Too many chromosomes

Quaking aspen is my favorite tree. In the autumn its leaves change color to a warm yellow, making a perfect contrast against its white and black bark. At sunset the forest glows with the light filtering through the stands of this species.

summit clone

I never stop learning new things about it, either. On a recent hike on Mt. Bigelow in Arizona, I came across two aspen saplings, both growing in a sunny meadow that was heavily disturbed by a recent wildfire. The two trunks were only a meter apart, but one had leaves the size of a coin, and the other, leaves the size of a dinner plate (photo credit: Kristine de Leon).


The high level of trait variation in aspen is something that has always fascinated me, and has been the subject of a few past papers. But this dimorphism was beyond what I had seen before – the closest example I could recall was a clone growing in the shadow of a cabin in Colorado, where the shade leaves were large and the sun leaves were small. But not this big and not this small.

Then I remembered a fact that my friend (and fellow ecologist) Burke Greer once mentioned to me. Aspen is sometimes triploid. This means it can have three copies of each chromosome instead of two, probably an outcome of reproduction between normal haploid gametes with abnormal diploid gametes that failed to separate during meiosis.

Why should that matter? Well, three copies of each chromosome mean a bigger genome, and a bigger genome requires a bigger cell to fit it in. All else being equal, that could cause bigger leaves and larger growth. The relationship between triploidy, leaf size, and trunk size was long-ago recognized in a related European species Populus tremula, and subsequently also found in our North American species Populus tremuloides (e.g. Ynge Melander’s 1938 report from Sweden).

This variation in traits between diploid and triploid individuals presumably has ecological consequences as well: variation in growth rates, water usage, thermal tolerance, and so on. A recent study by Mock et al. was able to show that triploidy is widespread in North American aspens, but more common in colder areas where (presumably) sexual reproduction may be more difficult. Not, in other words, Arizona.

This led me to wonder: if I had a diploid aspen in one hand, and a triploid in the other, with very different performance characteristics, what were they doing growing in the same place? I don’t know. Two answers suggest themselves: first, that one of the two clones may soon die, ending a brief evolutionary experiment; or second, that they actually aren’t diploid and triploid at all. I didn’t have the cytotyping resources on hand to be sure, and didn’t think to bring back leaf samples on my pleasure walk.

So – the mystery will have to wait until the next time someone more curious and better equipped walks that ridgeline!

Orographic lift


Last week the remnants of Hurricane Simon passed through Tucson. The city received just an inch of rain, but the story was different up in the mountains. The summit of Mt. Lemmon, rising almost 7000′ above the city, recorded five inches of precipitation in a single day. The reason is orographic lift – mountains force air to rise, where it cools and loses its water storage capacity, resulting in precipitation. It’s one of the reasons some of the wettest places in the world are on windward mountain slopes.


These mountains are primarily hard rock, with very little soil development. The result is that five inches of rain can’t be buffered underground, leading to surface flows, rockslides, and waterfalls.


One of the consequences of all this rain is a canyon ecosystem full of life, once the flash floods clear out.

canyon 1

Here is some milkweed (Apocynaceae) with a caterpillar on a branch – maybe a queen butterfly (Danaus sp.)?

canyon 2

In five years of living in the desert, I’ve never seen the mountain so dynamic, or shrouded in clouds. It was beautiful.

New paper on the latitudinal diversity gradient

One of best known patterns in ecology is the latitudinal gradient in biodiversity. Near the poles there tend to be fewer species than in the tropics. Here are two examples from my own travels.

First, a moist lowland forest on the Pacific slope of Costa Rica (9°N latitude). In a hectare of forest you can easily find one or two hundred species.


Second, a montane forest in central Norway (63°N latitude). Here a hectare of forest may only have one or two species.


So why this striking difference? This pattern underlies so much of the earth’s biodiversity, and has rightly fascinated ecologists for a century. But satisfying explanations have been lacking. Part of the trouble is that there are too many possible explanations, because most of the options (for example, warmer temperatures, available energy, longer time for evolution) correlate strongly with each other. This makes it hard to falsify any given hypothesis.

