Alright, I really hope you got here with me, as this is some pretty fun stuff. I know it’s math and acronym heavy (and cute animal light), but I find it deeply satisfying to quantify some of the more intuitive things about trees. With a couple more calculations, we can paint a very cool picture of the difference between sugar and striped maples.
What are Big Nights?
So to recap, sugar maple has many, many small leaves and in our example trees a crown almost twice as big as that of sugar maple (crown coverage is measured here as the crown projection area, or CPA). But CPA treats the crown as a two-dimensional ellipse rather than the complex three-dimensional shape with tiers of overlapping branches that trees actually are.
When I calculated the total surface area covered by each tree’s leaves (called the leaf surface area, or LSA), I found the exact opposite: that the striped maple covered a third more surface area than the sugar maple. Put another way, striped maple was producing far more leaf surface area of photosynthetic material in a smaller space than sugar maple.
Below is a summary of the various calculations used to compare the two trees:
| Striped Maple | Sugar Maple | |
| Number of leaves | 306 | 1060 |
| CPA, Crown projection area (ft²) | 59.1 | 112.2 |
| LSA, Leaf surface area (ft²) | 91.1 | 67.7 |
| Average weight of a leaf (g) | 0.688 | 0.188 |
| Total weight of leaves (g) | 210.5 | 199.3 |
| SLA, Specific leaf area (ft²/g) | 0.340 | 0.433 |
| LAI, Leaf area index | 0.393 | 0.288 |
Sugar maple covered less area with its leaves, but I wasn’t quite convinced that my sugar maple had lost the photosynthetic battle to striped maple. If you’ve ever held a striped maple leaf, you know just how thin they are. Sugar maple leaves, on the other hand, are both thicker and stiffer, and it was possible that while sugar maple’s smaller leaves covered less total surface area, the thickness of each leaf would result in more photosynthetic capacity per square foot.
Because all that stuff inside a leaf responsible for photosynthesis has a weight to it, we can quantify this total photosynthetic capacity by comparing the weight of leaves for striped and sugar maple relative to the surface area of the leaf (this ratio is called specific leaf area or SLA). To do this, I dried and weighed all the leaves on each tree and divided this by the LSA. This ratio gives a clearer sense sense than LSA of what the total surface area of all the photosynthetic material if laid out on a flat surface a single cell thick might be. And wouldn’t you know it, this put sugar maple back on top, winning the SLA contest by about almost 30%. Not bad.
What these differences mean
So here’s what we have so far:
- sugar maple produces lots of little leaves
- sugar maple leaves cover less total space than striped maple leaves
- sugar maple leaves are thicker and weigh more per square inch than striped maple leaves
- thicker leaves have more layers of photosynthetic cells than striped maple (these layers are called palisade mesophyll; see image below)
Interestingly, we see the same trend play out within a single tree. The leaves that grow up near the top where they are more exposed to the sun (called sun leaves) are thicker, have a thicker waxy cuticle, and are more deeply cut when compared to leaves that grow lower down in the shade (called shade leaves). This is most pronounced in the oaks (see image above), but can be seen more subtly in other species, including birches, aspens, maples, etc.
Cross section of (a) sun and (b) shade leaves of European beech, Fagus sylvatica (source)
Different life histories
Striped maples and sugar maples are quite different in how they approach life. While both are shade tolerant, sugar maple is long lived (up to 400 years), biding its time in the understory until an opportunity to open up in the canopy before ascending to their full stately stature (Vermont’s biggest sugar maple is about 6’ across, 73’ tall, with a crown spread of 62’; source). Striped maple on the other hand is an inveterate understory tree, rarely getting more than 20’ tall and very rarely more than 6” in diameter. It is unlikely to live more than 100 years.
Striped maple’s large leaves have just a single layer of palisade mesophyll because there’s less light available in the understory to capture. They cast a wide rather than deep net. While each striped maple leaf might have a lower SLA (weight to surface area ratio), their total weight is nearly five times more per leaf (average weight of sugar maple leaves was 9.2g vs 42.9g for striped maple).
Larger leaves are bigger energy investment, which makes each leaf more precious and riskier investment. While the canopy trees certainly create a shady environment, they also create a more stable, protected environment that makes it less likely the leaves will be damaged by frost, wind, or drought. The stable understory makes the cost of larger, energy/nutrient leaves worth the risk.
Green bark
While I didn’t factor this into my calculations, striped maple also has the major advantage of having photosynthetic bark. Our 12′ tall, 1″ diameter tree would add another 6.5 sq ft of surface area just along from the trunk (this doesn’t include all the twigs). This 10% increase in LSA is impressive, but this surface area has the added advantage of being able to photosynthesize not just in the summer but also during the fall and spring when the tree doesn’t have any leaves.
In the end, we have two different trees occupying nearly the same space in the understory but doing so in very different ways. Their lives are on very different trajectories and the adaptations we see in the leaves and bark reflect these differences.
