Walnut Trees and Plant Species Diversity

Natural History of The walnut trees

Walnut Fig 1

In North America, some of the most common members of the Juglandaceae family are Juglans cinerea and Juglans nigra, otherwise known as the Butternut or White Walnut and the Black Walnut or American Walnut respectively. These trees are found throughout eastern North America, and prefer deep, moist, fertile soils of bottomlands and gentle slopes, often occurring as an occasion tree amongst a deciduous forest of hardwoods (Juglans nigra (F) ), (Black Walnut: Juglans nigra). They are therefore a good object of study in the arboretum.

J. nigra is one of the most valuable hardwood trees in the United States, and has been used for everything from flooring to gunstocks throughout US and Canadian history. Because of its versatility, the species has become scarce and nearly extinct in some areas. Not native to the St. Lawrence Lowlands, the Black Walnut’s appearance in the Morgan Arboretum is of unknown origin. It is possible that the species was propagated from southern Ontario or the northeastern United States, due to its widespread use as a hardwood (Black Walnut: Juglans nigra).  A deciduous tree, the Black Walnut can be recognized by its pinnate leaves with 15-21 leaflets, as well as its dark, deeply furrowed bark (Juglans nigra (F) ) It can also be is typified by pedicellate (having a pedicel, a stem)   flowers that lack carpels (staminate)  (Stanford, Harden and Parks).

The Black Walnut Fruit

The Butternut tree is native to the St. Lawrence Lowlands and also minor component of deciduous forests in the area (Canada). It can be identified by its unique bark consisting of flat grey ridges arranged in a diamond or “X pattern” (Winter three identification) , as well as its compound leaves consisting of 11-17 leaflets ((Stanford, Harden and Parks). A somewhat fast growing and short lived tree, commonly dying after 75 years, starts producing fruit at about 20 years of age (Canada). These edible nuts are easily recognized by their hard jagged ridged shell covered by a green hairy husk (Stanford, Harden and Parks).

The Butternut Fruit

The Butternut Fruit

Perhaps the most defining characteristic of the Juglans Genus is the secretion of the chemical Juglone-C10H6O3– (Hejl, Einhellig, Rasmussen) into the soil surrounding the tree. Contained in the roots, leaves, fruit, and bark of the tree, this chemical inhibits the growth of many species of plants (Growing Hardwoods). Being that these trees are not shade tolerant, this chemical is Juglans’ way of eliminating competition in a deciduous hardwood forest. In fact, many trees planted beside the black walnut and this decline was attributed to Juglone (Hejl, Einhellig, an Rasmussen). However it should be noted that there are species such as beech, birch, and others that are tolerant to Juglone and are often times the only trees found surrounding a dense population Walnut trees (Appleton & Al.).

The secretion of Juglone is called an “allelopathic” interaction. In other words, the walnut can influence the growth and therefore the diversity of the plants in its surroundings by the mean of a chemical (Growing Hardwoods). Our goal as an undergraduate research team, was to verify if the presence of the walnut trees had a noticeable impact their environment, the Morgan Arboretum.

Our research question and our method

As we could not measure the very release of Juglone, we simply focused on the diversity of plants species found around the walnut trees in the arboretum. Therefore, we asked the following question:

“What is the diversity of plant species in the vicinity of the walnut trees in the Morgan Arboretum?”

We were more interested in the number of species and the broad category in which they fit, rather than the species themselves. The “broad categories” we used:

  •   A fern,
  •   A small plant of the understory or a shoot (less than 1 meter high, not woody),
  •   A shrub or a young tree (woody, more than 1 meter high),
  •   A mature tree (woody, cannot be hold tightly in the hand; e.i. diameter bigger than 10cm)

Mosses and fungi were excluded from the data as most mosses observed grew on rocks. Also, Fungi are not known to be affected by the chemical (Appleton & Al.).

