IAWA Journal,VoI.18(2), 1997: 139-146 DETERMINATION OF THE AGE OF PINUS OCCIDENTALIS IN LA CELESTINA, DOMINICAN REPUBLIC, BY THE USE OF GROWTH RINGS by Xander M. van der Burgt I Instituto Superior de Agricultura, Apartado 166, Santiago, Republica Dominicana SUMMARY The growth rings of Pinus occidentalis Swartz trees in La Celestina, Dominican Republic, show between-tree uniformity. With difficulty, two mean time series were made from ring widths of 1) all visible, includ­ ing intra-annual, rings and 2) groups of rings that were hypothesized to be annual. Both were compared with a 63-year range of rainfall data. An annual periodicity in wood formation is present, but obscured by many intra-annual rings. The annual periodicity of the trees may be a remnant of their possible origin from higher altitudes, where frosts may occur during the cold season. The youngest of the 7 investigated trees was about 39 years old; the oldest about 46 years. These seven trees contain be­ tween approximately 2 and 6 growth rings per year, with an average of about 3.5-4. Key words: Tropical trees, Pinus occidentalis, growth periodicity, rainfall, annual growth, intra-annual rings. INTRODUCTION The forestry and rural development project La Celestina is situated near San Jose de las Matas in the Dominican Republic. In La Celestina the only legal sawmill of the country can be found, surrounded by Pinus occidentalis forests. The exact age of the pine trees is unknown, because the forests were surveyed for the first time in 1980 (Plan Sierra 1991). For the benefit of a sustainable management of these forests it is crucial to know the age of the trees. Pinus occidentalis Swartz does not form distinct annual rings. "False or secondary rings that terminate some seasonal period of growth, are a characteristic of this spe­ cies" (Wellwood 1946). "Below 1200 m, (oo.) annual diameter growth creates several indistinct zones of increment. It is thus not possible to define accurately the ages ( ... ) of P. occidentalis trees from low altitudes" (Darrow & Zanoni 1990). In La Celestina, a varying number of growth rings is formed each year (Klotz & Torres 1991). Because growth is strongly influenced by soil moisture (Klotz & Torres 1991), a comparison of growth rings and rainfall data might give a method to determine the age of the trees. I) Present address: Department of Plant Taxonomy, P. O. Box 80 I 0,6700 ED Wageningen, The Netherlands. Downloaded from Brill.com10/01/2021 01:03:33PM via free access 140 IAWA Journal, VoL 18 (2),1997 The wood of P. occidentalis in La Celestina often contains groups of growth rings (Klotz & Torres 1991), One group consists of one or more rings of predominantly wide, thin-walled tracheids followed by one or more rings of mostly small, thick­ walled tracheids (Fig, lA), Considering these groups and the assumptions of Klotz and Torres (1991), it was hypothesized that there are two types of growth periodicity in the rings of P. occidentalis, A rainfall periodicity should be caused by an alteration of wet and dry periods, because during every wet period, of which there are several each year, an intra-annual ring should form. An annual periodicity should be induced by an an­ nual cambial dormancy, and should result in the formation of annual groups of rainfall rings. This hypothesis was tested with a set of wood samples taken at one point of time. STUDY SITE La Celestina is situated at 71 ° 01' Wand 19° 23' N and at c. 540 m altitude. The aver­ age annual rainfall in San Jose de las Matas, at 12 km from La Celestina, is 1239 mm (1931-1993). There are two wet and two moderately dry periods per year (Fig. 2), but wet or dry periods may occur year-round. Soils in La Celestina are superficial and have a low water table. The original forest of La Celestina probably was a deciduous forest. This forest was destroyed due to repeated burning by man, and replaced by a natural pine forest (Klotz & Torres 1991). Since 1983, the forest is managed with the aim of improving both wood production and the life of the inhabitants. During the present study, the management of La Celestina estimated the age of dominant trees between about 40 and 50 years, with the help of, among other observations, measurements of trees on 13 permanent plots. Older trees have not been found, but younger trees of all ages are abundant. MATERIAL AND METHODS A homogeneous forest stand was selected in La Celestina, and dominant trees in this stand were cored. All visible growth rings were measured with a portable ring-measur­ ing instrument, and synchronized Ccrossdated', Fritts 1976) by