ABSTRACT:

In order to determine potential causes for differing spine density on Pochota bark we measured soil moisture, spine density, spine size, canopy cover, soil types, tree height, temperature, DBH, and altitude for twelve trees. We discovered higher spine density at lower elevation and on alluvial soils. Also, trees with higher spine density had a higher percentage of canopy cover. There was no correlation between spine density and spine size, DBH, soil moisture or temperature.
 

INTRODUCTION

Palo Verde National Park protects a portion of the Rio Tempesque lowlands in Guancaste, Costa Rica. This area protects the 1% dry tropical forest left in Central America. The Park includes an amazingly diverse patch of habitats including fresh and salt water marshes, decidious, riparian and evergreen forests, and mangrove swamps.

The Pochote tree (Pochota quinata) is a Pleistocene tree that developed a formidable defense against the megafauna herbivores of that time. The trunks have tough sharp spines that have considerable length. They extended around the trunk, the height of the trunk and branches. These trees are common in Palo Verde, however, saplings were not observed. The leaves are palmately compound with five leaflets. The flowers are pollinated by the Hawk moth. They are 7-25 cm long with large number of stamins. The fruit is woody and five valved and the seeds vary from small and imbedded in Kapok to large and lacking Kapok. Kapok is the endocarpy surrounding the fleshy fruit.

Genetic diversity is the diversity in the genes of individuals of the same species. Genes are the component that determine an organism’s characteristics. In any healthy population of a species genetic variability exists among individuals. In order to preserve the genetic diversity of Pochote trees, if variation in spine density is genetic, then an altitudinal strip of land must be preserved.

Pochota trees have been noted to have variation in spine density and size. We set out to discover what may cause the differences. In order to determine the Pochota speciman that would be used for the study, a hike was taken on the trails of Palo Verde Biological Station. The trail taken was the Sendero Cerros Calizos. It is a limestone outcropping at the tip of the hill called El Marador. Many Pochota were found, marked and indentified for later examination. Phillip Hensel was intrumental in the identification of the trees. In addition, lower elevations along the alluvial plain were examined . Six examples were found.

METHODS AND MATERIALS:

1. At each tree these measurements were taken: Air temperature, Diameter at Breast Height (DBH), tree height, soil moisture, spine density , spine length, spine width, canopy cover, and altitude.
2. Equipment used: Armored alcohol thermometer, Loggers tape in centimeters, Clinometer (handmade of paper and cardboard), Altimeter, Vernier Caliper, Densiometer, shovel, Ziplock plastic bags, flagging, ruler, and 10 cm. Square cardboard grid.
3. Each group member was assigned a specific task. They performed these tasks at each tree to promote consistency of data collected.
4. DBH was found using Loggers tape which encircled the tree at either the highest elevation we could reach , or at the most cylindrical portion of the trunk. Data was then recorded in centimeters.
5. Spine density was determined randomly using a 10 centimeter square frame cut from cardboard. The form was placed on the trunk either above or below the DBH line. The number of spines found within the square was counted.
6. Spines are defined as the pointed projections from the trunk length. The length and width of the spines was determined using a ruler and Vernier calipers. Five spines were chosen for measurements within each square. The location of these five spines were : the spine at the center of the square, as well as the spines located at each of the four corners of the square. If fewer than 5 spines were found within the square, then all spines were measured regardless of the position within the square.
7. The height of the tree was determined using the following method. The distance was determined using Logger’s tape from the base of the tree to observer. A handmade Clinometer was used to determine the angle from the base of the tree to the top of the tree from the point of view of the observer. The formula was used to then determine the height of the tree.
8. Canopy cover was determined using a densiometer. It was made using the following materials and measurements. A PVC pipe measuring 34.5 cm in length and 6 cm in diameter was used. A grid made from an overhead transparency was placed over one end. The grid had 16 one cm squares drawn on it. Canopy cover was determined by holding the tube vertically at the top of the tube and allowing it to hang between 2 fingers and looking up through the tube. It is important the tube be vertical. The squares covered at least 50% by foliage were counted as one full square. If less than 50% was covered the square was not counted.
9. Soil samples were taken at each site at the base of each tree from an undisturbed area. Approximately three scoops of soil were placed in the sample bag. The bags were sealed and labeled. The bags were then massed and the data was recorded. The bags were opened and allowed to dry overnight, and then under the sun until noon the next day. The mass of the bags and the soil were then taken and recorded.

