Maliau bird nest vern project

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Are Asplenium spp. Suitable Indicators for Forest Disturbance?

Contents

Abstract

It has been suggested that epiphytic ferns of the genus Asplenium could serve as indicators of forest disturbance. In the Maliau Basin Conservation Area of Sabah, Malaysian Borneo, we examined the abundance, physical orientation, and size of Asplenium spp. on emergent trees in three forest classes -- undisturbed/primary, non-roadside selectively logged, and roadside logged. We discovered significant differences in fern abundance between undisturbed forest one one hand and both types of logged forest on the other. In terms of size, however, no significant differences were registered between undisturbed and selectively logged forest, nor between selectively logged and roadside logged forest. Correlation analysis, what's more, revealed that the diameter of the host tree significantly influences abundance. We conclude that Asplenium spp. are sensitive enough to logging disturbance to be employed as indicator species.

Introduction

Epiphytes are plants that live symbiotically on the trunks, branches and leaves of other plants (Richards, 1996). In rainforests, most epiphytes are small plants and only few are known to grow beyond several metres in height (Richards, 1996). Recent research has shown that these epiphytes have an important role in providing habitats to a rich variety of flora and fauna (Basset 2001, Ellwood et al. 2002, Ellwood & Foster 2004, Karasawa & Hijii 2006). The roots of epiphytes grow around the host stem or branches for anchorage. This network of roots obstruct the flow and reduce the erosivity of the waters, allowing accumulation of considerable quantities of debris. The resulting masses of humus that collected in these epiphytes provide nesting sites for many species of arboreal ants and invertebrates.

The bird's nest ferns, Asplenium nidus and other members of the genus, are perhaps among the most abundant and important epiphytes in tropical forests (Ozanne et al. 2003, Ellwood & Foster 2004). Freiberg and Turton (2007) demonstrated in a study that high temperatures and low humidity brought about by drought conditions led to the mortality of a high majority of A. nidus individuals in a North-eastern Australian rainforest. In a more recent study, Zhang et. al (2009) have linked the distribution of A.nidus to moisture availability. For these reasons, it was suggested that A. nidus might be a potential indicator species for forest disturbance (Andama et. al, 2003). However, no known studies have been conducted to explicitly test this theory.

Question

Are Asplenium spp. suitable indicators for forest disturbance?

Methods

In this project, forest disturbance are logging road and selective logging. We walked along the canopy walk (in undisturbed forest), down a logging road and made a transect across selectively logged forest. We documented the following features of all emergent trees with more than 40 cm diameter within 10 m to the left or right of the path: tree diameter, tree bark class (smooth or rough), number of ferns, and – for every individual fern – base diameter (<50 cm, 50-100 cm, 100-150 cm, >150 cm), vertical position, and horizontal position. Vertical position assigned by designating the space between the first projecting bough and the next as "A," that between the second and third as "B," and so on. Horizontal position will be inverse Strahler order: the first bough to project out from the trunk, or both boughs of a first trunk forking, assigned an “1,” and subsequent projections or multifurcations assigned sequentially higher integers. Refer to Fig. 1 for an idealized tree with both vertical and horizontal position labels.

Fig. 1: Tree schematic

Analysis

Using the statistical platform R, we applied a general linear model to compare abundance between the three forest classes and also to examine fern abundance as a function of bark classes. Chi square tests were utilized to examine fern size and abundance differences between these three groups. A correlation test was employed to determine the relationship between host tree diameter and fern abundance.

Results & Discussion

Statistical Results:

Our glm function assessing abundance yielded the following results: there is a significant difference in fern abundance between undisturbed forest and selectively logged non-roadside forest (ferns being more abundance in the undisturbed system: p= 2.12e-07). On other hand, there was no significant difference between selectively logged non-roadside forest and roadside logged forest (p=0.0866). Refer to Fig. 2 for a representation of this difference in abundance.

Fig. 2: Box-and-whisker plot of abundance across the three forest classes

Our glm function assessing bark class showed that there was no significant relationship (as defined by p threshold of 0.05) of fern abundance to bark category in all forest classes.

Our chisq.test results are as follows:

Differences of fern size (with ferns being largest in undisturbed forest):

       between undisturbed forest and logged forest: p=0.07594
       between logged forest and road side p=0.3191

Differences in vertical position (with ferns being highest up in undisturbed forest):

       between undisturbed and logged forest: p=0.0002892	
       between undisturbed and roadside forest: p=0.01023

Differences in horizontal position:

       between undisturbed and roadside forest: p=0.4974
       between undisturbed and selectively logged forest: p=0.0601

In terms of fern orientation, in short, the most pronounced difference between these sites is in vertical position. In the unlogged forest, specifically, the ferns tend to be positioned higher than in the logged forest.

Finally, our cor.test to identify whether there is a correlation between fern abundance and tree diameter revealed diameter to be significantly (positively) related to fern abundance (p=1.446e-5).

Conclusion:

In sum, our results tend to substantiate the idea that Asplenium spp. are negatively impacted by logging and could be used as an gauge for forest disturbance. Likely explanations for differences in abundance between the forest types we assessed include differences in the means and extremes of humidity, temperature, and the debris accumulation amongst branches which is necessary to sustain fern growth (presumably a function of tree density and leaf fall rates).

Future Research:

Future investigations should involve breakdown by species (e.g., the distinguishing of A. nidus and A. phyllitidis) to account for variance in ecological requirements and physiological tolerance within the genus Asplenium. This latter factor could be analyzed by subjecting captive ferns of various age classes to artificially-determined temperature and humidity regimes. Efforts ought to be made, in particular, to see whether there are differences between the conditions optimal for juvenile growth and those conducive to adult survival. A finer-scaled analysis of fern colonization in the wild ought to account for the abundances and spatial locations of potential competitors for space (i.e., Pandanus and other large epiphytes which occupy crotch regions in emergent trees).

Assessment of abundance and physical position would be assisted by a method for directly estimating the surface area available on each tree for Asplenium colonization, perhaps through modeling of tree form with industrial 3-d imaging equipment.

References

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Ellwood, M. D. F., & W. A. Foster. 2004. Doubling the estimate of invertebrate biomass in a rainforest canopy. Nature 429: 549–551.

Ellwood, M. D. F., D. T. Jones, & W. A. Foster. 2002. Canopy ferns in lowland dipterocarp forest support a prolific abundance of ants, termites, and other invertebrates. Biotropica 34: 575–583.

Freiberg, M., & S. M. Turton. 2007. Importance of drought on the distribution of the birds nest fern, Asplenium nidus, in the canopy of a lowland tropical rainforest in north-eastern Australia. Austral. Ecol. 32: 70–76.

Karasawa, S., & N. Hijii. 2006. Does the existence of bird’s nest ferns enhance the diversity of oribatid (Acari: Oribatida) communities in a subtropical forest? Biodiversity Conserv. 15: 4533–4553.

Ozanne, C. M. P., D. Anhuf, S. L. Boulter, M. Keller, R. L. Kitching, C. Korner, F. C. Meinzer, A. W. Mitchell, T. Nakashizuka, P. L. Silva Dias, N. E. Stork, S. J. Wright & M. Yoshimura. 2003. Biodiversity Meets the Atmosphere: A Global View of Forest Canopies. Science 301: 183–186.

Richards, P.W., 1996. The tropical rain forest, An ecological study, 2nd edition, Cambridge University Press, UK, 599 p.

Zhang, L & Nurvianto, S. 2009. Factors affecting the distribution and abundance of Asplenium nidus L. In a treopical lowland rain forest in Peninsular Malaysia. Biotropical. 1-6.