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Abstract Sustainable management of selectively logged tropical forests requiresthat felled trees are replaced through increased recruitment and growth. This studycompares road track and roadside regeneration with regeneration in unlogged andselectively logged humid tropical forest in north-eastern Bolivia. Some speciesbenefited from increased light intensities on abandoned logging roads. Others ben-efited from low densities of competing vegetation on roads with compacted soils.

This was the case for the small-seeded species Ficus boliviana C.C. Berg and Ter-minalia oblonga(Ruiz & Pav.) Steud. Some species, e.g. Hura crepitansL., displayedpatchy regeneration coinciding with the presence of adult trees. Our results suggestthat current management practices could be improved by intensifying logging insome areas to improve regeneration of light demanding species. Sufficient seed inputin logged areas should be ensured by interspersing large patches of unlogged forestwith logged areas. This may also assist regeneration of species that perform poorly indisturbed areas.

Keywords Bolivia Logging roads Microsite availability Seedling distribution

Soil disturbance

Soil compaction

Tropical forest management

Uneven-age management

J. Nabe-Nielsen W. Severiche T. FredericksenLouise I. Nabe-NielsenProyecto BOLFOR, Santa Cruz, Bolivia

J. Nabe-NielsenThe Royal Veterinary and Agricultural University (KVL), Copenhagen, Denmark

T. FredericksenFerrum College, Ferrum, VA, USA

New Forests (2007) 34:3140DOI 10.1007/s11056-006-9035-2

Timber tree regeneration along abandoned logging

roads in a tropical Bolivian forest

Jacob Nabe-Nielsen

Willy Severiche

Todd Fredericksen

Louise Imer Nabe-Nielsen

Received: 12 November 2005 / Accepted: 20 December 2006 / Published online: 25 January 2007 Springer Science+Business Media B.V. 2007


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Introduction

Selective logging in tropical forests is sustainable only when increased tree mortal-ities are balanced by increased recruitment and growth. Species that are used for

timber extraction are directly affected by logging, and the changed structure of alogged forest affects the abundance of other plant species (Molino and Sabatier2001; Hall et al. 2003). The search for sustainable logging strategies should aim toimprove the regeneration of the species whose stem densities are most reduced bylogging.

Tree regeneration is influenced by logging in several ways. Most importantly, thelight environment on the forest floor is altered in selectively logged forests, which islikely to affect the composition of the tree sapling communities (Nicotra et al. 1999;Molino and Sabatier2001). Soils are more disturbed in logged forests due to the useof heavy machinery for removing felled trees. This affects the regeneration of some

tree species (Guariguata and Dupuy 1997; Dickinson et al. 2000). Seed limitationmay also affect the regeneration in logged forests (Gullison et al. 1996; Ghazoul andMcLeish2001), partly because the distance between suitable regeneration-sites andthe nearest seed-producing trees are longer in logged than in unlogged forest.

The microhabitat-changes imposed by logging affect regeneration differently indifferent guilds of trees, and regeneration remains affected at least until the forestshave recovered the structure of unlogged forests. After one or two decades thelogged forests can possess a smaller number of large gaps than unlogged forests(Nicotra et al.1999) allowing for little regeneration of pioneer species. Even recent

disturbances caused by selective logging may be too small to allow for sufficientregeneration of economically valuable light demanding species in some forests(Mostacedo and Fredericksen1999; Hall et al.2003). The logging roads that are usedfor transportation of timber to the saw-mills are therefore likely to be important forthe regeneration of many species due to the large-scale, long-lasting alterations ofsoil and light environments (Guariguata and Dupuy 1997; Buckley et al.2003).

The aim of this paper is to investigate whether the environmental conditionsalong abandoned logging roads favour regeneration of the tree species that areextracted for timber. Specifically we assess (1) how small-scale environmentalvariations along abandoned logging roads and on roadsides differ from the ones in

unlogged forest, and how this is reflected in the probability of finding timber treerecruits. (2) We compare the density of timber tree recruits on abandoned loggingroads and in adjacent selectively logged forest and analyse how the number ofrecruits changes with time after abandoning the road. Together these analysesindicate how logging roads affect the microenvironmental conditions in parts of theforest and how this affects timber tree recruitment. This can help us planning timberextraction so that it affects the forest environment in a way that improves timber treeregeneration, thereby making logging more sustainable.

