Forest Physiognomy

I am old enough that I attended school at a time when educators still taught physiognomy to their students. I recall being attracted to the idea that you could predict someone’s character, criminal or violent inclinations, passions and general temperament by the location of bumps or indentations on the head, the shape of the nose, or the forward projection of the jaw. Dampening my enthusiasm, we were taught physiognomy as an example of pseudoscience, and that we should make sure to not embrace ideas simply because they were intuitively attractive. And this letdown came after I had spent several precious moments learning how to pronounce the word.

Two tranquil foreheads. Credit: Giambattista della Porta: De humana physiognomonia libri IIII. From website of the National Library of Medicine:

Later, I was delighted to learn in my plant communities class in graduate school that forests had physiognomy, and that reputable scientists actually studied it. Forest physiognomy is the general appearance of a forest, including the height, spacing and structural growth forms of its dominant species.  Michelle Spicer described to me that she went to Central America as an engineering undergraduate student, and became enraptured with tropical forests, including their physiognomy.

Tropical forest showing vast collection of lianas and a few epiphytes. Credit: Michelle Spicer.

Spicer switched from engineering to ecology, and as a graduate student realized that nobody had actually rigorously compared tropical and temperate forest physiognomy. Textbooks might talk about the importance of lianas (vines) and epiphytes (plants that grow on other plants and get nutrients from the air, water or debris lodged in their host plants) in tropical forests.  These same texts might also highlight the importance of the herbaceous layer in temperate forests. 

A temperate forest in the Smokey Mountains, USA. Credit: Michelle Spicer.

But there were few organized data to compare forest physiognomy in the two biomes. Spicer, an undergraduate student in her lab (Hannah Mellor), and her advisor (Walter Carson) chose to compare nine temperate forests and nine tropical forests, spreading across the Americas from Brazil to Canada. Each of these forests (studied by other researchers) had detailed downloadable plant species lists, which also included data about their height and reproductive status.  In total, the researchers went through over 100,000 records to create their dataset.

The figure below highlights the plant physiognomy concept. You can see that most of the species in temperate forests are herbs residing primarily in the forest floor layer.  In contrast, tropical forests have a much more even distribution of types of species, and location of growth.

The physiognomy of temperate and tropical forests. Credit: Jackie Spicer.

Quantitatively, 80% of temperate forest plant species are herbs, while only 7% are trees, and there are relatively few lianas and epiphytes.  In contrast, tropical forests boast a much more even distribution of each plant growth form.

Relative species richness of trees, shrubs, lianas, herbs and epiphytes in temperate and tropical forests.

Going along with the growth form distribution finding, most temperate plant species grow on the forest floor, while more tropical species are actually higher up (upshifted) in the understory – in part due to the prevalence of lianas and epiphytes in the understory layer.

Relative species richness of plants at different layers of temperate and tropical forests.

Spicer and her colleagues caution us that the up-shift in the tropical forest profile may be understated by the data, because even the best inventories are likely to miss epiphytes growing high in the canopy.

The tropical forest epiphyte Guzmania musaica. Credit: Michelle Spicer.

These findings have important implications for conservation and forest management.  Logging of tropical forests removes trees, but also removes lianas and epiphytes associated with trees. Lianas recover well from disturbance, but epiphytes take a long time to return following disturbance. Thus even relatively small-scale logging will significantly reduce biological diversity, not only in the plant communities, but in the many species of animals, fungi and microorganisms that interact with these plants. In contrast, temperate forests may be more resilient to logging, because the diverse herbaceous community can recover quickly, particularly if some canopy cover remains after logging.  Spicer and her colleagues argue that over-browsing by large ungulates, and changes in herbaceous species composition resulting from years of fire suppression are the two primary threats to the extensive biological diversity in the temperate forest herbaceous layer. With many species missing from the herbaceous plant community from these two sources, invasive species can take over, changing forest ecosystem functioning.  The researchers suggest that forest managers should prioritize managing the vast diversity of plant species that inhabit the temperate forest floor and understory.

note: the paper that describes this research is from the journal Ecology. The reference is Spicer, M. E., H. Mellor, and W. P. Carson. 2020. Seeing beyond the trees: a comparison of tropical and temperate plant growth-forms and their vertical distribution. Ecology 101(4):e02974. 10.1002/ecy. 2974.  Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2020 by the Ecological Society of America. All rights reserved.

It’s all happening at the ecotone

In an effort to make order out of the chaos of existence, scientists often resort to classifying stuff.  To make order of the natural world, ecologists classify different regions of the world into distinct biomes – large geographical areas with characteristic groups of organisms adapted to that particular environment.  Familiar examples of terrestrial biomes are tropical forests, temperate grasslands and desert, and in the aquatic world examples include open ocean, coral reefs and rivers. But what happens at ecotones, where two or more biomes come together? Research has shown that ecotones can be biodiversity hotspots, as the diverse habitats attract many different species, and may also attract edge specialists – species that are particularly adapted to conditions on the border between the two biomes.


