The Light Fantastic: Summoning Ghosts from the Past

Cutting-edge modern technology has brought us so far that scientists can now find ghostly prints from former human activities with breath-taking accuracy. Ghosts from the past, that is, and that modern technology is called LiDAR (light detection and ranging).

LiDAR feels like a magical way of looking at things and revealing hidden artefacts. It is basically a laser scan of an environment with extreme precision (often up to centimetres in accuracy), which allows us to recreate environments such as forests in 3 dimensions. LiDAR scans can be made from the ground or from the air, and in its simplest form consists of sending out a laser beam, The Light Fantastic, and capturing it again when it bounced back from an object.

The magic of LiDAR. Top left, a: principles of laser scanning from the air. The lower panels (d, e) visualize a cloud of raw LiDAR points extracted from both a stationary terrestrial LiDAR system and a drone, covering the exact same study area in the Aigoual forest (France). Upper-right panels (b, c): basic principles of time-of-flight vs. phase-shift LiDAR.

In a new review paper recently published in Journal of Ecology, we summarize the exciting potential of LiDAR for forest research. Indeed, when we can map a forest in 3D at the centimetre scale, we can find back structures that are impossible to see with the naked eye, and that hide former land uses, management practices or impacts of climate change.

An impressive example of this potential can be seen in the Compiègne forest, in northern France, where LiDAR data allows us to trace back hidden structures in the forest all the way to the Roman times, much and much further than historical maps can ever do.

An example of digging up ghosts from the past using aerial photography (1937-2000), old maps (18th-19th century) and finally LiDAR data to go back to antiquity. LiDAR scanning of the forest floor shows typical patterns of agricultural activities from the Middle Ages (bottom left), as well as linear microreliefs corresponding to a network of Gallo-Roman agrarian fields and secondary roads (bottom right). Data coming from a flight with an airplane over the forest in February 2014, getting a 3D image with on average 12 points per m²!

While digging up such ghosts of the past is obviously extremely fascinating, it has also important consequences for ecology; the main point we want to hammer home in this new paper. Indeed, such past management practices and land-use changes have big impacts on current species distributions. Basically, they can explain why you found certain plants in certain – sometimes weird – spots. Some barley on the forest floor? Perhaps there has been a medieval farm there!

These confounding effects of past land use can obscure the impacts of ongoing global changes such as climate change or atmospheric pollution on species distributions. And that is exactly why it is so critical to know about them. For example, Roman agricultural practices can still result in elevated nutrient concentrations in a forest soil, with consequently a higher presence of nitrogen-loving species in the forest understory. Without the technology to look back that far into the past, the presence of these species might mistakenly be attributed to nitrogen-deposition from the air during the 1980s, overestimating the impact of the latter on forest diversity.

Given how forest cover is increasing in Western Europe (France has seen its forest cover double since the 18th century!) there is bound to be a lot of historical artefacts and past land use hidden underneath our forests. Knowing these patterns will be critical for smart management decisions. Now LiDAR is there to reveal them.

LiDAR data (obtained on the left via aircraft, drone or terrestrial laser scanning) can be used to assess micro-topographic variation (e.g., skid trails) and forest structure (e.g., vertical layering of vegetation) at a landscape level, and thus highlight legacy effects still affecting the current composition of understory plant communities and their responses to macro-environmental changes through time-lag dynamics. For instance, it is possible to not only capture the imprints of historical forest management practices (e.g., ancient coppice-with-standards converted to high forests after World War II or the more recent intensification of heavy vehicles’ traffic to harvest timber) but also to unveil past land uses (e.g., ancient settlements or agricultural fields).
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The past can be hidden in forests in surprising ways: this lush green cover of quaking sedge (Carex brizoides) in the French Morman forest, for example, could be an important sign. Indeed, this species was well known in World War I, when it was used by German soldiers to make their mattresses. As a typical species from Central Europe, populations like this in France might very well have been established there unknowingly by German soldiers during the war. Picture by Jonathan Lenoir.

Reference:

Lenoir et al. (2022). Unveil the unseen: Using LiDAR to capture time-lag dynamics in the herbaceous layer of European temperate forests. https://doi.org/10.1111/1365-2745.13837

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