Identifying sustainable management strategies for near-natural temperate evergreen
rainforest in southern Chile – a simulation experiment
N. Rüger¹,*, Á.G. Gutiérrez², W.D. Kissling¹, J.J. Armesto²,³ and A. Huth¹
¹ UFZ - Centre for Environmental Research Leipzig-Halle, Department of Ecological
Modelling, P.O. Box 500 136, D-04301 Leipzig, Germany. * e-mail: nadja.rueger@ufz.de
² CMEB, Department of Ecology, Faculty of Sciences, University of Chile, Casilla 653,
Santiago, Chile.
³ Centre for Advanced Studies in Ecology & Biodiversity, Catholic University of Chile,
P.O. Box 114-D, Santiago CP 6513677, Chile.
_______________________________________________________________________
Abstract
Current forestry practices in southern Chile rely largely on the establishment of pure
plantations of exotic tree species after clear-cutting and burning of native old-growth
forests. There is little experience with silviculture of the species-rich native evergreen
rainforests. Nevertheless, conservationists, forest scientists and foresters call for
sustainable management of native forests to conserve its highly endemic biodiversity
and protect ecosystem services. We apply the process-based forest growth model
FORMIND to compare different management strategies in regard to their
sustainability. FORMIND is an individual-based forest growth model that simulates
the spatio-temporal dynamics of an uneven-aged mixed forest stand. For each
logging scenario we determine a sustainability index that combines economic
(amount and constancy of yield) and ecological (structural and compositional
similarity of logged and unlogged forest) criteria. Results show that high-intensity
logging (40-200 trees/ha) in long logging cycles (80-100 years) reaches highest
sustainability values. We suggest that our simulation results can provide general
guidelines for planning future sustainable management of native rainforests.
_______________________________________________________________________
Introduction
Temperate evergreen rainforest covers approximately 43000 km² restricted mainly to
lowland and mid-elevation areas in the western margin of southern South America (Donoso
1989). This remote and isolated eco-region has been classified as one of the most
vulnerable ecosystems of the world (Dinerstein et al. 1995). Large parts of these forests are
severely threatened by conversion to pastures or pure plantations of Pinus radiata or
Eucalyptus species. However, an initiative of Chilean foresters, biologists, and agronomists
calls for sustainable management of the remaining native forests to recover and maintain the
valuable ecological services that these forests provide (Lara et al. 2003). In the past, native
forests, particularly evergreen rain forests, were regarded as too complex to be managed
because of its species-rich canopy, uneven age structure, and high structural complexity.
Consequently, few studies have addressed its potential for sustainable use (Donoso 1989).
We apply the process-based forest growth model FORMIND to compare different logging
strategies and to determine management options that combine high timber yield with low
impact on forest structure and composition. FORMIND is an individual-based forest model
that allows for the detailed analysis of long-term implications of different logging strategies.
Similar modelling approaches have been applied to identify sustainable management options
for species-rich uneven-aged tropical lowland rainforests (e.g. Vanclay 1994, Huth & Ditzer
2001, van Gardingen et al. 2003). To enable a relative ranking of different logging scenarios
we define a sustainability index that includes both economic (amount and constancy of yield)
and ecological criteria (compositional and structural similarity of logged forest to unlogged
old-growth forest). A weight is attached to each criterion that can be adjusted according to
preferences of stakeholders and decision makers. The sustainability index facilitates the
determination of most suitable logging regimes according to given subjective priorities. This
study aims to show that sustainable management of native species-rich forests in southcentral Chile is possible. We suggest that our simulation results can provide guidelines for
future sustainable management, thereby providing opportunities for the conservation and
use of native biodiversity in forests outside protected areas (Armesto et al. 1998).
Methods
Study area
The model was parameterized for a coastal Valdivian temperate rainforest, as it occurs, for
example, in northern Chiloé Island, Chile, and along the coastal range of Chilean Lake
District (40-42°S). Floristically, this forest type is dominated by Eucryphia cordifolia
(Cunnoniaceae), a relatively light demanding emergent tree species, Aextoxicon punctatum
(only member of the endemic Aextoxicaceae) and Laureliopsis philippiana (Monimiaceae),
two shade-tolerant tree species which form the main canopy. In addition, several myrtaceous
tree species occur in the canopy and subcanopy. In the model, all myrtaceous species are
treated as one species because they share similar ecological characteristics (high shadetolerance, low maximum height). More details on model parameterization and validation can
be found in Rüger et al. (in prep.).
The model
The forest growth model FORMIND2.2 simulates the spatial and temporal dynamics of
uneven-aged, mixed species forest stands (Köhler and Huth 1998; Köhler 2000). It simulates
a forest (in annual time steps) of several hectares as a mosaic of interacting patches with a
size of 20 m × 20 m, corresponding to the crown size of mature trees. Within the patches, all
trees compete for light and space following the distance-independent gap model approach
(Shugart 1998). The carbon balance of each individual tree is modelled explicitly, including
the main physiological processes (photosynthesis and respiration). Allometric functions
relate above-ground biomass, stem diameter, tree height, crown diameter and stem volume.
Tree mortality can occur either through self-thinning in dense patches, senescence, or gap
formation by large falling trees. Gap formation links neighbouring patches. Regeneration
rates are effective rates regarding the recruitment of small trees at a diameter at breast
height (dbh) threshold of 1 cm, with seed loss through predation and other processes
already being implicitly incorporated.
