To what extent does plasticity of dipterocarp seedlings affect growth and survival in the tropical rainforest environment?

Authors Avatar

To what extent does plasticity of dipterocarp seedlings affect growth and survival in the tropical rainforest environment?

050139246

The Journal of Ecology (September 2007)

To what extent does plasticity of dipterocarp seedlings affect growth and survival in the tropical rainforest environment?

Abstract

This study focused on survival and growth of three dipterocarp seedlings in exposed areas compared with shaded areas of the tropical rainforest, in order to determine the role plasticity plays in this.  All species of dipterocarp seedlings showed some degree of morphological plasticity in response to the different environments.  Each of the three dipterocarp species showed equal plasticity in terms morphological traits when subjected to different light conditions.  Chlorophyll content was found to be a genetic trait rather than a trait of plasticity.  Herbivory was also found to be determined by species rather than environment.  Damage by herbivores was found to play a larger role in plant growth and survival than anticipated, perhaps more so than plasticity.  Further study is required to determine the extent herbivory affects the growth and survival of these seedlings.

Introduction

The importance of tropical rainforests and their role in supporting much of the world’s biodiversity is widely acknowledged, yet there is worldwide concern over the consequences of human activities on these ecosystems. (Myers et al, 2000)   The lowland forests of Sabah, Borneo are dominated by trees of Dipterocarpacae; a family which is both ecologically and economically important. (Ashton 1964, Whitmore 1984)  There are several species of Dipterocarps and they vary widely in physiology and shade tolerance (Ashton 1964, Barker et al 1997, Scholes et al 1997).  Dipterocarp timber has an enormous economic value, and as a result of logging there have been unparallelled reductions of these trees in forests of Sabah (Bischoff et al 2005).  Knowledge of dipterocarp regeneration is not only valuable for understanding the dynamic processes of the rainforest but it is essential for efforts to preserve and restore these species rich forests. However such knowledge is neither adequate nor complete (Leffler & Enquist, 2002).

Light availability in the rainforest understorey environment varies both temporally and spatially. (Tang et al 1999) An opening in the forest canopy, and consequently a light gap, is formed after a fall of a branch or tree.  Many studies have suggested that several rainforest tree species depend on canopy gaps for successful regeneration. (Brown 1993, Whitmore 1978, 1996)  The ability to adapt in response to a changing resource, such as light availability, is described as plasticity.  Plasticity may play a vital role in the growth and survival of tropical tree species, which are subjected to such a dynamic light environment.  Those tree species that exert high plasticity are assumed to grow and survive more successfully compared to less plastic species when a gap appears in the canopy (Rice & Bazzazz 1989, Givnish 2002).  

This study focused on survival and growth of three dipterocarp seedlings in gaps compared with shaded areas, in order to determine the role plasticity plays in this.  More specifically the following questions were addressed: (1) Do morphological traits and chlorophyll content of dipterocarp seedlings show plasticity in response to light, and if so, which traits are more flexible?  (2) Do shade tolerant and less tolerant species vary in their plasticity in response to light? (3) How vital is plasticity to the growth and survival of dipterocarp seedlings, and do other factors, such as herbivory play a role?

Methods

STUDY SITE AND SPECIES

The study was carried out in primary lowland dipterocarp rainforest surrounding Danum Valley Conservation Area, Sabah, East Malaysia, Borneo (4"58'N, 117'48'E).  The climate of these rainforests are characterised by two periods of heavy rainfall in May-June and October- January, with annual average rainfall of 2670 mm and annual mean temperature of 26.7°C (Wells 2007).  This study was undertaken between August 22nd and 31st 2007.

Three species of dipterocarp seedling were selected for study.  Hopea sangal, Dryobalanops lanceolata and Shorea macroptera were chosen because they co-exist in dipterocarp forest and have differing ecological characteristics (Ashton 1964).  S.macroptera is a light hard-wood fast growing species and requires high light intensity to regenerate. H.sangal is a heavy hard-wood slow growing species, whose seedlings are adapted to tolerate shade. D.lanceolata is a medium hard-wood whose seedlings can survive in a variety of light environments. These three species were not only singled out for study because of their differences in shade tolerance, but they are amongst the most logged dipterocarp species in Asia (Meijer & Wood 1964).  

Join now!

Using plots constructed in 2003 by a previous PHD student we were able to investigate the growth and survival of three species of dipterocarp seedlings in gaps and compare it to sites shaded by continuous canopy.  In total there were six plots, half in the shade and half in gaps.  There were two replicates of each plant in each plot.  The plots were standardised as they were planted four years ago at the same time and at similar age and heights, reducing the bias between seedlings.  

MEASUREMENTS

Several morphological traits and the chlorophyll content of each ...

This is a preview of the whole essay