logging scenarios. Following this, we discuss the two working steps
Model performance under undisturbed conditions
Although the structure and species compo-sition of mature forest shift continuously on a small scale (Wiens 1999), we think that a stability analysis is worth performing to evaluate model performance. In fact, all forest growth models known to the authors (cf. Liu & Ashton 1995) implicitly accept a so-called ”potential natural vegetation”
(PNV), which represents the steady state as a product of model structure and parametri-sation. None of them have tested the PNV in detail against field data of mature forest.
The critical point of the stability test was whether the site we categorised as mature forest was representative. In a few aspects, the mature forest lacked this representativ-ity. For example, no area was in gap phase and consequently early successional species were rare. We consider these points cru-cial for the stability analysis. However, a field inventory represents only one condition in space and time. A comparison with the time averaged dynamic as done in this anal-ysis suggests that these points have a minor influence on the quality of the model results.
For instance, the time averaged fraction of gap area in mature forest or the proportion of early successional species showed small differences to inital values (cf. Table 6.6).
Two further trends need to be discussed:
the decrease of most general variables be-low the 1.0 level (cf. Fig. 6.1), and the in-crease of most PFTs indices far apart from this level. The latter trend results from the fact that the initial species composition was fairly distinct from the averaged
mod-elled one. Field data of a larger sampling area might be needed to reflect the accu-racy of the modelled composition of plant functional types. From our model analysis, we know that recruitment is the most im-portant factor for species composition and needs to be modelled in more detail in fu-ture applications. The decline of the values of general variables could be explained by the missing gap phase. The initial data set suggests that the analysed mature forest is a well-structured stand showing high values in basal area, bole volume and leaf area in-dex. The stability index of general variables showed changes below 15%, indicating that the model and its parametrisation is stable and a suitable tool for further analysis of logging scenarios in the Caparo forest.
The sensitivity analysis was undertaken in realistic parameter ranges. This implies that the response in the result variables should not be too sensitive, otherwise a model with that many parameters is not suitable for a sound analysis. The results showed low sensitivity for most parameter variations. The sensitivity of the model to variations in the mortality parameters raises the question of whether these values were chosen properly. However, simulated mortality rates in the mature forest are within the range observed in other neotrop-ical forests (Condit et al. 1992; Carey et al.
1994).
Logging scenarios
Net timber volumes in the range of 30-60 m3 ha−1 cycle−1 assumed in the scenarios for the second and subsequent cutting cycles may be perceived as high having in mind the traditionally low intensity of wood re-moval in Latin America compared to South-East Asia (Plumptre 1996). In fact, the first cutting cycle is selective focusing on the most valuable species i.e. B. quinata, S.
macrophylla and C. odorata in areas where they occur clumped, resulting in low har-vest volumes, if referring to the whole
log-ging unit. As individuals of these species are found rarely below the minimum felling di-ameter MFD (Plonczak 1989; Kammesheidt 1994, 1998), logged-over stands are com-posed of potential commercial species with a considerably lower market value (cf. Cen-teno 1995). Further depletion of the most valuable species might result in an increase of log prices of formerly unlogged or rarely logged species. However, even an increase of log prices for less acceptable species would hardly offset the loss of valuable timber species so that a higher volume must be har-vested to keep the cost/benefit ratio of the first cutting cycle. To include all species above the legal size in the present logging scenario is reasonable because this is being done already in other concession areas in the western plains (J. Duque, pers. comm.).
Also, the currently applied MFD of 50 cm for medium-hardwood species, most of them with a mid-successional status constituting the bulk of commercial volume might be re-duced if sawable logs above this size should become scarce. Overall, harvest volume and MFD may vary within the range given in this study depending on the composition of commercial volume in the individual logging unit.
Unlike in other neotropical regions, some empirical data on long-term growth and yield are available to evaluate our results.
