My Review of Decurrent Trees

Just because it’s winter doesn’t mean there aren’t plenty of plants to look at. Deciduous trees and shrubs become particularly interesting during the winter months with their exposed branches and their growth habits made more obvious. The beauty of a tree’s “skeletal” structure is revealed when it’s stripped of its leaves and set against a winter sky. Winter is also a great time to prune certain trees and shrubs (when appropriate), partly because their branches are so easily viewed and “problem” areas readily reveal themselves. Whether you’re observing a tree’s branching structure simply for enjoyment-sake or because you plan to prune, you may find yourself noticing distinct differences in the growth habits of trees. Distinct growth habits can help you identify trees. They can also tell you something about a tree’s environment or growing conditions.

In the book, The Tree, Colin Tudge defines a tree as “a big plant with a stick up the middle.” Sometimes this “stick” runs straight up from the ground to the top of the tree without interruption and is the tallest portion of the plant. Other times, the “stick” reaches a certain height and branches out into multiple “sticks,” each one reaching out in a different direction – some heading more outward, while others continue to reach for the sky. This is the difference between excurrent and decurrent growth.

excurrent growth habit: dawn redwood (Taxodium distichum)

A tree with an excurrent growth habit has one central leader – or single trunk – that reaches all the way to the top of the tree. Side branches occur along the length of the trunk and generally get shorter as they move up the tree, producing a pyramidal or conical shape. Think of a typical Christmas tree. Many conifers exhibit excurrent growth, as do several deciduous trees such as sweetgum and pin oak, as well as aspens and other poplars. When a tree divides part way up the trunk, splitting into several large branches – none of which could be considered the dominant branch – it is exhibiting a decurrent growth habit. Trees that generally fall into this category include elms, maples, oaks, and ashes. The growth habit of a tree is largely a result of its genetics, but plants are known for their plasticity, taking on a wide variety of forms depending on their parentage and their circumstances. Trying to identify a tree based only its growth habit, isn’t likely to yield great results.

decurrent growth habit: oak (Quercus sp.)

The environment that a plant is growing in can have noticeable effects on the form the plant takes. A tree growing up in a forest thick with other trees will typically grow straight up in search of sunlight and will branch out very little until it can get up high enough to do so. That same species of tree growing in an open field might instead branch out extensively at a much lower height, taking advantage of the generous amount of space to stretch its branches out wide. As Tudge puts it in The Tree, “one form for the forest, another for the open ground.” Additionally, things can happen in a tree’s life that will drastically alter its form. If, for example, a storm comes through and breaks off a tree’s central leader, several side branches might grow out and upward to take its place, giving an otherwise excurrent tree a decurrent form. The pruning that humans often do (sometimes unwisely) to trees and shrubs, particularly in urban settings, can also alter a plant’s natural growth habit considerably. These are important considerations to make when assessing the forms of trees.

decurrent growth habit: golden rain tree (Koelreuteria paniculata)

I give decurrent growth habits five stars. No shade on excurrent trees. They’re also beautiful. But while trees with excurrent growth habits have otherwise predictable forms, decurrent trees are full of surprises. Their broad and rounded forms provided by their deliquescent branching structures are endlessly interesting, and their capacious canopies ensure that no two trees are alike.

decurrent growth habit: Malus sp. (I presume)


The Discovery of a Living Fossil

In the early 1940’s, the genus Metasequoia was only known scientifically in fossil form. It had, in its day, been a widespread genus, found commonly in many areas across the Northern Hemisphere. It thrived among the dinosaurs. However, sometime during the Pliocene, the genus was thought to have died out. Thousands of fossils were left behind, and that would have been the end of the story had a member of its genus not been discovered still alive in a Chinese province later that decade. Its discovery is easily one of the greatest botanical stories of the 20th century, fascinating in its own right. The circumstances surrounding its scientific description, as it turns out, are equally interesting.

In the January 2016 issue of Landscape Architecture Magazine, Kyna Rubin details the event in an article entitled The Metasequoia Mystery. It’s the type of story that you almost need a crazy wall to sort out. A broad cast of characters interacted at various levels in order to make this profound discovery during a tumultuous time when the world was at war and China was being invaded by Japan.

