A Temperate Deciduous Monster--Too Big?

Question

What is a flower or shrub back home is a tree in an alternate Earth, and vice versa. That is something worth exploring.

For example, Taraxacum officinale can grow from stems typically up to 5-40 centimeters tall, but sometimes up to 70 centimeters back home. In this alternate Earth, it grows from trunks up to 5-40 meters tall, sometimes 70. Bellis, the daisy genus, grows from 5-20 centimeters (2-8 inches) back home, whereas it grows from 5-20 meters (16-65.6 feet) in this alternate Earth. Dahlia, on this alternate Earth, grows from 6-30 meters (19.7-98.4 feet) tall. Gaillardia, the blanket flower, reaches a maximum height around 80 meters (262 feet!)

This all sounds amazing, but there is a snag, pun intended--such massive heights back home are usually found in rainforests, tropical or temperate. In temperate rainforests, it is usually the conifers that reach such heights. In an ordinary temperate forest, like in New York, Maine, France, Germany or even Poland's famous Białowieża Forest, can broad-leaved angiosperms reach such monstrous heights? Or would certain environmental factors make this unlikely?

Answer 1

No, you can't just scale up a daisy.

Or.

The Square-Cube Law strikes again.

The Square-Cube Law says that as you increase the size of a thing, it's volume grows faster than its surface area. Specifically, the volume is cubed while the surface area is squared.

Think about a daisy as a basically a big tube, a cylinder. Let's say 10 cm high and 1 cm in radius. That has a volume of 31 $cm^3$ and a surface area of 69 $cm^2$. Now double its size: 20 cm high and 2 cm radius. That has a volume of 251 $cm^3$ and a surface area of 276 $cm^2$.

Notice something? The ratio of the surface area to volume dropped. It went from $\frac{69}{31} = 2.2$ to $\frac{276}{251} = 1.1$. If we double the size again to 40 cm by 4 cm, it drops again to $\frac{1106}{2011} = 0.55$. Every time you double the size you halve the ratio of surface area to volume. At 10 m high and 1 m around (or 1000 cm x 100 cm) you're down to $\frac{691150}{31400000} = 0.022$. 100 times taller, $\frac{1}{100}$th the surface area-to-volume ratio.

Ok, so what? A lot of biology depends on this ratio is high. For example, photosynthesis depends on the surface area of the leaves. A 10 cm high daisy has plenty of surface area to fuel its volume. But a 10 m high daisy now has to fuel 1,000,000 times more daisy (volume) with only 10,000 times more photosynthesis (surface area). It will starve. Large plants have developed more and smaller leaves (smaller in comparison to their size) to increase their surface area. This is why you'll see small plants with a few large leaves, but large plants like trees with many, many small leaves.

Simple circulatory systems also depend on a high surface-to-volume ratio. Rather than having a complex system that delivers food, carbon dioxide, and nutrients directly to the cell, many depend on simple diffusion to get these molecules from outside the plant to the innermost cells. Or from a plant's vascular tissue to every cell. This works in a small plant with relatively few layers of small cells. But when you scale up the distances become too great for diffusion to be effective and the cells further from the surface of the plant will suffocate and starve. A more complex vascular system is necessary. Or, in the case of trees, reducing your volume of living cells by allowing your interior cells to die and form heartwood.

Then there's pumping water and nutrients up from the soil to the top of the plant. In a small plant this is easy and requires little specialized biology, the distance is small, the tubes are narrow, so capillary action will handle it. But at 10 m tall this is not enough. Specialized mechanisms for getting water to flow that high are necessary which trees have but daisies do not.

Structurally as well, a daisy will collapse under its own weight. Trees and woody plants differentiate their trunk into bark for strength and protection, phloem for transporting nutrients, xylem for transporting water, and the heartwood made of old mineralized xylem cells to protect the center of the tree and provide more strength. A daisy lacks these specialized structures, or they're not so well developed. At its size, it does not need them. Much of its strength comes from water pressure, this is why small plants droop when they aren't watered, but trees do not.

And so on. In order for this to work your giant daisies have to have evolved to do all the things trees do. Then they wouldn't look much like daisies, they'd look like trees.

You can worry about all that for your story. Or you can just decide it's cool and hang a lantern on it like Atomic Robo does with equally implausible giant insects.

Answer 2

It is possible, except maybe for that supertall one.

/In an ordinary temperate forest, like in New York, Maine, France, Germany or even Poland's famous Białowieża Forest, can broad-leaved angiosperms reach such monstrous heights?/

Oaks and alder are broad-leafed angiosperms which are in temperate rainforests. Species in each can grow to 30meters+. That covers your examples except for the 80m+ one.

Question 2: could nonwoody angiosperms attain tree-like heights? I will assert yes, because of bamboo. Bamboo is a grass, and so an angiosperm. Some bamboo species can reach 30 meters+. Plants can use lignin to stabilize cellulose into rigid, flexible structures capable of supporting a plant to height. Technically bamboo is not wood, but a different evolutionary solution to the same problem of developing structural materials using lignin and cellulose. If bamboo can do it and trees can do it, other angiosperms should be able to do it. Lignin is evolutionarily ancient and all plants have it. Ancient plants which were neither angiosperms nor gymnosperms were able to attain heights of 30+meters, presumably with structural solutions that differed both from wood and bamboo.

I do not know why the very tallest trees are gymnosperms. I do not think it is known.

Predicting why I will get the downvote - my answer is not limited to rainforests. I would argue that tall plants do not necessitate rainforests. There are very tall eucalypts which are not in rainforests. The giant sequoia is not in a rainforest.

Answer 3

I don't see any mention of Scalesia yet, so here's the link to the wikipedia page. TL;DR Scalesia is a daisy-tree. It's a member of asteracea (daisies, dandelions, sunflowers, etc.) that evolved on Galapagos. As Schwern predicted, they look like trees. The largest are 20 meters tall.