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.