Sara Weinstein collects data at the ocean to land ecotone. Credit: Anand Varma.

Sara Weinstein’s graduate research explored the ecology and transmission of raccoon roundworm, Baylisascaris procyonis, a widespread raccoon parasite that causes severe disease in other animals (including humans).  She was dissecting raccoons to study infection patterns and as she describes “it would have been a waste of perfectly good raccoon guts to not also examine the rest of the parasite community.”  This examination would allow her to determine whether the generalization that ecotones are biodiversity hotspots for terrestrial and aquatic organisms also applies to the much more murky world of gut parasites.


A raccoon poses next to a culvert. Credit: SB Weinstein.

Working with four other researchers, Weinstein compiled a database of published accounts of gastrointestinal parasites from surveys of 256 raccoon populations.  They then used this database to classify parasites as either core or satellite.  Core parasites are locally abundant, common over a large region and can occupy a broad ecological niche.  Satellite parasites are rare, restricted to a small portion of a region and have narrow ecological niches.


Microphallus sp. – a group of relatively rare satellite trematodes collected from a raccoon gut. Credit: SB Weinstein.

Weinstein and her colleagues found that the data divided raccoon gut parasites into two distinct groups.


Top graph. Parasite frequency across raccoon populations. Most parasite genera were found in less than 10% of the raccoon populations.  Dashed line indicates 30% cutoff between satellite and core genera.  Bottom graph. Proportion of raccoons infected with each parasite  in relation to range-wide prevalence.  Larger data points indicate more populations surveyed for a given parasite.


There were eight taxa (genera) that were found in more than 40% of raccoon populations. In contrast there were 51 genera that were found in fewer than 30% of raccoon populations, with the vast majority of these found in fewer than 10% of raccoon populations in the survey (top graph on left).  The eight common taxa – core parasites – also tended to be present in more individuals within each population than did the 51 less common genera of satellite parasites (bottom graph on left).


Having defined core and satellite parasites, the researchers then did a thorough analysis of the gut contents of 180 raccoon collected by trappers and animal control agents in Santa Barbara County between 2012 – 2015. They hypothesized that the prevalence of core parasites should not be overly affected by ecotones.  In contrast, satellite parasites should increase in ecotones, because ecotones provide unique environmental conditions that would be suitable to some of the less common species in the parasite community.


In Santa Barbara County, Weinstein and her colleagues identified four core parasites and nine satellite parasites within the population, with a mean of 2.24 parasite species per raccoon. Racoons nearer to the marine ecotone harbored more parasite species than did raccoons more distant from the marine ecotone, a result of much greater richness of satellite species (left graph below). The story was very different for the freshwater ecotone.  Overall, parasite richness was relatively constant in relation to distance from the freshwater ecotone.  There were actually fewer core parasites but more satellite parasites near the freshwater ecotone (right graph below).


Left graph. Total parasite richness (orange line) in relation to distance from shore.  Satellites (orange fill) increased in abundance near the shore, while core parasites (maroon line) were steady. Right graph. Total parasite richness in relation to distance from freshwater.

Why did core parasite richness decline near the freshwater ecotone?  Weinstein and her colleagues believe that diet may play an important role.  For example, the core parasites Atriotaenia procyonis and Physoloptera rara were more common in raccoons far from freshwater, probably because racoons are infected by these two parasites as a result of eating terrestrial (but not aquatic) insect species that are intermediate hosts for these two parasite species.  As it turns out, these intermediate insect hosts prefer upland habitats that tend to be located relatively distant from the freshwater ecotone.

Increased abundance of rare parasites at ecotones has important implications for human health.  Several emerging infectious diseases, such as lyme disease, yellow fever and Nipoh virus are associated with ecotones. Habitat development by the expanding human population is causing increased habitat fragmentation, creating more ecotones, and potentially increasing the prevalence of these and other, equally unfriendly, parasites.

note: the paper that describes this research is from the journal Ecology. The reference is Weinstein, S. B., J. C. Van Wert, M. Kinsella, V. V. Tkach, and K. D. Lafferty. 2019. Infection at an ecotone: cross-system foraging increases satellite parasites but decreases core parasites in raccoons. Ecology 100(9):e02808. 10.1002/ecy.2808.  Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.


What grows up must go down: plant species richness and soils below.

