So my mom asks me this question all the time.She's like, what's the best tree to plant? And I'm like, what do you mean? Like, you live in the middle of the desert in Texas and it's dry. You should plant native, but are you planting in palm trees? Are you planting in the middle of the scrubland? What do you want to plant? And she's like, no. Like, what's the best tree to plant for everything? I'm like, well, do you want fruit? Do you want shade? Like, what do you want? There really is no best tree to plant, the best tree to plant is a native one that would be growing there already, and then the second best tree to plant is the one that will survive in your yard.

My name is Devlin Grewal. I am a master's student in Michelle Tseng’s lab at the BRC. My thesis looks at the insect communities on urban trees, on the UBC campus. Our big question was whether or not native trees supported greater richness and abundance, then introduced trees. And there's some pretty good evidence in the literature that suggests that they would. Abundance is the number of physical insects that are on a tree, just how many there were and then, diversity or in our case, richness is the number of distinct species that were there.

You can kind of think of the, tree as like a familiar food. So the abundance and richness, of the insects that live on the tree is kind of a function of the shared ecological history of the tree and the insect. So if a tree hasn't been in an area for very long, then native insects to this area might not recognize that it's a food source. They may not be accustomed to the secondary compounds; These are like defences that make the tree, taste unpleasant for insect herbivores. Or they might not recognize the tree's morphology. So it could be within a similar genera or family, but they just, like, haven't interacted with this tree before, so they don't know how to eat it. For this reason, we thought that native trees would support a greater richness and abundance, of insects than introduced trees.

We selected 16 species of trees that are, prevalent across the UBC campus. these represented more than 30% of the tree species that are planted, in Vancouver. From these 16 species, we had five replicates of each species. 80 trees total. Then our team of undergrads, which was, Savi, Julie and Oliver went out almost every day in the summer, went to each tree, and collected insect specimens from it via beat sheet technique. Which is where you get a sheet. Or in our case, we use an inverted umbrella. And then, we physically agitated the branches that these insects would be living on, by beating them with a stick. When the insects fall out and onto this umbrella, we're able to then collect them, using a paintbrush and then put them into our collection jars that are filled with ethanol.

After we collected the specimens, we took them back to the lab. I used, resources from our Beaty Spencer entomological collection, as well as online guides and other resources to ID almost 1500 specimens. Out of these 1500 insects that we collected, a really big portion of them were, herbivorous. These are things like aphids that feed on juices from the leaf stems and leaves. We also found a number of predatory insects, like ladybugs that feed on things like the aphids and then things called detritivores, which are like the trash men of the trees. They feed on, the, detritus and, organic matter that builds up on the leaves.

What we found was that native trees supported a greater richness of insects on their leaves than introduced trees. However, introduced trees supported a similar abundance, of insects to native trees. So on one hand, we have more species that are present on native trees, but on the other it seems like they're supporting the same number of bugs overall.

We also wanted to know how insect communities differ between coniferous and deciduous trees. Coniferous are things like pine. Your needle bearing trees. Deciduous are your broadleaf trees. There's a lot of evidence (that) suggests they would have different communities and that coniferous trees, because they have these like, tight bundles of, needles, would be able to support more small bodied arthropods than deciduous trees, which have a larger kind of, like, flat area. We were able to find that coniferous trees supported a similar richness and abundance to their deciduous tree neighbours. However, deciduous trees and coniferous trees supported different communities of insects overall.

Our big recommendations are that native trees support greater richness than introduced trees. This is really important. We're in the middle of massive declines of insects globally. Urbanization, is happening globally. There’s supposed to be more than 5 billion people living in cities and other urban areas by 2030. More and more the diversity and the species that we interact with in cities are the diversity and species that we interact with globally.

The majority of trees in, urban areas are planted because they're ornamental and because they have short stature and they're easy to, maintain and grow. What that preferentially selects for is a bunch of introduced species that are short in stature and pretty, but not really like ecologically useful. It’s really important for us to be preserving Biodiversity in cities not just for us to interact with them, but also for ecosystem services. Things like, thermal amelioration, making the cities cooler, stormwater sequestration, making runoff and the floods less severe, but also for the biotic services they provide to other insects and, species that live in the city.

For these reasons, we should putting native to try and conserve as many insects as possible. What we recommend is that we should be planting native trees, ideally a mix of deciduous and coniferous trees. That way you get both of the communities. You should also vary the species that you're planting. Like don't just choose one native tree and plant it across the whole city. Look at things that are native to your area at different growth forms with different fruiting and flowering times.

Our top three richness trees were. Pseudotsuga menziesii, Tsuga heterophylla and Pinus contorta. My recommendation would be like, you know, go see what's available and then think about why you want the tree, what it's going to be there for, kind of like look into how much land or like how much space you have to grow it and then plant native. And plant widely and as diversely as possible, so you can then support a wide variety of insect species and biodiversity.