Methods

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is 250 m. The soils include oxisols, ultisols and inceptisols. During the JuneOctoberdry season wildfires occur, and the area has a long history of anthropogenic dis-turbances (at least 400 years; Paz2003). The logging techniques used in La Chontaare similar to the ones used elsewhere in Bolivia, where logs are hauled to the

nearest logging road using rubber-tired skidders. Logging roads are often locatedseveral hundred metres from the place where the tree was felled. On the road thelogs are either collected or loaded directly onto large trucks and transported to thesawmill. This results in a heavy and uniform compaction of the soil on the roads,which persists after the 23 year period where the road is used. The logging protocolin La Chonta ensures that at least 20% of the harvestable trees are retained as seedtrees and that regenerating trees are damaged as little as possible. The protocolconforms to the standards of the Forest Stewardship Council (www.fsc.org) and theforest is therefore comparable to other forests where a low impact logging scheme ispracticed.

We studied the distribution of timber trees on abandoned logging roads using twodifferent experimental designs, one for investigating the effect of fine-scale envi-ronmental variation, the other for comparing species abundances on road tracks withspecies abundances in the neighbouring selectively logged forest. Species are listedin Table1. Only genus names will be used hereafter.

In the first part of the study we used sixty, 5 5-m plots (0.15 ha in total). Half ofthe plots were placed at random in a 450 300-m unlogged forest plot. Theremaining 30 plots were distributed evenly among roads in three logging compart-ments that were selectively logged 1, 2 and 4 years prior to the study. The three

study sites were located less than two km apart. Prior to logging the forest hadsimilar stature and approximately the same species abundances in the three areas.The plots in each compartment were placed 30 m apart. Every second plot wasplaced on the road track and the following one was placed on the road side in areasthat had been heavily disturbed when roads were constructed. Road track plots wereplaced at least 1 m from the roadside in areas with a level, homogeneous road

Table 1 Timber tree species found in the study and their frequency in the 5 5-m plots. Plots wereplaced on logging road tracks (RT; 15 plots in total), roadsides (RS; 15 plots) and in unlogged forest

(UL; 30 plots)Species Family No. of plots

RT RS UL

Ampelocera ruizii Klotzsch Ulmaceae 4 2 6Aspidosperma cylindrocarpon Mull. Arg. Apocynaceae 1 0 0Cariniana estrellensis (Raddi) Kuntze Lecythidaceae 1 0 0Cariniana ianeirensis R. Knuth Lecythidaceae 1 0 0Cedrela fissilis Vell. Meliaceae 0 1 0Ficus boliviana C.C. Berg Moraceae 3 0 0Gallesia integrifolia (Spreng.) Harms Phytolaccaceae 0 1 0Hura crepitansL. Euphorbiaceae 1 2 13Pouteria nemorosa Baehni Sapotaceae 0 0 6Pseudolmedia laevis (Ruiz & Pav ) J F Macbr Moraceae 0 2 14

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surface. In each plot we identified all timber trees >20 cm high but with diameter atbreast height (dbh) 3 plots in either unlogged forestor along the roads.

In the second part of the study, we recorded the number of saplings (30150 cmhigh) and small poles (>150 cm high but


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Other species appeared to regenerate more frequently in unlogged forest thanalong the roads.Pouteriawas not found in the 30 road plots, whereas it was commonin the unlogged reference plots. Similarly, Pseudolmedia was most abundant inunlogged forest and least abundant along the most recently abandoned loggingroads. Most of the Pseudolmedia individuals in the forest plots were 20 cm high but


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plots (df = 1, v2 = 6.69, P< 0.01). When comparing all pairs of environmentalvariables, the only significant correlation was found between undergrowth densityand canopy openness, with higher canopy openness scores in the less dense plots(r= 0.47, P< 0.01, n = 60; Bonferroni-corrected test). As the density of the un-

derstorey vegetation was closely related to canopy openness, we did not analyse itseffect on plant presence directly.

