The study:

One of the most consistent findings arising from 20 years of study in our lab is that wood frogs seem to adapt to life in cold, dark ponds. In general, cold-blooded animals like reptiles and amphibians are not suited for the cold and function much better in warmer conditions. So, wood frogs that live in colder ponds should have a harder time competing against their neighbors in warmer ponds.

In response, cold-pond wood frogs seem to have developed adaptations that level the playing field. In separate experiments, we’ve found that wood frog tadpoles in cold-ponds tend to seek out warmer water (like in sunflecks) and have lower tolerance to extremely warm temperatures. Most importantly, they can mature faster as eggs and larvae.

But I’ve always struggled with a lingering question: if cold pond frogs have adapted these beneficial adaptation to compete with warm-pond frogs, what is keeping those genes out of the warm-ponds? Shouldn’t cold-pond genes in a warm pond mean double the benefits? One would expect the extrinsic environmental influence and the intrinsically elevated growth rates to produce super tadpoles that metamorphose and leave the ponds long before all the others.

Kaija, who was an undergrad in the lab at the time, decide to tackle that question for her senior thesis.

We hypothesized that there might be a cost to developing too quickly. Studies in fish suggested that the trade-off could be between development and performance. The idea is that, like building Ikea furniture, if you build the tissue of a tadpole too quickly, the price is loss of performance.

So we collected eggs from 10 frog ponds that spanned the gradient from sunny and warm to dark and cold. We split clutches across two incubators that we set to bracket the warmest and coldest of the ponds.

Then we played parents to 400 tadpoles, feeding and changing water in 400 jars two to three times a week.

We reared the tadpoles to an appropriate age (Gosner stage 35ish). Those in the warm incubator developed about 68% faster than those in the cold incubator. In addition to our lab-reared tadpoles, we also captured tadpoles from the same ponds in the wild as a comparison. Development rates in the lab perfectly bounded those in the wild.

Once they reached an appropriate size, we put them to the test. We simulated a predator attack by a dragon fly naiad by poking them in the tale. Dragonfly naiads are fast, fierce, tadpole-eating machines and a tadpole’s fast-twitch flight response is a good indicator of their chance of evading their insect hunters. It’s a measure of performance that directly relates to a tadpole’s fitness.

Above the test arenas, we positioned highspeed cameras to capture the tadpoles’ burst responses. We recorded 1245 trials, to be exact—way more than we ever wanted to track by hand. Fortunately, Kaija is a wiz at coding; and with a bit of help, she was able to write a Matlab script that could identify the centroid of a tadpole and record its position 60 times per second.

We measured the tadpoles’ speed during the first half second of their burst response and looked for an association with their developmental rates. One complicating factor is that a tadpole’s fin and body shape can influence burst speeds. So, a weak tadpole with a giant fin might have a similar burst speed to a super fit tadpole with a small fin. To account for this, we took photos of each tadpole and ran a separate analysis mapping their morphometry and included body shape into our models.

As we had hypothesized, tadpoles reared at warmer temperatures show much slower burst speed than their genetic half-sibling reared in the cold incubator. We even saw a similar, but weaker effect for the tadpoles that were allowed to develop in their natal ponds. It seems that developing too fast reduces performance.

Thus, it certainly seems that the counter-gradient pattern we see of faster development in cold-pond populations, but not in warm-pond populations, is at least partially driven by the trade-off between development rate and performance.

In fact, it may even be the case that we’ve been viewing the pattern backwards all along. Perhaps instead we should consider if warm-pond populations have developed adaptively slower development rates to avoid the performance cost. This especially makes sense given the range of wood frogs. Our populations are at the warm, southern end of the range. Maybe this tradeoff is also a factor constraining wood frogs to the cold north of the continent?

If warm weather and faster development are a real liability for wood frogs, it is only going to get worse in the future. We know from another recent study that our ponds have been warming quickly, especially during the late spring and early summer months. But climate change is also causing snow to fall later in the winter forcing frogs to breed later. The net result is that wood frogs may be forced to develop fast intrinsic developmental rates in response to a contracting developmental window, while at the same time, extrinsic forces drive development even faster. That’s a double whammy in the trade-off with performance. And might lead to too many “Ikea furniture mistakes” at the cellular level.