Speaking of Japan, let’s start there. In 1941, Japanese paleobotanist, Shigeru Miki, published research describing fossils that for decades were thought to be either Sequoia or Taxodium as a new genus, Metasequoia. As Rubin points out, due to the war, “not every Chinese botanist would have had access to recent international research, let alone articles by botanists of an enemy country.” This could explain why in 1943 when Zhan Wang – a professer of forestry at Beijing University and the forest administrator for the Ministry of Agriculture and Forestry – was introduced to a living Metasequoia by an old classmate and local villagers in the Hubei Province, he wasn’t sure what he was looking at.

The tree was obviously important to the local people. They called it shuisa (water fir) and had built a shrine around it. Wang collected several branches and some cones that had fallen on a rooftop. At the time he identified it as Glyptostrobus pensilis (water pine), a tree common to the area; but he may have wondered if this was correct.

Eventually Wang’s samples and the details of his collection were brought to the attention of Wanjun Zheng, a dendrologist at the National Central University. Intrigued, Zheng sent his graduate student, Jiru Xue, to collect more samples from the same tree that Wang had encountered. These samples were more complete, and when they were presented to Xiansu Hu – the director of Fan Memorial Institute of Biology in Beijing – the mystery was solved. Hu had access to Miki’s research and concluded that what they had was a living fossil.

In 1948, Hu and Zheng published a paper describing the species and giving it the official name, Metasequoia glyptostroboides. The discovery ignited the botanical community as well as the general public, and soon seeds of what became commonly known as dawn redwood were being disseminated across the globe. Unfortunately, Wang’s contribution was not mentioned in the original paper, and the exact account of the discovery became convoluted.

photo credit: wikimedia commons

Dawn redwood (Metasequoia glytostroboides) is a deciduous, medium to large tree. Its cones are round and about 1 inch long. Its leaves are oppositely arranged and have a feather-like appearance. Its bark is fibrous, stringy, and red-brown to gray in color.  (photo credit: wikimedia commons)

At some point, a discussion between Zheng and a forester named Duo Gan (also known as Toh Kan) revealed that Gan had come across the tree in 1941, but he did not make any collections. Despite Zheng learning of Gan’s encounter after Zheng and Hu’s original paper had been published, Gan’s story became prominent, further obscuring the role that Wang played.

It’s important to note that none of Wang’s original collections were used as the type specimen – the particular specimen of an organism to which the scientific name is formally attached and is referred to in the scientific literature. The type specimen was collected by Xue. This is not uncommon, as initial collections may not always be in the best condition and may not include all the parts and pieces necessary to identify and describe a new species. But, as Rubin notes, “it was Wang’s specimens [that Zheng and others] had first examined and those specimens brought the tree to their attention to begin with.” So Wang’s contribution is an important part of the story.

Thanks to Wang’s former students, his role in the discovery has received greater exposure. Jinshuang Ma in particular has made it his mission to highlight the part that Wang played in the event. Apart from maintaining a website all about Metasequoia, Ma also spent several years searching for a lost herbarium specimen collected by Wang, which he found in an abandoned herbarium in Nanjing. You can read about his find in this article from the August 2003 issue of the journal Taxon. (Ma’s well researched summary of the events surrounding the Metasequoia discovery is also worth reading.)

Failure to acknowledge Wang’s contribution (at least initially) perhaps didn’t make waves outside of China, but in Rubin’s words, “the omission of Wang’s contribution sparked immediate hullabaloo inside China’s botanical circles in the late 1940’s.” Power and class differences likely played a big role. Hu and Zheng were established scholars that had received their educations in the United States and France respectively. Wang was young, from a remote village, and had not studied abroad. While Wang “went on to become one of China’s most distinguished forestry experts and botanists,” he was early in his career at the time of the Metasequoia discovery.

A deep respect for the elders in his field may be the reason that Wang’s students claim that he “never complained” about his treatment. His students go on to say that Wang “was not interested in personal gain,” and instead was simply satisfied to see that Metasequoia “was now growing successfully all over the world and was better protected.” It is listed as endangered on the IUCN Red List and would likely be extinct in its shrunken native range had awareness of its existence not come about when it did.

Fossil of Metasequoia occidentalis - photo credit: wikimedia commons

Fossil of Metasequoia occidentalis – photo credit: wikimedia commons

There are plenty of other interesting details to this story. Read the full article and check out the links on to learn more. The account of Jiru Xue (also known as Hsueh Chi-Ju), the graduate student who collected the type specimens, is particularly interesting. Suprisingly, the tree Wang and Xue took their collections from is still alive today and is estimated to be over 400 years old.

Other longform article reviews on Awkward Botany: