If you’ve been out wandering under the oaks in the last few weeks and looked up, you may have noticed tree branches sagging with the weight of huge crops of acorns. Or perhaps now you’re cleaning them up in your yard! But what gives? Why and how do these “bumper crops” of acorns form, and what can they tell us about climate, forest health, and natural cycles? Let’s dive in to the science of “masting”.
What is Masting and How does it Work?
Masting is the process of (1)”regionally-synchronous” (meaning all at the same time in a given place) production of (2) large seed crops at (3) semi-regular intervals. Simply put…it is the word for when all of the trees of one species, like live oak, produce large numbers of seeds (acorns), but also repeatedly do so every few years.
This isn’t a new phenomenon, both in terms of its occurrence in nature and us humans taking advantage of it! For example, images created in the 14th century show farmers hitting trees with sticks to knock off acorns for their pigs during mast years. Further back still, there is evidence of indigenous people in regions with oaks observing and benefitting from the patterns of masting—for example more than 10,000 years ago in California.
So why and how do trees do this? Lets start with why trees mast, using the numbered components in the first paragraph above.
1. Synchrony
Species like oaks and pines are wind-pollinated, as opposed to pollinated by animals like many non-tree species or species with large, attractive flowers. Wind pollination depends on releasing all of that lovely pollen we all know and love, but also on recipients of that pollen being in the right place at the right time.
So if you’re a tree, how do you end up in the right place at the right time for pollen? For one, you just get lucky. Evolution through natural selection is based on fairly random selection by the environment of individuals that happen to display the traits beneficial to living in current conditions; nature “selects” the lucky ones, so those trees that happened to receive pollen at the same time that another tree releases it are “selected” (by natural selection) to continue to propagate by producing acorns.
So that knocks out one of the three components of masting: synchony, or many trees in a region matching timing of their pollination and seed production. There are various cues that may drive trees to produce pollen, but being a tree that responds to those cues in the same way as those around you leads to synchronous seed production!
2. Large Seed Crop Production
Time to learn some fancy terms! The tree species listed above (most oaks and pines) are monoecious (meaning “in one house”). This means the produce both female and male reproductive structures (think pollen and seed) on the same tree. Some trees that do this can self-pollinate, thus making new seeds. But, for some reason, trees that do this seem to make less seeds and seeds that don’t last as long. Because of this, nature has once again selected (over many generations) trees that outcross (the process of successfully reproducing by sending your pollen out and creating new seed). Further, some pines like Ponderosa pine have actually become so specialized for wind pollination that they only make cones/seeds in the upper branches, where they can catch the most pollen, and only make male cones (pollen producers) in lower branches, preventing any self-pollination.
So how does this all come together? If I’m a tree and I’m making lots of pollen as conditions are right (more on that later), then I can probably “bet” that other trees will do the same, but not too much, and ensure I invest in staying alive (in fact, in biology we have a term for this, called “bet hedging“…so it turns out even trees have “behaviors”!), so I’ll make my female reproductive structures at the same time, betting that even if I don’t self-pollinate, trees around me are likely making a lot of pollen now too. But, there are tradeoffs to this…
Making seeds is hard! Reproductive structures like seeds can cost as much as one-tenth of the resources created by a tree through photosynthesis (like sugars and other carbohydrates) in a given year.
In fact, making seeds can be so difficult that making a lot of seed in one year can really deplete resources for the following year. This means that for a “mast” to occur, not only do trees need the perfect climate for making and pollinating seeds, they also must have not made a lot of seeds the prior year, instead saving some carbon from the prior year in their roots.
And this behavior may be what drives the “regular intervals” part!
3. Masting at Semi-regular Intervals
How do trees “know” when to reproduce to keep masting cycles? The answer is still not entirely clear, but the majority of evidence (remember, in science we simply look at where the majority of the evidence points, but that could change!) points to microclimate and how trees respond to that microclimate.
Once again lets put ourselves in the point of view of the tree. If my main way of spreading pollen is on the wind, would I be more successful if I released all of my pollen in a cold, wet year, or in a warm, dry year?
If you said warm and dry, you’ve successfully outcrossed!
But its more complicated than that (as these things usually are). Like we discussed earlier, producing pollen and seed is VERY expensive. If you think of roots and other structures in the bottom of a tree as a bank, with carbon and carbohydrates as your cash, reproducing essentially draws down your account, betting that it will be replenished by a good (moist, sunny) growing year. So while a warm, dry, windy season may be good for spreading pollen, that season has to come after a good growing year with lots of moisture left over. This allows trees to “save up” for the reproductive cycle.
