Field Research at Palo Verde

By Claire Jackan

        We spent the last week and a half at the Palo Verde Biological Station in the Guanacaste province of Costa Rica.  The faculty-led projects culminated our experience and have helped us develop and improve our understanding of the research process, particularly regarding field methods and data analysis. We spent two days in the field and one day preparing an extended abstract and group presentation to share with the class. One of the two field days was dedicated to Professor David’s project involving the relationship between Guazuma ulmifolia fruit size and parasitic mistletoe, and the other was focused on Professor Mariano’s project about the mutualist defense response of ant colonies to Acacia herbivory. 

            My group presented the mistletoe project.  We learned that mistletoe (Phoradendron quadrangulare) is a parasitic plant that is dispersed by birds and is introduced into the branches of many host tree species, including Guazuma ulmifolia. In some studies, mistletoe has been shown to reduce host performance in the form of reproduction and growth and has also shown increased host death rates. We expected that Guazuma ulmifolia trees with greater percent mistletoe cover would have depleted resources caused by the parasite, which would result in smaller fruit size. Our sampling was difficult to control in terms of representing all levels of mistletoe parasitism equally because we were only able to use unripe, accessible fruit (within 7.5 meters from the ground using tree pruners). For a few of the taller trees, one of the students in our group showed off her tree-climbing skills by scaling several branches in order to reach the high fruits. When the branch was clipped off with the tree pruners, two of us (the ‘catchers’) would quickly collect the falling fruit and add it to our data collection.  At the end of the two days in the field, Professor David was the only one to have suffered the painful ant bites as he accidentally bumped into the Acacia trees several times during our collecting.

            Since we were not able to test causation in this study, we used JMP Pro to instead determine correlations between mistletoe percent coverage and fruit length, width, and mass. None of these tests found any significant correlation between fruit size and percent mistletoe; therefore the data did not support the hypothesis that percent mistletoe would have an inverse relationship with smaller values for length, width, or mass of fruit due to the parasitic nature of the mistletoe. This could be because mistletoe does not actually have an effect on fruit size, or because we were unable to account for confounding variables such as the proximity of fruit to the mistletoe, proximity of the branch to sunlight, or proximity of the tree to the marsh. Overall, the faculty-led project about the relationship between mistletoe parasitism and fruit size was a great opportunity to learn more about plant symbiotic relationships in the tropical dry forest and it was helpful to practice presenting this information in front of the class as well.