Red Mangoves

2009 The Correlation between Trunk Circumference and Number of Prop Roots in Rhizophora mangle

Alan Bohn, Virginia Conover, Heather Hill, & Mike Mastropole NCSSM 3/6/2009

Abstract: The purpose of this study was to find a correlation between the number of aerial prop roots and the circumference of the Rhizophora mangle on Wee Wee Caye, Belize. We did this by measuring the circumference and counting the number of prop roots of 130 trees. We collected data over 13 sites around the caye in order to investigate environmental differences. We found that there were more roots on trees closer to water and where there was more wind. We also made observations that the nutrients from the outhouses may produce stronger roots. A large frequency of trees with seven to nine roots represented the population of the trees. Introduction: Wee Wee Caye is an island off the coast of Dangriga, Belize. It is one of the dozens of mangrove cayes in the lagoon of the largest continuous barrier reef in the Western Hemisphere (Farnsworth, 1996). The substrate that formed the caye is ground up dead coral. Above the coral rubble, the mangrove swamps have laid down debris and peat, about one meter deep (Shave, 2009). The focus of this study is Red Mangrove, Rhizophora mangle, which tends to lean out over the water, is a pioneer species that will, over time, increase the size of the caye. The mangrove sends out aerial prop roots in order to support itself when growing horizontally to reach more sunlight. The prop roots extend only to the hydrogen sulfide layer and allow the tree to exchange gases for aerobic respiration (Rutzler, 1996). Since the tidal amplitude is approximately 30 cm(Farnsworth, 1996), the mangrove roots create a unique habitat for many organisms. The mangroves also prevent erosion and buffer storm systems. Red mangroves are viviparous (Rutzler,1996), meaning that the seeds germinate and mature while on the branch of the parent. These seeds are one of the most successful drift seeds, contributing to the Red Mangroves’ abundance on the coasts of Belizean cayes (Shave, 2009) . For this study, the goal will be to determine a relationship between circumference of the trunk and the number of prop roots for the Red

Mangrove. Our initial hypothesis is that we will find a positive correlation where the number of roots

increases as the circumference increases. We also plan to investigate different environmental factors such as wind and water coverage to see how it affects the number of prop roots. Methods and Materials: For the study, a Garmin GPS and two Kestrel wind meters were brought from the North Carolina School of Science and Mathematics. Three measuring tapes were borrowed from Wee Wee Caye Marine Biological Station. In order to best represent the mangroves around the island, thirteen sites were chosen, eleven around the island and two inland. Ten trees were sampled from each site which should be a sufficient number to show a correlation while taking into account the limited time available for data collection. While it is sometimes difficult to distinguish an individual tree from a clone, we decided to count the roots of clones if it clearly had an individual trunk with its own prop roots. We observed that prop roots often grew off of other prop roots; however we only are including prop roots in our count which were directly connected to the trunk of the tree. Circumference was measured right above the highest aerial prop root.

Results:

Figure 1: A map of Wee Wee Caye showing the 13 study sites

Figure 2:

ResultsSite

GPS Coordinates

1

16.76412: N 88.14284: W

2

16.76412: N 88.14230: W

3

16.76424: N 88.14236: W

4

16.76381: N 88.14191: W

5

16.76416: N 88.14173: W

6

16.76460: N 88.14171: W

7

16.76500: N 88.14173: W

8

16.76477: N 88.14314: W

9

16.76534: N 88.14323: W

10

16.76583: N 88.14330: W

11

16.76581: N 88.14277: W

12

16.76552: N 88.14169: W

13

16.76495: N 88.14275: W

GPS Coordinated for each site

Figures 1 and 2 show the locations of the sites where we measured trees. The map was created using Google Earth 4.3.7284.

Circumference (cm) Figure 3:

All site data comparing the circumference to the number of roots

Circumference (cm)

Circumference (cm) Figure 4: Sites 4, 5, and 10

Circumference Linear(Circumference)

Figure 5:

Sites 2 and 3

Figure 4 represents the trees that are located on the coast and are in areas that

experience a greater amount of wind. Figure 5 represents the sites that are located on the inner areas of the caye.

