A walk into the past

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The New Forest is not very new any longer. King William I established it in 1079 C.E. and it has persisted in southern England through the present day. I had the pleasure of walking through some of it last week on an ecological tour with Jonathan Spencer and Jane Smith (land managers and very good scientists both). I felt the entire time that I was reading a palimpsest on which layers of history had been written, partly erased, then written over again.

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One of the most striking features of the forest is the open heathland, where many forest ponies are grazed by commoners, and have been for hundreds of years. I was surprised to find out that the heaths do not occur in the absence of humans. It is not so much the grazing that maintains them, but the regular low-level fires that are used as a management technique to prevent forest encroachment. The evidence for fire extends several thousand years, so it is not so surprising that these landscapes appear to be natural.

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Another surprising feature was the predominance of oak-dominated forests (Quercus robur). These trees are found especially in enclosed areas where commoners have no grazing rights. I learned that 17th century military needs drove this pattern, beginning with King William III attempting to ensure timber supplies for the navy via the passage of the Enclosure Act, “For the Increase and Preservation of Timber in the New Forest”. More oaks were planted in subsequent centuries, but of course the demand for shipbuilding timber has decreased rapidly by now. Still, many trees remain standing as testament to this policy.

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And finally, a surprising pattern appeared in many of the hollies (Ilex aquifolium). Low to the ground, its leaves have prominent teeth. The tree hurts when you brush up against it. But above three meters or so, its leaves lose their teeth entirely, and become almost oval-shaped. That height is further up than any modern herbivore can reach, calling into question the selective pressure for the phenotype. One idea is that the spines are an adaptation to deter long-extinct Pleistocene megafauna (see Obeso (1997) and Cooper and Owen-Smith (1986), as well as Crowley and Godfrey (2013) for a Madagascan perspective). A sign of their eating habits remains written on these plants even today.

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The forest is not a very wild place, but it is a well-managed place where many different uses have found a way to coexist over thousands of years. It feels very different than any of the national forests in the western United States, where land use intensity has increased so dramatically in just the last two hundred years. We are at the beginning of a much longer journey this forest has already taken.

Measuring the pines

I recently escaped from the heat of Tucson to the Catalina Mountains, five thousand feet above, and ecologically a world away. Our lab installed a set of new forest plots near the summit of Mt. Bigelow, and I was glad to help with the fieldwork.

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Here we are carrying 100-meter measuring tapes up the slope. The forest is dominated by conifers, with an open understorey maintained by wildfire.

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Brian taught John how to use a compass to sight bearing lines and lay out the plot on the landscape. It was an easy place to work – no snakes, no large obstacles, no tangle of vines anywhere.

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The worst obstacles we encountered were a few large treefalls. Here Sean is tagging a sapling that was crushed by the larger tree but managed to survive, bending under the weight and sending out new branches into the light-filled gap.

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All the trees received a metal tag – here Brian is marking a white fir, Abies concolor.

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I was surprised how many species of trees there were on the mountaintop. I had expected only one or two – a pine and a fir – but we found a few types of each, as well as some other broadleaved species. It is a pleasure when a forest reveals its diversity, especially on a cool sunny morning when the hot desert lies far below and far out of mind.

Traveling through time in a slot canyon

Canyon lands guard well their secrets. Canyons themselves are not difficult to find – they form where water would run – but entrances and exits are more difficult to come by. And once inside a canyon, the world changes.

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This canyon is hard to notice at first, cut into the base of some otherwise unremarkable sandstone ridges. It seems a small thing, a few feet wide – but it grows, draining into Buckskin Gulch, then the Paria River, after which it reaches the Colorado River, and some hundreds of miles later, the Pacific Ocean. This narrow chasm holds more secrets than a glance from the surface might suggest.

Descending this canyon, one travels back in time. The water has carved a route through some of the oldest parts of the Grand Staircase. The surface rocks are Navajo sandstone, only early Jurassic in age, but the canyon quickly cuts down through the Hermit Shale, Coconino Sandstone, Toroweap Formation, and Kaibab Limestone – rocks more than 270 million years old, rocks formed in shallow seas and tidal flats, when dinosaurs roamed the earth.

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These layers of history are evident when navigating the canyon’s depths. The walls are often separated by a distance no wider than than my outspread arms, with the surface visible hundreds of feet up, if visible at all. Descending into this stratigraphic history, I felt very young.

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Only a dim and diffuse light reaches the canyon bottom. On this day the surface saw sunny weather with air temperatures near 90 °F – nearly thirty degrees warmer than the canyon bottom at mid-day.

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In this darkness there remain stagnant pools of water, protected from evaporation. Mud and sand line the canyon bottom. An occasional plant or bird can be found, but the primary users of this water seem to be flies and other insects.

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The presence of water is a reminder of the canyon’s origin, and the dramatic processes that have shaped its formation. Buckskin Gulch drains a watershed more than thirty miles long, in many places capped by largely impermeable rocks. A storm dozens of miles away can cause a flash flood here. There is ample evidence of such flooding in the logjams and debris piles emplaced in improbable locations.

