Touching Effects: Visiting Plants In The Field May Alter Their Future

James Cahill (now at the University of Alberta, formerly at the University of Delaware) together with Jeff Castelli and Brenda Casper (University of Pennsylvania) were studying plants in an abandoned hayfield and along a forest floor when they noticed that plants they had marked for study were experiencing extremely high rates of attack by insects. The scientists hypothesized that they, the human visitors, were somehow causing this to occur.

To test their theory, the ecologists marked 605 plants within 12 plots in an abandoned hayfield in Pennsylvania's Brandywine Valley. Six plots were visited weekly, while the remaining six plots were left unvisited as controls. When plants were visited, they were stroked once from base to tip, with care taken not to damage the plant body. This handling was designed to mimic what occurs when scientists typically take repeated measurements of plants in field studies.

One of the species studied, Indianhemp (Apocynum cannabinum) was negatively affected by visitation, experiencing high rates of leaf area loss due to insects. A second species, Sulphur Cinquefoil (Potentilla recta), seemed to benefit, as the plants experienced less leaf area loss when visited than when unvisited. The third species, commonly known as Butter and Eggs (Linaria vulgaris), also tended to fare better when visited. Fewer plants of this species died when visited than did their unvisited control counterparts.

The remaining species in the study, Carolina Horsenettle (Solanum carolinense), Canada Thistle (Cirsium arvense), and Kentucky Bluegrass (Poa pratensis), were not affected either positively or negatively by visitation.

"The long-standing assumption that field researchers are benign observers is fundamentally flawed," says Cahill. "The very act of conducting an experiment can alter experimental results, and the potential effects that researchers may have when visiting plants must be addressed in future field studies."

The researchers hypothesize that touch-activated plant responses may be the cause of some of the effects experienced by the plants in the study. These responses, which have long been documented by ecologists, can include many physical and chemical reactions to handling. For instance, touching a plant may result in changes to its structure, architecture and the toughness of its leaves. It may also result in a release of volatile insect-attracting chemicals from the plants themselves or from neighboring vegetation.

The scientists also note that when nearby plants were trampled, the plants which were being studied received more light, which could prove beneficial to plants. Competition for light can reduce the growth of many plant species and increased amounts of light can make some plants more vigorous. Trampling the neighboring vegetation could, however, also make the plants in question more visible to plant-eating insects. The ecologists were unable to find any clear pattern of traits which would make an individual species more or less vulnerable after visitation. They suggest that further studies should examine whether the effects of visitation are common in variety of plant communities, and also whether the observed effects were due to the act of touching or merely approaching plants. They also suggest additional investigations into how field visitations affect insect community structures.

"Although questions remain about visitation effects," Cahill says, "we believe it is clear that field biologists working with plants can no longer assume that their activities in the field do not alter the biology of study organisms."

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