As a separate part of this study, we also measured metabolic rates in out tadpoles in hopes of understanding the relationship between developmental rates, performance, and cellular respiration. I’m in the process of analyzing those data, so stay tuned for more!

Update (Nov. 2020): And, I’m especially thrilled that our piece will be the cover article for the journal, featuring a pair of breeding wood frogs from our population! My hands nearly froze trying to get this underwater shot, so I’m glad it was worth the effort!

Over the past 20+ years, our lab has been monitoring over 50 populations of wood frogs at Yale Myers Forest. Each year in early spring, we listen for the duck-like clucks of the male frogs which means that they have emerged from under the snow and moved into the breeding ponds. Shortly afterward, we head out into the freezing ponds to count the egg masses as a way to monitor population density over time.

In this study, we looked at how the oviposition date (the day on which frogs deposited eggs) has changed over time. As climates warm, we usually expect for the timing life-history events (like oviposition, emergence from hibernation, flowering time, etc.) called ‘phenology’ to advance in the year as winters get shorter. That’s just what most species do. And the trend of advancing phenology is strongest for amphibians.

Given that annual temperatures at our field site have increased by almost 0.6 C in the past two decades, we expected frogs to breed and lay eggs earlier. If our frogs were like other amphibians, we might expect oviposition to come around 6 days earlier.

Surprisingly, we found the opposite. Our frogs seem to be breeding 3 days LATER.

To figure out what might be going on with our frogs, we decided to look more closely at climate across the season, not just annual averages. It turns out that most of the increase in annual temperatures are felt later in the summer, but relatively less when frogs are breeding. Snowpack, on the other hand, is actually accumulating later and lasting longer. In the figure (Fig. 3 from the paper) below, you can see these trends. On the left are the comparisons between temperature, precipitation, and snowpack between 1980 and 2018. On the right, we plot only the difference in trends over time. At the top-right, we plot the oviposition dates to show how seasonal changes in climate line up with frog breeding.

We also looked at how the timing of oviposition correlated with climate across the season. We found that breeding occurs later when there is more snow at the beginning of the breeding window. Also colder temperatures just before breeding correlate with delayed oviposition (which makes sense if colder temps mean more snow and less melting).

So, we think that frogs may be kind of stuck. Persistent snowpack might be keeping them from breeding earlier. But at the same time, warmer summer temperatures might be drying up their ponds faster. If so, this could be a big problem for tadpoles that need to maximize their time for development. The figure below shows that frogs tend to breed earlier when winter and early spring air temperature are high. As we’d expect, more snowpack correlates with later breeding. High precipitation during the spring delays breeding (probably because it is falling as snow).

Twenty years is a long time to be collecting ecological data, but it is a pretty short window into the evolutionary history of wood frogs. And, we don’t know how long snow and temperatures may have been working against these frogs. So, as a final piece of our analysis, we used a machine learning technique called a random forest to predict oviposition dates backwards in time an additional 20 years. It doesn’t seem like much has changed over the past half-century or so. In one way, that could be good news in that at least things don’t seem to be getting any worse.

The big question is, how will frogs cope with these climate changes? If tadpoles are faced with an ever-shrinking window of time to develop into frogs, will they be able to keep up? Or, will they lose the race and end up as tadpole-shaped raisins in our ponds?

I won’t give away any spoilers, but I’m looking at our long-term larval datasets to ask that question next.

Here’s my talk from the conference, “Evolution of Intrinsic Rates: Can adaptation counteract environmental change?“:

As part of my dissertation research, I spent the spring monitoring the development of wood frog eggs. Some of the eggs I brought to the lab, removed from the jelly coat and reared in incubators. I left the rest of the eggs in the ponds and checked on them daily until they hatched. Although not part of my research, I thought it would be interesting to take photos comparing the wild eggs and algae symbionts to the lab-reared eggs without. The relationship between this alga species and only a handful of amphibians is one of the most unique stories in biology. To the best of my ability, I’ve chronicled the Green Egg symbiosis story below.