In the second part of the study the relative abundances of the different timber treespecies was roughly the same as in the plots from the first part of the study (Fig. 2).TerminaliaandPouteriawere among the species with the highest overall abundance,and they were much more abundant on logging roads than in the neighbouringlogged forest. These species were also the most abundant in the first part of ourstudy, but in that part,Pouteriawas found in unlogged forest only, and only as smallindividuals. Ficus was found in high densities on some of the logging roads in thesecond part of the study whereas Hurawas more abundant inside the forest than on

the roads. The pioneer species Schizolobium was also more common in the selec-tively logged forest than on the roads. The lower tree densities in the second part ofthe study were mainly caused by the exclusion of plants 2030 cm high. In the firstpart of the study, 37% of the saplings belonged to this category and for Hura andPouteriamore than half of the saplings were 2030 cm tall.

Hura

seudolmedia

Term

inalia

Po

uteria

.estrellensis

Schizolobium

0

80

160

240400

600

Plantsperha

small polessaplings

0

80

160

240400

600

Plantsperha

1 years old

Selectivelyloggedfore

st

Hura

seudolmedia

Term

inalia

Ficus

Po

uteria

.estrellensis

Schizolobium

Swietenia

Ce

drela

2 years old

Hura

seudolmedia

Term

inalia

Ficus

Po

uteria

.estrellensis

Schizolobium

Swietenia

Cedrela

4 years old

Roadtrack

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Microhabitats of road tracks and roadsides differed in several aspects. The soilwas more compacted on road tracks (1.54 g cm3) than on roadsides or in selectivelylogged forest (1.17 g cm3 and 1.14 g cm3), respectively (P< 0.001, F= 54.2; two-way Anova). Soil compaction did not differ among transects that had been aban-

doned for different periods. Canopy openness was elevated (CIE > 1) more often onroad tracks (in 93% of the plots) than on the roadside (40%) (P= 0.001,G2 = 10.7;likelihood-ratio test). In the unlogged forest 33% of the plots had elevated canopyopenness, which was not significantly less than on the roadsides. The vegetation onthe roadsides was much denser than on the roads. It was dominated by Urareaspp.and Heliconia spp. in some areas. Terminalia and Ampelocera were nearly exclu-sively found in road track plots and Ficus was never found in roadside plots(Table1).

Discussion

Timber tree regeneration and growth were improved on abandoned logging roadsfor some of the studied species only. BothTerminaliaandAmpeloceraoccured morefrequently along roads than in unlogged forest. These species were by far the mostabundant in our study, and their distributions were strongly affected by environ-mental variation. Ampelocera was most frequently found in areas with tall canopy,possibly because this is where seed trees were most likely to occur. Both species werefound more often on road tracks than in the dense vegetation of the roadsides, even

though Terminalia was strongly associated with plots with little canopy openness.This suggests that the species is a poor competitor, and that small individuals areeasily killed by strong direct light. Inside the forest, Terminalia grows in nearly thesame densities in scarified and in non-scarified parts of logging gaps (Fredericksenand Pariona, 2002). High sapling densities on roads are therefore likely to resultfrom the decreased competition from pioneer species rather than from any directeffect of soil disturbance. Reduced pioneer regeneration on compacted soils is notunique to the present study (see e.g. Aide et al. 1995).

Ficuswas exclusively found on abandoned logging roads. Inside the forest in LaChonta, the species is uncommon even in large gaps. Pariona et al. (2003) found only

10.7 saplings per ha in logging gaps in La Chonta. Both Ficus and Terminalia arerelatively small-seeded. Seedlings of small seeded species may not be able toestablish on a thick litter layer (Grubb1996). Limited seed reserves may also causeseedlings to establish relatively slowly, which in turn causes them to be poor com-petitors. The relatively frequent regeneration ofFicus on road tracks is, therefore,likely to be economically very important.

Some species were equally abundant in the forest and along the logging roads, yetthey appeared to grow better by the roads. This was the case for Hura, that onlyattained a height >150 cm on roads. The species also grows better in scarified than in

non-scarified parts of logging gaps (Fredericksen and Pariona 2002). In contrast toTerminalia, it appeared to benefit from high light intensities, even though it was ableto establish under low-light conditions in the unlogged forest. Both Hura and

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dispersal is often a major limitation to tree recruitment following human distur-bances (Gullison et al. 1996; Slik et al.2002; Chazdon2003).

