As a result of different types of disturbance, forests are a
mixture of stands at different stages of ecological succession. Successional
stage is likely to influence forest productivity and carbon storage, linking
the degree of forest disturbance to the global carbon cycle and climate.
Although tropical montane forests are an important part of tropical forest
ecosystems (ca. 8 %, elevation > 1000 m a.s.l.), there are still
significant knowledge gaps regarding the carbon dynamics and stocks of these
forests, and how these differ between early (ES) and late successional (LS)
stages. This study examines the carbon (C) stock, relative growth rate (RGR)
and net primary production (NPP) of ES and LS forest stands in an Afromontane
tropical rainforest using data from inventories of quantitatively important
ecosystem compartments in fifteen 0.5 ha plots in Nyungwe National Park in
Rwanda.
The total C stock was 35 % larger in LS compared to ES plots due to
significantly larger above-ground biomass (AGB; 185 and 76 Mg C ha−1
in LS and ES plots), while the soil and root C stock (down to
45 cm depth in the mineral soil) did not significantly differ between the
two successional stages (178 and 204 Mg C ha−1 in LS and ES plots). The main reasons for the difference in AGB were that ES trees
had significantly lower stature and wood density compared to LS trees.
However, ES and LS stands had similar total NPP (canopy, wood and roots of
all plots ∼ 9.4 Mg C ha−1) due to counterbalancing effects of
differences in AGB (higher in LS stands) and RGR (higher in ES stands). The
AGB in the LS plots was considerably higher than the average value reported
for old-growth tropical montane forest of south-east Asia and Central and
South America at similar elevations and temperatures, and of the same
magnitude as in tropical lowland forest of these regions.
The results of this study highlight the importance of accounting for
disturbance regimes and differences in wood density and allometry of tree
species dominating at different successional stages in an attempt to quantify
the C stock and sink strength of tropical montane forests and how they may
differ among continents