Few prairie plants stand out more than the compass plant (Silphium laciniatum). With its uniquely lobed leaves and a flower stalk that rises well above the rest of the vegetation, it is nearly impossible to miss. It is also quite easy to identify. Seeing a population in full bloom is truly a sight to behold but the ecology of this species makes appreciating its splendor all the more enjoyable. Today I would like to introduce you to this wonderful member of the aster family.
Any discussion about this species inevitably turns to its common name. Why compass plant? It all has to do with those lovely lobed leaves. When they first develop, the leaves of the compass plant are arranged randomly. However, within 2 to 3 weeks, the leaves will orient themselves so that their flat surfaces face east and west. They also stand vertically. This is such a reliable feature of the plant that past generations have learned to use it as a reliable way in which to orient themselves.
Photo by peganum licensed under CC BY-SA 2.0
Of course, helping humans find their way is not why this feature evolved. The answer to their orientation has to do with surviving in the open habitats in which they grow. Anyone who has ever spent time hiking around in prairie-like habitats will tell you that the sun can be punishing and temperatures get hot. What's more, the range of this species overlaps with much of the rain shadow produced by the Rocky Mountains meaning water can often be in short supply.
By orienting their leaves in a vertical position with the flat surfaces face east and west, the plants are able to maximize their carbon gain as well as their water use efficiency. At the same time, the vertical orientation limits the amount of direct solar radiation hitting the leaf. In essence, compass plant leaf orientation has evolved in response to the stresses of their environment. Research has shown that the sun's position in early morning is the stimulus that the plant cues in on during leaf growth.
Aside from its fascinating biology, the compass plant is also ecologically important. Myriad pollinators visit its large composite flowers and many different species of birds feed on their seeds. However, it is the insect community supported by the compass plant that is most impressive. Surveys have shown that nearly 80 different species of insect can be found living on or in it stems. Many of these are gall making wasps and their respective parasitoids. With individual plants producing up to 12 stems each, these numbers soon become overwhelming. Needless to say, this is one of the cornerstone plant species anywhere it grows naturally.
Photo Credit: [1]