Lammas growth

With fall approaching (or already upon us?), trees are making their last minute deposits of starch and getting ready to drop their leaves. While the first signs of the impending winter are just starting to paint our hillsides, the trees have quietly been making preparations for the last 3 months. Winter buds, for example, contain the leaves that will sprout this coming spring. These are produced and have been laying dormant since mid-summer in most species.

There are two big advantages to creating buds for next year in mid-summer. The first is that energetically, next year’s growth is already paid for (primarily by starches stored from the previous year) as summer insects emerge and conditions turn hot and dry. Even if the tree loses all of its leaves to hungry caterpillars (as with the oaks this summer), they won’t be any farther behind in preparing for winter. Compare this to fruits and nuts, which ripen in the fall (red oak acorns take 2 years to mature). In any given year, reproduction is essentially a bonus activity. In the wake of a stressful summer, a stressed tree can halt production of fruits. I’ve been on an apple kick and noticed that hard hit apples were nearly barren this year while those that escaped the caterpillars have been incredibly productive.

The second advantage is that in a pinch, the buds from this year can be coopted to produce new leaves in the summer to recover from damage. And this is exactly what happened with our oaks.

Back in post-caterpillar August, the terminal buds from many oaks burst open, about 9 months before expected. This abnormal flush of late season new growth – typically a lime green with hints of red protective pigments – is called lammas growth, and is common in oaks (and conifers like jack pine, Sitka spruce, and douglas fir). It doesn’t occur every year, but is most common in response to defoliation from insects (or heavy deer browse or even heat stress) earlier in the summer. It’s a built-in failsafe.

Lammas growth occurs around (and takes its name from) Lammas Day, or Loaf Mass, a Christian holiday derived from pagan traditions that celebrated the very first harvest of the year around August 1st.

I wanted  to briefly mention two other more common growth patterns for winter buds and compare/contrast them to the flexibility of lammas growth. In some species, like sugar maples or white ash, a bud contains tiny versions of every single leaf that will be in the canopy in the following year. If the bud has 11 tiny sugar maple leaves within it, then next year, the emerging branch will have 11 leaves and no more. Every branch ends in a terminal bud that marks the end of growth for the current year. Again, buds are produced in mid-summer and so a tree with this growth pattern is betting that it will have everything in place by the end of June for dealing with challenges that might arise during the rest of the growing season. Trees with this determinate growth pattern tend to grow in stable environments.

But some environments are not so stable. A disturbance may knock down a neighboring tree, damage a branch, or even defoliate the canopy. Being plastic (with indeterminate growth) means that a tree can continue to produce new leaves throughout the growing season. Because the number of leaves is not fixed, these trees never produce a terminal bud. The terminal bud acts as a book mark at the tip of growth, and it has no leaf at the base of it (so has no leaf scar). Instead, just beyond the bud the twig abruptly ends with a leaf scar and sometimes a small knob of growth extending past the bud (e.g. elms and black locust). You can see this on the leaves during the summer which grade from a light green (the newer growth) to the darker greens of the early spring growth. There’s often a distinct difference in shape too. Spring leaves of boxelder tend to have 3 leaflets, while those produced later in the summer often have 5-7 (or can even be doubly compound).