Why are there so few tree species in Europe?

Photo by Susulyka licensed under CC BY-SA 4.0

Take a look at a list of tree species from temperate Europe, North America, and Asia and you will notice a glaring disparity. Whereas North America and Asia are home to something like 1000 tree species each, Europe is home to just about 500 species. Why is this?

The answer may lie partly in the glacial history of the Northern Hemisphere as well as in some quirks of geology. Starting in the late Pliocene, roughly 3 million years ago, the Earth began to cool. As our planet entered into a epoch dominated by massive, continent-wide glaciers, life was responding accordingly.

Historically it was assumed that Europe lost many of its temperate tree species thanks to the east-west orientation of its mountain ranges. As glaciers advanced from the north, species were pushed farther and farther south until they hit physical barriers in the terrain like the Alps. With nowhere to go but up, many species that couldn’t handle either the rate of climate change or the altitude adjustment simply winked out of existence. Fossil evidence from Europe provides plenty of evidence that this region was once home to far more tree species, including relatives of sweetgum (Liquidambar spp.) and tulip trees (Liriodendron spp.) that are still present in North America, and umbrella pines (Sciadopitys spp.), which still exists in Asia. Many temperate tree species in North America and Asia were spared this fate because there were far fewer barriers to successful southern migrations.

This all sounds a bit too simple and indeed, recent studies suggest that it is. Though climate change, glaciers, and mountains certainly played a role in the differential extinction rates of European trees, the story is a bit more complicated than that. It turns out that the European mountain ranges don’t present as impenetrable of a barrier to plant migrations as was once thought. The fact that southern Europe and northern Africa share many similar taxa is proof of this. Instead, the amount of suitable habitat and land area available to trees migrating down from northern Europe may have played an even larger role in the extinction rate of European trees.

Extent of glacial coverage (blue) during the last ice age. Map by Hannse Grobe licensed under CC-BY-2.5

It is a well documented phenomenon in ecology that smaller areas of land support smaller numbers of species. This is the case for Pleistocene Europe. Suitable habitat for temperate tree species during this time would have largely consisted of three peninsulas (Iberia, Italy, and the Balkans) separated by the Mediterranian Sea. Each of these peninsulas boast mountain chains that would have offered small bands of suitable microclimates for temperate tree species to find refuge during glacial advance.

Pushed into tiny pockets of refugia, Europe’s temperate tree species would have been more vulnerable to extinction than tree species in North America and Asia, which had far more suitable habitat available to them in the southern portions of those continents. By looking at which taxa survived and which went extinct, patterns do start to emerge. Tree species that are widespread in Europe today are descendants of trees that were far more tolerant of cooler growing seasons and harsh winters than genera that went extinct. This likely reflects the fact that their ancestors were those species that found refuge high up in the mountains.

Alternatively, present-day Europe also boasts small pockets of what are termed “relictual genera,” which is a fancy way of saying species that were once more common in the past than they are today. These so-called relictual taxa have been found to be far more tolerant of drought than genera that went extinct. This likely reflects the fact that their ancestors found refuge in warmer, low-elevation habitats in southern Europe.

It appears that species on either end of the tolerance curves were the ones that won out in Europe’s extinction lottery. By tolerating either extreme cold or extreme drought, “stress tolerators” were able to not only survive repeated glaciation events, but also provide seed sources for those lineages following glacial retreat.

Only the species that were able to find suitable habitats in southern Europe’s glacial refugia were the ones that were able to recolonize the region after the Ice Age had ended. At this point in time, these are some of the best pieces of evidence we have in explaining the disparity in tree diversity between Europe, North America, and Asia. What’s more, I find disturbing trends in such extinctions because it wasn’t like the glaciers always wiped out species immediately. Instead, many species were able to survive glaciation but were pushed into smaller and smaller pockets of suitable habitat until relatively small disturbances pushed them over the edge.

Today, we humans are changing Earth’s climates at a rate that hasn’t been seen in over 50 million years and all the while we are fragmenting habitats more and more. What is going to happen to species living today in these tiny pockets?

