Allelopathy and Other Forms of Plant Supression Under Trees

[Brett Chedzoy]

Every late summer and early fall as I walk along the edges of our pastures adjoining the woods I notice in the drier years a significant difference between forages growing inside and outside the influence of the trees' roots.  The browning and stunting of grass growth appears to be limited to a band along the edge.  Further inside the woods and silvopastures, the effect isn't nearly as noticeable.  I have some theories on why this is so, but they're just guesses at this point.  What is clear, however, is that the suppression is (probably) largely a reflection of competition for soil moisture.  In some cases, I think trees can help increase available soil moisture through things like: shading and microclimate buffering; hydraulic lift; and interception/infiltration (stem flow and crown drip).  Much, however, is species and site specific. In some ways, trees can enhance the growth of forage plants - and in other ways, they can compete and or limit their growth.

So sunlight interception aside, two other significant ways in which trees can have a suppressing effect on understory plant growth are: allelopathy and competition for rooting space.   These are factors that we need to pay attention to when selecting trees to favor in our silvopasture thinnings.

Allelopathy is the suppression of one plant by another through chemical secretions.  A common example is juglone in some species of Juglans (walnuts).  I came across a short article on this in a recent newsletter:

http://www.wvagriculture.org/market_bulletin/Past_Issues/2014/10-14... 

Cereal rye is another plant known for it's allelopathic properties, so care must be taken when following a rye crop with something else.  Time (> 45 days) and disturbance of the soil surface seem to be remedies in the case of rye.  Allelopathy from trees may take much longer to dissipate.  I've heard stories of gardeners who still can't grow tomatoes even a few seasons after removing the large walnut tree.

My guess is that there are many plants commonly found in silvopasture environments that exhibit some degree of allelopathy on their neighbors.  In most cases, the effects are too small to notice or even be detected.  I also feel that many plants have not been well-studied in this regard.  Allelopathy in walnut is well-know because it's a common and important tree - especially in residential areas where it can conflict with other horticultural endeavors.  But a plant like European Buckthorn (Rhamnus cathartica) on the other hand has not received the same degree of attention and study.   I haven't been able to find anything in the literature on allelopathy in buckthorn, but looking at the suppression of most herbaceous plants under its drip line (~ rooting zone) suggests that there is clearly some degree of allelopathy being exhibited.  Below are a couple of pictures showing some mature buckthorn on our farm.  The first shows bare soil under living buckthorns, despite plenty of side lighting.  The second shows a nearby stand of buckthorn that was poisoned (girdled, and a systemic herbicide sprayed into the girdle cut at the base of the stem).  Within a season of killing the mature trees, the existing seedbank appears to have fully expressed itself.

The other commonly observed form of forage suppression beneath some trees is due to root competition.   Just as not all trees are equal in their crown density and the amount of sunlight that they intercept, neither are they the same in the way that their roots grow and compete with other plants.  Beech and hemlock are two species that appear to have very dense and superficial roots that suppress other plants from growing directly beneath them (at least out to the drip line of the crown, which roughly coincides with the main root system).  The picture below illustrates the effect of the root system of a hemlock tree on forage growth and composition, despite considerable side lighting.  Although beech, hemlock and possibly other species like sugar maple may reduce forage growth in close proximity, there may well be other reasons to leave a component of these species in the silvopasture.

In the case of the above picture, a clear difference can be seen in the foreground beneath the hemlock (no grass, lots of violets) vs. the background with a maple overstory (grass).  Although root competition may be the primary factor in this difference, solar energy levels directly beneath the hemlock - and even pH in the soil surface are factors that probably also contribute to the difference in vegetation composition and growth.