Picture worth a thousand words: updating repeat photography for 21st century ecologists

By William M. Hammond, Marie E.B. Stone, and Paul A. Stone. Read the full paper here.

When you ask an ecologist to describe a landscape—they will often begin by describing the organisms or features of that landscape that are most familiar. A forest ecologist may begin by describing the trees, while an ecohydrologist may first describe the watershed. Photographs, unlike ecologists, are not encumbered by passions and myriad other biases. Thus, the ancient adage “A picture is worth a thousand words” still holds—photographs carefully document both the features of a landscape one is interested in, but also the many important details of a landscape that are not noticed by the photographer. While landscapes appear immutable within a single photograph, comparing repeated photographs of the same landscape can reveal change through time. In the Peloncillo Mountains of southwestern New Mexico, we conducted a 3-year repeat photographic survey, aiming to modernize methods established in the era of film cameras. Specifically, we tested the precision of re-locating and re-capturing photographs without requiring placement of permanent markers in the landscape (a current state-of-the-art in repeat photography) at our remote field site. Yet, in the middle of establishing our survey, two wildfires burned through more than half of our monitoring stations. We quickly multiplied our aims to also include the immediate effects and subsequent recovery from wildfire of a system we had just carefully documented with photographs a few months prior. Additionally, the wildfire allowed us to test the mettle of our updated methods against dramatic landscape disturbance. Without photographs, we would only have our memories and field notes—encumbered by the biases of a herpetology lab—to remember the landscape’s prior state. Therein lies the beauty of repeat photographic points. By establishing set points on landscapes of interest, and repeating the same photographs across time, a careful comparison for both expected and unexpected landscape changes may be undertaken.

Figure 1: Repeat photographs of a landscape dominated by Pinus leiophylla, Chihuahuan pine. Initial repeat photograph was established in May 2015, prior to the June 2015 wildfire that burned this woodland. In August, the canopies were still scorched, but by the following summer the needles had fallen. In the understory, the increased availability of light and nutrients can be seen in a rapid recovery of grass and forb species following fire. Many of these canopy-dominant pines died during the intense Hog Canyon wildfire.

When establishing repeat photographic points in May 2015, we expected to capture short-term differences including seasonal phenology, or signs of grazing (the land is cattle-leased by the USFS). However, the wildfire put us in the unique position of having landscape photographs taken a month prior to burning (Figure 2). Often, recovery monitoring starts after a major landscape disturbance, so the value of having the prior-to-disturbance state of these Sky Island landscapes was not lost on us.

Figure 2. Repeat photographs taken in May 2015 and May 2016 of an ephemeral pool in Blackwater Canyon, Peloncillo Mountains, NM. In May 2015, the pool was full of water, and nearby trees healthy. In June of 2015 the area burned during wildfire. By May of 2016, clear signs of tree mortality accompany a dry canyon.

We documented systems where fire had burned (compared to where it had not), and unintentionally this built additional confidence in our modernized method of repeat photography. Even without permanent markers, in a landscape greatly disturbed by recent wildfire, we were able to achieve a high percent similarity (a self-conceived metric of overlap between two time-separated photographs), indistinguishable from retakes at unburned sites. We continued monitoring through the summer of 2017 to document the landscape’s transformation post-fire, establishing new repeat photographic points in recently burned areas (Figure 3).

Figure 3. Repeat photograph pair taken in August 2015 (left, following a June wildfire) and May 2016 (right, nearly a year after wildfire). While canopies were not consumed by the wildfire—scorched foliage of evergreen trees (reddish brown in Aug. 2015, left) had fallen, revealing widespread tree mortality in burned areas following the wildfire.

Repeat photography has a rich history that began in the 19th century, along with the technological invention of the first cameras. Sebastian Finsterwalder, a German mathematician, used these early cameras to document geologic structure and glacial movement in the Alps. Since then, advances in technology have brought what was once a heavy array of equipment into a device that fits in your pocket. Modern smartphones include high-resolution digital cameras, relatively accurate GPS sensors, compasses to document camera bearing, and accelerometers to aid in leveling of images. By testing digital repeat photography using similar technologies, we have shown that repeat photography need not be limited by heavy equipment or the installation of permanent landscape markers, and that high-quality repeat photography can be conducted on a modern smartphone. To that end, we are continuing to develop a smartphone repeat photography application—and are seeking collaborators for this and future efforts to establish a long-term repeat photographic point repository. Such a collection should allow the on-line and ongoing contribution of a wide audience of citizen scientists, to document the current baseline state of landscapes and provide a point-of-comparison through carefully documenting the necessary data (as demonstrated via our method) for a precise future photographic recapture. After all, if a picture is worth a thousand words, a global collection of repeated landscape photos surely is priceless.

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