Figure 6:

Represents frequency of trees per number of roots

Figure 6 represents a Gaussian distribution curve and shows the whole population of trees. The peak represents the area of greatest frequency. Discussion: All sites on the east side of the island have strong winds. Sites 2, 3, and 13 were further inland and therefore had relatively low winds. We noticed that in sites 5 and 6, the trunks of the trees are particularly parallel to the ground while the inland sites are more vertical. The trees in site 1 are younger since hurricane Keith destroyed many of the trees in fall of 2000 (Shave, 2009). At site 12, we observed trees leaning into the island, which is the opposite of what we observed at all the other sites. When talking with Paul Shave, the caretaker of the island, we learned that site 12, given the name Wee Wee

Wee Caye, started as a single red mangrove the sprouted a few meters off the coast. After several years, it grew larger and more trees sprouted, eventually connecting it to the Wee Wee Caye’s coast. This explains what we observed since the trees on Wee Wee Wee Caye would have originally leaned out in all directions, including towards the Wee Wee Caye. In observing Figure 3, there is positive relationship, proving our hypothesis that the number of roots increases as the circumference increases. We decided to separate the groups based on environmental factors. We observed mangroves where the water reached further inland tended to have more roots. When we looked at inland sites versus costal sites, there were noticeable differences caused by wind and water. Sites 4, 5, and 10, are examples of where the trees were surrounded by water and tended to have more roots. This was proven as the slope in Figure 4 was greater than the slope in Figure 5, showing that the trees in the water grew a greater number of roots as circumference increased. In locations near the outhouses, we observed sturdier roots which are probably due to a higher level of nutrients. Using all of the tree data, we created a distribution curve based off a Gaussian Regression in Excel 07. The resulting graph is shown above in Figure 6. This demonstrates that out of the 130 trees that we measured, the highest frequency occurred for seven to nine roots. We found this interesting because it shows 45.38% of the trees are within that range.

Literature Cited: Clark, M. and M. Lee. 2008. Distribution of Mangrove Seedlings in Wee Wee Caye, Belize. Unpublished.

Farnsworth, E.J. and A.M. Ellision. 1996. Scale-Dependent Spatial and Temporal Variability in Biogeography of Mangrove Root Epibiont Communities. Ecological Monographs 66(1): 45-66.

Rutzler, K. and I.C. Feller. 1996. Caribbean Mangrove Swamps. Scientific American: 94-99.

Shave, Paul. Personal Communication. February 26- March 5, 2009.

Sweetser, M.R., R.V. Zucker, M.A. Ahearn, and G.A. Boggs. 2005. Distribution of Rhiozphora mangle Seedlings in Wee Wee Caye mangal. Unpublished.

Appendix: Site

#Roots 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4

3 6 6 7 8 9 10 11 13 15 4 4 5 5 6 7 10 10 11 13 5 5 5 7 7 8 8 11 11 11 6 8 8 9 9 10 10 11 12 12

Circumference 13 22 48 41 43 49 59 23 59 40 49 57 28 32 34 29 28 70 69 53 29 48 66 17 34 34 36 50 62 77 48 29 46 34 34 43 45 37 46 52

8 8 8 8 8 8 8 8 8 8 9 9 9 9 9 9 9 9 9 9 10 10 10 10 10 10 10 10 10 10 11 11 11 11 11 11 11 11 11

7 7 8 8 8 9 10 10 12 15 6 7 7 7 8 9 9 10 12 20 5 5 6 7 8 9 9 10 13 15 5 6 8 8 9 9 9 10 15

24 26 22 28 29 18 24 30 18 26 39 28 33 40 29 28 54 19 39 72 24 58 35 37 64 39 59 81 61 60 20 27 38 54 47 56 63 30 71

5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7

6 7 7 9 9 12 12 14 17 19 7 7 8 8 8 9 11 11 13 15 6 6 7 7 7 8 9 9 9 18

33 34 39 39 41 40 70 54 77 44 53 56 45 54 57 44 40 59 62 64 23 27 30 31 37 36 36 43 46 33

11 12 12 12 12 12 12 12 12 12 12 13 13 13 13 13 13 13 13 13 13

15 4 7 7 7 8 9 11 12 12 18 4 5 6 6 6 6 7 8 8 9

75 14 16 21 49 15 18 42 60 89 42 39 47 25 30 45 54 40 43 46 56