Green Eggs

Folks have been fascinated by the coincidence of algae and amphibian embryos for a long time. One of the first notes was published in 1888. Orr was studying amphibian skeletal and nervous system development. He needed embryos and had collected egg masses of a few local species. He was surprised to see that the salamander eggs he procured seemed “to present a remarkable case of symbiosis.” He noticed that there were algae cells situated inside of the egg membrane.

This is surprising because that membrane, called the vitelline membrane, is impermeable to everything but ions and small molecules, which certainly excludes entire algae cells. So the only other option is that the algae entered the embryo before the vitelline membrane had formed. But this would be no less surprising because, like basically all animals, the vitelline membrane is formed in the ovaries, and in amphibians, extra layers of jelly are added in the oviducts, long before the egg is exposed to pond water.

In the figure above (from Gilbert 1942) depicting the layers surrounding a salamander egg, it is clear to see that there are many layers acting as barriers between the pond water and the egg.

Which came first–the algae or the egg?

So, Orr was left with the plaguing question: Which came first—the algae or the egg? Perplexed, he wrote, “I have not discovered how the Algae enter the membrane, nor what physiological effect they have on the respiration of the embryo, but it seems probable that in the latter respect they may have an important influence.”

It turns out that Orr was right, algae do have an important influence on the embryos, but it was not demonstrated until the 1940s.  The phenomenon continued to perplex scientists for decades. By the 1940s, algae had been reported in the eggs of multiple salamander species and also wood frogs across the continent from California to New England and Virginia. Gilbert (1942) was the first to explore how the algae came to enter the egg and also studied the importance of the algae-egg relationship.

As to the entrance of the algae into the egg, he first assumed that the algae were present in the ovitract of the female salamander. He washed the oviducts of salamanders and tried to culture the solution, but no algae developed. If algae were not present in the ovitract, then they enter from the pond after the eggs are deposited. To confirm, Gilbert took a recently inseminated female and allowed her to lay eggs in an aquarium with pond water then removed her and allowed her to lay another clutch in an aquarium with algae-free tap water. Algal cells were present in the eggs laid in pond water within hours, but no algae grew within the tap water eggs after a few days. At the end of this experiment, Gilbert put the eggs from the tap water aquarium in a natural pond. Within a few days, algae had penetrated the jelly membrane.

By closely studying the eggs, he found that the algae transformed as they penetrated the egg. When free-swimming in the pond, the algal cells are long and oval with flagella used for locomotion. As the algae penetrate the egg layers, they grow in size, become spherical in shape, and lose their flagella. It is these large, non-mobile cell-types that rest on the inner membrane and create the “verdant blanket” (as Gilbert describes it; 1942, p. 220).

To demonstrate symbiosis, he compared hatching rates in egg masses kept in light and dark to prevent photosynthesis, finding that light and algae increased hatching success. Although algalogistis (yep, that’s a thing) hadn’t officially recognized the alga species, folks who worked with amphibian eggs called it Oophila amblystoma (“Oo” and “phila” come from the Greek words “egg” and “love,” and “amblystoma” is the genus name of the salamander species the algae cohabitates with. So, the name just means “an algae that likes salamander eggs”).

Just a few years ago, researchers took a closer look at the Oophila algae. They wondered if this was really one species or a clade of algal species, and if it the latter, did different algal species associate with specific amphibian species? To answer those questions, they sequenced the genome of algae from four species across the continent.

It turned out that the Oophila are closely related, close enough to be considered a single clade or species, but within the group there are subclades that tend to associate with each host. However, the researchers also found evidence that different subclades of the algae occasionally defect and switch to other host species.

Friends or just roommates?

Since Gilbert’s era, we’ve learned a lot about the economy of the biological transactions between the algae and the embryo. Using microelectrodes inserted within the egg membrane, Bachmann et al. (1986) measured oxygen concentration within the egg in contrast to the outside water in both light and dark. In the light, the algae worked overtime, producing so much oxygen through photosynthesis that the oxygen production exceeded that needed by the embryo and the environment inside of the egg became super-oxygenated, even when the surrounding water was anoxic (oxygen-poor).

