บทคัดย่อ :
It is difficult to estimate the age and growth rate of tropical tree species because most lack visible tree rings or have only indistinct and irregular ring patterns in the stem cross section. Tree ring analysis can help us understand the forest development process and recovery rate after disturbances. Therefore, tree ring analysis is particularly important in tropical regions, where forest degradation is accelerating. In addition, the analysis will enable reconstruction of past climate patterns in these regions, where instrumental data on climate before the 20th century are scarce. Such data are becoming important to predict forest response to climate change.
Several methods have been applied to estimate the age and radial growth rate of tropical trees (Worbes 1995, 2002). Based on the nature of the approaches, they are classified into two categories: (1) direct measurement of radial growth increments and (2) detection of periodicity of wood formation. The former includes analysis via dendrometer (Ohashi et al. 2001, Baker et al. 2002, da Silva et al. 2002) and cambial marking (Mariaux 1967, Shiokura 1989, Nobuchi et al. 1995, Sass et al. 1995b), and the latter includes wood anatomical (e.g. Stahle et al. 1999, Fichtler et al. 2003, Schongart et al. 2006, Steenkamp et al. 2008) and isotope analyses (Evans and Schrag 2004, Poussart et al. 2004, Verheyden et al. 2004). Annual increment in radial growth can be measured with dendrometers (or dendrobands) or the cambial marking method. The dendrometer enables continuous measurement of diameter growth and provides information on growth rhythms, However, since the increment measured with a dendrometer includes that of the bark, the effect of the bark in determining annual growth of the xylem cannot be ignored. Cambial markings create visible marks on the xylem, thus providing information on growth rate during a known time (marked) period. However, when strong wound reactions cause deformations and irregularities in the xylem, the cambial markings cannot be used for accurate annual growth estimations (Sass et al. 1995). Unfortunately, the above methods cannot provide information regarding the tree s growth rate prior to the marking period. Tree age can also be estimated by its age-diameter relationship (Therrell et al. 2007, Steenkamp et al. 2008), but the establishment of the relationship requires massive data on trees of known age and radius. In addition, tree size and age are often not related in multi-aged or multi-species forests (Oliver and Larson 1996). Therefore, its application in species-rich tropical forests is problematic.
Despite the long-accepted belief that tropical trees do not form visible annual rings, recent anatomical and chemical studies reported periodicity of wood formation. Periodical changes in the environment, such as short drought periods or long-lasting moisture inundation, induce the rhythmic growth of trees (Worbes 2002). The marginal parenchyma band formed in some tree species is one annual-ring indicator (o.g., Stahle et al. 1999, Fichtler et al. 2003, Schongart et al. 2006, Steenkamp et al. 2008); other anatomical annual ring indicators include vessel size and distribution (Stahle et al. 1999), wood density variations, pattoms of parenchyma bands, and variations in vessel distribution (Fichtler et al. 2003). Unfortunately, the visible and apparent features in xylem are usually restricted to particular specics and arcas having climatic extremes. In contrast, an isotopic approach can be applied to ringless species (Poussart et al. 2004, Verheyden et al. 2004). The 8C and S O values in wood document climate-related variables, such as precipitation (Saurer et al. 1997), temperature, and irradiance (McCarroll and Loader 2004). However, since isotope discrimination in plants is so complex and not yet fully understood, there are some uncertainties in interpreting the results. In addition, the cost- and time-consuming nature of isotope analysis limits the application to long and high-resolution chronology. Another important potential ring indicator in
เอกสารอ้างอิง :
Ohashi, S (2012) Detecting invisible annual rings of tropical tree species