Almost 20 years ago, Dorota Porazinska was a postdoctoral researcher investigating whether plant diversity influenced the diversity of organisms that lived in the soil below these plants, including bacteria, protists, fungi and nematodes (collectively known as soil biota).  Surprisingly, she and her colleagues discovered no linkages between aboveground and belowground species diversity.  She suspected that two issues were responsible for this lack of linkage. First, the early study lumped related species into functional groups – for example nematodes that eat bacteria, or nematodes that eat fungi.  Lumping simplifies data collection but loses a lot of data because individual species are not distinguished.  Back in those days, identifying species with DNA analysis was time-consuming, expensive, and often impractical. The second issue was that even if aboveground-belowground diversity was linked, it might be difficult to detect.  Ecosystems are very complex, and many belowground species make a living off of legacies of carbon or other nutrients that are the remains of organisms that lived many generations ago.   These legacy organic nutrient pools allow for indirect (and thus more difficult to detect) linkages between aboveground and belowground species.

Porazinska and her colleagues reasoned that if there were aboveground/belowground relationships, they would be easiest to detect in the simplest ecosystems that lacked significant pools of legacy nutrients. They also used molecular techniques that were not readily available for earlier studies to identify distinct species based on DNA analysis. The researchers established 98 1-m radius circular plots at the Niwot Ridge Long Term Ecological Research Site in the Colorado, USA Rocky Mountains. At each plot, they identified and counted each vascular plant, and recorded the presence of moss and lichen.  They also censused soil biota by using a variety of DNA amplification and isolation techniques that allowed them to identify bacteria, archaea, protists, fungi and nematodes to species.

PorazinskaOpening9256 Photo

Field assistant Jarred Huxley surveys plants in a high species richness plot. Credit Dorota L. Porazinska.

As expected in this alpine environment, plant species richness was quite low, averaging only 8 species per plot (range = 0 – 27).  In contrast to what had been found in other ecosystems, high plant diversity was associated with high diversity of soil biota.


Relationship between plant richness (x-axis) and soil biota richness (y-axis) for (A) bacteria, (B) eukaryotes (excluding fungi and nematodes), (C) fungi, and (D) nematodes.  OTUs are operational taxonomic units, which represent organisms with very similar or identical DNA sequences on a marker gene.  For our purposes, they represent distinct species.

Looking at the graphs above, you can see that different groups responded to different degrees; nematodes had the strongest response to increases in plant richness while fungi had the weakest response.  When viewed at a finer level, some groups of soil organisms, including photosynthetic microorganisms such as cyanobacteria and green algae actually decreased, presumably in response to competition with aboveground plants for light and possibly nutrients.

Given the strong relationship between plant species richness and soil biota richness, Porazinska and her colleagues next explored whether high plant richness was associated with soil nutrient levels (nutrient pools).  In general, there was a strong correlation between plant species richness and nutrient pools (see graphs below).  But soil moisture, and the ability of soil to hold moisture were the two most important factors associated with nutrient pools.


Amount (micrograms per gram of soil) of carbon (left graph) and nitrogen (right graph) in relation to plant species richness.

Ecologists studying soil processes can measure the rates at which microorganisms are metabolizing nutrients such as carbon, phosphorus and nitrogen.  The expectation was that if high plant species richness was associated with higher soil biota richness, and larger soil nutrient pools, then the activity of enzymes that metabolize soil nutrients should proportionally increase with these factors.  The researchers found that enzyme activity was very low where plants were absent or rare, and greatest in complex plant communities.  But the most important factors influencing enzyme activity were the amount of organic carbon present within the soil, and the ability of the soil to hold water.


Patchy vegetation at the field site. Credit: Cliffton P. Bueno de Mesquita.

Porazinska and her colleagues hypothesize that the relationship between plant species richness, soil biota richness, nutrient pools, and soil processes such as enzyme activity, exist in most ecosystems, but are obscured by indirect linkages between these different levels.  They hypothesize that these relationships in other ecosystems such as grasslands and forests are difficult to observe.  In these more complex ecosystems, carbon inputs into the soil form large legacy carbon pools. These carbon pools, and the ability of the soil to hold nutrient pools, fundamentally influence the abundance and richness of soil biota. In contrast, in nutrient-poor soils, such as high Rocky Mountain alpine meadows, legacy carbon pools are rare and small. Consequently, plants and soil biota interact more directly, and correlations between plant species diversity and soil biota diversity are much easier to detect.

note: the paper that describes this research is from the journal Ecology. The reference is Porazinska, D. L., Farrer, E. C., Spasojevic, M. J., Bueno de Mesquita, C. P., Sartwell, S. A., Smith, J. G., White, C. T., King, A. J., Suding, K. N. and Schmidt, S. K. (2018), Plant diversity and density predict belowground diversity and function in an early successional alpine ecosystem. Ecology, 99: 1942-1952. doi:10.1002/ecy.2420. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.