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.

Seagrass scourge: when nutrient enrichment reaches the tipping point

Sean Connell has watched as south Australia has lost vast expanses of kelp forest and seagrasses over the past years. One of the primary culprits associated with loss of seagrass meadows is excessive nutrients, particularly nitrogen, which enters the ecosystem with runoff, and causes an increase in algal epiphytes (epiphytes are small plants that grow on other plants). Epiphytes can negatively affect seagrass by blocking sunlight needed for photosynthesis, and indirectly, by increasing the rate of cellular respiration within the ecosystem, thus using up oxygen needed by seagrass for metabolic processes.


Two dolphins swim above a bed of seagrass off the south Australian coast.

Connell and his colleagues noticed that seagrass loss was often sudden; a large seagrass meadow would appear to be in good shape, and then it would abruptly disappear. They suggested that there might be a threshold effect in nutrient levels that seagrasses can tolerate; that these systems function well until a certain threshold in nutrient levels is crossed, above which there is an abrupt loss of seagrasses. They tested this hypothesis by subjecting plots of the seagrass Amphibolis antarctica to seven different concentrations of dissolved inorganic nitrogen (DIN) over a 10 month period, and monitored the abundance of epiphytes and seagrass over that timespan.

The meadows were about two km offshore from Lady Bay, Fleurieu Penninsula, Australia, in about 5 meters of water. Different amounts of nitrogen fertilizer were wrapped in nylon bags (for slow continuous release of DIN) and staked to the ocean floor. Amphibolis antarctica grows by producing new leaves at the top of each leaf cluster, but at the same time it drops old leaves. Leaf turnover, the researchers’ measure of growth, is simply new leaf production minus old leaf drop. The researchers tied on a small nylon cable at known locations on selected plants, noted how many leaves were above and below each tie at the beginning of the experiment, and recounted leaf number 10 months later. Finally, the researchers measured epiphyte growth by microscopically viewing a sample of seagrass leaves, and counting the number seagrass leaf cells that were covered by epiphytes.

Seagrass growth was relatively unaffected by all tested DIN levels.


Leaf production per day in relation to concentration of DIN.

However, leaf drop showed a strong threshold effect; leaf drop rates increased sharply between 0.13 – 0.15 mg/L of DIN.


Leaf drop per day in relation to concentration of DIN.

Putting these two graphs together, you can see (below) that leaf turnover switched from positive to negative at 0.13 – 0.15 mg/L of DIN. Negative leaf turnover translates to a sudden loss of seagrass at that threshold. At least in this system, at this location, 0.13 – 0.15 mg/L of DIN is the tipping point, beyond which the seagrass system suddenly goes into decline.


Leaf turnover per day (left y-axis and red data), and Epiphyte cover (% – right y-axis and green data), in relation to concentration of dissolved inorganic nitrogen.

The graph also shows that the tipping point coincides with an epiphyte cover of approximately 60%. It is possible that increased epiphyte cover may reduce seagrass photosynthetic rates (particularly in lower leaves), so that leaf turnover suddenly shifts into the negative zone, but the study was not designed to identify the underlying mechanism.

Seagrass meadows perform important ecosystem services, such as absorbing excess nutrients from the sediment, and providing habitat and food for a diverse group of grazers and indirectly, for their consumers. Thus seagrass conservation is vital. The danger here is that moderate levels of nutrients do not appear to have much effect on seagrass populations, but there is an abrupt shift to seagrass loss once the nutrient threshold is crossed. This makes the system very difficult to manage, because the loss occurs without warning. Australian ecologists have repeatedly failed to restore lost seagrass meadows, as simply reducing nutrient levels does not reverse the process. Thus anticipating seagrass loss before it happens is the most viable management solution for this critical ecosystem.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Connell, S. D., Fernandes, M., Burnell, O. W., Doubleday, Z. A., Griffin, K. J., Irving, A. D., Leung, J. Y.S., Owen, S., Russell, B. D. and Falkenberg, L. J. (2017), Testing for thresholds of ecosystem collapse in seagrass meadows. Conservation Biology, 31: 1196–1201. doi:10.1111/cobi.12951. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2017 by the Society for Conservation Biology. All rights reserved.