Maliau drip tips project

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Role of the drip-tips in Tropical Rain Forests: Association of Leaf drip-tips with other leaf morphological characters to reduce the growth of epiphylls and fungi.

Douwe Yntema, Mohd Ridwan A. Rahman and Imesh Nuwan Bandara

Contents

Abstract

Our study was conducted in order to determine the importance of leaf drip-tips on epiphylls and fungi covering in understory trees in the tropical rain forest. Leaf drip-tips are common feature of understory trees in wet lowland rain forests but their function has yet to be properly understood. They hasten the drying time of leaves and prevent easy colonization by epiphylls and fungi. Random sampling of trees, including shaded and gap areas, was conducted. We observed various leaf morphological characteristics as well as leaf age, which was determined by the distance away from the stem and subsequently given a number ranking. We measured both leaf length and drip tip length, estimated epiphyllic cover, and counted lichen species richness for each leaf. Twenty-four leaves were randomly selected from age ranks 1-6 in order to conduct analysis. We found a positive relationship between epyphyll percentage cover and leaf age. We found a negative relationship between epiphyll percentage cover and drip tip rati0. We also found a positive relationship between lichen species richness and leaf age. We found that leaves with larger drip tip ratios had less epiphyll and fungi percentage covering. Moreover very strong relationship between leaf age and epiphyll and fungi percentage covering was determined and found to increase significantly over time. Drip tips are a significant, albeit small, cost to trees and may thus be assumed to have some purpose or significance. Our result shows that it may be advantageous for an understory tree to produce large drip tips.

Introduction

Drip tips, or long narrow apices of leaves, are common in wet lowland rain forests (Leigh 1975); however, their function has yet to be properly understood. A long standing hypothesis is that they both hasten the drying time of leaves (Stahl 1893; Dean and Smith 1978; Ivey and De Silva 2001) and prevent easy colonization by epiphylls and fungi (cite) which may cause reduced photosynthetic rates. Ivey and Desilva (2001) found "no evidence that drip tips reduced occurrence of epiphylls or that drip tips helped to shed debris from leaf surfaces" but found that they may reduce fungal growth. Burd (2007) found similar results but noted that the possibility that the small cost of a drip tips which may cause temporary reductions in epiphyll growth may provide a sufficient explanation for their evolution. Current studies have yet to effectively determine the true function or evolutionary significance of drip tips. Further study may increase our understanding of drip tips' function and give better insight into their evolutionary significance.

Question

Does drip tip size in relation to leaf length influence the growth of epiphylls, fungi, and other leaf covering? We believe this is important to understand considering the possible trade-offs of varying leaf morphology to trees. Studies to date have used artificial leaves and leaves with removed drip tips. To our knowledge, this study is the first to compare varying relative drip tip size in relation to other characteristics.


Methods

Field Methods

Our study was conducted between 13 July and 17 July 2009 in the Maliau Basin Conservation Area. Random sampling of trees was conducted by way of six 50 meter transects. One sample was taken every 5 meters alternating to the left or right of the transect path for a total of 60 branches and 391 leaves (leaves with severe damage were not included in the sample). Transect locations were chosen by walking 10 minutes or more along forest paths. A stopping point was chosen and then a coin was flipped to determine whether the sample would be taken to the left or right of the path. After our path side was chosen, we walked 30 meters into the forest. We then chose a straight path with the aid of a compass. All samples collected were found nearest to the transect under 2 meters height. Samples included both shaded and gap areas.

We observed various leaf morphological characteristics including arrangement, shape, apex, margin, and lamina. Leaf age was determined by the distance away from the stem (i.e. the youngest leaves are furthest away) and given a number ranking. We measured both leaf length and drip tip length; drip tip length was measured from the inflexion point. In order to estimate epiphyllic cover, we placed a transparent plastic sheet marked with twenty 5mm X 5mm squares over each leaf. The percentage occurrence of epiphylls was calculated by multiplying the proportion of occupied squares by 20. A lichen species richness count was also taken for each leaf.

Analysis

Our statistical hypothesis is that there is a strong relationship between drip tip ratio (drip tip length divided by leaf length) and epiphyll percentage cover. There is also a positive relationship between leaf age and lichen species richness. Our null hypothesis is that there is neither a relationship between drip tip ratio and epiphyllic cover no leaf age and lichen species richness.

We randomly selected 24 leaves from age ranks 1-6 in order to conduct analysis based on R results. Leaf age ranks over six were not included due to insufficient availability. Linear models were used to determine the relationships between epiphyll percentage cover and leaf age, epiphyll percentage cover and drip tip ratio, and lichen species richness and leaf age.

Results

We found a positive relationship between epyphyll percentage cover and leaf age (slope: 4.28; p-value 0.0006). There is a negative relationship between epiphyll percentage cover and drip tip ratio(slope: -1.08; p-value 0.005). We also found a positive relationship between lichen species richness and leaf age (slope: 0.36; p-value: 0.00006).















Discussion

We found, perhaps in contradiction of previous studies, that leaves with larger drip tip ratios had less epiphyll and fungi percentage covering. In order to help test acceptable sampling and help confirm our primary result, a simple yet very strong relationship between leaf age and epiphyll and fungi percentage covering was determined and found to increase significantly over time. Drip tips are a significant, albeit small, cost to trees and may thus be assumed to have some purpose or significance. Our result shows that it may be advantageous for an understory tree to produce large drip tips in order to increase photosynthetic rates.

We found that lichen species richness increases as a function of leaf age. While previous studies have indicated that lichen abundance does not increase with the presence of drip tips, our study shows that lichen richness does increase over time on leaves and that larger drip tips are negatively associated with lichen species richness. This again indicates the potential benefits of proportionally larger drip tips to trees.

This study is by no means comprehensive. Further study into our data needs to be conducted. For example, Burd (2007) noted a stronger correlation between opposite leaves and epiphyll covering than between the presence or absence of drip tips and epipyll covering. Similarly it is possible that other leaf morphological characteristics such as shape, apex, margin, and lamina strongly influence epiphyll and fungi covering. Our study measures all of these characteristics, but has yet to analyze their relationships to epiphyll and fungi covering.

References

  • Dean, J.M., and Smith, A.P. (1978), ‘Behavioral and morphological adaptations of a tropical plant to high rainfall’, BIOTROPICA 10:152-154.
  • Ivey, C. T. and DeSilva, N. (2001), ‘A Test of the Function of Drip Tips’, BIOTROPICA 33(1), 188-191.
  • Leigh, E.G. (1975), 'Structure and climate in tropical rain forests. Ann. Rev. Ecol. Syst. 6:66-68.
  • Stahl, E. (1893), ‘Regenfall and Blattestalt. Ann. Jard. Bot. Buitenz. 11: 98-182.