This extra photosynthetic boost is more than just a little helpful. Most amphibian embryos get oxygen by diffusion across the egg membrane from surrounding water or air. Rinder and Friet (1994) wondered if diffusion of oxygen from the surrounding water alone could support amphibian species that had evolved with algal symbionts. They measured oxygen gradients across the cell membrane and injected dye in egg masses to see how much water would be able to flow past the eggs. In wood frogs, they found that the egg mass was loose enough and contained enough channels that water diffusion alone could support respiration. However, spotted salamander egg masses are dense and water does not penetrate to the interior eggs. In their case, the innermost eggs would suffocate without their personal algal oxygen factories. (Note: Hutchinson and Hammen (1958) inferred this relationship many decades before through experimentation rather than directly measuring O² within the eggs.)

This plant-animal relationship is a multifaceted exchange. The reciprocity extends past simple O² and CO² cycling. The algae also benefit from the nitrogen waste produced by the embryo, which limits the ammonia buildup in the egg which is toxic to embryos (Goff and Stein 1978).

Friends or more-than-just-friends?

This symbiotic relationship is certainly interesting, but not altogether surprising. After all, lots of animals host symbiotic microbes, even humans house a panoply in our guts and all over our skin (Gilbert et al. 2018). These types of ectosymbionts, commensal organisms that live in and on other organims’ bodies but outside of the tissue are common.

But the green egg story gets even more fascinating. In 2011, Kerney et al. used a technique that binds only to algae DNA and causes it to glow under fluorescent lights (fluorescent in situ hybridization (FISH)). In doing so, they could see algae cells inside(!) of the salamander tissue and cells. This makes this salamander-algae relationship the only example of endosymbiosis in a vertebrate ever seen! Previously, this type of animal-plant union was thought to only be possible in simpler organisms like coral and mollusks.

Friends or frenemies?

So, not only do these free-swimming algae manage to invade the egg membrane, but they even make it all the way inside of the embryonic cells. To some extent, this makes sense for the algae. Why would you want to live out in the pond water with variable temperatures and predators when you could live inside a nice cozy egg with abundant CO2 and nutrients? But once inside the egg, why would algae invade inside an animal cell where there is less access to light for photosynthesis?

A team of researchers (Burns et al. 2017), including Kerney, wanted to figure out the riddle. They isolated and sequenced the genes expressed by salamander cells with and without algae cells from inside and outside of the animal cell. Inside the animal cells, algae seemed to be under more stress, to the point that they switch from photosynthesis to fermentative energy production, something that would normally only happen in highly unfavorable environments. On the other hand, salamander cells with algae were unfazed and even seemed to suppress their immune response which would ordinarily attack foreign cells, perhaps to keep the algae within. For algae, then, there appears to be a risk-benefit balance that drives them inside of the egg where conditions are favorable, but risk being captured inside of the embryonic animal cells.

Better than Seuss

After over a century of investigation, the story of the Green Eggs has gotten more and more interesting with finer and finer resolution. The saga goes to show how the naturalist’s intrigue can inspire a cascade of illuminating research.

Understanding this complex relationship can also help us understand threats to amphibian populations. Herbicides can kill the symbiotic algae and result in lower hatching success and reduced developmental rates. Olivier and Moon (2009) tested the effect of the herbicide atrazine on spotted salamander egg masses. They found that even low concentrations of the chemical killed the algae. In addition to the direct effects of the chemical, the loss of the symbiont had negative repercussions.

References:

Bachmann, M. D., Carlton, R. G., Burkholder, J. M., and Wetzel, R. G. (1986). Symbiosis between salamander eggs and green algae: microelectrode measurements inside eggs demonstrate effect of photosynthesis on oxygen concentration. Can. J. Zool. 64, 1586–1588.

Burns, J. A., Zhang, H., Hill, E., Kim, E., and Kerney, R. (2017). Transcriptome analysis illuminates the nature of the intracellular interaction in a vertebrate-algal symbiosis. Elife 6. doi:10.7554/eLife.22054.

