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Do leaf characteristics of Anaxagorea crassipetola change with respect to light intensity?

Canopy Chart    Leaf Area Chart    Length/Width Chart   Stomata Chart    Herbivory Chart

Kim Walsh Bev Mowrer Thomas Philip Jenelle Hopkins

Abstract - Leaf characteristics of Anaxagorea crassipetola were studied in relation to the light environment.  Leaves in gaps (higher light) showed a difference in number and location of stomata, but little other differences were observed. 

Introduction - Environmental change due to logging reduces canopy cover and introduces light gaps into the rainforests.  This study will investigate possible differences in leaf characteristics by comparing the leaves of Anaxagorea crassipetola found under canopy cover and in the gaps. Anaxagorea crassipetola occupies an under-story habitat in portions of the primary forest and is frequently found on sloping terrain. The hypothesis is that there will be variations in leaf characteristics in the Anaxagorea crassipetola due to differences of light intensity found between trees growing under the canopy verses those growing in a gap area. 

Background - Studies have shown differences in shade tolerant species of trees and gap associated species.  (King 1991)  Shade tolerant saplings shift most of their above-ground growth to leaf replacement when heavily shaded.  As natural gap openings occur because of tree fall, the trees then will allocate two-thirds of above-ground growth to stem and branches.  This allows the trees to quickly reach canopy height where sunlight competition decreases.  These trees preferentially allocate photosynthate to leaf growth, then to roots, storage, stem growth as carbon availability increases (King, 1991). 

A study area in the La Selva Biological Station showed the average density of Anaxagorea crassipetola trees to be 433 trees per hectare. The trees ranged from 1.5 – 8.8 centimeters diameter at breast height and 4-10 meters tall. Trees averaged 3.2 meters to the nearest neighbor. The bark of the Anaxagorea crassipetolais is red in color, as is the main leaf vein. The leaves are alternate, narrow, and coriaceous. They grow along one plane and are characterized by minute, stellate trichomes. (Gentry, 1993).

Young leaves tend to be glossier and a lighter green in color. The leaves are readily recognized by a unique vein pattern found along the outer edge of the under-side.  The outer waxy layer, leaf toughness and trichomes of the Anaxagorea crassipetola help reduce herbivory.

Water can wash essential minerals and other chemicals from leaves.  The protective waxy coating acts to retard water loss.  The coating also discourages herbivory and fungi growth.  Drip tips are another mechanisms that helps reduce leaching by speeding water run-off from the leaf surface.(Kricher, 1997).

Weevils of the Cyrionyx genera, Drosophilidae fruit flies, and Nitidulidae beetles pollinates Anaxagorea crassipetola. (Armstrong et al, 1997).  Seed dispersal in this plant is quite limited and is accomplished mechanically by up to 18 follicular fruitlets.  When ripen the fruitlets snap open and fling 2 smooth hard seeds a few meters away from the tree.  Other dispersal agents include pocket mice and other seed-eating rodents.

Methods - This study was conducted at the Organization for Tropical Studies field station at La Selva, Sarapiquí, Costa Rica. The tree used in this study was Anaxagorea crassipetola (family Annonacae).  Sites were chosen approximately every fifty meters progressing into the primary forest along the CES trail.  Five sites were sampled.  For each site, one tree growing under the canopy and one growing in a gap area were selected.  Tree heights ranged from 3 to 16 meters, with an average height of 6.5 meters.  Three leaves were removed from each tree by selecting a singular branch closest to two meters from the ground.  For each branch, the leaf closest to the trunk, a mid-branch leaf and the terminal leaf were removed, stacked in leaf removal order, then they were tagged and bagged.  The tree height was approximated.  Light intensity was measured in two methods.  An electronic photometer reading was taken and four densiometer reading were taken at each sample site.  Time of day was also recorded.  Data are listed in Appendix Table 1. 

The 30 sample leaves were processed through the L1-1600 Steady State Porometer, which recorded each leaf’s surface area.  The leaves with herbivore damage were later “repaired” by covering the chew-holes with duck tape and re-processing them.  This provided the area of herbivory.  Length and width measurements were also recorded. Data are listed in Appendix Table 2.

 Due to the high accumulation of epiphytic flora and fauna on the older and mid-point leaves, only the terminal leaves were chosen for stomata counts. In order to observe stomata density, a one-centimeter square area on the top surface and underside near the primary vein in the center of each leaf was painted with clear nail polish.  The nail polish was then peeled off and observed under a 1000x magnification. Stomata were counted in the field of view.  Data are listed in Appendix Table 3.

The tips of the leaves were examined for differences.  Each leaf tip was traced approximately four centimeters from the end.  Visual comparisons were made between old, medium and young leaves, and between light gap and canopy leaves.  No obvious differences were observed.

 Data- The following charts illustrate the findings.

Click on a thumbnail to see the enlarged chart
Canopy Cover	Leaf Area	Length and Width
Stomata		Percent Herbivory

Analysis - Our results show that there is more canopy cover in the forest and less canopy cover in the gap.  We can infer from this that there is less light in the canopy than in the gap.

Stomata was found on the tops and bottoms of the gap leaves but only on the bottoms of the canopy leaves.  The greatest herbivory was found in the older leaves of the gap trees.  Our t-test analysis shows these results to significant at less than 6% error.  t=2.21, df=8, P=.058.  Older leaves also had more epiphytic coverage. No morphological trends were noted in leaf width, although leaf length appears to be related with the age of the leaf, with the younger leaves being the longest. 

Conclusion - This study indicates that there are changes in leaf structure with respect to location within the rain forest environment.  These changes should be taken into consideration when disturbing the rain forest ecosystem.  Some plants may be able to make quick adaptations when forced to go from a canopy cover to an open edge, but some plants may not.  Also, fauna associated with the particular plant may also be impacted by the new abiotic conditions.   A complex web of interactions may irrevoccably be destroyed.  Further studies are indicated.

 Seasons may effect herbivory by influencing the abundance of insects and the presence of the tree’s reproductive unit being the flower Any single season field study is a snapshot in ecological time.

Bibliography

1.                  Armstrong, J. 1997. Floral Herbivory, Floral Phenology, Visitation Rate, and Fruit Set in Anaxagorea crassipetola, a lowland rainforest tree of Costa Rica.  Journal of the Torrey Botanical Society.  124(3) pp. 228-235. 

2.                  Gentry, A.  1993. Woody Plants of Northwest South America.  University of Chicago Press.  pp. 227-228. 

3.                  King, D.A.  1991. Correlations Between Biomass Allocation, Relative Growth Rate and Light Environment in Tropical Forest Saplings.  Functional Ecology (5). pp. 485-492. 

4.                  Kricher, John 1997.  A Neotropical Companion; An Introduction to the Animals, Plants, & Ecosystems of the New World Tropics.  Princeton University Press.  pp. 49 & 149.

Acknowledgments: 

            The team would like to thank La Selva Field Station Orlando Vargas for his assistance in selecting the tree and site locations.  We would also like to thank graduate research assistant Crystal Wenderberger for showing us the technique used for collecting leaf cells for the stomata count.