Photo Credits: [1] [2]

Further Reading: [1] [2] [3] [4]

Germinating a Seed After 32,000 Years

[SOURCE]

What you are looking at are plants that were grown from seeds buried in permafrost for nearly 32,000 years. The seeds were discovered on the banks of the Kolyma River in Siberia. The river is constantly eroding into the permafrost and uncovering frozen Pleistocene relics. Upon their discovery, researchers took the seeds and did the unthinkable - they grew them into adult plants. To date, this is the oldest resurrected plant material. 

The key to their extreme longevity lies in the permafrost. They were found inside the frozen burrow of an Arctic ground squirrel. The state of the burrow suggests that everything froze quite rapidly. As such, the seeds remained in a state of suspended animation for 32,000 years. This is not the first time viable plant materials have been recovered from Pleistocene permafrost. Spores, mosses, as well as seeds of other flowering plants have been rejuvenated to some degree in the past but none of these were grown to maturity. 

Using micropropagation techniques coupled with tissue cultures, researchers were able to grow and flower the 32,000 year old seeds. What they discovered was that these seeds belonged to a plant that can still be found in the Arctic today. It is a small species in the family Caryophyllaceae called Silene stenophylla. However, there were some interesting differences. 

As it turns out, the seeds taken from the burrow proved to be a phenotype quite distinct from extant S. stenophylla populations. For instance, their flowers were thinner and less dissected than extant populations. Also, whereas the flowers of extant populations are all bisexual, individuals grown from the ancient seeds first produced only female flowers followed by fewer bisexual flowers towards the end of their blooming period.Though there are many possible reasons for this, it certainly hints at the different environmental parameters faced by this species through time. What's more, such findings allow us a unique window into the world of seed dormancy. Researchers are now looking at such cases to better inform how we can preserve seeds for longer periods of time. 

Photo Credit: Svetlana Yashinaa, Stanislav Gubin, Stanislav Maksimovich, Alexandra Yashina, Edith Gakhova, and David Gilichinsky

Further Reading: [1]

Osage Orange

As a kid I used to get a kick out of a couple trees without ever giving any thought towards what it was. My friend's neighbor had a some Osage orange trees (Maclura pomifera) growing at the end of his driveway. Their houses were situated atop a large hill and the road was pretty much a straight drop down into a small river valley. After school on fall afternoons, we would hang out in my friends front yard and watch as the large "hedge apples" would fall from the tree, bounce off the hood of his neighbor's car (why he insisted on parking there is beyond me) and go rolling down the hill. I never would have guessed that almost two decades later the Osage orange would bring intrigue into my life yet again. This time, however, it would be because of the evolutionary conundrum it presents to those interested in a paleontological mystery...

The fruit of this tree are strange. They are about the size of a softball, they are green and wrinkly, and their insides are filled with small seeds encased in a rather fibrous pulp that oozes with slightly toxic white sap. No wild animal alive today regularly nibbles on these fruits besides the occasional squirrel and certainly none can swallow one whole. Why then would the tree go through so much energy to produce them when all they do anymore is fall off and rot on the ground? The answer lies in the recently extinct Pleistocene megafauna. 

The tree is named after the Osage tribe who used to travel great distances to the only known natural range of this tree in order to gather wood from it for making arrows. It only grew in a small range within the Red River region of Texas. When settlers made it to this continent, they too utilized this tree for things like hedgerows and natural fences. 

What is even stranger is that recent fossil evidence shows that Maclura once had a much greater distribution. Fossils have been found all the way up into Ontario, Canada. In fact, it is believed that there were once 7 different species of Maclura. It was quickly realized that this tree did quite well far outside of its current natural range. Why then was it so limited in distribution? Without the Pleistocene megafauna to distribute seeds, the tree had to rely on flood events to carry the large fruit any great distance. With a little luck, a few seeds would be able to germinate out of the rotting pulp. Botanists agree that the Red River region was a the last stronghold for this once wide ranging species until modern man came on the scene. 