This gets even more complex in conifers like Ponderosa pine, where cone production can take up to 3 YEARS! While oaks can have a good mast year by experiencing a good wet winter and a warm spring, pine trees need: good growing season (wet but sunny) in year 1 to “prime” cone production and build up resources, a dry year in year 2 to maximize pollination, and a wet year in year 3 to ensure they don’t run out of carbon before the cones are produced.
And running out does happen…all the time! This is called cone abortion, and was discussed in depth in a paper we have on our website’s new publications page! In that paper, we discuss how climatic stresses like drought may dramatically influence reproductive capacity in trees. This is further exacerbated by neighborhood crowding, bark beetle, and…anything, really that stresses out a tree!
So when considering the primary 3 components that define masting, we can say the following things are required:
- A tree receiving a lot of pollen produced by another tree was “selected” to produce female reproductive parts at a given time, based on some shared cue, reinforcing synchrony.
- In order to maximize reproductive effort, that same tree (in cases where the tree is monoecious) would likely be selected to produce lots of pollen while also being able to receive pollen, ensuring successful outcrossing and production of the next generation—a behavior that was reinforced, again through natural selection.
- Doing all of this is hard and resource-intensive, so I better get it right and align with my neighbors, as if I don’t, I’ll probably miss all of the pollen and my opportunity to outrcoss. This tradeoff reinforces synchrony, the mass production of reproductive structures, and resource storing and bet hedging to align with climate and neighborhood masting cycles.
What can this tell us about forest health, climate change, and what to do next?
Now that we’re a little more comfortable with what drives masting, how can we examine patterns in masting to learn more about climate change? Or, more simply, what does climate change mean for bumper crops?
As discussed above, warmer Springs are typically associated with mast years in oaks. However, a series of warm and dry years can stress trees, reducing potential investment in reproduction versus survival or drought defense. Research on trends in masting have found that indeed, masting frequency is increasing over time. However, perhaps more interestingly, the variability in this masting (how often it happens, how regular the cycles are, etc) is increasing as well.
In other words, warming and general climate change (which is also associated with increased variability beyond just warming) are driving potential changes in the regularity of masting cycles. It remains to be seen how this will influence large-scale forest health and tree survival, but we may expect decreasing synchrony if trees have to increasingly “bet hedge” with their resources under drought, extreme winters, pest stress, and general changes in forest health.
One aspect of forest health that we can control fairly immediately that influences reproductive behavior is forest crowding or density. A tree that has more competing neighbors has to “bet hedge” more, potentially reducing masting likelihood. Further, it has been shown that large trees with little competition produce more seeds in general. Similarly, trees in less dense forests have access to more resources, increasing their resilience to all of the stressors mentioned above, from drought to pests.
This is why prescribed fire, and processes and projects that are restoring forests to the condition in which they can burn at low severity relatively frequently, are so important! But we will save that for another blog, or revisit some of our older ones on fire and forestry in our region!
So now that you have a better understanding of masting, we hope that next time you look at a bumper crop of acorns, you consider the years, climate, carbon, and complex network of conditions and behaviors that led to that crop. Happy trees make happy bumper crops!
References and additional reading
Note that many references and resources can be referred to in the masting literature, but here is a small subset directly used in the writing of this article).
- Koenig Walter D. 2021. A brief history of masting researchPhil. Trans. R. Soc. B37620200423 ↩︎
- Pearse, I.S., Koenig, W.D. and Kelly, D. (2016), Mechanisms of mast seeding: resources, weather, cues, and selection. New Phytol, 212: 546-562. https://doi.org/10.1111/nph.14114 ↩︎
- Koenig, W.D., Knops, J.M.H., Carmen, W.J. and Pearse, I.S. (2015), What drives masting? The phenological synchrony hypothesis. Ecology, 96: 184-192. https://doi.org/10.1890/14-0819.1 ↩︎
- Lauder, J.D., Moran, E.V., Hart, S.C. Fight or flight? Potential tradeoffs between drought defense and reproduction in conifers, Tree Physiology, Volume 39, Issue 7, July 2019, Pages 1071–1085, https://doi.org/10.1093/treephys/tpz031 ↩︎
- Pearse Ian S., LaMontagne Jalene M. and, Koenig Walter D. 2017. Inter-annual variation in seed production has increased over time (1900–2014)Proc. R. Soc. B.28420171666 ↩︎