hand. Many rings ap­ peared to be obvious on some cores, and vague or absent on others. If a ring was present on less than about 70% of all cores, the ring was considered to be invisible. The best cores were selected during synchronization. The result was checked with program DPL-COFECHA (Holmes et al. 1995). A mean time series (Fritts 1976) was calculated from all intra-annual and annual rings with program ARSTAN (Holmes et al. 1995). This mean time series was compared with rainfall data in order to test the presence of a rainfall periodicity and determine the age of the trees. The mean time series and the monthly rainfall data were filtered with various low-pass statistical filters (Fritts 1976). It was assumed that the age of the trees was between 20 and 80 years; sizes of filters were chosen in accordance with this assumption. Graphs of the filtered data were compared in search of similarity. Downloaded from Brill.com10/01/2021 01:03:33PM via free access Van der Burgt - Annual and intra-annual rings in Pinus occidentalis 141 Trees in the selected stand contained a sequence of seven clear groups of growth rings, near the pith (Fig. IA), and many less clear groups. These groups were compar­ ed with rainfall data in order to test the presence of an annual periodicity and deter­ mine the age of the trees. All groups of growth rings that matched the description given in the introduction were considered to be annual groups. On core sections with only growth rings visible but no groups, group boundaries were chosen at growth-ring boun­ daries with the help of a correlation with annual rainfall. A second mean time series was made from the widths of these annual groups of rings, and also compared graphi­ cally with rainfall. After dating of the time series, the ages of the study trees were determined. The first mean time series that includes the intra-annual rings was compared graphically (with­ out filtering) with monthly and seasonal rainfall of the established period, to test again the presence of a rainfall periodicity. Correlations and multiple regressions were cal­ culated between the second (annual) mean time series and monthly and seasonal rain­ fall, with program SPSS/PC+, to determine the relationship between rainfall and growth. A B 'I~!'''''J'' ; , ii-~:-~--~~-~~~~,.'.ft:.''n.-~. """.~~'~.. ' .f1·"'-?i~il£-',:~t::""~_"."7'~.'.'-.-'.'~ ...... -.' --~·f~'".:'-.~.'- . :I' 'j'" 1- t 'l ---~.:!;'".. ' ,- ,,...•. - .. - ~::::." '. c ;. · fi.oJ r~: ,'f!' ,·f: . ='* '. <..--=-; . 1 ~ ~-J- JI_ _ , _ .ll.c.Ll._.... <- .....;Q...~ Fig. L Growth rings of Pinus occidentalis. - A: A sequence of7 groups of growth rings, formed in 1959-1965. - B: An example of uniformity between growth rings, formed in 1981-1984, of two trees in the same forest stand, The uniformity is not complete; the group of rings with much light-coloured wood consists of about 5 rings and 9 rings respectively. - C: An example of a core section that could not be synchronized. Length of core sections 34, 17, 17 and 18 mm. Downloaded from Brill.com10/01/2021 01:03:33PM via free access 142 IAWA Journal, Vol. 18 (2),1997 300 60 _250 50 E U -S200 40 e...­ c ~ o :::J 30 iii ~ 150 Q; '0.. a. '0 100 20 E Q) ~ a: 50 10 o II , II , II , I I , I I , I I, II, I I , II , I I , I I , II , II , II , II , II , I I , II , I I , I I , I I , II , I I , II , II 0 J F M A M J J A SON D J F M A M J J A SON D Fig. 2. Annual distribution of rainfall (bars) and temperature (line) in San Jose de las Matas. Mean annual temperature is 24.1 °C; mean annual sum of rainfall amounts to 1239 mm. 7 E6 ;5E -'0 ~4 01 §3 ~2 o ~1 o 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Growth-ring number Fig. 3. Synchronization of ring-width measurements (including intra-annual rings) of 15 growth­ ring series that were used for a mean time series. RESULTS In the selected stand, IS Pinus occidentalis trees were cored at four sides of the stem, during May and June 1993. Synchronization of the growth-ring time series was diffi­ cult and time-consuming, although the uniformity of growth rings from different trees was often very clear (Fig. IB). Cores were excluded if their growth rings could not be synchronized completely (Fig. lC). The time series of 15 cores of 7 trees (3 cores of 4 trees each, 1 core of 3 trees each) were considered suitable for a mean time series (Fig. 3). These cores contained 2476 growth rings; 465 (18.8%) were rejected, 110 (4.4%) were invisible and had to be estimated. It is impossible to make an objective decision about which vague intra-annual ring should be included and which not (Fig.