An average annual bole volume increment of 3.8 m3 ha−1 y−1 (SD = 4.2; trees ≥ 10 cm d.b.h.) for the 30 years cutting cycle with conventional logging is a conservative growth rate compared with Veillon’s (1985) mean figure of 4.4 m3 ha−1 y−1 (SD = 0.5) measured in a stand 15-32 years after light logging. The basal area increment in the first 10 years after logging over the simula-tion period under convensimula-tional or reduced-impact logging with 30 years cutting cycles and a removal of 30 and 6 0 m3ha−1cycle−1, respectively was 0.3 and 1.1 m2 ha−1 y−1. Lozada (1998) found similar basal area in-crement rates in the first ten years in exper-imentally cut stands with a basal area
re-moval of 20-84%. By simulating 240 years, early successional species accounted on av-erage with 83-95% to the basal area incre-ment at any initial phase (1-10 yr) after logging in 30-years intervals, while Lozada (1998) found a mean percentage of 45. This suggests that by shorting cutting cycles over a longer period of time early successional species can become predominant after dis-turbance.
Ingrowth and mortality declined with in-creasing length of cutting cycles (cf. Ta-ble 6.5). In the 60 year cutting cycle with reduced-impact logging both param-eters reached average values in the range of 2.2-3.8% which, is similar to turn-over rates of trees ≥ 30 cm d.b.h. in old-growth forest in Panama (Condit et al. 1992). We assumed in our simulation a continuous in-put of seeds. This might have been too optimistic in case of short cutting cycles.
In combination with a low minimum felling diameter at least some commercial species will be harvested before they have attained their reproductive stage. Thus, stand com-position will either shift to common species capable of early reproduction or overall re-generation will decline.
We simulated the spatial pattern of dis-turbance associated with logging. Differ-ences between logging methods in terms of area damaged could be even more pro-nounced taking into account that cut trees are expected to fall swiftly to the ground if vines are cut well before logging operation as assumed in our model. Conventional log-ging methods do not consider vine cutting;
this results in tree tangles which extend the canopy gap area (cf. Johns et al. 1996).
Vine cutting may become an even more im-portant measure in logged-over stands be-cause these areas support a proliferation of lianas (Kammesheidt 1999). With reduced-impact and conventional logging, respec-tively 23-35% and 50-73% of the simulated area was damaged (i.e. log-landings, felling and skidding areas), corresponding to a basal area removal of 2.5-5 m2 ha−1 cycle−1.
By contrast, Hendrison (1990) found in Suriname with a basal area removal of 4 m2 ha−1 cycle−1 that 22 and 36%, respec-tively of the forest area was damaged under controlled and uncontrolled logging. The much higher damage level with conventional logging in Venezuela highlights the careless logging methods in the study area.
The data set for simulation combined well-drained and poorly drained sites. The latter sites might show a slower rate of tree establishment and lower diameter in-crement rates than the well-drained site ow-ing to different soil water availability in the rainy and dry season. This may result in a different speed of succession. To date, how-ever, no study has tested this hypothesis.
Simulations were made under the as-sumption of no disturbance other than log-ging and gap creation owing to tree fall.
Fire, for example, is a real hazard because of the considerable increase in fuel mass af-ter logging (cf. Nepstad et al. 1999) which is easily inflammable during the pronounced dry season. Particularly short cutting cycles with conventional logging methods, leaving large tracts of open forest, increase the sus-ceptibility to fire.
Conclusions
Both the stability and sensitivity analy-sis showed that Formind2.0 simulates the stand dynamics of Caparo forest within re-alistic limits. The model’s capability to simulate the spatial heterogeneity of stands with high resolution makes the model useful for simulating growth and yield of logged-over forest. Whether cutting cycles identi-fied as sustainable in terms of timber yield, are economically viable in the long run will strongly depend on species composition and log quality of merchantable trees. Reliable forecasts to this end will offer new chal-lenges to forest modelling.
Acknowledgements
We thank F. Hapla, G¨ottingen University, who determined the wood density of se-lected tree species. H. Bossel, J. Chave, A.R. Watkinson, and an anonymous re-viewer provided helpful comments on the manuscript. P. K¨ohler was funded by the Otto-Braun-Foundation of the University of Kassel.