RESULTS

We found no statistical correlation between soil moisture and spine density. (y = 5.9 -.06 x, R² =.0012, df =10, F =.01, P =.92) A two-tailed t-test did show a slightly significant difference between the average spine density of plants on alluvial soil (6.2/cm²) and plants on limestone soil (4.0/cm²). (R² = 0.164, t value = 1.405, d.f. = 10, p = 0.08) In addition, spine density also correlates with altitude. Lower spine density occurs at higher elevation. (y = -2.9 ln x +15.8, R² =.33, df =11, F= 4.8, P =.05). (Fig.1) Spine diameter, however, increases slightly with an increase in altitude. But there is no statistical correlation between spine density and spine diameter. (Y=1.36-.02x,R^2=.03,df=11,F=.01, P=.59) (y=.0078x+.8509, R² = .29, df =11, F = 4.1, P = .07). (Fig. 2). We also found some interesting relationships with canopy cover. Canopy cover increased logistically with an increase in spine density. (y = 62.87+ 6.77log x, R²=.38, df =10, F=5.4, P=.04). (Fig. 3). Canopy cover was also higher even when spines were smaller. ( y= -15.8 + 93.1, R² = .52, df = 10, F = 10.97, P = .008). (Fig. 4 and 5). The canopy cover difference is not due to tree size. There was a lot of variation between canopy cover and tree size (measured by Diameter-Breast-Height (DBH) ( y = 1.9 x - 57.0, R²= .23, df = 11, F = 3.0, P = .11) as well as no visible altitudinal trend in tree size (also measured by DBH) (y = 148.5 -19.0 log x, df = 11, R² = .13, F = 1.5, P = .25) (Fig. 6). Canopy cover decreased with an increase in altitude. (y= -.21 x + 83.9, R² = .45, df = 11, F = 8.18, P = .02) (Fig. 7) Qualitative observation indicated that trees found under 20 meters were on growing on alluvial soil while trees at higher elevation were on very rocky limestone. In addition, spine density was not equally distributed on individual trees. Ten out of twelve trees had the spines concentrated on the south sides of the trees, on buttresses and on the tops of branches.
 

DISCUSSION

We studied a limited number of samples, but were still able to see some interesting trends. There was no significant correlation between soil moisture and spine density, or between DBH and altitude. So, the decreasing density of spines with increasing altitude is not due to size. Also, there was also no statistical correlation between DBH and canopy cover. Therefore, variation in canopy cover is also due to factors other than size. However, there was a significant correlation between canopy cover and spine density. Canopy cover was lower with trees with higher spine density. This may indicate the adaptive significance of the spines having reduced herbivory. Spine density also decreased logistically with an increase in elevation. Perhaps this is related to historical cattle exposure at lower elevations. Spine diameter was higher at higher elevations. This may have contributed to lower spine density since fewer spines would fit in a 10 centimeter square. However, spine density did not significantly correlate with spine diameter. Additionally, spine density appeared to correlate to soil type. Alluvial soils had a slightly higher spine density.

This hypothesis about selective pressure from herbivory is dependent upon maintaining genetic differences between trees found on the floodplain and those found on the limestone outcrop. Since the trees are wind and bat pollinated, you would expect a mixing of genetic material as there are no physical barriers between the two populations. Further study of genetic incompatibility and pollination would be necessary to determine if the two populations are genetically distinct. If cross pollination does occurs then this would indicate the increased significance of environmental factors like soil and nutrient availability. In either case, no Pochota saplings were seen, indicating either a pollination, dispersal or establishment problem. In order to maintain the Pochota population and thus diversity of the Dry Forest, understanding these relationships are essential.

An additional qualitative factor observed was varying distribution of spines on individual trees. Ten out of twelve trees measured showed higher densities of spines on the south facing side than on the north facing side. Others had spines on the top of the branches but not on the bottom. We believe this observation warrants further study as this may indicate a potential relationship to southern light exposure.

In short, 16% of variation of spine density could be attributed to soil type while 32% may be related to elevation. Elevation may contribute to differing spine density due to differing herbivory pressures. In order to preserve genetic diversity of Pochote trees, if variation in spine density is genetic, then an altitudinal strip of land must be preserved.
 

ACKNOWLEGEMENTS

Gentry, A.H. 1993. A Field Guide to the Families and Genera of Woody Plants on Northwest South America(Columbia, Ecuador, Peru) with Supplementary Notes on Herbaceous Taxa, p.284. University of Chicago Press, Chicago, IL

Understanding Biodiversity: The Biodiversity Debate: Exploring the Issues, World Wildlife Fund, 1997, pp.1-4