Begonia's Hawaiian Cousin

Photo by Forest and Kim Starr licensed under CC BY 2.0

Begoniaceae is a strange family. It consists of two genera - Begonia, which comprises roughly 1,400 species, and Hillebrandia, which consists of a single species endemic to Hawai'i (Symbegonia has since been placed back into Begonia). Although I adore the entire family, its that single genus that is the focus of our attention today. Far from being a strange one-off, Hillebrandia has a fascinating evolutionary history.

The sole species, Hillebrandia sandwicensis, is the only member of the family native to Hawai'i. It differs from the genus Begonia in characters such as its petals, which are more numerous and more differentiated, its ovaries, which do not completely close, as well as various morphological characteristics of its fruit and pollen, which I won't go into here. It occurs naturally only on the islands of Kauai, Maui, and Molokai where it inhabits wet ravines in montane rainforest zones. Nowhere is this species considered abundant. 

Since its discovery in 1866, H. sandwicensis has been the object of much curiosity. Where did it originate? How old of a species is it? How did it get to Hawai'i? Thanks to some molecular work, a few of these questions are becoming a bit more clear. For starters, we can now confidently say that Hillebrandia is a sister lineage to all other Begonias. This in turn has provided a crucial step in our understanding of its biogeography.

Photo by John Game licensed under CC BY 2.0

Molecular dating techniques place the genus Hillebrandia at about 51–65 million years old, much older than any of the Hawaiian islands. As such, it is likely that this lineage is not the results of an adaptive radiation like we see in most of the archipelago's flora and fauna. Instead, it is now believed that H. sandwicensis is the only known relict species in Hawaiian flora. In other words, the ancestor of H. sandwicensis did not arrive at Hawai'i and then evolve into the species we know today. Instead, it is more likely that the lineage arose elsewhere and then, through a random long-distance seed dispersal event, made it to Hawai'i's oldest islands some 30 million years ago and has been island hopping to younger islands ever since. 

Although its recent history and geographic origins are still open to much speculation, the story of this unique genus has gotten a bit clearer. Its status as Hawai'i's only known relict plant species is quite exciting to say the least. What we can say for sure is that its history was likely full of serendipity that should be celebrated each time someone has an encounter with this lovely Hawaiian plant.

Photo Credits: [1] [2]

Further Reading: [1]

"The Ghosts of Cultivation Past"

Photograph © Andrew Dunn, 13 September 2005. Website: http://www.andrewdunnphoto.com/ licensed under CC BY-SA 2.0

All too often we think of a species' niche as a sort of address. Species will be present in suitable habitat and absent from unsuitable habitat. Certainly this oversimplification has been useful to us, however, it often ignores context. Species, especially long lived ones, can often be found in unsuitable habitat. Similarly, biotic interactions such as pollinators and seed dispersers are regularly overlooked when considering "suitable habitat." The absence of factors such as this can leave plants stranded in suboptimal conditions. 

A recent paper published in PLOS One tackles this very idea by looking at a species of tree many of us will be familiar with - the honey locust (Gleditsia triacanthos). This central North American legume is widely planted as a street/landscape tree all over the United States. Ecologically speaking, honey locusts can be found growing wild in open xeric upland sites. In places like the southern Appalachian Mountains, however, they can also be found growing in mesic bottomlands. Regardless of where it is found, the honey locust seems to be severely dispersal limited (except in cases where cattle and other livestock have been introduced). 

Before modern times, honey locust likely relied on Pleistocene megafauna to get around. The end of the Pleistocene marked the end of these large mammals. Left behind were many different plant species that had evolved alongside them. For a small handful of these plants, humans were a saving grace. Such is the case for the honey locust. Inside the honey locust pods there is a sugary pulp, which in southern Appalachia, the Cherokee were quite fond of. The Cherokee also used the tree for making weapons and gamesticks. As such, the honey locust holds great cultural significance, so much so that the Cherokee named at least one settlement "Kulsetsiyi" (more commonly known today as Cullasaja), which translates to "honey locust place." 

Author, Dr. Robert Warren, noticed that in southern Appalachia, "Every time I saw a honey locust, I could throw a rock and hit an archaeological site.” What's more, the trees were not recruiting well unless cattle or some other form of human disturbance was present. This species seemed to be a prime candidate for testing persistent legacy effects in tree distributions. 

Using seed germination experiments and lots of mapping, Dr. Warren was able to demonstrate that honey locust distributions in the southern Appalachian region are more closely tied to Cherokee settlements than its own niche requirements. The germination experiments strengthened this correlation by showing that mesic bottomlands had the lowest germination and survival rates. 

Additionally, these sites are well known as former sites of Cherokee settlement and agriculture. Because this tree held such significance to their culture, it is quite likely that in lieu of Pleistocene megafauna, Native Americans, and eventually European livestock, allowed the honey locust to reclaim some of its former glory. Of course, today it is a staple of horticulture. Still, the point is that despite being found growing in a variety of habitat types, the honey locust is very often found in unsuitable habitat where it cannot reproduce without a helping hand. In the southern Appalachian region, honey locust distributions are more a reflection of Native American cultural practices.

Photo Credit: Cambridge Botanic Garden

Further Reading:
http://bit.ly/27SySpq

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.

Leaf Credit: http://www.oaknames.org/

Further Reading:
http://www.sciencedirect.com/science/article/pii/S0378112713006580