Gilbert, J. A., Blaser, M. J., Caporaso, J. G., Jansson, J. K., Lynch, S. V., and Knight, R. (2018). Current understanding of the human microbiome. Nat. Med. 24, 392–400.

Gilbert, P. W. (1942). Observations on the Eggs of Ambystoma Maculatum with Especial Reference to the Green Algae Found Within the Egg Envelopes. Ecology 23, 215–227.

Goff, L. J., and Stein, J. R. (1978). Ammonia: basis for algal symbiosis in salamander egg masses. Life Sci. 22, 1463–1468.

Hutchison, V. H., and Hammen, C. S. (1958). Oxygen utilization in the symbiosis of embryos of the salamander, Ambystoma maculatum and the alga, Oophila amblystomatis. Biol. Bull. 115, 483–489.

Kim, E., Lin, Y., Kerney, R., Blumenberg, L., and Bishop, C. (2014). Phylogenetic Analysis of Algal Symbionts Associated with Four North American Amphibian Egg Masses. PLoS One 9, e108915.

Olivier, H. M., and Moon, B. R. (2010). The effects of atrazine on spotted salamander embryos and their symbiotic alga. Ecotoxicology 19, 654–661.

Orr, H. (1888). Memoirs: Note on the Development of Amphibians, chiefly concerning the Central Nervous System; with Additional Observations on the Hypophysis, Mouth, and the Appendages and Skeleton of the Head. J. Cell Sci. s2-29, 295–324.

Pinder, A., and Friet, S. (1994). Oxygen transport in egg masses of the amphibians Rana sylvatica and Ambystoma maculatum: convection, diffusion and oxygen production by algae. J. Exp. Biol. 197, 17–30.

29 March 2018

My first wood frog of the season, hopping slowly from one snowy bank to the other across the mist-dampened road. This is the first warm night of the spring. The air has condensed against the cold ground into eerie sheets of mists that look like they are getting tangled in the trees and spilling out over the more open ponds.

I pick up this first little frog caught in my headlights to move her on toward her destination and away from tires. I can feel the gritty snow crystals on her skin, which I imagine must be terribly uncomfortable on her soft underbelly. She is sluggish but clear-eyed. Females generally are redder in coloration, but she seems brighter and more sanguine than any wood frog I’ve ever seen. Maybe her ruddy complexion is due to the profusion of blood just recently mobilized and coursing through the capillaries of her skin—blood that until just a few hours ago had been frozen in her cells.

30 March 2018

Noon – The ponds are mostly quiet. A few bold peepers chirp in the warmer ponds with more open canopies. I see wood frogs listlessly floating at the surface and racing for the leaves at the pond bottom when I step into the water, but none of them are vocalizing, yet.

5 PM – The warm day and persistent drizzle this evening were a clear call for the lethargic amphibians to join the early wood frogs in the ponds. Spotted salamanders and wood frogs are out in waves, but other species are on the move, too. I saw one crayfish scuttling across the road from forest to a large tussock-filled pond and two four-toed salamanders apparently racing side by side.

9 PM – Driving around tonight as slowly as possible yet still swerving to miss late-recognized salamanders, I realized that spotted salamanders are a great object lesson against teleological evolution. Evolution is obviously not forward-looking if it resulted in a slow-moving, soft-bodied animal that so perfectly matched the color or wet pavement. Even the bright yellow spots that one might think would stand out in the headlights are indistinguishable from the bright flecks in the chip-and-seal road and just act as further camouflage.

The frogs have found their voices and the choruses are erupting in vernal pockets across the forest. In one pond, I caught a mass of six or more wood frogs clamoring together in a tangle of splayed hindlegs. It is clear that these frogs are driven by a severe case of FOMO incomparable to even the most angst-ridden teenager—all are too concerning with missing out on the chance for breeding none have realized that they’ve engage not a gravid female wood frog, but a male spotted salamander! The poor salamander is helpless in the trap-like bear hugs of the frogs.

