Southern Beeches and Biogeography

If you spend any time learning about paleontology, you are bound to come across at least one reference to the southern beeches (genus Nothofagus). This remarkable and ecologically important group of trees can be found growing throughout the Southern Hemisphere at high latitudes in South America, Australia, New Zealand, New Guinea, and New Caledonia. Not only are they prominent players in the forests in which they grow, their fossil history has provided scientists with invaluable data on plate tectonics and biogeography.

Southern beeches may not be the tallest trees in any forest but that doesn’t mean they aren’t impressive. Numbering around 37 species, southern beeches have conquered a range of climate zones from temperate to tropical. Those living in lowland tropical forests tend to be evergreen, holding onto their leaves throughout the year whereas those living in temperate or montane habitats have evolved a deciduous habit. Some species of southern beech are also known for their longevity, with individuals estimated to be in excess of 500 years in age.

Nothofagus alpina

Anyone familiar with the true beeches (genus Fagus) will quickly recognize many similarities among these genera. From their toothy leaves to their triangular nuts, these trees are strikingly similar in appearance. Indeed, for much of their botanical history, southern beeches were included in the beech family (Fagaceae). However, recent molecular work has revealed that the southern beeches are genetically distinct enough to warrant their own family - Nothofagaceae.

The beech-like fruits of Nothofagus obliqua var. macrocarpa

As mentioned, the southern beeches, both extant and extinct, have been important players in our understanding of plate tectonics. Their modern day distribution throughout the Southern Hemisphere seems to hint at a more concentrated distribution at some point in the past. All of the continents and islands on which they are found today were once part of the supercontinent of Gondwana, which has led many to suggest that the southern beech family arose before Gondwana broke apart during the Jurassic, with ancestors of extant species riding the southern land masses to their modern day positions. Indeed, the paleo record seems to support this quite well.

Fall colors of Nothofagus cunninghamii.

The southern beeches have an impressive fossil record that dates back some 80 million years to the late Cretaceous. Their fossils have been found throughout many of the Southern Hemisphere continents including the now-frozen Antarctica. It would seem that the modern distribution of these trees is the result of plate tectonics rather than the movement of seeds across oceans. This is bolstered by lines of evidence such as seed dispersal. Southern beech nuts are fairly large and do not show any adaptations for long distance dispersal, leading many to suggest that they simply cannot ocean hop without serious help from other forms of life.

Nothofagus fusca

However, life is rarely so simple. Recent molecular work suggests that continental drift can’t explain the distribution of every southern beech species. By studying trees growing in New Zealand and comparing them to those growing in Australia and Tasmania, scientists have discovered that these lineages are far too young to have originated before the breakup of Gondwana. As such, the southern beeches of Austrialasia more likely got to their current distributions via long distance dispersal events. Exactly what allowed their seeds to cross the Tasman Sea is up for debate, but certainly not impossible given the expanse of time available for rare events to occur. Regardless of where anyone stands on this recent evidence, it nonetheless suggests that the biogeographic history of the southern beech family isn’t as clear cut as once thought.

Nothofagus fusca

Unfortunately, while southern beeches hold a prominent place in the minds of naturalists, the same cannot be said for the rest of the world. Little care has been given to their scientific and ecological importance and massive quantities of these trees are logged each and every year. Today it is estimated that 30% of all southern beech species are threatened with extinction. Luckily, large portions of the remaining populations for these trees are growing on protected lands. Also, because of their scientific importance, numerous southern beeches can be found growing in botanical collections and their seeds are well represented in seed banks. Still, southern beeches and the forests they comprise are worthy of respect and protection.

Photo Credits: [1] [2] [3] [4] [5] [6]

Further Reading: [1] [2] [3]

History of Grass Evolution Written in Dinosaur Poop

Photo by Sugeesh licensed by CC BY-SA 3.0

Grasses dominate our planet today but that has not always been the case. Because of both their ecological and cultural importance , the origin and diversification of grasses has long been a hot topic in biology. We know that grasses really hit their stride following the extinction of the dinosaurs, and that they changed herbivore anatomy in a big way, but their origins remain shrouded in mystery. Recently, a discovery made in fossilized dinosaur poop has shone a surprisingly bright light into the history of grasses on our planet.

Prior to this discovery, the earliest evidence of grasses came in the form of fossilized pollen and tiny pieces of silica called phytoliths. Phytoliths are essentially tiny pieces of glass that serve as a form of defense against herbivores. Because they are made of silica and fossilize well, phytoliths turn up frequently in the fossil record. This makes them extremely useful for finding evidence of grasses even where whole-plant fossilization is unlikely.

Illustration by Nobu Tamura (http://spinops.blogspot.com) licensed by CC BY-NC-ND 3.0

Whereas phytoliths are not unique to grasses, their form is often taxon-specific. With a good eye and a bit of training, one can look at a phytolith under a microscope and tell you what type of plant it came from. This is where the dinosaur poop comes into the picture. By examining fossilized dinosaur poop from India, paleontologists can get an idea of what dinosaurs were eating.

By examining the fossilized poop of a group of large herbivorous dinosaurs called Titanosaurs, paleontologists now have a better idea of grass diversity in the late Cretaceous. They have uncovered a surprising diversity of phytoliths, which demonstrate that at least 5 distinct grass taxa that we would recognize today were alive and well some 100.5 to 66 million years ago. These include extant groups like Oryzoideae (think rice and bamboo), Puelioideae, and Pooideae (think wheat, barley, oat, rye, and many lawn and pasture grasses). There were other lesser known lineages mixed in there as well.