Another clue comes from the toxicity of the fruits. Small animals cannot eat much of it without being poisoned. This makes sense if you are a Maclura relying on large animals as dispersers. You would want to arm your fruit just enough to discourage little, inefficient fruit thieves from making a wasteful meal out of your reproductive effort. However, by limiting the amount of toxins produced in the fruit, Maclura was still able to rely on large bodied animals that can eat a lot more fruit without getting poisoned. Today, with the introduction of domesticated megafauna such as horses and cows, we can once again observe how well these fruits perform in the presence of large mammals. 

Finally, for anyone familiar with Maclura, you will notice that the tree is armed with large spines. Why the heck does a large tree need to arm itself so extravagantly all the way to the top? Again, if you need things like mammoths or giant ground sloths to disperse your seeds, you may want to take some extra precautions to make sure they aren't snacking on you as well. It takes energy to produce spines so it is reasonable to assume that the tree would not go through so much trouble to protect even its crown if there once wasn't animals large enough to reach that high. The Pleistocene megafauna went extinct in what is evolutionarily speaking only the blink of an eye. Trees like the Osage orange have not had time to adapt accordingly. As such, without the helping hand of humans, this tree would still be hanging on to a mere fraction of its former range down in the Red River region of Texas.

Further Reading:
http://plants.usda.gov/core/profile?symbol=MAPO

http://www.americanforests.org/magazine/article/trees-that-miss-the-mammoths/

http://www.plosone.org/article/info:doi%2F10.1371%2Fjournal.pone.0001745

Southern Tundra

One would hardly consider the southern half of North America to be a tundra-like environment but even so, some tundra plants exist there today...

Up until about 11,000 years ago, much of North America was covered in massive glaciers that were, in some places, upwards of a mile thick. These colossal ice sheets scoured the land over millennia as they advanced and retreated throughout the Pleistocene. Where they covered the land, nothing except some mosses survived. A vast majority of plants were either wiped out or were forced to survive in what are referred to as glacial refugia.

Refugia are ice free areas either within the range of the ice sheets, such as mountain tops, or areas just outside of the ice sheets. Many of North America's plant species took refuge to the south of the glaciers in what is now the Appalachian Mountains. Echos of these plant communities still exist in the southern US today. Some of which are quite isolated from the current distribution of their species. These plant communities are considered disjunct and coming across them is like seeing back in time.

One such plant is the three-toothed cinquefoil (Sibbaldiopsis tridentata). This species is mainly found in northern Canada and Greenland and is considered a tundra species. It needs cold temperatures and is easily out competed in all but the most hostile environments. Why then can you find this lovely cinquefoil growing as far south as Georgia?

The answer are mountains. A combination of high elevation, punishing winds, and lower than average temperatures, means that the peaks of the Appalachian Mountains have more in common with the tundras found much farther north on the continent. As a result of these conditions, plants like S. tridentata have been able to survive into the present while the majority of their tundra associates migrated north with the retreat of the glaciers.

Because of their isolated existence in the Appalachians, S. tridentata is considered endangered in many southern states. Being able to see this plant without having to visit the tundra is quite a unique and humbling experience. It is amazing to consider the series of events that, over thousands of years, have caused this species to end up living on top of these mountains. It is one of those things that one must really stop and mull over for a bit in order to fully appreciate.

Further Reading:
http://plants.usda.gov/core/profile?symbol=sitr3

http://onlinelibrary.wiley.com/…/j.1365-2699.1998.…/abstract

http://www.castaneajournal.org/doi/abs/10.2179/10-039.1

http://instaar.colorado.edu/AW/abstract_details.php?abstract_id=16

A Temporary Inland Sea in Northeastern North America

There are many species of small, nondescript spurge out there. All too often they go completely unnoticed, even by plant lovers like myself. As I have come to learn time and time again, every species has an interesting story to tell. That is why I started In Defense of Plants in the first place. The story I want to tell you today came to me from a chance encounter I had while exploring a beach on Lake Erie. I was musing over some tumbleweed I had found when I noticed some small spurge barely poking out of the sand around me. I took some pictures and moved on. Had I realized what I would come to learn from this spurge, I probably would have spent more time admiring it.

