Chapter 2

SPATIALVARIATIONINTREE

SPECIESCOMPOSITIONACROSS

TROPICALFORESTS:PATTERN

ANDPROCESS

Jérôme Chave

OVERVIEW

Understandin gthe causes of spatial variation in floristic composition is one of the overarchin g goals of plant ecolo gy.
This goal has been challenged by the difficulty of unfolding the spatial component of biodiversity, and of interpreting
it biologically, especially in the tropics. Hence until recently, virtually nothing was known about the real impact of
land-use change on tropical biodiversity, in spite of the rapid rates of tropical deforestation and habitat loss. This
picture has changed dramatically over the past few years, with the development of large-scale inventory projects and
the implementation of methods for quantitative analysis of floristic data. Here, I provide an overview of the definitions
of spatial floristic turnover, or beta-diversity, and a statistical toolkit for the analysis of beta-diversity. I also contrast
ecological theories which underlie the statistical tests. I then review recent empirical studies on plant beta-diversity in
tropical forests. This panorama shows that a consensus on field and analytical methods is now bein greached. There is
a need for careful reinterpretations of published ecological patterns in light of well-formulated ecological hypotheses.
Only through ambitious field studies and collaborative approaches will further progress be achieved in this fascinating
research area.

INTRODUCTION

If the traveller notices a particular species and

wishes to find more like it, he may often turn his

eyes in vain in every direction. Trees of varied

forms, dimensions and colours are around him,

but he rarely sees any one of them repeated. Time

after time he goes towards a tree which looks like

the one he seeks, but a closer examination proves

it to be distinct. He may at length, perhaps, meet

with a second specimen half a mile off, or may fail

altogether, till on another occasion he stumbles on

one by accident.

Ever since Wallace’s (1895) description of the

diversity in tropical tree species, this outstanding

variety of form has been regarded as a curiosity

and a scientific challenge. Over the past decade,

record-settin glevels of tree diversity have been

reported, where, in one hectare of old-growth

forest, every other tree represents a new species

(Valencia et al. 1994, de Oliveira and Mori

1999). Recent diversity mappin gprojects have

also demonstrated the great variability of tree

species richness in the Amazon (ter Steegeet al.

2003) and in Borneo (Sliket al. 2003). What

explains these strikin gfloristic chan ges in tropical

forests at both local and regional scales?To answer

this question, it is necessary to examine changes

in biological diversity at all scales, for they are

caused by processes that, themselves, operate