3.5. Carbon content in aboveground tree compartments
Table 4 shows the means multiple comparisons for aboveground
biomass C content according to tree compartments
and species On the average for all the species C content
follows the pattern: Stems > Branches > Foliage > Bark. The
conifer by far is the species with the highest values of C
content in all compartments surpassing the 50% except for
foliage (49.6%). All the broadleaf species have average C values
minor than 50%. The smallest C values are V. guatemalensis for
foliage (41%) and T. amazonia for bark (40.2%). On the average,
C content for all the broadleaf species shows similar values for
branches (47.9%) and stems (48.6%), although there are
statistical significant differences among the species.
4. Discussion of results
4.1. Species growth
The results of the present study provide valuable information
to support the establishment and management of plantations
of four native and two introduced species in pure designs
along an environmental gradient in the southern region of
Costa Rica. Native species cannot compete in growth and
adaptability against the two introduced in very unfertile acid
soil with prolong dry season (Eco-region 1). As soil fertility and
soil moisture improves the native species become more
competitive but in most cases they hardly surpass the
productivity indices of the two introduced species, except in
very fertile and humid sites (Eco-region 4). The point to
emphasize here is that in Costa Rica there is a strong tendency
to establish forest plantation in unfertile or degraded sites [12]
and hence the promotion of using native species instead of
introduced species must be strongly reconsidered. Further
discussions about this aspect are better treated in [13].
4.2. Aboveground biomass and C content
As expected the two introduced species had the highest
aboveground biomass (and hence carbon sequestration) due
to its high adaptability and growth on acid unfertile soils (Ecoregion
2). But as discussed before these differences could be
reduced as site fertility and soil moisture improve. The
information about C content and the set of equations to estimate
aboveground biomass by tree compartments and whole
tree, for each of the species or group of species, are significant
contributions for further studies of nutrient cycling, site
nutrient management and carbon sequestration. Caution
must be taken when using these equations; their application
should be preferably along the sampled range of DBH of each
of the study species. As found in prior publications, this study
confirm that steam C content was appreciably higher in
conifers (P. caribaea) than the selected broadleaf species (50.8%
vs. 48.2%, respectively). Our results indicate that the widely
used 50% wood C content figure is a fair rule of thumb when
there is lack of information. We recognized that our estimation
method of C content did not include chemical forms of
volatile C. As shown by [40] the volatile C fraction is nonnegligible,
averaging 2.2% with a high variation among species
in China.