What Triggers Bud Break? Monthly Maple REVIEW

A brief introduction to this new feature – Monthly Maple Review – we review a research article once each month to spotlight key findings, investigate curiosities, and uncover important implications for Ohio’s maple producers.  Please comment below if you have thoughts, ideas, insights, or questions.  And if you stumble on to a new maple article and want to see it highlighted in a Monthly Maple Review, please reach out to me via email – karns.36@osu.edu.

Bud Break in Sugar Maple Submitted to Changing Conditions Simulating a Northward Migration” by Ping Ren and colleagues.  This article was published in 2021 in the journal Canadian Journal of Forest Research.

In our first Monthly Maple Review, we looked at producer attitudes and behaviors regarding climate change and its projected impact on maple.  For this our second installment, we focus on how a simulation experiment predicts climate change will effect bud break in sugar maples.

As climate shifts and range-restricting thresholds follow, plants and animals must adapt and keep up with changes or risk being left behind.  Many organisms are well-suited, at least from a mobility standpoint, for keeping up – take birds and their gift of flight for instance.  Other species likely face serious challenges; the American pika is commonly pointed to as an example.  Pika are small, marmot- or groundhog-like creatures that live in treeless alpine habitat in the Rocky Mountains.  It is easy to imagine pika being literally stranded on mountain peaks above timberline unable to migrate and keep up with shifts in suitable range.  Many plants are also considered less adaptive to shifting conditions and may not be able to move into higher latitudes or elevations necessary to keep up with suitable growing conditions; sugar maples are no exception.

In emergency scenarios, assisted migration is a solution whereby humans literally help other organisms keep up with shifting climate conditions.  Already, experiments have been conducted with many plant species, including some trees such as the whitebark pine, to verify suitability of growing conditions beyond the current limits of the species distribution.  Will sugar maple or other maple species need a special assist from us?  No one knows for sure, but studying what factors drive bud break is a small step to understanding if they are likely to need our help in the future.

The essence of Ping Ren and team’s experiment was to examine bud break under controlled conditions while varying temperature and photoperiod (also known as day length).  The experiment’s most basic hypothesis was that “photoperiod outweights temperature in initiating bud break when the chilling requirement in unfulfilled.”

To understand the study’s results, we first need to wrap our minds around 3 main environmental factors, or signals – the variables we believe most plants are responding to when they wake from winter dormancy and start to stir towards bud break.  First, winter chilling – more intense and longer periods of cold during the heart of winter contribute to chilling.  This deep freeze is what resets the annual clock of trees and influences the trigger of growth reactivation.  Perhaps it is worthwhile to think of chilling as similar to a human experiencing a prolonged session of deep sleep.  Second, spring temperature – this is just what it sounds like.  In a simple system, cooler spring temperatures may wake plants from deep sleep more slowly than a rapidly warming and sudden onset of spring (check out a series we did in winter 2022 on growing degree days to better under the role of spring temperature).  And finally, photoperiod – more commonly known as day length.  The most important thing to note on this final factor is that while any given year might vary in terms of winter chilling or spring temperature, length of day is fixed and will always be fixed regardless of where climate change takes us.

While each factor in isolation is relatively easy to understand, it is the complicated interactions between winter chilling, spring temperature, and photoperiod that likely determine the actual timing of bud break in a species.  This study ran 2 experiments that essentially confirmed the hypothesis that sugar maple bud break is more determined by photoperiod than by spring temperature when the requirement for chilling is not met.  Let’s put that another way – during winters that do not put sugar maples into a deep sleep for long enough (winter chilling), day length has more of an effect on bud break timing than how cool or warm spring temperatures are.  In other words, the experiments confirmed the authors’ central hypothesis.

Let’s unpack that a bit more and talk about some take home messages.

Resetting a sugar maple’s internal clock is accomplished primarily by meeting the chilling requirement – being cold enough for long enough.  When that chilling requirement is not met, it takes additional and louder signals to wake up a tree from dormancy to initiate bud break.  While this might sound a bit counterintuitive, the fact is that waking up a tree from a deep sleep is easier and more predictable than trying to wake up a tree that has been tossing and turning in its winter bed.  Under changing climatic conditions, warmer winters may result in unmet chilling requirements that ultimately result in delayed bud break thereby shortening growing seasons.  But remember, winter chilling is just the first consideration.  What about spring temperatures?

At face value, most sugarmakers understand the effect of a warm spring – trees break bud faster.  In a cool spring, buds stay closed longer and the sap season might last a bit longer too.  In a worst case scenario, climate change wreaks complete havoc on winter weather not allowing sugar maples to adequately chill and temperatures jump back to springtime highs so quickly that any sap season is effectively crowded right off the calendar.  That’s where day length seems to play a crucial and important role.

Think of day length/photoperiod as a speed governor on a go-cart.  I hate so-called governors growing up.  I wanted to ride my go-cart at top and dangerous speeds, but my parents set the speed governor so that I could only drive certain speed limits.  When spring temperatures warm abruptly and it seems that the sugar maples might break bud extraordinarily early, length of day pumps the brakes and slows down that process regulating it closer to normal.  Essentially, photoperiod may be a crucial regulating factor to keep sugar maple bud phenology more on track than would be expected otherwise.  In the authors’ own words – “Because day length will not change under climate warming, photoperiod becomes ultimately limiting when bud break in sugar maple occurs too early.”

