!