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Debris piles and rockfalls make for a tight squeeze in some cases – and on this sunny day, also build a healthy respect for clouds. I saw several logs jammed across the canyon wall more than a hundred feet overhead. The Buckskin Gulch – Paria Canyon system is some thirty-eight miles long – and aside from the top and bottom, there is only one other exit.

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The canyon’s history is not limited to ancient geology and recent flash flooding. As it attracted me, it has also attracted other people. A few of the smoother walls are inscribed with petroglyphs, telling a story I am not prepared to interpret. Yet, exploring in this place, it is clear that it is a special place, a closely-kept secret in the spare landscapes of the west.

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For the curious, more information about the geology of the Grand Staircase – Escalante National Monument region can be found in this Bureau of Land Management report.

School is out

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How does it feel to be done with school, forever? My PhD dissertation is defended, submitted, and approved, and a diploma will arrive in the mail in a few weeks. It has been a long road to get here, and it feels worthwhile to reflect on the experience.

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Graduate school was a five-year experience for me, one that I almost never entered. I was teaching science in central Idaho beforehand, and the Arizona state legislature decided cut the funding for my PhD a few months before I was due to move. I nearly decided against changing life-paths because of this, but some alternative state and then federal funding came through at the last minute. At this moment, going to graduate school feels like it was a good decision. I feel that I now have the tools, the experience, and the connections to begin making my own mark in ecology. I see the world through very different eyes now, and will forever be grateful for the chance to broaden my viewpoints in these ways.

Looking back, I am struck by how much of a difference having research money has made in terms of being able to finish projects. I never had to spend much time working on other peoples’ projects, or teaching in areas that did not interest me. I owe much of that to two factors: first, the encouragement of my supervisor, Brian Enquist, and second, the availability of consistent funding support for my research and travel. Most graduate students receive far less funding and independence than I was able to find, and I don’t think that this difference has anything to do with merit. I think instead that small successes have a snowball effect, with the chance of getting fellowships and grants strongly reinforced by having had a fellowship or grant. The experience is a lot harder for students who aren’t lucky enough to escape a poorly-paid teaching assistant position, or those without US citizenship who can’t apply for many federal funding sources. I don’t know what the solution should be, but the current situation is demonstrably unfair.

The actual experience of doing a dissertation was not very helpful for me. The final document comprises a set of papers, all of which are either published or in review at scientific journals. These papers will be widely read and discussed by the scientific community, because they are searchable and available on the public internet. The actual dissertation, on the other hand, will probably languish unread in a university library for the next several decades. I was required to spend a large amount of time formatting and collecting chapters for this document. I think that process was a waste of time that will not benefit myself, the university, or the wider scientific community, but which costs a large amount of administrative time as well as fees paid to private publishing and printing companies.

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Someone asked me if it feels different to have a degree rather than to be a student. It does and it doesn’t. I think about science questions and work on manuscripts just as before. I get more respect from some people for no very good reason, other than that a few letters are now attached to my name. I feel like an unwanted barrier has been placed between me and some friends who are earlier along in their graduate school experiences. But there is much joy in having accomplished something difficult, and I look forward to being able to operate with more independence and chase after my own sources of research funding.

My degree will be conferred this Saturday, but I won’t be attending the ceremony. I’ll be teaching a program at the Sky School, and will then be disappearing into the outdoors. Exploring these natural worlds is what keeps me excited about science, and I wouldn’t have it any other way.

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Drying up

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Even a little water is worth fighting over. The summer is coming early this year, and what little surface water remains in many of the canyons is beginning to dry up. While walking in Finger Rock Canyon the other weekend, deep within a mesquite bosque, I found just a single small seep coming from a fissure in a boulder.

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The water was being put to good use. Honeybees (Apis sp.) crowded the seep, drinking what little was available. Occasionally a yellowjacket (Vespula sp.) would fly in, and easily be accommodated. But the scene cleared out immediately when a large wasp or two (Polistes sp.) appeared.

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I didn’t see any direct conflict, but it was very clear that wasps could dominate the resource despite their more limited numbers. I wonder how this conflict will play out as the heat increases, the water slowly disappears, and all creatures wait in hopes of the summer monsoon rains…

Fighting over science

Most academic publications go ignored and poorly cited; fewer make an impact or controversy. I recently had the dubious honor of two papers being ‘debunked’ in the scientific literature.

My papers were focused on explaining the leaf economics spectrum – a global pattern that describes leaves use resources like carbon and nitrogen (Wright et al., Nature 2004).

My central hypothesis was that the leaf’s venation network is important in determining multiple aspects of the leaf’s functioning. One paper was published in Ecology Letters in 2011, and the other in Journal of Ecology in 2013

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A few months ago, Lawren Sack and colleagues published a paper in the Journal of Experimental Botany, in which they argued based on both empirical and theoretical grounds that venation networks were not a useful explanation for the leaf economics spectrum.