31 March 2018

A handful of early clutches were generated in last night’s excitement. I collected a few eggs from each clutch, carefully recorded the water chemistry, and installed a temperature logger at the oviposition site. As the eggs develop, I’ll be back to check their progress and monitor the water conditions, comparing the wild embryos to their brethren back in the lab.

1 April 2018

It is a cloudy day but dry and breezy. A few ponds are chorusing, but they are skittish and clam-up any time I get near. Only a few new egg masses appeared overnight. The forecast is calling for snow tonight, so it may remain quiet for the next couple of days.

Despite the fact that foresters have been estimating forest canopy characteristics for over a century or more, the techniques and interpretation for these measurements is surprisingly inconsistent. As part of my dissertation research, I wanted to ask what I thought was a simple question: “How much light reaches different vernal pools over the season?” It took a lot of literature searching, a lot of emails, and a lot of trial and error to discover that this is actually not as simple of a question as I originally thought.

But in the end, I’ve developed what I think is a very sound workflow. In the hopes of saving other researchers a journey through the Wild West of canopy-based light estimates, I decided to publish my notes in a series of blog posts.

• In this first post, I’ll cover the rationale behind hemispherical photo measurements.
• The second post will compare hardware and address measuring/calibrating lens projections.
• The third post will focus on capturing images in the field.
• Finally, the fourth post will be a detailed walk-through of my software and analysis pipeline, including automation for batch processing multiple images.

Why measure light with hemispherical photos?

Light is an important environmental variable. Obviously the light profile in the forest drives photosynthesis in understory plants, but it is also important for exothermic wildlife, and can also impact abiotic factors like snow retention. An ecologist interested in any of these processes should be interested in the light environment at specific points in the habitat. Although light intensity at a point can be measured directly with photocells (Getz 1968), this requires continuous measurement at each point of interest over the entire seasonal window of interest. For most applications, this would be intractable. A more common method involves measuring the canopy above a point and estimating the amount of sky that is obstructed in order to infer the amount of light incident at that location.

Fortunately, since both foresters and ecologists are interested in the canopy, the field has produced a multitude of resources for measuring it. However, foresters are more often interested in estimating the character of the canopy itself and the canopy metrics they use are not always directly relatable to questions in ecology.

For instance, while foresters are generally interested in canopy COVER, ecologists might be more interested in canopy CLOSURE. These terms are similar enough that they are often (incorrectly) used interchangeably. According to Korhonen (2007) and Jennings et al. (1999), canopy cover is “the proportion of the forest floor covered by the vertical projection of the tree crowns” while canopy closure is “the proportion of sky hemisphere obscured by vegetation when viewed from a single point” (see the figure below).

The choice between measuring cover versus closure depends on both the scale and perspective of your research. For instance, forest-wide measurements of photosynthetic surface will probably be estimated from remote sensing data which is a vertical projection (i.e. canopy cover). However, if you are interested in the amount of light reaching a specific understory plant or a vernal pool, canopy closure is more relevant.

The disparity in perspective can be an issue in downstream analysis too. For instance, some analysis procedures consider only the outer edge of a tree crown or contiguous canopy and ignore gaps within this envelope. This kind of analysis is much less useful if your interest is in the light filtering through canopy gaps.

Canopy estimation:

There are three main ways to estimate canopy characteristics: direct field measurements, indirect modelling from other parameters, and remote sensing. The choice of measurement methods employed will largely be determined by the scope of the question and the scale-accuracy required. For the purpose of my research, I am interested in estimating light environments for a specific set of vernal pools; so, direct measurements are most useful. However, if I wanted to know how different tree composition influences light environments of ponds on average across forest habitats, I might want to try modelling that relationship followed by ground-truthing with direct measurements.

The rest of this post will focus on directly measuring canopy closure to estimate light environments.

Measurement methods:

The most popular methods for directly measuring closure are hemispherical photography and spherical densiometer. A spherical densiometer (Lemmon 1956) is basically a concave or convex mirror that reflects the entire canopy. A grid is drawn over the mirror and researchers can count the number of quadrants of sky versus canopy.