So where does that leave us?  Will sugar maple need our help in assisted migration as conditions change faster and faster into this and coming centuries?  Time will tell, but if this study teaches you nothing else – you can certainly walk away with 2 big takeaways.  First, trees are remarkably complex organisms.  And second, trees have a few tricks up their sleeves!

Maple Buds and the Story They Tell (Part 2)

We have all heard it (or said it) – once the maple trees ‘bud out’ the sap collection season is done.  “Done” meaning the sap has become buddy and making syrup for the season is over.  Our eyes see it and we know to expect it, but we all hold out for just one more day of collection before the tide turns.  That day or the very next, the sweet smell of sap turns sour.

The good news is you don’t have to rely completely on your nose when boiling that last batch of sap.  Keep a close eye on how the tree buds develop as the season progresses.  When you start the season the buds are tight.  As the season progresses and the weather changes towards spring, the tree buds tell the story. We can use our eyes to track bud development as the season progresses, the weather warms, and making syrup draws to a close.  The good news is that this progression is somewhat predictable if one understands how something called growing degree days (GDDs for short) correlate.  Read Part 1 released last week to learn how you can start tracking GDDs and incorporate them into your season planning.

Using a 60-power spotting scope we took pictures of our trees on March 17th, 23rd and 27th during the 2021 sap season.  The bud progression is for sugar maple and a red x silver native hybrid maple that are present at the Ohio State Mansfield sugarbush.  For reference, we deemed our sap no longer worth collecting on March 21st.

First the red x silver “mystery” maple – in the March 17th photos, the hybrid’s buds are noticeably swollen but the flowers have not burst forth yet.  In the branch I’m holding, you can see the flower buds cracking open with the leaf bud still tightly closed in the center (black arrow).  This is crucial to understand because trees have both flower buds and growth buds which break at different times and have different impacts on the sugaring season.

By March 23rd, despite the difficult lighting, the buds have clearly flowered.

This is even more apparent on March 27th when I took the last set of photographs.  Examine the leaf bud (in the black circle) which is protruding more but still closed surrounded by the bright red maple flowers.  Our operation’s sap edged towards being “buddy” in the last 2 days preceding the red maple buds popping completely out.

For the sugar maple photos, the differences are more subtle and the progression is slower – a timeline we talked about in Part 1 last week.  In the March 17th pictures, the buds were barely noticeable at the ends of the uppermost twigs of the trees; however, buds were more prominent 6 days later.

By March 27th and nearly a week after we had closed down the sugarbush, sugar maple buds were elongated and swollen and obviously scaled but not yet officially burst open.

All in all, this was a great exercise to watch how trees go through the season’s progression as the weather changes.  A set of binoculars is a handy tool for the sugar maker; pick some key trees in your woods and watch their buds next year.  Better yet, keep detailed notes and be a studious observer of 3 primary things: sap quality, tree bud development, and those GDDs we mentioned earlier.  Once you are familiar with what the bud progression looks like relative to your tree’s sap production, you will have information to align alongside GDDs for anticipating when the end of the season is near.

The Ohio State Phenology Calendar: Understanding Nature’s Biological Clock (Part 1)

A special thanks to Denise Ellsworth from OSU’s Department of Entomology for contributing her phenology expertise that makes this article possible!

Phenology, sometimes referred to as the world’s oldest science, is the study of recurring biological events and their relationship to weather and climate. Examples of phenological events include bird migration, flowering of plants, and the seasonal appearance of insects. Because the growth and development of plants depend on temperatures, phenological events of plants, such as bud swelling or flowering time, may be useful for monitoring short-term weather patterns. Likewise, scientists can detect long-term changes due to climate change by tracking the pattern of phenological events over many years.

Insects emerge earlier in warmer years than in cooler years, and plants bloom earlier too. The critical assumption in the use of plant phenology to predict other biological events is that the phenological sequence (the order in which events occur) remains constant from year to year even when weather patterns differ greatly. It is no mystery, even to a novice sugar maker, why plant phenology matters in maple. The quality of maple syrup is at stake! Once the phenological calendar for a sequence is established, the biological calendar is easily monitored to anticipate when maple syrup quality drops. If phenology can be grasped, this can greatly simplify the logistics of planning and scheduling monitoring programs, post-season clean-up and sanitization, and other critical activities. And using phenological sequence is valuable to a whole host of applications beyond just maple—beekeepers, naturalists, and gardeners also use the predictable patterns of nature to predict plant bloom and other biological activity.