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They state in their abstract:

Leaf vein traits are implicated in the determination of gas exchange rates and plant performance. These traits are increasingly considered as causal factors affecting the ‘leaf economic spectrum’ (LES), which includes the light- saturated rate of photosynthesis, dark respiration, foliar nitrogen concentration, leaf dry mass per area (LMA) and leaf longevity. This article reviews the support for two contrasting hypotheses regarding a key vein trait, vein length per unit leaf area (VLA). Recently, Blonder et al. (2011, 2013) proposed that vein traits, including VLA, can be described as the ‘origin’ of the LES by structurally determining LMA and leaf thickness, and thereby vein traits would predict LES traits according to specific equations. Careful re-examination of leaf anatomy, published datasets, and a newly compiled global database for diverse species did not support the ‘vein origin’ hypothesis, and moreover showed that the apparent power of those equations to predict LES traits arose from circularity.

I’m not much for controversy, and this paper caught me by surprise – our research groups hadn’t had any behind-the-scenes discussions beforehand. But scientific criticism is healthy – it helps a field determine which ideas are worthwhile and supported by evidence. We read the criticism carefully, worried that it raised some points that would undermine our work. Our worry was not about our egos – more concern that we had done something unhelpful for the field of ecology.

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After a lot of thinking and re-analysis, we decided that Sack et al. raised some important points, but that their major criticisms were unfounded. We disagree about how to interpret empirical data, and also how to build a useful model – and we disagree about how to carry out certain calculations.

We just published a response in the same journal, saying:


Our model for the worldwide leaf economics spectrum (LES) based on venation networks (Blonder et al., 2011, 2013) was strongly criticized by Sack et al. (2013) in this journal. Here, we show that the majority of criticisms by Sack et al. are based on mathematical and conceptual misunderstandings. Using empirical data from both our original study as well as others in the literature, we show support for our original hypothesis, that venation networks provide predictive power and conceptual unification for the LES. In an effort to reconcile differing viewpoints related to the role of leaf venation traits for the LES, we highlight several lines of further investigation.

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Check out our response in the Journal of Experimental Botany and decide for yourself. We’re glad to share the debate with the whole field.

New paper: predicting climate from leaf venation networks

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If you travel from the alpine zone to the lowland tropics, more than just the climate will change. Species composition follows changing environments: there is a reason why a Cyathea tree fern doesn’t live in Colorado but does live in Costa Rica. (Note of course that many millions of years ago, when the earth was warmer, rain forest could be found even in what is now Antarctica!).

What is it about a tree fern that prevents it from living in a cold environment? A central idea in modern ecology is that measuring properties that reflect an individual’s function, performance (and ultimately fitness) can explain this pattern. My main hypothesis has been that leaf venation networks are the key ‘trait’ to measure. Here’s why. Plant growth requires water loss through transpiration in the leaves, and transpiration requires water supply, which is provided by the leaf veins. If there is a preferred growth rate and water loss rate set by the environment, then only certain leaf venation networks should be viable in each environment.

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I just published a paper that tests this idea. I had noticed early on that different vein networks were associated with colder and warmer, or wetter and drier environments, as you see above. To see how general these patterns were, I went in search of leaves from a broad range of climates.

First, Dr. Brad Boyle led a botanical expedition on the western slope of Costa Rica. We started in lowland moist forest, as you see below. Access to one field site required hiking past a beautiful beach, and it was very tempting to ditch our equipment and sweaty clothes in favor of a late-afternoon swim.

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We collected plants all the way up to cloud forest and páramo vegetation, across an elevation gradient of more than 3000 meters.

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Second, I made collections in the Colorado Rocky Mountains. Here, sites ranged from high desert to subalpine meadow.

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My favorite sites were in aspen forest, where Neill Prohaska helped me with tree climbing (aspen bark is very slippery).

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We found an intriguing pattern – vein density (length of veins per unit area) decreases with elevation, meaning that lowland sites have higher resource fluxes than montane sites. But the slope of the line wasn’t constant: the relationship depended on whether we were in the tropics or not.

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I decided to try to explain this pattern based on plant physiology: perhaps the slope difference reflects differences in climate, i.e. growing season temperature, carbon dioxide availability, or intensity of solar radiation. We developed a mathematical model that uses vein density to quantitatively predict these climate variables, based on the idea I wrote about at the beginning of this piece: there is an optimal physiology for a given environment, so that the water supply in a leaf matches the water supply in the environment.

I was surprised by how well it all worked. Below is an observed-predicted plot: we compare observed values of climate to values predicted from measurements of vein density, such that a perfect fit falls on the diagonal.

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You can see that we sometimes over-predict or under-predict – but in all cases, the model explains nearly all the variation in the empirical data, with trends in the correct direction. I think this means that the model is capturing some important aspects of reality. The paper’s main contribution is showing that leaf veins provide an important explanation for why species are only found in certain climates around the world.

You can read the paper in New Phytologist (link, or free PDF reprint). It’s been more than three years between conceiving the project, doing the fieldwork, doing the math, and seeing this published. Science isn’t always fast. We certainly had setbacks, and I’ll close by showing you one of them. Visualization of leaf veins requires some chemical work in the lab, and we had a very exciting hotplate malfunction one day. Fortunately everyone’s eyes and fingers survived the accident, and our dataset is just a little bit smaller as a result!

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