Exploring urban tree planting sites in the Ottawa-Gatineau region for use in bird research

i) What is the distribution of site ages?

Let’s start with a brief overview of the summary statistics for site ages.

summary(treedata_trimmed$site_age)

   Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
  1.000   2.250   6.000   7.024  11.000  19.000 

So, the newest site is only about 1 year old, while the oldest site is 19. The mean age of sites is 7 years.

But how is this distributed?

Overall, it seems there is a skew towards newer sites! It may be that tree planting is gaining more popularity in Ottawa, or our sample may just be skewed by a large number of new sites for some other reason. For my field work, I may have to consider trimming out some of the newer sites to have a more even distribution of site ages.

ii) What is the distribution of planting strategies?

There are much more Standard plantings than Tiny Forest plantings in this data set. This makes sense, as Tiny Forests were only planted by FCF and are a relatively new approach to tree planting in Ottawa. I should keep this in mind when doing further analyses.

iii) What is the distribution of total trees planted at the sites?

It seems that the majority of the sites have relatively small numbers of trees planted. There are particular greater amounts of sites with ~100 trees, ~300 trees, and ~600 trees, and there are only a few outliers with > 600 trees. When selecting sites for field work, I should keep this distribution in mind, particularly if I want to have a sample with mostly similarly sized sites.

iv) What is the distribution in species diversity at the sites?

Here we see a somewhat normal distribution, with a bit of a left skew. The majority of sites have around 10 species of tree, but a few sites have much higher species diversity scores. This could be the Tiny Forest sites, which typically have higher diversity due to the principles of Miyawaki method / Tiny Forest planting. We will investigate this further in part 3.

How about variation in diversity within functional groups? Is there a large variety of trees from each group, or do some groups have more diversity than others?

Interestingly, it seems that diversity is the most varied within the deciduous trees. In other words, most sites plant only a few different types of conifers or shrubs, but sites have a wide range of diversity in decidious trees. Some have only a few different species of deciduous, while others have many different species of deciduous. Note that there is only one lumped group for “Other”, so we can’t really tell what the variation in diversity is within those non-trees. Tree Canada and FCF only plant native species, so this most likely just reflects which types of trees are native to the Ottawa-Gatineau region and are available from tree suppliers.

i) What are the most and least commonly planted species?

Let’s start with a ranked list of all of the species at the sites.

Wow, that’s a lot of different species! It’s hard to get much out of this chart, but “Other” clears the rest as the most planted thing… This is likely because of the many non-tree understory plants that are planted at Tiny Forest sites.

Let’s exclude “Other” and look again at the top 10 most planted species.

So, the top ten most planted species includes 6 deciduous trees, 2 conifers and 2 shrubs. The most planted overall is Acer saccharinum (Silver maple), the most planted conifer is Larix laracina (Tamarack) and the most planted shrub is Viburnum lentago (Nannyberry). All very iconic Ottawa trees.

But what about the least planted species?

Interesting! The least planted species are mostly deciduous trees, as well as no conifers and two shrubs. There is a tie for 10th, as Liriodendron tulipfera (Tulip tree) and Castanea dentata (American chestnut) both have eleven occurrences, so 11 species are listed instead of a bottom 10. The least planted overall is Asimina triloba (Pawpaw) with only 2 trees in total across all of the sites.

Now, on average, what proportion of a site is shrubs vs trees vs conifers vs other?

It seems like on average, sites plant mostly deciduous trees, followed by shrubs, then conifers, then other plants. However, if we look at the individual data points, we can see there are a few sites that have a large proportion of conifers, as well as a few sites that only contain deciduous trees. “Other” also has a wide variation, with about half of the sites containing few to no other plants, and the other half having about 1/3 of its total planted count be other plants. This is likely showing the difference between Standard plantings and Tiny Forests.

ii) Have the same types of trees been planted consistently over time?

Next, I’m interested in knowing if the types of trees planted has remained consistent over time. For simplicity, let’s look again at our top 10 most planted species.

Most of these graphs seem to be all over the place from year to year, which makes it hard to extrapolate any real trends from these data. We can see that many of these trees are planted in burst years though- such as a large spike of Acer saccharinum (Silver maple) and Acer rubrum (Red maple) in ~2017. This could be a reflection of supply availability, or just years where more planting happened in general. It may have even been from a single site where a very large volume of maple trees were planted!

iii) Has the number of trees planted at sites changed over time?

It seems like there’s been a slight increase in the number of trees planted per site over time. This might be skewed by Tiny Forest plantings in recent years; the Tiny Forest method involves a lot of dense, high tree count plantings, and have only been planted in the last few years in this data set. Regardless, it would seem that the number of trees planted on average has not changed significantly over time. This is good to know, as it will make it easier to select a subset of sites for field work that come from a wide range of years while still having similar numbers of trees.

i) Do older plantings have more or less diversity than recent plantings?

It seems like newer sites have greater species diversity than older ones. Again, this could be skewed by Tiny Forests in recent years. Let’s remove Tiny Forests from the data and re-plot.

Aha! It was skewed by Tiny Forests. This is good to know; species diversity at standard sites has NOT changed over time.

ii) How do Standard plantings vary in structure compared to Tiny Forests?

Our analysis for the last question showed that Tiny Forests are more diverse than Standard plantings. Let’s plot that difference in another way.

Clearly, Tiny Forests on average have much higher species diversity than Standard plantings. This is reflective of the Miyawaki method, which demands high density, high diversity plantings. We should also check if this is partially because Tiny Forests simply have more trees compared to Standard plantings.

Well, look at that. Despite being much more diverse, on average, Tiny Forests do not actually have that many more trees in total than Standard plantings. This makes sense again- they have roughly the same amount of trees on average, just packed into a much smaller area, and with many more types of trees and plants included.

How about functional group composition- do Tiny Forests and Standard plantings share similar proportions of tree types?

As expected, there is a lot more “other” in Tiny Forests compared to Standard plantings, due to the planting of understory plants in Tiny Forests. Interestingly, there also appear to be proportionally few conifers at Tiny Forests. If we look at the variance within each group, we can also see that the proportions of each functional group vary a lot more in Standard plantings, while these proportions are more consistent in Tiny Forests. This is likely because the Standard plantings were done by several different contractors in different contexts, while Tiny Forests were always done by FCF following a stricter set of planting guidelines.

iii) Do plantings with high planted tree counts have higher diversity?

It would appear that generally, plantings with more trees are also more diverse. Makes sense.

Since we found that on average, Tiny Forests typically have roughly the same number of trees as Standard plantings, this trend should hold up even if we remove Tiny Forests from the data set:

As expected, the trend holds. This will be important to note when selecting a subset of sites for field work. That concludes our analysis!