The landscape and taxonomic distribution of wood density in a Bornean rain forest

1.       Introduction
Wood density (or, as usually measured, wood specific gravity) is a key plant trait which is inversely related to mean growth rates and also tends to correlate with drought tolerance and shade tolerance. As part of a larger study on the ecological and biogeographic distribution of tree species across the Indonesian archipelago, we measured twig wood for all 257 morphotypes tree species occurring as individuals (> 10 cm DBH) in six 0.25-ha plots in lowland dipterocarp forest in the Gunung Palung National Park in West Kalimantan, Indonesian.

2.       Methods

Six plots of 0.25 ha were set up in lowland dipterocarp forest (100-300 m ASL) near the Riam Parit area of the park. Three plots were in the valleys, on rich alluvial soil, and three plots were on nearby ridges, with shallower, sandier soil.  All trees were measured for diameter and marked permanently with tree tags. We collected sterile (or fertile where possible) twigs for identification and these were subsequently matched at Herbarium Bogoriense.  Two living twigs of 1 cm minimum diameter were collected from a representative tree for each species by tree limbers, during the preparation of botanical and DNA vouchers, and cut to 10 cm lengths.

Following Cornelissen et al. (2003), in the lab, we submerged the twig sections in water in a container on a balance to determine displaced volume, and then dried the sections in an oven and weighed them.  From the two samples per species, we calculated a mean wood specific gravity.

3.       Results
The mean wood density for all 257 species was 0.6135 (SE = 0.0067), with the least dense species being Durio oxleyanus Griff. (at 0.313) and the densest being Reinwardtiodendron humile (Hassk.) Mabb. (at 0.859) (Table 1). The mean of the least dense family was 0.445, for Podocarpaceae, and the most dense was 0.810 for Ochnaceae (Table 2). There was asignificant difference in wood density among the trees classed by plot (Fig. 2), but not when classed by site (high ridge, vs low valley; Fig. 3).

4.       Conclusion
We expect the more drought stressed sites to have trees with slower growing, more drought tolerant species. While there was no overall significant difference between trees in high and low plots, the one real ‘real’ hilltop plot (plot B), with shallow, rocky soil, had very high wood density (Fig. 2).  The finding that at least some of the hilltop plots had high wood density has an implication for carbon storage in the large degraded areas of Borneo: in these degraded areas, there is often no primary forest left except small patches on the tops of hills.  These are likely to have a relatively high carbon density for their area, and should be preserved if at all possible.

5.       References
        J. H. C. Cornelissen, S. Lavorel, E. Garnier, S. Díaz, N. Buchmann, D. E. Gurvich, P. B. Reich, H. Ter Steege, H. D. Morgan, M. G. A. vander Heijden, J. G. Pausas and H. Poorter. 2003. A handbook of protocols for standardised and easy measurement of plant functional traits world wide. Australian Journal of Botany 51 (4) 335-380.

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 Table 1. Least and most dense woods, by species.

Family                      Species                                                   Mean          SE

Malvaceae               Durio oxleyanus Griff.                                                 0.313                 NA

Anacardiaceae         Parishia insignis Hook.f.                                              0.341                 NA

Dipterocarpaceae    Shorea  domatiosa P. S. Ashton                                   0.359                 NA

Myrtaceae               Syzygium leptostemon (Korth.) Merr.&Perry             0.366                 NA

Apocynaceae           Dyera polyphylla (Miq.) Steenis                                  0.379                  NA

NA                           NA                                                                                  0.385                  NA

Burseraceae           Santiria tomentosa Bl.                                                   0.392                  NA

Dipterocarpaceae  Shorea parvistipulata Heim.                                          0.395                  NA

Myrtaceae             Aromadendron elegans Blume                                       0.402                  NA

Dipterocarpaceae  Dipterocarpus tempehes Sloot.                                      0.415                  NA

...
Ochnaceae             Gomphia serrata (Gaertn.) Kanis                                  0.811                  NA

Fabaceae                Dialium                                                                           0.813                  NA

Melastomataceae  Memecylon myrsinoides Bl.                                          0.815                  0.002

Clusiaceae              Garcinia  rostrata (Hassk.) Miq.                                  0.815                  0.006

Clusiaceae              Garcinia  balicca Miq.                                                   0.819                  NA

Clusiaceae              Garcinia rigida Miq.                                                      0.824                  NA

Rubiaceae               Ixora elliptica R. Br.et Ridl.                                         0.830                  NA

Myrtaceae              Rhodamnia cinerea Jack.                                             0.832                  NA

Polygalaceae          Xanthophyllum stipitatum A. W. Benn                         0.842                  NA

Meliaceae              Reinwardtiodendron humile (Hassk.) Mabb.               0.860                  NA

Fig. 1. Distribution of wood densities for 257 taxa.

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