1. INTRODUCTION
1.1. Background on tree dating methods
Knowledge of the age of trees has a number of implications.
Firstly, tree dating, combined with knowledge on stand structure can give
information about forest disturbance. Therefore, it is often used to study the
forest dynamics (Mundo et al., 2007). It also prevents the loss of
genetic diversity and allows foresters to develop sustainable harvest pr
actices (Roel, 2005). The knowledge on age and increment growth of the trees
also sheds new light on global climate models (Gerhard et al., 2004;
Walter, 2004; Bouriaud et al., 2005; Schöngart et al.,
2006; Bütgen et al., 2007; Thomas, 2007). To estimate tree a
ge, scientists use two major methods. The first is the relative dating that
regroups the periodic annual increment (PAI) method and the crown class model
(Backer, 2003). These methods are not always accurate tree dating. They are
based on diameter growth without taking in consideration that many other
factors like soil fertility could influence tree-growth. Therefore, they can
overestimate or underestimate the age of trees.
To address this deficiency, dendrochronology also called
Tree-ring analysis was proposed in 1901 by Ellicott Douglas (1867-1962). He was
the first to remark that each year, trees add a layer of wood to its trunk and
branches and then producing annual rings. This approach has a lot of advantages
for forest management studies (Brienen , 2005). First, tree-ring
analysis gives information on the real age of a tree and the lifetime growth
rates and is therefore more effective than relative methods. It can be used to
reconstruct past disturbance (Brienen et al., 2007; Patrick et
al., 2008). For example, tree-rings are often used to reconstruct fire
history from fire scars (Welsberg & swanson, 2001; Guyette &
Stambaught, 2004; Van Horne & Fule, 2006; Hall, 2008). Another advantage of
the ring analysis is the possibility it offers to quantify variation in growth
among individuals over long periods of time (Desta et al., 2003).
Tree-ring analysis is also a good complementary tool to permanent sample plot
measurements. It needs only short time to provide many d ata on tree life.
Finally, tree-ring is used to reconstruct atmospheric gas concentration over
the past (Kennichi et al., 2004; XingYun et al., 2006;
Kristopher et al., 2007; Louise et al., 2008).
Despite all these advantages, this method has sometime s
limited applications in the tropics
because of invisible rings in certain
species (Pascale et al., 2004). Also, it is only useful for
trees
that are less than 600 years old (Worbes, 2002). However, it has been
demonstrated in
recent reports that some trees live more than 1000 years (Miguel
& Elena, 1998; Patrut et al., 2007).
Therefore, an alternative absolute dating method like
Radiocarbon is necessary. This approach was developed by Willard in 1946. The
radiocarbon method is more effective than all other approaches (Ramsey, 2007).
The dating method was used to explain some ecological and paleoclimatological
phenomena (Anouk et al., 2004; Patrut et al., 2007). But its
application requires well equipped laboratory and thus, it is more expensive.
According to Worbes (2002), it is better to use dendrochronology when tr ees
show visible rings. Many recent studies have also reported the presence of
annual ring in tropical species that may allow the use of tree-ring analysis as
dating tool (Miguel & Elena, 1998; Worbes, 2002; Dezzeo et
al., 2003; Fichtler et al., 2006; Patrut et al.,
2007).
In the present study we established the growth performance
from sapling s to trees for five savanna species (Isoberlinia doka
Craib and Stapf, Pterocarpus erinaceus Poir., Anogeissus
leiocarpa (DC.) Guill. & Perr., Detarium microcarpum Guill
& Perr., and Diospyros abyssinica (Hiern) F. White) of
West Africa. All of these species are found in open natural stands that were
subject to periodical bush fire. The main research question in these savanna
stands is related to the time the trees species, especially the endangered ones
needs to become fire-resistant. All of the five targeted species showed visible
annual tree -rings (Nigärd et al., 2004; Tarhule &
Leavitt, 2004; Poussart et al., 2006; Shöngart et al.,
2006) and allowed the use of tree-ring analysis for the study.
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