Experimentation can be tricky for landscape ecologists, especially if we're considering landscapes at the human scale (it's a bit easier
at the beetle scale [pdf]). The logistic constraints of studies at large spatial and temporal scales mean we frequently use models and modelling. However, every-now-and-then certain events afford us the opportunity for a 'natural experiment' - situations that are not controlled by an experimenter but approximate controlled experimental conditions. In her opening plenary at
ESA 2009,
Prof. Monica Turner used one such natural experiment - the
Yellowstone fires of 1988 - as an exemple to discuss how disturbance affects landscape dynamics and ecosystem processes. Although this is a great example for landscapes with limited human activity, it is not such a useful tool for considering human-dominated landscapes.
Landsat satellite image of the Yellowstone fires on 23rd August 1988. The image is approximately 50 miles (80 km) across and shows light from the green, short-wave infrared, and near infrared bands of the spectrum. The fires glow bright pink, recently burned land is dark red, and smoke is light blue.
Before getting into the details, one of the first things Turner did was to define
disturbance (drawing largely on
Pickett and White) and an idea that she views as critical to landscape dynamics - the
shifting mosaic steady state. The shifting mosaic steady state, as described by
Borman and Likens, is a product of the processes of vegetation disturbance and
succession. Although these processes mean that vegetation will change through time at individual points, when measured over a larger area the proportion of the landscape in each seral stage (of succession) remains relatively constant. Consequently, over large areas and long time intervals the landscape can be considered to be in equilibrium (but this
isn't necessarily always the case).
Other key ideas Turner emphasised were:
- disturbance is a key component in ecosystems across many scales,
- disturbance regimes are changing rapidly but the effects are difficult to predict,
- disturbance and heterogeneity have reciprocal effects.
Landscape Dynamics
In contrast to what you might expect, very large disturbances generally increase landscape heterogeneity. For example, the 1988 Yellowstone fires burned
1/
3 of the park in all forest types and ages but
burn severity varied spatially. Turner highlighted that environmental thresholds may determine whether landscape pattern constrains fire spread. For instance, in very dry years spatial pattern will likely have less effect than years where rainfall has produced greater spatial variation in fuel conditions.
Turner and her colleagues have also found that burn severity, patch size and geographic location affected early succession in the years following the Yellowstone fires. Lodgepole pine regeneration varied enormously across the burned landscape because of the spatial variation in serotiny and burn severity. Subsequently, the size, shape and configuration of disturbed patches influenced succession trajectories. Turner also highlighted that
succession is generally more predictable in small patches, when disturbances are infrequent, and when disturbance severity/intensity is low (and vice versa).
Ecosystem Processes
One of the questions landscape ecologists have been using the Yellowstone fires to examine is;
do post-disturbance patterns affect ecosystem processes?
Net Primary Production varies a lot with tree density (e.g.,
density of lodgepole pine following fire) and the post-fire patterns of tree density have produced a landscape mosaic of ecosystem process
rates. For example,
Kashian and colleagues found spatial legacy effects of the post-fire mosaic can last for centuries. Furthermore, this spatial variation in ecosystem process rates is greater than temporal variation and the fires produced a mosaic of different functional trajectories (a 'functional mosaic').
Another point Turner was keen to make was that the Yellowstone fires were not the result of fire suppression as is commonly attributed, but instead they were driven by climate (particularly hot and dry conditions). Later in the presentation she used the ecosystem process examples above to argue that the Yellowstone fires were not an ecological disaster and that the ecosystem has proven resilient. However, she stressed that fire will continue to be an important disturbance and that the fire regimes is likely to change rapidly if climate does. For example, Turner highlighted the study by
Westerling and colleagues that showed that increased fire activity in the western US in recent decades is a result of increasing temperatures, earlier spring snowmelt and subsequent increases in vegetation moisture deficit. If climate change projections of warming are realised, by 2100 the climate of 1988 (which was extreme) could become the norm and events like the Yellowstone fires will be much more frequent. For example, using a spatio-temporal state-space diagram (seebelow),
Turner and colleagues [pdf] found that fires in Yellowstone during the 15 years previous to 1988 had relatively little impact on landscape dynamics (shown in green in the lower left of the diagram). However, the extent of the 1988 fires pushed the disturbance regime up into an area of the state-space not characteristic of a shifting-mosaic steady state (shown in red).
