The simulation of forest fragments of differ-ent sizes showed three differdiffer-ent directions in which forest remnants might develop (ex-amples for size were taken from results of scenario fragmented 3): (i) collaps of the forest structure (1 ha); (ii): a forest with a high fraction of early successional species
(9–64 ha); (iii): rain forest with a species composition of an undisturbed forest (81–
100 ha). A size of at least 80 ha is necessary for maintaining a total biomass and species composition, which is similar to a primary forest. It might be that even in these large scale areas a shift in the species composi-tion within the different species groups oc-curs. Turner & Corlett (1996) found in a rain forest fragment of 100 ha in Southeast-Asia no change in standing biomass over 50 years, but strong shifts in the species com-position.
So far, little modelling effort has been focused towards attempting to understand the interplay between fragmentation pro-cesses and natural regeneration of the for-est. A probable explanation for this is that
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Figure 8.6: Patterns with a fraction of the landscape being regrowed with secondary succession after abandoned of cleared ares (same scenarios as Fig.8.5 butt= 301−600 y).Above-ground biomass as function of cleared fraction (CF) and pattern formation (random, clustered 1−3).
Bars show time averages (including SD), crosses correspond to values att= 600 y.Total biomass (1st row), and fraction of early- and late-successional species (2nd and 3rd row).
the problem of fragmentation is spatially ex-plicit. Laurance et al. (1998) estimated with the model of Laurance & Yensen (1991) when forest fragments were dominated by edge effects. According to their analyses strong and moderate effects should rise once the fragment size falls below 90–500 ha. Af-ter our simulations strong effects on stand-ing biomass and species composition are only seen at the lower end of this range.
Ferreira & Laurance (1997) stated that even in forest fragments of 1000 ha a substantial impact of fragmentation will be expected, because 22−42 % of the border area was influenced by edges. With our simulations the structural characteristics of the forest are not expected to change substantial for that large areas. The estimate of impacts of fragmentation over the ratio of the
bor-der area seems to overestimate the effects largely.
Therefore, it requires forest growth simu-lators able to describe the spatial structure of the landscape. Such quantitative models have only recently become available (Bossel
& Krieger 1994, Pacala et al. 1996, K¨ohler
& Huth 1998a, Chave 1999b).
The structure of the forest matrix has been repeatedly shown to be an important indicator of the faunal diversity (Gascon et al. 1999; Hanski & Ovaskainen 2000; Jor-dan 2000). Indeed, the three-dimensional spatial heterogeneity of forest communities is the main mechanism which allows for the observed diversity of heterotrophic species.
The species composition is strongly affected by the invasion of ecotonal species, which
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Figure 8.7: Time variation of the above-ground biomass for selected fragmentation scenarios (random, clustered 1−3) and cleared fraction (CF = 0−90 %).Total biomass (solid bold line), biomass for early successional species (solid line), for mid-successional species (broken dotted line), and forlate successional species (broken line).Savanna species are always below 1 Mg ha−1 and are not shown.Land-uses were abandoned at year 300.The biomass was computed only within the forest plots fromt= 0 y tot= 300 y, and over the whole landscape aftert= 300 y, whence the apparent discontinuity in the biomass curve.
might outcompete the species of the closed rain forest. Our simulations show that the indicators of forest structure such as the within-plot maximal canopy height are indeed strongly affected by fragmentation.
The underlying mechanism in our simula-tions is the increased mortality rate for large edge trees. Since the ecotone area increases quickly with fragmented fraction, the strong positive correlation between canopy height and fragmented fraction is unsurprising.
Above-ground biomass estimates for neotropical forests still rely upon scarce data, and the models used to relate the dbh with the tree biomass (e.g. Brown 1997; Higuchi et al. 1998) have a lim-ited predictive power (Brown et al. 1995;
Chave et al. 2000c). At the Piste de Saint Elie 5 ha dataset, Brown’s (1997) allometric equation gave an estimate of 210.7 Mg ha−1, Higuchi-Santos’s (1998) equation yielded a high 358.9 Mg ha−1, and the fit proposed in Chave et al. (2000) gave Figure 8.8: (opposite page): Spatial distri-bution of dominant trees as function of clear-ing fraction (CF) and of spatial clustering (random, clustered 1−3) of selected scenar-ios for different times.Top: time t = 300 y, just before meadows are abandoned; bottom:
t = 600 y.Simulation area was 1 km2, each pixel corresponds to one patch (20 m×20 m).
Pixels inform about size and successional group of the dominat tree of the patches according to legend.
t= 300 y
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H = 10-30m H > 30m
Figure 8.8: Captions are found on opposite page.
245.2 ± 30 Mg ha−1. An in situ biomass experiment (0.25 ha plot where all trees were felled and weighed) predicted values above 400 Mg ha−1 more consistent with Higuchi-Santos’s predictions, but probably skewed by the choice for the experiment plot. Therefore, although it is quite diffi-cult to assess the quality of the values pre-dicted by our model (445 ± 15 Mg ha−1 in the control run), it is probable that the real number is smaller. We have found that after 300 years the regenerating forest reached significantly smaller figures than in the control run (59−68 %). This suggests a long-lasting effect of disturbances upon the carbon pool of forests, as observed in field experiments (Saldarriaga et al. 1988).
Another explanation is that natural distur-bances operating over large areas (such as large treefalls) maintain the system below its expected biomass capacity.
Plant functional diversity was strongly affected by fragmentation in all the sim-ulations. The fraction of early succes-sional species which were maintained was significantly larger over the whole land-scape, which had to be expected because of the increased area available for this PFT.
However, the increase was also significant within the forest fragments (up to 30 % for CF = 90 %). Regeneration after land use change showed that the forest recovered only after 100 years. In clustered fragmen-tation scenarios, this transient regime was longer (> 200 y), which is a consequence of seed-dispersal limitation for late succes-sional species. A recent study has found that species composition took even longer to recover (Ferreira & Prance 1999). Undis-turbed forest had 147 species per ha while the secondary forest had a mere 89 species per ha, but the estimated biomass was com-parable. This picture constrasts with that of Saldarriaga et al. (1988), in part because the disturbance type were clearly different in the two forests, but also because of an unsufficient taxonomic effort in the latter survey (only 75 % of the trees were
identi-fied to the species level).