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Too warm, too early: the peril of summer in spring

Plants and trees may struggle, biologist warns

Forsythia blooms on the St. George campus (Photo by Caz Zyvatkauskas)

Research Communications Director Paul Fraumeni talks with acclaimed plant biologist (and U of T Scarborough’s vice-principal, research) Professor Malcolm Campbell about the dangers of weather that lets us wear shorts in March but threatens the healthy growth of plants and trees – and the entire ecosystem.

We’ve just experienced one of the warmest winters and early springs in Southern Ontario history. How has this affected our natural world?

The way in which the temperature has changed will have a profound effect on the way in which plants respond.

Plants will respond to the irregularly high temperatures we’ve experienced. We can already see that. There’s been a greening of grass and an early opening of buds. Normally, the forsythia, for example, would be just starting to flower at this time, but now the forsythia flowers have sprouted fully and a number of other plants are already bursting out in buds. There are hedges that should be sitting dormant at this time but are already looking green.

Is this a bad thing?

There are a number of reasons this can be a bad thing.

If the temperatures become cold again, plants that would normally not have leaves in the presence of cold temperatures can be susceptible to the cold.

Trees like maple are able to deal with the cold after a warm spell because they have remarkably high sugar content in their sap. That sugar allows for the sap to stay fluid even under lower temperatures. It lowers the freezing temperature for that sap, so it stays liquid even when it’s cold.

Other trees aren’t able to do that.

Now, get ready, because this gets complicated. The sap in those trees actually freezes and a process known as cavitation takes place. Cavitation is a breaking of the water column that goes from the roots up to the leaves starting with an introduction of air bubbles. When the ice forms in that water column something has to make up the place of the liquid that was displaced by the air pocket. And as ice, liquid water takes up a smaller amount of space than the size of the air pocket. So air remains in that extra space and the water column cracks.

Then the sap thaws and there are now bubbles in it. As in a capillary in your body, if you have a bubble in the middle of a capillary, you can no longer draw liquid up through it. So, now the tree has lost its capacity to transport its water from the roots up to the aerial tissue.

How cold does it have to get after the warm spell for cavitation to occur?

Any temperature below zero after a warm spell is going to put the plant increasingly at risk. It depends on the species of tree or plant. Farmers often talk about how many frost-free days there are in the spring. You want to avoid days where it’s been warm and then you go into sub-zero frosty days again.

How else can the warm spell be bad?

Another important potential problem for the plant is the appearance of pests.

Just as the warmer temperatures are good for getting the plant out of its dormant state, the warmth can be good for getting pests and pathogens out of their dormant state as well.

The plant has to contend with pests and pathogens during the season anyway, but it may be that the things that prey on the insects are not around at the right time. So now, with the abnormally warmer weather, you have the emergence of insects at a time when you wouldn’t normally have them and there’s nothing to prey on them and the plant is unprotected.

I’ve heard the early warm temperatures could improve pests’ ability to reproduce in greater numbers.

Some pests will have double or even more opportunities to reproduce over the normal growing season, thereby increasing their numbers. In the past, the number of frost-free or cold-free days in the western part of Canada has meant that things like the mountain pine beetle, which carries a devastating fungal disease, has now had two opportunities for reproduction over a normal growing season, thereby increasing their numbers and being better able to transmit that fungal disease far and wide.

So the warmer weather in March increased the number of opportunities for a pest or pathogen to attack the plant and potentially do it at a time where the pests’ and pathogens’ enemies are not around.

Finally, because the higher temperatures can induce the plants flower earlier, their flowering cycle might be off with, for example, either migratory pollinators or with pollinators that require different conditions to be around. They would be out of sync with their natural ability to reproduce.

So the warm weather throws the whole natural cycle off.

Exactly. Hummingbird-pollinated plant species are normally timing their flowering for when the hummingbirds are here. But it may be that the hummingbirds have not yet had an opportunity to make their way up to Canada from their wintering home in the Gulf of Mexico to be around to pollinate at an appropriate time that has now been induced by the early warming.

These huge weather fluctuations must really wreak havoc on the agricultural industry – professional flower growers and fruit farmers, for example.

That’s right. Farmers always have to contend with the weather, but unexpected variations upset their planning. Still, there can be positives too for crops, such as those used for foraging, like hay. With the early warming, you’ve extended the growing season for those crops. Just as you’ll have to mow your lawn more frequently this year, now you’ll have greater opportunities to be cutting crops like hay species, like timothy or alfalfa. It might well be possible to get an extra cut off the field before the end of the season.

And it could be a good thing for some fruit production. For wind-pollinated plants, they may have their grains available earlier in the seasonal harvest. But of course that can be problematic too for farmers who have made sure that they’ve timed their harvests so that they can do other things. Everything is phased in farming and warmer weather earlier can throw everything off.

This brings up the ecosystem. It really is an integrated system, where every component works off of each other.

That’s an excellent point. You can imagine that after making a long voyage up from the Gulf of Mexico to this part of the world, if the plants are out of sync, hummingbirds no longer have the food source they’ve become dependent upon in this region.

People who are birdwatchers in North America have noticed two things recently. One is that species that are further south have been pushed northward, maybe because foodstuffs that would normally be available to them in the south are no longer there because those foodstuffs are now available further north because of the warmer weather and so their expanding their range northward.

Similarly, and this is really intriguing, there has been an inordinate number of sightings of snowy owls in southern latitudes this winter. This is thought to be due to diminished lemming numbers in the north that would normally be taking advantage of factors like the snowpack. Now those species are in a state of decline because of warmer weather and it’s pushing the snowy owl further south in search of prey.

Is this global warming?

Good question. There are some results that just came out in Nature GeoScience that the frequency at which we observe these extreme weather events is attributable to climate change. So with this warm winter and early spring, maybe it’s an outlier, but the frequency and distribution with which we observe these record-breaking temperatures and weather is indicative of something that may well be attributable to a larger overriding system — climate and changes in the climate.

There’s a lot to understand in this complex process, isn’t there?

Yes, but we are tackling all that we’ve talked about today in our research at U of T. There are groups throughout U of T that use the tools of genetics, biology, and ecology to better understand how plants respond to the environment and how this impinges on other organisms in the environment.