12 October 2018

Leaf shape as a marker of average annual temperature

Plants in temperate climates tend to have leaves with serrated margins, i.e. they have jagged edges; plants in warmer and more humid climates tend to have what are known in botanical jargon as entire margins, that is, smooth and unserrated. The difference is illustrated in the photographs.

Rather than there being a sharp cut-off between the temperate and tropical styles of leaves, there is a continuous relationship between the climate and the mix of leaf types found in it: that is, as the climate gets a little hotter and wetter, the proportion of entire margins increases a little. This means that looking at a single leaf doesn't tell us that we are looking at a temperate or tropical climate; but
looking at a whole lot of species will allow us to do something a whole lot better than simply dividing climates into tropical or temperate: we can actually estimate the average annual temperature.

We can establish by observation that the ratio of temperate to tropical leaf styles is a surprisingly good indicator of average annual temperature, as illustrated by the graph [right], showing the relationship between floras and temperature in the forests of East Asia.
More at the link (and source credit for the graph).  That article didn't address the question of "why."  I found some related discussion in a Smithsonian article about fossilized leaves:
Scientists are still trying to understand the exact basis for this relationship, but they think it’s because plants in colder climates need to get a jump-start on converting sunlight to energy (photosynthesis) in the spring. Having more teeth enables more water to move out of the leaves, increasing the flow of sap and ramping up photosynthesis. This is important if you need to start photosynthesizing lots of food quickly, say because you only have a brief growing season before the cold comes. If you’re in a warm climate though, jagged edges do more harm than good: losing water can be dangerous to the leaf and to the whole plant, especially when it’s hot. This set of tradeoffs makes one leaf shape more favorable (and thus more predominant) at certain temperatures.


  1. I was thinking of magnolia tree leaves, as well as citrus leaves (by the way, the orange blossom is, in my opinion, the most beautiful scent in creation), and how they inhabit the southern reaches of the 48 contiguous states...which is where our more temperate climate begins to slowly blend into a sub-tropical/tropical climate. Those trees tend to only live in warmer areas, so maybe....

  2. Do you happen to know if leaf size has anything to do with the amount of sunlight a plant prefers? For my houseplants I always use the rule of thumb "large leaves => plant tries to catch as much sunlight as possible, hence is used to an environment where sunlight is sparse, ergo, do not place in direct sunlight", but I realize you could just as well say that about a plant that has small leaves but lots of them. Any ideas?

    1. I've often thought of large leaves and thick canopies as defensive measures (shade the ground around the stem to prevent competition from growing there). Light capture might be relevant - though I think with hosta it's the color of the leaves that indicates happiness with full shade vs part shade.

    2. Ah, I hadn't thought of that, interesting!

  3. The additional surface area (maybe what I would call the "fractal circumference") for serrated leaves seems to be the key, since greater surface area would permit more in and more out of whatever the subject may be.

    I also wonder about the "waxiness" of leaves. It seems that some, like magnolia, are virtually waterproof.

  4. A florist bit-o-wisdom is "the darker the leaf the less light it needs".


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