I recently had a paper come out that tries to find a way forward. It was just published in PNAS, co-authored with Christine Lamanna, Cyrille Violle, and many other scientists from the BIEN eco-informatics initiative. In the paper, we argue that focusing on species diversity is less useful than focusing on functional diversity. Rather than counting numbers of species, we should be counting the number of ecological strategies available to species. We argue that this shift is useful, because many theories of biodiversities can be naturally expressed in terms of functional variation. So we recast several major theories in terms of functional traits, which are properties of species that reflect their ecological strategies. Then we collected or compiled data for hundreds of forests throughout the New World – surveys of species and of traits. Here are a few photographs from some fieldwork at a site in Costa Rica.

cr 1

cr 2

In the end, what we showed is that none of the major biodiversity theories make predictions that are consistent with observed data. This is a big challenge to our current understandings of the pattern. The biggest issue seems to be understanding why functional diversity peaks where it does. You might imagine that tropical regions can support more species because there are more available strategies – whereas closer to the poles, environments are so extreme that there are fewer viable strategies available to plants. It turns out this isn’t true! We find that temperate regions support more strategies that tropical regions, even though tropical regions have more species. In the below movie, you can see the three-dimensional overlap between temperate and tropical functional traits (made with the hypervolume R package). The temperate region is larger – and you can confirm this quantitatively in Figure 3 of the paper).


We don’t understand why the world works this way yet – but it’s hard to argue with data. I hope explaining this diversity mismatch will help us get just a bit closer to understanding the fundamental and elusive latitudinal diversity gradient.

Hiding under the lichens

seedlings 3

I spent the better part of September inside of a cloud, not moving. Recording data for a seedling census isn’t the most glamorous job, especially in the mountains when the cold wet autumn air inevitably finds its way through all the layers you might be wearing.

seedlings 5

Here is Dovrefjell, home to my friend Kristin Odden Nystuen‘s dissertation research. Her study focuses on understanding ecological change as more willow shrubs begin to encroach on the meadows and heaths of the mountains. A key part of the question is about how seedlings will germinate and grow in changing environments, so off I went to write down numbers and learn Norwegian names for plants.

seedlings 1

Her sites are located above the treeline in these mountains, so every day of field work meant a beautiful hike through mires and forests.

seedlings 2

At higher elevation, the landscapes are dominated by dwarf shrubs (Vaccinium, Empetrum, Betula, and so on), but also by lichens. I had never seen a landscape with such a thick carpet of these organisms – in some cases, ten or twenty centimeters deep. Here you can see the lichen Cladonia stellaris choking out the shrub Betula nana. On a wet day, walking on these landscapes feels like walking on a sponge cake – or at least how I imagine walking on a cake would be like. On a dry day, the lichens dehydrate and crunch underfoot. (I winced every time I crushed dozens of years of growth, but these systems are certainly used to regular trampling from large ungulates.)

seedlings 6

With that much lichen on the ground, I didn’t expect that we would find many seedlings able to grow through. Yet somehow, they were there. Here’s a Dryas octopetala (center) and a Pinus sylvestris (top) doing their best to grow. It will take Kristin another year to find out how well they survive, but I’m impressed they even got this far! Where there are available resources, somehow life always finds a way to use them.

seedlings 4

The story of a mass extinction

dmns 1

PLoS Biology just published my paper on how the meteorite that killed the dinosaurs affected plants. You can read the paper here (Plant Ecological Strategies Shift Across the Cretaceous–Paleogene Boundary), or read an excellent popular summary here (A Plant’s Guide to Surviving the Chicxulub Impact). There’s also media coverage in Newsweek, The Daily Mail, and several other outlets.

Instead of repeating these stories about the science, I want to share a little about how this project came together. It has been a long road. The project was first conceived some time in late 2009, when I was in a first-year graduate school class on ecology, and had the good fortunate to hear a paleobiology lecture from Karl Flessa while also reading on my own about fossilized leaves. I was already interested in learning about plant functioning from leaves, so it wasn’t much of a jump to start thinking about analyzing fossil leaves.

