Tropical Oaks - Lessons in Biogeography from a Giant Acorn


Seeing the nut of Quercus insignis in person for the first time was a peculiar experience. I didn't know acorns came that big! What was even stranger was encountering this species in the tropics. I thought that in leaving my temperate home behind, I had left trees such as oaks behind as well. Thus, picking up this gigantic acorn was a challenge to my ignorance of tropical forest diversity. What it did for me was ignite a fury of questions regarding the biogeography of the genus Quercus.

Quercus insignis is native from Mexico to Panama. It is a member of the white oak grouping and, despite having one of the largest acorns of any oak species, relatively little is known about this species. What we do know is that it is in trouble. It is considered critically endangered in Mexico and near threatened in Guatemala and Panama with a remaining stronghold in Nicaragua. Habitat loss and changing environmental conditions seem to be at the core of its disappearance.

One big question was looming over me. What was an oak doing this far south? Call it a northern bias but I have always associated oaks with more temperate climes. I needed to get over this. My investigation lead me to some very interesting work done on the family to which oaks belong - Fagaceae. Based on some incredible paleontological and genetic detective work, we now know that Fagaceae originated in Asia. The first fossil evidence of a member of this family dates back some 100 million years, during the early part of the Cretaceous.

At this time, the continents of Asia, Europe, and North America were still connected. Some 60 million years ago, the genus Quercus diverged from Castanea. They were also starting to radiate across the Northern Hemisphere. The first fossil evidence of oaks in North America comes from Paleogene deposits dated to 55 to 50 million years before present. This is when the oaks really started to hit their stride.

Between 22 and 3 million years ago the genus Quercus underwent numerous speciation events. The new terrain of North America must have presented countless opportunities for oaks because they quickly became the most specious genus of all the Fagaceae. This radiation was particularly fruitful in what would become the U.S. and Mexico. Of the roughly 220 species that exist in this region today, 160 occur in Mexico, and of those, 90 species are endemic.

This brings us to the tropics. Evergreen and semi-evergreen oaks have done quite well in this region. However, their astounding diversity quickly drops once you hit the isthmus of Panama. South America is home to only one species of oak. What happened that limited the oaks reign south of the equator?

To put it simply, geology happened. For much of the Earth's history, North and South America shared no connection. Though the exact time frame is debated, tectonic forces joined the two continents some 4.5 million years ago. The Great American Interchange had begun. The two continents were able to freely exchange flora and fauna like never before. The migrations are thought to have been a bit lopsided. Tropical flora and fauna did not do as well farther north but temperate flora and fauna seemed to find warmer climes more favorable. As such, South America gained disproportionately more biodiversity as a result.

This pattern did not hold true for everything though. For the oaks, only one species (Quercus humboldtii) made it through. As such, the genus remains a dominant fixture of the Northern Hemisphere. Sadly, much of this diversity is at serious risk of being lost forever. Like the magnificent Quercus insignis, many of the world's oaks are on the decline. Disease, habitat loss, and countless other issues plague this genus. A 100 million year old journey is quickly being undone in less than two centuries. The hand of man is time and again proving to be a force unrivaled in the biological world.

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Conifer Leaf Drop

It's that time of year when evergreen trees become quite apparent. The most obvious are the conifers. These trees hold steady while everything else seems to be in a mad rush for winter. Despite the term "evergreen" the conifers are nonetheless preparing for winter as well, though on a much more subtle level. Anyone paying close attention will see some color changes happening to them too. Despite the designation as "evergreen" the conifers do shed leaves.

Timescales are everything for us humans. We tend to notice things that happen relatively fast, like an entire forest turning color in only a few weeks. The conifers have adopted a strategy that isn't as in tune with our perception. Conifers, for the most part, specialize in harsh habitats. Excelling in poor soils and extreme cold, they tend to invest in the long term. Needles are one such adaptation. Their minimal surface area and structural integrity make up for their costly production in nutrient poor conditions. When a conifer produces needles, they need to last for a while.

And that is exactly what they do. The average conifer needle has a lifetime of roughly 2 years (with some exceptions of course). It doesn't make sense for them to bank on a whole new set leaves every year. Because of the way they grow, conifers usually shed their leaves from the inside out. New leaves are produced at the tips of branches and, as older leaves get shaded out, conifers cut their losses and drop them. If you take a close look at conifers during the fall, this pattern becomes readily apparent.

Leaf drop doesn't always happen quickly either. They are often spaced out over time. One of the reasons I like plants so much is that they operate on vastly different timescales than we do. As you become more and more familiar with different species, plants can teach you to start looking at things a bit different than you are used to. Get outside and find some needle dropping conifers of your own.

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