Hemispherical photos use a similar principle. A true hemispherical lens projects a 180 degree picture of the canopy that can then be processed to determine percentage of sky versus canopy.

Advantages of densiometer are that they are cheap, easy to operate, and can function in any light conditions. The exact converse is true of hemispherical photography which can be expensive, difficult to operate, and requires very narrow light conditions (I’ll get into the particulars in post 3).

So, why would anyone use hemispherical photos over densiometers?

The main disadvantage of a densiometer is that the estimates are instantaneous, whereas hemispherical photos can be integrated to estimate continuous measurements. It is easiest to explain this with an example.

Imagine you want to know how much light is received by a specific plant on the forest floor at the very edge of a clearcut (see figure below). A closure estimate from a densiometer would indicate 50% closure no matter the orientation of the tree line. However, if that little plant is in the northern hemisphere, we know that it will receive much less light if the clearcut lies to the north than if the clearcut lies to the south due to the angle and orientation of the sun.

The advantage of hemispherical photos (if taken with known orientation and location) is that they can be used to integrate light values over time with respect to the direction of the sun relative to gaps in the canopy. This means that with a single photo (or two photos for deciduous canopies) one can calculate the path of the sun and estimate the total light received by the plant in our example at any point in time or cumulatively over a range of time.

An ancillary advantage of photos is that they can be archived along with the scripts used for processing, which makes the entire analysis easily reproducible.

I’ll go much further in depth in later posts, but as a preview, here is a general overview of how light estimates are calculated from hemispherical photos:

1. Hemispherical photos are captured in the field at a specific location and known compass orientation.
2. Images are converted to binary, such that each pixel is either black or white using thresholding algorithms.
3. Software can then be used to project a sun path onto the image.
4. Models parameterized with average direct radiation, indirect radiation, and cloudiness can then estimate the total radiation filtering through the gaps represented as white pixels in the photo at any point in time or averaged over time ranges.
5. The result is a remarkably accurate point estimate of incident light at any time in the season.

I’ll be drafting subsequent post in the coming weeks so be sure to check back in!

Be sure to check out my other posts about canopy research that cover the theory, hardware, field sampling, and analysis pipelines for hemispherical photos.

Also, check out my new method of canopy photography with smartphones and my tips for taking spherical panoramas.

References:

Getz, L. L. (1968). A Method for Measuring Light Intensity Under Dense Vegetation. Ecology 49, 1168–1169. doi:10.2307/1934505.

Jennings, S. B., Brown, N. D., and Sheil, D. (1999). Assessing forest canopies and understorey illumination: canopy closure, canopy cover and other measures. Forestry 72, 59–74. doi:10.1093/forestry/72.1.59.

Korhonen, L., Korhonen, K. T., Rautiainen, M., and Stenberg, P. (2006). Estimation of forest canopy cover: a comparison of field measurement techniques. Available at: http://www.metla.fi/silvafennica/full/sf40/sf404577.pdf.

Lemmon, P. E. (1956). A Spherical Densiometer For Estimating Forest Overstory Density. For. Sci. 2, 314–320. doi:10.1093/forestscience/2.4.314.

It might be surprising, but tree canopies can have major impacts on aquatic amphibians. The greater the canopy cover, the less light will reach a ponds surface. Wood frog tadpoles are adapted for rapid development and every small increase in water temp from every small stream of sunlight can make or break a tadpole’s survival. In addition to temperature, the amount of light incident on a pond dictates the amount of algae growth, which in turn dictates the concentration of oxygen in the water. Also, the amount of algae can dictate how much food competition exists between tadpoles, and how bold tadpoles must be to seek out food. In a pond full of predators, being bold can have dire consequences.

More leaves in the canopy also means more evapotranspiration, which in turn means greater water demand from tree roots. As tree suck water out of the ground, the water levels in the pool drops and eventually dries out completely. Amphibians are in a race against time to metamorph before that happens.

So, as you can see, tadpoles and trees have a surprisingly close relationship. All of these compounding factors allow us to tell a lot about wood frog evolution, just by looking up.