The highs and lows of cone production in white spruce
Background
In many species of trees, particularly those that are wind pollinated, some years trees produce a huge abundance of seeds, or cones in conifer trees (Fig. 1), and then a few years will go by with very low or no reproduction at all. It is generally thought that there is a high level of synchrony among trees within a population - meaning the trees all tend to produce lots of seeds at the same time, or few seeds at the same time - and this can span large areas up to thousands of miles. This phenomenon is called 'mast seeding'. The overall research question that I'm pursuing is: 'What are the patterns, drivers, and consequences of mast seeding across spatial scales?'
Figure 1: Seeding-bearing white spruce cones in the fall of a mast year are brown when they open up and can cover much of the crown of a tree. (Photo credit: Jalene LaMontagne)
There are evolutionary reasons why mast seeding may occur, with the leading ideas being that 1.) synchronized high levels of reproduction leads to satiating seed predators or 2.) synchronization increases the efficiency of wind pollination in trees, both of which could lead to more survival of seeds. In terms of what controls mast seeding, weather conditions, and in particular summer temperature one and two years before the seeds are produced seem to be key. However, despite the highly levels of synchrony, there is also considerable variability among individuals. Individual trees can be doing different things in the same year that we do not yet fully understand.
Understanding the patterns and drivers of mast seeding is key to understanding the dynamics of many other species as well, from very local to even continental scales. As many things are in nature, it's all connected.
Figure 2: New white spruce cones during a mast seeding event. (Photo credit: Jalene LaMontagne)
White spruce (Picea glauca) is the main species that my lab studies (although we've started to study other conifer species as well) and it has a distribution across the North American boreal forest. White spruce has a strong ecological role, providing seeds for numerous species of animals, including seed-eating birds, small mammals, and insects. The pulse of food produced by white spruce in a high-reproduction 'mast event' or 'mast year' (Fig. 2) has cascading effects in the ecosystem. White spruce is also an economically important species as its wood is used for lumber and paper products, and the seeds are valuable for forest regeneration (Fig. 3). Understanding the patterns and drivers of mast seeding is key to understanding the dynamics of many other species as well, from very local to even continental scales. As many things are in nature, it's all connected.
Figure 3: White spruce seedling. (Photo credit: Jalene LaMontagne)
Methods
Trees are great to study for a lot of reasons, including that they generally stay in the same spot where you left them year after year, although the occasional tree falls over. In 2012, I started a field research program on mast seeding in white spruce in the Upper Peninsula of Michigan and northern Wisconsin. In 2015 I added trees at the Cloquet Forestry Center (CFC) in Minnesota. In total now there are nearly 1,000 individually tagged white spruce trees distributed across 18 sites involved in the study.
Trees are great to study for a lot of reasons, including that they generally stay in the same spot where you left them year after year...
Figure 4: LaMontagne counting cones on trees. (Photo credit: Abby Leeper)
Every year my research team and I go up to the five sites we have at the CFC to count cones on almost 200 tagged trees. When we count the cones on the individual trees, I use binoculars and bring a camera with a telephoto lens. If there are fewer than about 200 cones on a tree, I just count them (Fig. 4), but if there are about 200 cones or more then I take a photo of the top of the tree where the cones are. Then we use an open source computer software called imageJ and a student technician will count the cones on the computer screen. We call those cones that we can see a "cone index", and then we convert the index number to an estimated 'total cones' using an equation derived back when I was a PhD student (LaMontagne et al. 2005). Every two to three years we measure the diameter at breast height of the trees, we keep data on tree height, and we have data loggers for temperature at each of the sites (Fig. 5). In addition to this, I work with a collaborator – David Greene with Humboldt State University – and we record data on white spruce cone production on trees at sites spanning over 3,000 km (1,800 miles). I am also working with Ben Zuckerberg at University of Wisconsin – Madison and Court Strong at the University of Utah , on a large-scale National Science Foundation-funded project linking patterns of climate variability to tree reproduction and animal population dynamics at National Ecological Observatory Network (NEON) sites across the United States, including Alaska.
Figure 5: Abby Leeper downloading data from a temperature logger on a tagged tree. (Photo credit: Jalene LaMontagne)
Findings
The most interesting finding to date is that white spruce reproduction on trees at the five sites at the CFC are different than those for trees at sites in northern Wisconsin and Michigan. That means that when it is a high cone production year at CFC (Fig. 6), that there are few cones on trees in Wisconsin and Michigan, and vice versa. This is consistently the case going back to 2015.