Fossilized dinosaur poop or “coprolite.” USGS Public Domain

These findings are exciting for a variety of reasons. For one, it tells us that despite lacking teeth specialized for eating grasses, large herbivorous dinosaurs like the Titanosaurs were nonetheless incorporating these plants into their diet. It also tells us that grasses were already quite diverse by the late Cretaceous. The fact that modern clades of grass were around back then sets back grass evolution many millions of years. It also tells us something about grass biogeography. It suggests that grasses were already wide spread across the supercontinent of Gondwana long before India broke away. Finally, it tells us that grasses evolved silicate phytoliths long before more recognizable grass-eating herbivores came onto the scene.

I am always blown away by the details paleontologists are able to extract from such tiny fossils. Who knew dinosaur poop could tell us so much?

Photo Credits: [1] [2] [3] [4]

Further Reading: [1]

Meeting Amborella trichopoda

When I found out I would be seeing a living Amborella, a lump formed in my throat. There I was standing in one of the tropical houses at the Atlanta Botanical Garden trying to keep my cool. No amount of patience was ample enough to quell my excitement. How was I going to react? How big were these plants? Would I see flowers? Could I touch them? What were they growing in? My curiosity was through the roof.

Naturally this sort of excitement is reserved for those of us familiar with Amborella trichopoda. This strange shrub is not something that would readily stand out against a backdrop of tropical flora. However, if life history and ecology were to be translated into outward appearances, Amborella would likely be one of the most gaudy plants on this planet. What I was about the lay eyes on is the only member of the sole genus belonging to the family Amborellaceae, which is the sole member of the order Amborellales.

Even more exciting is its position on the angiosperm family tree. As flowering plants go, Amborella is thought to be the oldest alive today. Okay, so maybe this shrub isn't the oldest flowering plant in the world. It is likely that at one time, many millions of years ago, there were more representatives of Amborellaceae growing on this planet. Until we turn up more fossil evidence it is nearly impossible to say. Still, Amborella's place in the story of flowering plant evolution is consistently located at the base.

That is not to say that this shrub is by any means primitive. I think the first thing that shocked me about these plants is just how "normal" they appear. Sans flowers, I didn't see much out of the ordinary about them. They certainly look like they belong on our timeline. Without proper training in plant anatomy and physiology, there is little one could deduce about their evolutionary position. Regardless of my ignorance on plant morphology, there is plenty to look at on Amborella.

For starters, Amborella has tracheids but no vessel elements, making its vascular system more like that of a gymnosperm than an angiosperm. Its small flowers are borne in the axils of the evergreen leaves. It has no petals, only bracts arranged into a spiral of tepals. The female flowers consist of 4 to 8 free carpels and do not produce a style. Male flowers look like nothing more than a spiral cluster of stamens borne on short filaments.

If plant anatomy isn't enough to convince you, then the genetic analyses tell a much more compelling story. DNA sequencing consistently places Amborella at the base of the flowering plant family tree. Again, this is not to say that this shrub is by any means "primitive" but rather its lineage diverged long before what we would readily recognize as a flowering plant evolved. As such, Amborella offers us a window into the early days of flowering plants. By comparing traits present in more derived angiosperms to those of Amborella, researchers are able to better understand how the most dominant group of plants found their place in this world.

Another interesting thing happened when researchers looked at the DNA of Amborella. What they found was more than just Amborella genes. Inside the mitochondrial DNA are an unprecedented amount of foreign DNA from algae, lichens and mosses. In fact, an entire chunk of DNA corresponded to an entire mitochondrial genome of a moss! Researchers now believe that this is a case of extreme horizontal gene transfer between Amborella and its neighbors both growing on and around it. Both in the wild and in cultivation, Amborella is covered in a sort of "biofilm." Whether or not such gene transfer has assisted in the conservatism of this lineage over time remains to be seen.

At this point you may be asking how this lineage has persisted for over 130 million years. For the most part, it is probably due to chance. However, there is one aspect of its ecology that really stands out in this debate and that is its geographic distribution. Amborella is endemic to Grande Terre, the main island of New Caledonia. This is a very special place for biodiversity.

New Caledonia is a small fragment of the once great super-continent Gondwana. New Caledonia, which was part of Australia at that time, broke away from Gondwana when the super-continent began to break up some 200-180 million years ago. New Caledonia then broke away from Australia some 66 million years ago and has not been connected to another land mass since. A warm, stable climate has allowed some of the most unique flora and fauna to persist for all that time. Amborella is but one of the myriad endemic plants that call New Caledonia home. For instance, 43 species of tropical conifers that grow on these small islands are found nowhere else in the world. The whole region is a refugia of a long lost world.

Being a biodiversity hot spot has not spared New Caledonia from the threats of modern man. Mining, agriculture, urbanization, and climate change are all threatening to undo much of what makes this place so unique. The loss of a species like Amborella would be a serious blow to biodiversity, conservation, and the world as whole. We cannot allow this species to exist only in cultivation. New Caledonia is one place we must desperately try to conserve. Meeting this species has left a mark on me. Being able to observe living Amborella up close and personal is something I will never forget as my chances of seeing this species in the wild are quite slim. I am so happy to know that places like the Atlanta Botanical Garden are committed to understanding and conserving this species both in the wild and in cultivation. For now Amborella is here to stay. Long may it be that way.

Further Reading:

http://bit.ly/29MuMuw

http://bit.ly/29MuML0

http://bit.ly/29ZKNJS