Logging scenarios
We selected two logging strategies for analysis, which are part of current silvicultural
protocols in the region (Lara & Donoso 1999); low and high intensity selective logging with a
minimum cutting threshold of 50 cm dbh. Logging intensity is varied from 2 to 20 trees/ha (in
steps of 2 trees/ha) and from 20 to 200 trees/ha (in steps of 20 trees/ha) for the low and high
intensity logging scenarios, respectively. Time between two harvesting operations (logging
cycle) was varied from 2 to 10 years (in steps of 2 years) and from 20 to 100 years (in steps
of 20 years). We use inventory data from the study area described above as initial situation.
Logging started after 100 years – to allow the model to reach steady state – and was
performed over 400 years. We assume reduced-impact logging where falling trees are
directed to existing gaps if possible. Additional vegetation damage due to skidding is
considered to increase linearly with felling intensity from 5 to 23% for 2 to 20 logged
trees/ha, and from 23 to 50% for 20 to 200 logged trees/ha. No damage occurs to trees > 50
cm dbh.
Sustainability index
The sustainability index (SI) combines five criteria, namely total yield (Y), constancy of yield
(CV), an index of structural change (ISC), an index of compositional change (ICC), and leaf
area index (LAI) for the stand. All five criteria are normalized in relation to the maximum
value obtained from all logging scenarios (Ymax, CVmax, ISCmax, ICCmax) or the value of a
5
control scenario (LAICtrl), weighted (w1-5,
∑w
i =1
SI = w1
i
= 1 ), and finally summed up:
Y
CV
ISC
ICC
LAI
+ w2 (1 −
) + w3 (1 −
) + w4 (1 −
) + w5 (
)
Ymax
CVmax
ISC max
ICC max
LAI Ctrl
This results in SI values ranging from 0 to 1 with higher values indicating more sustainable
logging scenarios. Y was calculated as the total logged stem volume over the entire logging
period. CV was determined as the coefficient of variation of Y. ISC measures the change in
mean numbers of trees in three diameter classes (5-50 cm, 50-100 cm, and >100 cm dbh) of
a simulated logged forest in comparison to an unlogged forest. ICC indicates the change in
ecological dominance of a logged forest in relation to an unlogged forest based on
importance values of trees ≥ 5 cm dbh (i.e. the sum of relative basal area and relative
density). LAI values are directly determined from model output and averaged over the
simulation time 400-500. For more details see Rüger et al. (in prep.).
Figure 1: Sustainability index (SI) of different selective logging scenarios varying in logging
intensity and logging cycle.
Results
Under the chosen weighting of the five criteria (w1=2, w2-5=1), high-intensity logging (40-200
trees/ha) in long logging cycles (80-100 years) yield highest sustainability values (Fig. 1).
Among the low-intensity logging scenarios those with relatively low intensities and long
cycles reach highest sustainability values. Highest timber yield is obtained for high logging
intensities (60-200 trees/ha) and logging cycles longer than 40 years (cf. Rüger et al. in
prep.). Greatest structural and compositional similarity to unlogged forest is achieved for low
logging intensities and long logging cycles (2 ind/ha every 10 years or 20 ind/ha every 100
years). To ensure the conservation of animal species that depend on old trees (> 100 cm
dbh) as habitat (Diaz et al. 2005), it is recommended to explicitly leave some old trees in the
forest since most studied logging scenarios fail to keep them. This effect has also been
observed for commonly applied logging practises (Armesto et al. 1999). Mean LAI values
seem sufficiently high (>2 m²/m²) in all logging scenarios to prevent soil erosion due to heavy
rain that is typical for this area. Even directly after a logging operation they usually do not fall
below 1 m²/m² (cf. Rüger et al. in prep.).
Discussion
We aimed at comparing currently used logging strategies with different logging intensities
and logging cycles in terms of their sustainability. We measured sustainability of a logging
scenario as a combination of amount and constancy of yield on the one hand, and ecological
impact on forest structure and composition on the other hand. Maximum annual stem volume
increments for the simulated logging scenarios range between 6.3 and 13.6 m³/ha. These
values are considerably lower than estimates for single-species plantations of Nothofagus
dombeyi that reached mean annual increments of up to 26 m³/ha (Donoso et al. 1993).
However, despite this difference, our estimates show that the native old-growth forests can
provide a valuable source for constant and sustainable timber harvest. For a wide range of
logging intensities and cycles, sustainability values are very similar, because a surplus in
yield is accompanied by a decrease in “naturalness” (similarity to an unlogged forest) of the
forest. This means that the ultimate choice of a management strategy strongly depends on
the decision makers’ priorities and goals. Nevertheless, high-intensity logging (40-200
trees/ha) with long logging cycles (80-100 years) seems to offer an acceptable compromise
between economic and ecological criteria. FORMIND has proven to be a useful tool to study
long-term implications of anthropogenic intervention in species-rich uneven-aged native
forests. We suggest that our simulation results can provide guidelines for decisions about
sustainable annual felling rates, maintaining low ecological impacts on forests, as demanded
by Lara et al. (2003). Sustainable management of remaining native forests in coastal and
lowland areas of south-central Chile can support the conservation of biodiversity outside
protected areas.
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