Reconsidering rhododendron

Jens-Christian Svenning (at Aarhus University in Denmark) had some important comments on my last blog post that are worth sharing:

Interestingly, Rhodendron ponticum is not truly biogeographically non-native to NW Europe. It now has a relict distribution in the W Palearctic and was formerly much more widespread. Most notably, it reached the British Isles on its own in the Hoxnian/Holsteinian Interglacial – and guess what – became very abundant. It’s also worth noting that Rhododendron ponticum can be dominant within its current southern range – just as its close relative relative, R. maximum, can be in E North America. So, overall Rp in the UK is not really a truly exotic invader and it’s just behaving normally (given the ideal climatic conditions) (although necessarily benignly from a (local?) biodiversity perspective). In this way it seems more comparable to other native dominating species such as bracken (in the UK).

This longer-term view challenges my notion of what ‘normal’ should be. This invasion will come but it will also go. The Earth’s history is very long, and an interglacial climate excursion or human perturbation are but small things upon it.

Battling a monoculture

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A beautiful view from the coastal cliffs in Exmoor – but not if you look more closely. The right side of the photograph is dominated by a dense yellow-green canopy extending up and down the cliffs for kilometers in each direction. It looks like a scar, or a cancer. What is it?

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If you walk England’s Southwest Coast Path, you’ll pass through this area near the Glenthorne Estate to the west of Porlock Weir. Today it would be difficult walking were it not for the path chainsawed through the landscape. The problem is Rhododendron ponticum (Ericaceae), native to southern Europe and western Asia. It has spread into Britain, where it is enjoying considerable success. Where it grows, it generates a monoculture, outcompeting all other plants with its dense canopy and tangling branches. Where Rhododendron grows there is no understorey. There are no insects, and there are scarcely any birds. Native species can’t eat it. The area feels haunted and sinister.

For Rhododendron, this monoculture is a success story. It has been able to grow better than anything else in the landscape, and now dominates. The only difference between it and the other native species is that it is more successful. Why? Perhaps because it has escaped its natural enemies, growing in a strange land far from home. Or perhaps because its growth has an immediate negative impact on other plants. Or perhaps because it has been helped by people, who first planted it for its showy flowers and ability to provide cover for game animals. There are many ways to achieve a high evolutionary fitness, but in the end some plants are winners and some are losers.

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Just because it has been successful does not mean it is wanted. We humans get to choose the landscapes we want, and the plants of last century have lost their appeal. The European Union has provided funding in partnership with Great Britain to restore this landscape to one dominated by native species. I saw only a small part of the restoration efforts, but it looks like difficult work – pruning and drying and stacking and removing a huge amount of tangled biomass, then returning to the site to plant native trees in their place. I think it will be many years before they succeed – and this story is being repeated all across the country.

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As a comparison, here is a view from a deep Exmoor combe. This landscape is also a patchwork of species resulting from centuries of land use change. But somehow it feels a much more peaceful and functional system than that Rhododendron wasteland.

It would be easier to let the non-natives win – their dominance is in many ways natural – but that is not an outcome many want to see. Certainly I did not want it, walking through that pale imitation of a coastal forest.

Every mountain and hill shall be brought low

Can you guess what this is a photograph of?

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It’s an algal bloom on the surface of a lake, seen from above. The algae are growing so well because of eutrophication due to shallow depths, low flow, and high nutrient inputs. The pattern is beautiful, but the lake itself is a mess. When I say lake, I should say river – in this case, the River Glyme, flowing through the grounds of Blenheim Palace in Oxfordshire. In the 1760s, it was dammed by “Capability” Brown as part of the park landscape.

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The result is elegant (at least if you ignore the algae), but leaves much to be desired from the standpoint of ecological functionality. The lake is a pale imitation of a natural one, and the gardens require far more maintenance than the forests and pastures they replaced.

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These engineered landscapes make me think of one particularly controversial area of ecology: the relationship between biodiversity and ecosystem functioning. A basic premise of much conservation biology is that more species result in better functioning, more robust ecosystems – this is one of the key reasons to fight species loss. Yet here is an example of a site with far more species than the woodland it replaced, yet demonstrably lower levels of functionality, and far more fragility. The palace is thus a counterexample. Is this phenomenon common? And if it is, does this undermine this key assumption? In support of this, Vellend et al. showed that over the past century there has actually been no systematic loss in biodiversity in local study areas. Their conclusion held regardless of what happened over that century – climate change, pollution, species invasions, introduction of grazing, fire, and so on. The one exception (unsurprisingly) was the conversion of natural ecosystems to monoculture agriculture. As ecosystems degrade, they don’t necessarily lose species. Rather it seems to be the case that species from across regions spread out, equalizing the composition of different local sites.

So what should we take away from this story? That conserving biodiversity is not useful? I wasn’t sure until I walked through the grounds of that palace. A better interpretation – the one taken by Vellend et al., is that the number of species in a site is itself not a useful indicator of functioning. The performance of these species, and their response to unforeseen changes, matters more. The high diversity of garden plants and algae at Blenheim Palace should not be our measure of functionality. Conservation to keep up species numbers will not succeed, but conservation to keep up functionality may. It is a more nuanced message, but one that more ought to hear.

(Properly defining functionality, of course, becomes the next major issue – and that will have to wait for another post.)

The top of Wytham

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The woods at Wytham are do not look very ordinary. They are cut through by roads and instrumentation, because this forest has been a home for ecological research at Oxford for over sixty years. This week I had the chance to make the short cycle trip from the city center to the top of the forest, where my host Yadvinder Malhi has begun to study carbon dynamics and functional ecology. This part of the forest hides a tower, built of steel scaffolding and wooden planks, installed to give researchers access to the canopy of the forest.

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We went up, a climb by ladder of some thirty or forty feet. The dark light of the forest floor changes to a harsher glow as one reaches the level of the highest leaves, and epiphytes like lichens begin to appear. It is difficult to navigate the top platform for all of the recently-grown branches obstructing the path.

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Our goal for the day was to test out a gas exchange instrument (a LiCor 6400XT) for canopy photosynthesis measurements. These instruments take up as much space as a large suitcase, weigh the same as a sack of concrete, and are unbelievably expensive. Too dangerous to hand-carry it up the ladder, where in any case it would not fit through the scaffolding.

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So we brought it up by rope and pulley – a short journey, thankfully free of any accidents, and we were shortly ready to go with measurements for the afternoon.

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These towers exceed the expectations set by any childhood treehouse dream. There is something magical about climbing up through layers of forest, only to emerge into a whole new world where the wind blows, the sun streams in, the floor sways, and the leaves respond. These are days that hardly feel like work at all.

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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.


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