PouteriaandPseudolmediawere more abundant inside the forest than on roads,and both species occurred in a large fraction of the unlogged forest plots. Pouteria

was associated with plots with intermediate canopy openness and relatively dry,level ground. The abundance of large Pouteria (>150 cm tall) on the four-year oldlogging roads suggests that the species needs elevated light intensities to grow up.The same may be the case for Pseudolmedia. Both Pseudolmedia and Carinianaestrellensis regenerated more often as resprouts than from seeds in the loggedareas (W. Severiche, pers. obs.).

Although regeneration appears to be frequent on logging roads for many of thestudied timber species, it is difficult to determine whether increased tree densitieswill eventually result in successful replacement of the trees that have been felled.The effects of increased juvenile densities may be counterbalanced by, e.g.,

decreased growth rates at later stages, and a complete life-cycle analysis is generallynecessary to conclude if a species regenerates sufficiently (Condit et al.1997; Caswell2001). It is only safe to conclude that species that display a complete lack ofregeneration in particular habitat types regenerate insufficiently, provided that thecurrent regeneration pattern does not vary through time. Nevertheless, we believethat the species that appear to benefit from decreased competition on logging roadsin La Chonta, particularly Terminalia and Ficus, will continue to have increasedpopulation densities on logging roads relative to closed forest. The logging roadstherefore seem to have a positive, long-lasting effect on the regeneration of some of

the species that are most intensively logged.Reduced competition on logging roads appears to be the main reason for the highlevels of road track regeneration for many timber tree species in La Chonta. Thereduced competition can, at least in part, be attributed to poor pioneer establish-ment on compacted soils. Some of the pioneer species may also have germinatedwhile the roads were still in use, which caused an exhaustion of the soil seed bankbefore it was possible for the plants to establish. This was the case forSchizolobium(T. Fredericksen, pers. obs.), the only pioneer species that was included in our study.The species was more abundant in selectively logged forest than on roads, althoughit is known to benefit strongly from increased soil disturbance (Fredericksen and

Pariona2002). We did not record any regeneration of the species in unlogged forest.The abundant regeneration and high diversity on old logging roads in La Chonta

contrasts with the findings of Guariguata and Dupuy (1997). They found lower treespecies richness on logging roads than beside roads or inside the forest in a study inCosta Rica. There was evidence that soil compaction and initial lack of on-site plantpropagules resulted in slower recovery of stem densities and species richness on theroads. Soil compaction at the level found in the present study may generally impederoot growth, especially in dry soils (Dias and Nortcliff1985). This appeared not to bethe case for several of the timber species in La Chonta: Species with relatively high

sapling densities on logging roads were generally present as large individuals on theroads as well.The best way to improve regeneration of selectively logged species may be to

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Previous land use is sometimes reflected by the tree community composition forcenturies (Sheil and Burslem2003). In abandoned agricultural areas where pioneerswere eliminated with the aid of fire, poor competitors like Terminaliamay have beenfavoured in the same way as they are favoured on abandoned logging roads today.

The species composition in other logged forests does not necessarily converge as fastto the composition of the unlogged forest as we believe it does in La Chonta (cf.Kuusipalo et al.1996; Webb1998), especially where the forest disturbance intensityhas historically been low.

Conclusion

We found evidence that several timber tree species regenerated better in areas withincreased soil disturbance on abandoned logging roads. The most abundant of them,

Terminalia, benefits from the reduced competition on the road tracks that is partlycaused by reduced pioneer regeneration on compacted soils. Some of the speciesthat display increased sapling growth on roads, especially Hura, appear to sufferfrom seed limitation. To ensure sufficient regeneration of disturbance-adaptedspecies, species that suffer from recruitment limitation and of species that areassociated with unlogged forest, we suggest that the current management practicecan be improved by logging and disturbing the soil more intensively in some patches,while ensuring that nearby patches of unlogged forest are left intact.

Acknowledgements Many thanks to Joaqun M. Justiniano for assistance with the field work and tothe people at BOLFOR for logistical assistance. JNN wishes to thank WWF/Novo Nordisk A/S andFiedlers Grant for financial support.

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