Figure 6: High cone production on a white spruce tree at the Cloquet Forestry Center in July 2020. (Photo credit: Jalene LaMontagne)
The differences between these regions or states are reflected in some recent work published in Nature Plants that showed a breakdown in where large reproduction mast events occurred in white spruce across North America with the divide centered on the western side of Lake Superior (Fig. 7; LaMontagne et al. 2020). In addition, our data show that approximately 20% of the individual trees within sites have patterns of reproduction that are different from what their overall population is doing. The plan is for this to be a long-term research study, and I anticipate that we will know more about what creates the variability in the patterns that we see. I want to understand what it is that makes sites in Minnesota and west of Lake Superior different from sites in the east, and why we see so much variability among individuals at the same site. My hunch is that this is where there is a persistent breakdown in climatic patterns that drive white spruce mast seeding across its range.
The differences between these regions or states [MN, WI, and MI] are reflected in some recent work published in Nature Plants that showed a breakdown in where large reproduction mast events occurred in white spruce across North America with the divide centered on the western side of Lake Superior.
Figure 7: High reproduction mast events at sites for white spruce (red) were clustered in eastern North America in 2006, breaking down near the western side of Lake Superior. (Adapted from LaMontagne et al. 2020)
Why this project?
I've been interested in questions about emergent patterns across scales, and why there is so much variability in nature within a species, dating back to my time as an undergraduate and a graduate student. When I was a PhD student doing research in the forests of northern Canada, I had noticed that some individual spruce trees had cone production patterns that differed from the population (Fig. 8), and that sites even 5 km (3 miles) apart could be different (LaMontagne and Boutin 2007). That was a bit of a surprise, because the main drivers for mast seeding have been linked to weather conditions and over these small distances conditions should be pretty similar. Also, there are thought to be strong evolutionary reasons for trees to be synchronous with other trees, including starving and then satiating seed predators, and increasing pollination efficiency. Most studies on mast seeding back then didn't really look at individual trees, so I thought that maybe I was onto something.
...there are thought to be strong evolutionary reasons for trees to be synchronous with other trees, including starving and then satiating seed predators, and increasing pollination efficiency. Most studies on mast seeding back then didn't really look at individual trees, so I thought that maybe I was onto something.
Figure 8: During a mast event, many individual white spruce trees have high levels of cone production, but there is often some asynchrony among trees within a study site. (Image Credit: Jalene LaMontagne)
In 2011, I started my faculty job at DePaul University , and I had decided that mast seeding would be part of my research program. This area of research is fascinating to me, and fills a lot of the buckets of what I'm interested in as a population ecologist. There are connections to a variety of wildlife species impacted by seed production (including birds like crossbills, and small mammals like red squirrels, seed-eating insects, and their predators) and, I love being out in the woods, so that's always a fantastic perk of doing this kind of work.
Why the CFC?
The Cloquet Forestry Center is an ideal location to conduct this research, it has been really excellent and I look forward to coming back every year. When I was looking to set up more sites in 2015, I was looking for a place with good numbers of accessible white spruce trees, and CFC could offer that (Figure 9). Little did I know then that the location of CFC would be so ideal for asking the questions I have about broad-scale patterns in mast seeding (being on the western side of the geographic split in mast-seeding synchrony). Importantly, the staff at CFC have been so helpful over the years and a pleasure to work with.
Figure 9: The quiet forest roads of the UMN Cloquet Forestry Center offer rapid access to 200 scattered white spruce trees tagged and monitored annually since 2015. (Image Credit: Jalene LaMontagne)
Project Feature Author: Jalene LaMontagne, Associate Professor, DePaul University, Email: jlamont1@depaul.edu; Twitter: @lamontagnelab ; Website: lamontagnelab.weebly.com .
Project Collaborators: A number of current and former students have assisted with this research over the years, including Abby Leeper, Cristian Corona, Jillian Sterman, and Jessica Barton.
Funding: This research has been supported by National Science Foundation grant DEB-1745496, the DePaul University Research Council, and the CSH Faculty Summer Research grants programs.
References
LaMontagne, J.M., S. Peters, and S. Boutin. 2005. A visual index for estimating cone production for individual white spruce trees. Canadian Journal of Forest Research 35: 3020–3026. https://doi.org/10.1139/x05-210
LaMontagne, J.M. and S. Boutin. 2007. Local-scale synchrony and variability in mast seed production patterns of Picea glauca. Journal of Ecology 95: 991-1000. https://doi.org/10.1111/j.1365-2745.2007.01266.x
LaMontagne, J.M., I.S. Pearse, D. Greene, and W.D. Koenig. 2020. Mast seeding patterns are asynchronous at a continental scale. Nature Plants 6: 460-465. https://doi.org/10.1038/s41477-020-0647-x
