Phenology in the Forest
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
In the forest, like much of life, timing is everything! It’s why most animals have their young in the spring or early summer when food is abundant. It’s why most plants don’t bloom in December, when there’s a good chance that their flowers would be killed by a subsequent frost. The study of the timing of different biological events is called “phenology.”
What’s a Phenology Study?
A phenology study involves identifying when different organisms enter different stages of their life, or behave in particular ways. My phenology study focuses largely on plants. Plant phenology frequently involves studying the behavior of selected individuals over the course of several years. Some “events” in a plant’s life that can easily be tracked are, for example, when the veins on the new leaves are first visible (aka, bud break), or the first time you can see the sex organs inside of a flower.
I started the phenology study at Tryon Creek State Natural Area (TCSNA) early in 2013. For this study I tracked ten different species growing along 4 different trails at the park; Red Fox, Old Main, Cedar/West Horse Loop (referenced here as “Cedar”) and Middle Creek/Big Fir (referenced here as “Middle Creek”). For perennial species with above ground parts, I tagged the plants and followed them each year. For annual plants, or those species arising from underground organs, I identified a given patch of ground and studied plants at that location. As time went by, I started including observations on a few other species like a delightful patch of bleeding hearts (Dicentra formosa), and some spittlebug nymphs (suborder: Auchenorrhyncha).
I made observations on a weekly basis, with a few exceptions caused by vacations, and extreme weather conditions. For a couple of reasons, mostly related to “learning curve” issues, I starting collecting useable data in 2013 part way through the growing season.
One of the challenges in conducting a phenology study is the issue of when you should report the results. In this case, the differences I observed between 2016 and 2017 are dramatic enough that it is time to provide you with a report. This probably won’t be my last phenology report, “God willing and the creek don’t rise” (to use an old expression).
The Drivers of Plant Development
Plant development is driven by several factors, key among them being day-length, temperature and moisture availability. When it comes to spring budburst in our area of the world, temperatures probably are the primary driver.
The temperature plays two important, and quite different, roles in bud break. The plant needs to hold off on bud break until the threat of a killing frost is past. Thus most perennial plants in our area have a “chilling requirement.” This means that the buds have to experience a certain amount of chilling before they can start growing. Secondly, the buds have a “forcing requirement” which is a certain amount of warm temperatures to get the buds growing after the chilling requirement has been met. As anyone who has ever walked through the forest in the spring knows, these requirements vary dramatically between different species of plants. If the plants receive less than the normal amount of chilling curing the winter, they will need a greater amount of warm “forcing” in the spring. Although it is clear from the diagram that there is at least some minimal amount of chilling needed to ensure that the buds will eventually open.
The diagram below shows the generalized nature of the relationship of chilling and forcing for both Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla). While the curves have a basically similar shape, it is apparent that with low levels of chilling, the hemlock will break bud first, but with large amounts of chilling, the Douglas-fir will break bud first.
Chilling and forcing requirements of Douglas-fir and western hemlock1
In reviewing these bud break results, please be aware that there is very little agreement on the exact temperature that separates the “chilling” and “forcing” functions. I believe most scientists would think 50 degrees is little bit too high, but to be blunt, this is the base I used because it is the best database to I have available. As you can see in the chart below, there have been dramatic differences between years in the number of growing degree days in the first three months of the year, primarily that 2017 has a much cooler spring.
*Data from the Aurora Airport, approximately 15 miles from TCSNA.
So What Happened?
Presenting even a summary of all the data I collected would be a sure cure for insomnia, so I’ve picked out a couple of examples from the study which are fairly typical of the general trends. The first example is to look at the behavior of the Pacific waterleaf (Hydrophyllum tenuipes). This is a plant that has underground roots, stems and buds.
The graph below shows the results for the years 2015 through 2017. On average, the appearance of the first leaves in spring 2017 was delayed an average of 2.5 weeks from the first appearance of leaves in the prior two years. (And yes, the absence of data for the week of Feb 11, 2015 is unfortunate!) The date of first flowering in 2017 was on average 3.0 weeks later than first flowering in 2016. The primary lesson here is that both budburst and flowering in 2017 was much delayed compared to the two prior years. Interestingly, the average date of when the last leaves died was nearly identical in 2016 (33.0 weeks) and 2017 (33.75 weeks).
Similarly, the leafing out of the vine maple (Acer circinatum) was also delayed about 3 weeks in 2017, as seen in the chart below. For the vine maple, so few of the plants I followed produced flowers on a regular basis that the data is probably not worth presenting, although what data there is follows the same general pattern as the Indian plum above.
At first glance, these lines all look fairly similar. However, looking at Week 20, for example, the number of growing degree days in 2016 is about double the number for that same date in 2017. This is a huge difference!
The final set of plant data that I will include here is for snowberry (Symphoricarpos albus), a reasonably common, but not abundant shrub at TCSNA. Here again, both the budburst and flowering of these plants is three to five weeks later in 2017 compared to 2016, as seen in the graphs below:
And it’s not just plants
Below is a chart of the sightings for two years of spittlebugs. In the beginning of 2016, if there were no spittlebugs seen, I just left the space on my datasheet blank. Midway through that year, I recognized the folly of that approach, and started making a clear record showing that no spittlebugs were seen. My notes on the March 2016 spittlebug indicate that it was just one individual bug, and a small one at that. Sometimes Mother Nature will show off one specimen of something way out of season, but that probably doesn’t really mean the season has started. No matter what you think about the March 2016 outlier, it is abundantly clear that the spittle bug, like many of its botanical associates, was late in 2017 by at least three weeks.
The Connection to Global Warming
As documented here in the differences between the various years, the organisms in the forest are sensitive to environmental temperatures. This will be very important as we consider global warming, and what we should do about it. I love the fact that Mother Nature provided us with a great example of the fact that the trend of global warming is not a straight linear process, but will have some hiccups like 2017. By the way, the early readings from this year (2018) show that some plants are starting growth way earlier than they did in 2017. But that’s grist for another note at least a year away. A fascinating aspect to this, which we may be many years away from experiencing, is that almost all woody plants require a certain amount of chilling before they break bud. If global warming ever gets to the point where the winter temperatures are not adequate to chill the buds, already completed research tells us bud break will be significantly delayed. Then we could have real problems. But for now, enjoy the forest that we have!
1Harrington, Connie and Peter Gould. 2016. Rise and Shine: How Do Northwest Trees Know
When Winter Is Over? Science Findings, Issue 183. USDA Pacific Northwest Research Station.