On The Ohio State University Phenology Calendar website, degree-day data and related plant bloom and pest emergence sequences are accessible for any location in Ohio.  A degree-day is a measure of the amount of heat that accumulates above a specified base temperature during a 24-hour period. A degree-day is also referred to as a growing degree-day (GDD), heat unit, or thermal unit. One GDD accumulates for each degree the average temperature remains above a specified base-temperature over those 24 hours. Several degree-days can accumulate during a 24-hour period.  However, it is important to understand that degree-days have meaning only in relation to the base temperature that has been specified. The Ohio State Phenology Calendar uses 50 degrees F as the base temperature. To provide an example, if the average temperature over a 24-hour period is only 47 degrees F with a base temperature of 50 F, no GDDs would accumulate. However, if the 24-hour average temperature was 55 degrees F, 5 GDDs would be added to the phenology calendar (more on degree day calculation here).

To inform The Ohio State Phenology Calendar, daily temperature data from 12 OARDC Research Stations and three USDA-ARS weather stations located throughout Ohio are used to calculate cumulative GDD in real-time.  Calculations for locations between weather stations are extrapolated from climatic isotherms for Ohio.  Upon entering a date and any Ohio zip code, degree-day accumulation for that location is calculated, and the user is directed to the appropriate spot on the phenology calendar to determine what plants are blooming and what pests are active in their locale.  By scrolling through the full phenological calendar, it is possible to see what blooming and pest events have already occurred, as well as what has yet to occur.  And by clicking on the Summary tab, you can get a year-by-year breakdown of GDD count for the same date and zip code location across the past 6 years.

It is important to define a couple terms as we launch into species-specific phenology.  First bloom is defined as the first flower opening to expose sexual parts. Full bloom is when just one out of twenty buds is still closed while all others are open to expose sexual parts.

Of particular interest to maple producers, silver maple is listed first with 34 GDD at first bloom. A bit further down the sequence, silver maple reappears with full bloom at 42 GDD.  Red maple first bloom follows at 44 GDD just after silver maple full bloom. Red maple full bloom averages 75 GDD.  Sugar maple is not currently listed on the GDD calendar; however, it is believed that sugar maple tracks very closely with black maple – another of the “hard” maples.  While there is some uncertainty about the exact GDD timing for sugar maples, they are definitely “late bloomers” as compared to their “soft” maple counterparts.

The consistency in phenological sequence from year to year demonstrates that even one year of observation is useful to expand the phenological sequence to other plants or insects not included on the OSU calendar. This means that users can readily create, expand, and customize their own biological calendars by observing plants in first or full bloom and taking note of the GDD for that date on the OSU calendar. Many observers use a journal or excel file to track plant and insect activity from year to year, adding in new plants or insects of interest. These calculations can even be made by referring to photographs that show first bloom or full bloom; the photo’s date and location can be entered on the OSU calendar to determine the GDD for that event. Insect observations should be of developmental stages, such as egg hatch or adult emergence.

For the maple producer, understanding the predictability of nature’s patterns is crucial for better anticipating the end of each maple season. For years and years, sugar maple bud break was the traditional visual signal to take down buckets and end the sap season. Unfortunately, lots of poor-quality sap was made waiting for those first buds to break. Now we know that physiological changes occur within the tree prior to actual bud break that bring seasons to a close earlier. And sanitation issues that result in “sour” sap (due to bacterial build-up) halt most sap seasons before “buddy” sap is rampant. While we are excited to continue tracking sugar maple performance relative to GDDs, keeping an eye on the 100 GDDs mark is a rough indicator for when things are winding down. Some woods will shut down earlier and others will stretch a bit later, but when the Forsythia approaches full bloom in your yard – which occurs right around 100 GDDs depending on variety – you can be sure the end of your sugaring season is nigh.

New Article Series Launches Next Monday

This short post will serve as a sort of guidepost, a table of contents or roadmap if you will, for the next month or so worth of content.  We are excited to bring you a 4-article series on maple phenology.  Phenology is a fancy word for describing nature’s calendar.  We’ll discuss one of the most practical and accessible tools for tracking phenology – the growing degree day, or GDD for short.  Second, we’ll seek to understand and document how GDD is related to species-specific patterns in maple bud and bloom timing and why that matters for maple producers.  Then over the course of two installations, Les Ober will break down why an improvement of one’s understanding of maple season timing is particularly important towards the season’s end and how you can minimize and prevent unwanted bouts with “sour” or “buddy” sap.  After all, our main goal is promoting sustainable production of high quality maple syrup!

Upcoming Webinar (11/19): Climate Change Impacts and Risks to Southern Maple Production

The maple syrup industry is impacted by both seasonal weather and long-term changes in climate. While the short-term conditions impact annual production cycles and quality, long-term changes in climate are having an impact as well. Temperatures across the maple syrup production areas of the US are warming, and climate change extends well beyond just temperature to include shifts in seasonal precipitation patterns and increasingly extreme events. Projections of future climate pose significant risks to the future of maple production across southern zones. Join the webinar (Register HERE) to explore the influence of weather and climate change on the maple industry and discuss the implications for the future.

Speaker: Aaron B. Wilson – Aaron is an Atmospheric Research Scientist at The Ohio State University, holding a joint appointment with the Byrd Polar & Climate Research Center and OSU Extension. He is also with the State Climate Office of Ohio.

The webinar is part of the Out of the Woods series hosted by Ohio State University, Future Generations University, and Penn State University.