Over the next year or two I was able to refine some ideas, with good support from Brian Enquist and my dissertation committee, and started calling around to different museums asking about their fossil collections. Peter Wilf pointed me to Kirk Johnson at the Denver Museum of Nature and Science, where there existed an exquisite collection of fossils spanning the Cretaceous-Paleogene boundary. Perfect.

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I flew up there for the first time in early 2012, and spent several hours examining the collection. I still remember waiting for two hours for a bus in east Denver on a cold snowy night. It looked good, so I wrote a grant to the Geological Society of America and got some funding to come back a second time. That summer I spent a week at the museum, passing long days in the museum basement, examining every single fossil in the collection, then photographing many of the better-preserved one.

dmns 2

Analyzing the images took the better part of the summer, and then the first manuscript appeared later in the year. It went through dozens of revisions, was joined to another project, then split up again. A bloodbath of Track-Changes in Microsoft Word.

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Then Dana Royer got in touch in 2013. He was interested in the same topic and had already measured some complementary things on the same fossils. We joined our datasets, re-wrote the paper, did a dozen more revisions. We submitted it to Science and made it through one round of review, but were rejected after some pushback on the dataset. We tried again at Nature, where we were rejected without review.

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Some more revision followed, and the paper was next submitted to PLoS Biology. We went through a few rounds of peer-review to address some questions of bias in the dataset, and finally got the paper accepted over the summer. The last few weeks have been filled with correcting proofs, final figures, coordinating popular press articles, and getting in touch with the media.

It’s wonderful to see this project finally finished. The final project is nothing like I had first imagined it, but I have learned so much about paleobiology and met some wonderful scientists along the way. And I’m glad to be done with the endless manuscript revisions!

Where the musk ox live

Winter is coming to Dovrefjell. The high peaks are already covered in snow, and the plants of the lowlands have changed to turn gold and red for autumn. On this landscape the musk ox (Ovibos moschatus) is also preparing for winter.

winter 2

For me the musk ox has always been a mythical sort of animal, a reclusive inhabitant of far-flung northern lands. On this fieldwork trip to Norway I have finally had a chance to see them. They are shaggy brown creatures, difficult to spot at a distance in the heaths and meadows of these glacially-carved mountains. Up close they look like they are wearing a rug, because their long hair sways around their legs as they walk. I can’t imagine what it is like for them in the depths of the snowy winter, but they are still engaged in the important business of eating the season’s plants and lichens.

winter 1

The musk ox has not been in Norway for more than a century. It was re-introduced from a Greenland population in the late 1940s and early 1950s, but perhaps deserves to call this area its home. The species once had a far wider range, reaching in the Pleistocene as far south as New Mexico. Since that time its range had dramatically contracted until this active human intervention. So why has the musk ox disappeared?

Two major hypotheses are climate change (we are no longer in an ice age) and human over-hunting (I have had it in sausage form, and can report it tastes very nice). Anyway, the discussion is very controversial, but a recent paper by Lorenzen et al. has suggested that climate change is a sufficient explanation for the disappearance of this species. There are many areas in today’s world where the animal might be able to live, but which it has not yet re-colonized – one justification for the last century’s reintroduction. Certainly these landscapes feel cold enough to me – while the ice age is long over, there are still glaciers and ice nearby.

winter 3

The existence of this animal here challenges me to think about the shadow of the past, and the roles of climate change and human action in shaping these landscapes. We live in the shadow of a very different earlier world.

BMC Ecology photo contest – landscape ecology

bmc death valley 2

I recently won a photography contest that I forgot I had entered. The image is of an endorheic lake basin at the Racetrack Playa in Death Valley, California. The contest is run through the academic journal, BMC Ecology. They say:

Section Editor Michel Baguette was equally impressed by the variety of ecosystems that the image depicts in this inhospitable part of the world, and the adaptations that species must exhibit in order to survive there:

“I am puzzled by the regular organization of the vegetation, typical of plants growing on a poor soil. I also like the juxtaposition of different ecosystems (desert, lake, mountains).”

It’s a beautiful and stark place. Here’s another image from the rock outcrop in the center of the basin. It is an endless vista of dry mud flats.

bmc death valley 1

You can read about all the other winning entries at the BMC Ecology website – there are some great images and stories!


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