Our story begins roughly 18,000 years ago during the height of the last glacial period. Much of northern North America was buried under a massive glacial ice sheet. This was unlike anything we can witness on the continent today. In some spots the ice was well over a mile thick. The weight of that much ice on the land caused the bedrock underneath to compress, not unlike a mattress compresses under the weight of a human body. This compression pushed much of northeastern North America lower than sea level. Unlike a mattress, however, rock can take a very long time to rebound after the weight has been lifted. Around 13,000 years ago when the glaciers began to retreat, the land was still compressed below sea level. 

Map via Orbitale licensed under CC BY-SA 3.0

With the ice gone, the ocean quickly rushed in to fill what is now the St. Lawrence and Ottawa River valleys as well as Lake Champlain. A salty inland lake coined the Champlain Sea was the result of this influx of ocean water. For some time, the Champlain Sea provided seemingly out of place maritime habitat until isostatic rebound caused the land to rise enough to drain it some 10,000 years ago. During this period, the Champlain Sea was home to animals typically seen in the northern Atlantic today including whales, whose fossils have been found in parts of Montreal and Ottawa. Coastal plant communities formed along the shores of the Champlain Sea, which brings me back to my little spurge friend. 

Inland beach pea (Lathyrus japonicus). Photo by Alastair Rae licensed under CC BY-SA 2.0

Sea rocket (Cakile edentula)

The species in question is Chamaesyce polygonifolia, the seaside spurge. By no means rare, this obscure little plant is more typically found along the coast of the Atlantic. Along with other species like the inland beach pea (Lathyrus japonicus) and sea rocket (Cakile edentula), these plants followed the shores of the Champlain Sea and remained here in sandy, disturbed habitats ever since. These species are echoes of a brief period of time when North America was going through a lot of changes. Again, had I known this at the time, I don't know if I would have left the beach so quickly that day. I love to be reminded of how small we really are, how fleeting our existence really is. I love meeting species that are players in a much bigger story and Chamaesyce polygonifolia and company are just that. 

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

Echoes of a Glacial Past

Climate change is often talked about in the context of direct effects on species. However, as John Muir so eloquently put it, "When we try to pick out anything by itself, we find it hitched to everything else in the Universe." In essence, nothing is ever black and white and the research I am writing about today illustrates this fact quite well.

Ants and plants have some very intricate interactions. A multitude of plant species rely on ants as their seed dispersers. Many of these plant species are spring ephemerals that take advantage of the fact that there is little else for ants to eat in the early spring by attaching fatty capsules to their seeds that are very attractive to foraging ant species. We refer to seed dispersal by ants as “myrmecochory.”

There are two big players in the foraging ant communities of eastern North America, the warm adapted Aphaenogaster rudis and the cold adapted Aphaenogaster picea. The cold adapted A. picea emerges from winter dormancy early in the spring while the warm adapted species emerges from dormancy much later in the spring. In the southern portions of their range, A. rudis outcompetes A. picea.

What is the big deal? Well, the researchers looked at two plant species that rely on these ants for seed dispersal, Hepatica nobilis and Hexastylis arifolia. Hepatica nobilis sets seed early in the spring, relying on ant species like A. picea to disperse its seed whereas Hexastylis arifolia sets seed late in spring, which is prime time for A. rudis. Researchers noticed that, in the southern portions of their range where A. picea had been displaced, Hepatica has a very clumped and patchy growth habit where farther north it did not. Hexastylis on the other hand seemed to have a more normal growth pattern in the south.

By performing some transplanting experiments and examining foraging and seed dispersal, they found that the absence of A. picea in the south spelled ecological disaster for Hepatica. It continues to set seed but because A. rudis emerges long after seed set, it is not filling the gap left by the missing A. picea. Hexastylis, which only grows in the south and sets seed much later, does just fine with the warm adapted A. rudis. Farther north where A. picea still rules, Hepatica has no trouble with seed dispersal but Hexastylis drops out of the ecosystem entirely. In essence, because of warming climate trends since the end of the Pleistocene, Hepatica is falling out of sync with its mutualistic ant partner in the southern portions of its range and, in time, may become extirpated.

Further Reading: [1]