The spatio-temporal state-space diagram used by Turner and colleagues [pdf] to describe potential landscape disturbance dynamics. On the horizontal x-axis is the ratio of disturbance extent (area) to the landscape area and on the vertical y-axis is the ratio of disturbance interval (time) to recovery interval. Landscapes in the upper left of the diagram will appear to an observer as relatively constant in time with little disturbance impact; those in the lower right are dominated by disturbance.
Remaining Questions
Turner finished her presentation by highlighting what she sees as key questions for studying disturbance and landscape dynamics in a changing world:
- How will disturbance interact with one another?
- How will disturbances interact with other drivers?
- What conditions will cause qualitative shifts in disturbance regimes (like that shown in the diagram above)?
It was comforting to hear that a leader in the field identified these points as important as many of them relate closely to what I've been working on thinking about. For example, the
integrated ecological-economic forest modelling project I'm working on here in Michigan explicitly considers the interaction of two disturbances - human timber harvest and deer herbivory. The work I initiated
during my PhD relates to the second question - how does human land use/cover change interact and drive changes in the wildfire regime of a landscape in central Spain? And recently,
I reviewed a new book on threshold modelling in ecological restoration for
Landscape Ecology.
Much of Turner's presentation and discussion
applied to American landscapes with limited human activity. This not surprising of course, given the context of the presentation (at the
Ecological Society of America) and the location of her study areas (all in the USA). But although natural experiments like the 1988 Yellowstone fires may be useful as an analogue to understand processes and dynamics in similar systems, it is also interesting (and important) to think about how other systems potentially differ from this examplar. For example, the Yellowstone fires natural experiment has little to say about disturbance in human-dominated landscapes that are prevalent in many areas of the world (such as the Mediterranean Basin). In the future, research and
models of landscape succession-disturbance dynamics will need to focus as much attention on human drivers of change as environmental drivers.
Turner concluded her plenary by emphasising that ecologists must increase their efforts to understand and anticipate the effects of changing disturbance regimes. This is important not only in the context of climate as driver of change, but also because of the influence of a growing human population.
Labels: Academic, Forests, Landscapes, Wildfire
The ancient trees of Petrified Forest National Monument - preserved as quartz crystal moulds of trees buried by sediments before they decomposed - are over 200 million years old. 
At that time, in the Late Triassic, northeastern Arizona was located near the equator resulting in a tropical climate and vegetation. The climate and landscape couldn't be much more different now and the sheer scale of change (both time and location) are hard to comprehend looking out over the desert sunset.
The physical size of the Grand Canyon isn't much easier to comprehend, even when you're stood at the very edge of the southern rim. 
By the time the Colorado river had begun carving the canyon a mere 17 million years ago, the processes leading to its formation had already been at work for around 2,000 million years (the lowest sediments at the bottom of the Inner Gorge date to around that time). Sunset here is no less timeless than in the Petrified Forest.
Compared with the forest and the gorge the Barringer Crater was created in the blink of an eye. But the 300,000 ton meteor that hit earth 50,000 years ago had probably being travelling on that collision course for a much longer time. 
That these awesome features remain - so huge in time and space - reminds us how fleeting our biological landscapes are.
The ancient trees of Petrified Forest National Monument - preserved as quartz crystal moulds of trees buried by sediments before they decomposed - are over 200 million years old. 
At that time, in the Late Triassic, northeastern Arizona was located near the equator resulting in a tropical climate and vegetation. The climate and landscape couldn't be much more different now and the sheer scale of change (both time and location) are hard to comprehend looking out over the desert sunset.
The physical size of the Grand Canyon isn't much easier to comprehend, even when you're stood at the very edge of the southern rim. 
By the time the Colorado river had begun carving the canyon a mere 17 million years ago, the processes leading to its formation had already been at work for around 2,000 million years (the lowest sediments at the bottom of the Inner Gorge date to around that time). Sunset here is no less timeless than in the Petrified Forest.
Compared with the forest and the gorge the Barringer Crater was created in the blink of an eye. But the 300,000 ton meteor that hit earth 50,000 years ago had probably being travelling on that collision course for a much longer time. 
That these awesome features remain - so huge in time and space - reminds us how fleeting our biological landscapes are.
