The greater Calgary urban forest, both privately and city owned, is not sustainable without the regular addition of water. The rain and snow we receive are not enough for the urban forest, which is here because of the ongoing regular addition of water into the root zones of the trees. Trees in Calgary suburbs that never receive extra water will die. Calgary is not naturally a treed area. It is located in a grassland, and three of the four native trees, the Douglas fir, the white spruce, and the balsam poplar were found predominantly in the river valley. Only the fourth, the trembling aspen, ventured out onto surrounding hillsides.
As the city continues to expand, the naturally rich underlying topsoil, usually over a foot in depth, has been removed. Poorer, thinner soils are installed over the underlying clay layer in suburban community developments. These soils do not have the water-holding capacity of the native soils. Thinner poor quality soils without much humus dry out much faster than native soils in our highly evaporative environment.
Trees are by nature not efficient water users such as cactus. Trees’ long evolution has prepared them as well as possible for adversity and drought, but there are limits. When we look at the tree’s body and its three main organs, roots, stem or trunk, and leaves there are major differences in how each organ is prepared to deal with water shortages. The older roots are covered with a corky outer bark that is very good at keeping water inside. The tiny root hairs and non-woody roots, which do all of the water and nutrient absorption from the soil, are mostly safe from the sun, being a little deeper in the soil.
The trunk is covered with the outer bark, again a waxy corky waterproof barrier to evaporation. As trees mature the outer bark is a strong preventative to water loss.
The leaves are the tree’s weak area when it comes to water loss. To fulfill their role as energy capturers, the process of photosynthesis, the tree needs to have its leaf stomata open to allow the free movement of CO2 gas from the atmosphere into the inner leaf spaces. Stomata are the pores, tiny openings in the leaf surface that allow the free movement of CO2 gas and water vapor in and out of the leaf. These stomata are the openings in the leaf that allow capture of carbon that is used as the building blocks for simple sugars that fuel all the life processes of the tree. These sugars are all that trees eat.
When water shortage is sensed by the tree, too much water loss from leaves, opposed to water gain from the roots, the tree closes its stomata, doing a good job of reducing water evaporation.
But this closure happens at a great cost; the leaf with its stomata closed is no longer photosynthetically active; it’s waiting, waiting for water, meaning that you can starve your tree by not watering.
Trees like all living things have a basic economy; for trees in Calgary they are only productive, making and storing sugars, for less than six months of the year. But the tree is alive every second of the whole year, meaning that it needs to work very hard when it can, in the months when the leaves won’t freeze. Trees store lots of energy, and then spend it to survive during the six plus cold months.
They can only survive, or better thrive, when they receive adequate water, which allows them to produce the energy they need for the processes of their ongoing lives.
What to do when another recurring drought or worse hits the prairies? This is nature’s way of belt tightening, and we will have to modify and prioritize. As water availability for suburban trees drops we need to make some hard choices. Perhaps we already have too many trees in Calgary? There is no good in giving every tree a little water; that’s a waste that accomplishes nothing. Better to reduce the number of trees and keep those reduced numbers healthy. The city needs to prioritize what to keep and what to lose. Water has in the past been seen as an abundant, endless resource, but now this is not so. The strongest factor is the ongoing expansion of the city and its population. There is only one Bow River.
The city also needs to take a hard look at tree priorities. This applies equally to all private tree owners. Keeping the strongest, best adapted to our changing conditions and water availability, may be the rule in the future. The city can’t continue to plant. Their ongoing cry is that the urban forest is still not big enough , but they don’t maintain what we already have. Tree loss on public lands continues to increase as the drought goes on. We need to make hard decisions about what to keep and where, and then carry on as good stewards. Choose what to keep healthy with regular watering and remove the rest, which are dead or dying.
Species selection is very important and some of our favorite trees are not going to make the cut. The best example of this is the cutleaf weeping birch. This tree has the highest water needs of all of Calgary’s trees. Look at the hundreds of them standing dead around the city, They rot very quickly and are dangerous to people and property below. Other water-loving trees also need to be looked at closely; willows and poplars top the list. Nature never lies, trees that naturally grow near abundant water sources are heavy water users. Others such as conifers, spruce, fir and pines are good examples of trees that are much better suited to our ongoing drought, having evolved in drier, harsher conditions. In our thin soils they still need watering, but nowhere near as much as a birch, which is a swamp tree.
Let’s choose what to keep based on health and drought adaptability; let’s use higher quality and thicker layers of soil in suburban developments; let’s choose more conifers, evergreens which are much more efficient water users; let’s fertilize our suburban properties less, as fertilizers act like salts in the soil, tying up and bonding strongly with water that trees should get. Let’s support our city in the hard choices ahead and choose to take good care of the trees deemed worthy of the water they need to survive.