That being said, we sampled thoroughly the number of plant species around the walnut trees:

  •   Step one: Identifying the walnut trees. Because of the time of the year, most leaves were gone on the trees part of the Juglan family. So, they were identified on the basis of two criteria: the bark and the branching. The butternut trees have a light bark with an “X pattern” made of flat, white ridges (Winter three identification). The black walnut has a dark bark with deep furrows. It is dark brown in the inside (Winter three identification). This bark can be confused with the bark of the ash tree. But the ash tree has opposite branching, whereas the walnuts have alternate branching (c.f. pictures and drawing). We also looked for fruits (nuts) below the litter.
  •   Second step: Setting up a distance of sampling. We chose the distance of 7 meters from the bark as it corresponds to the branch cover (the “Drip Line”) and the estimated root radius of most walnut trees in the Morgan Arboretum.
  •   Third Step:  Sampling in circular zone–7 meters in diameter– around the bark of the trees with the use of a measuring tape. The type of plants (described above) was noted and its distance from the tree was recorded as well. We took a picture of every species and labeled them with a number on the picture to be able to recognize them from tree to tree.

Thirteen trees were sampled. A total of six walnut trees were found in different part of the forest (Juglans cinerea trees and three Juglans nigra). Same sampling methods for the same number of “control trees” (oak, maple, and shagbark trees chosen randomly in the same part of the forest than their corresponding walnut) was done. The three black walnuts (Juglans nigra) we sampled were found in a patch of the Arboretum composed almost uniquely of black walnut trees. Since this patch was crowded with black walnuts, we could not sample a control tree in this area and thus sampled one control tree for all three black walnut in an area nearby.

Data sampling with the use of measuring tape

Data sampling with the use of measuring tape

Problems encountered

Two major problems were encountered. Firstly, finding a significant number of butternut trees in the Morgan Arboretum was a challenge. Hence, our research question, initially focused on white walnut, was broadened to walnut trees (black walnuts were more abundant in the Arboretum). Secondly, the time of year caused problems. In fact, between the first and second session of data sampling (one week apart), most of the plants from the understory had wilted because of the cold. It is why on the second lab, we sampled a lot more shrubs and trees than small plants. Also, the walnuts were harder to identify as the leaves had fallen.

Works Cited

Alice M. Stanford, Rachel Harden, and Clifford R. Parks. “Phylogeny and  Biogeography of  Juglans” American Journal of Botany 87(6): 872–882.  June 2000. University of North Carolina, Chapel Hill, North Carolina 27599 USA. American Journal of Botany Online. Web 04 November. 2014.

Angela M. Hejl, Frank A.  Einhellig, and James A. Rasmussen.“ Effects of Juglone on Growth, Photosynthesis and Respiration.” The Journal of Chemical Ecology. March 1993, Vol 19, Issue 3, pp 559-568. Sprinker link. Web. 04 November. 2014.

“Black Walnut: Juglans nigra” 4-H Forest Resources. Environmental Education at SFFC, n.d. Web. Nov 06 2014.

Bonnie Appleton, Roger Berrier, Roger Harris, Dawn Alleman, and Lynnette Swanson. “The Walnut Tree: Allelopathic Effects and Tolerant Plants”. Virginia Tech Dept. of Horticulture, Norfolk VCE, and Chesapeake VCE. Virginia Cooperative Extention. Web 04 Nov. 2014.

Canada. Dept. of Environment. Species at Risk Public Registry: Butternut. Ottawa. mod. Oct. 21 2014. Web. Nov 06 2014.  

“Growing Hardwoods: About black walnut”. Walnut Council. Write Forestry Center. 2011. Edited Nov 06, 2014. Web. Nov. 08, 2014.

“Juglans nigra (F)”. Royal Horticultural Society. N.p 2014. Web Nov. 06 2014.

“Winter three identification”. Three bark ID. N.p, n.d. Web. Oct 19 2014.


About Crystal Ernst

Hakai Institute postdoctoral scholar at Simon Fraser University (B.C.)

3 comments on “Walnut Trees and Plant Species Diversity

  1. Awesome post! We love the simple and straight forward methods! The post also excellently describes how to identify the Walnut tree and we will be looking for them next time in the Arboretum! We were wondering if the Walnut is native to North America? Since some trees are tolerant to Juglone, we were perhaps thinking this is evidence to coevolution!

  2. That’s a good question, the Black Walnut is not native to the Lowlands, it has been introduced for its good properties as a hardwood tree.
    And about the coevolution, that’s a good guess, but nothing has been proved (so far!) about it. The beech tree is very resistant to Juglone. It might have co-evolved with the butternut (which is native to the Lowlands) and acquired a resistance to juglone that allows it ti survive among black walnuts too.

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