Timber tree regeneration along abandoned logging roads in a tropical Bolivian forest Jacob Nabe-Nielsen1,2,*, Willy Severiche1, Todd Fredericksen1,3, Louise Imer Nabe-Nielsen1 Published in New Forests (2007) 34:31–40 DOI 10.1007/s11056-006-9035-2 Abstract Sustainable management of selectively logged tropical forests requires that felled trees are replaced through increased recruitment and growth. This study compares road track and roadside regeneration with regeneration in unlogged and selectively logged humid tropical forest in north-eastern Bolivia. Some species benefited from increased light intensities on abandoned logging roads. Others benefited 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 Terminalia oblonga (Ruiz & Pav.) Steud. Some species, e.g. Hura crepitans L., displayed patchy regeneration coinciding with the presence of adult trees. Our results suggest that current management practices could be improved by intensifying logging in some areas to improve regeneration of light demanding species. Sufficient seed input in logged areas should be ensured by interspersing large patches of unlogged forest with logged areas. This may also assist regeneration of species that perform poorly in disturbed areas. Keywords Bolivia, Logging roads, Microsite availability, Seedling distribution, Soil disturbance, Soil compaction, Tropical forest management, Uneven-age management 1
Proyecto BOLFOR, Santa Cruz, Bolivia The Royal Veterinary and Agricultural University (KVL), Copenhagen, Denmark 3 Ferrum College, Ferrum, VA, USA * Present e-mail address (in 2008):
[email protected] 2
Introduction Selective logging in tropical forests is sustainable only when increased tree mortalities are balanced by increased recruitment and growth. Species that are used for timber extraction are directly affected by logging, and the changed structure of a logged forest affects the abundance of other plant species (Molino and Sabatier 2001; Hall et al. 2003). The search for sustainable logging strategies should aim to improve the regeneration of the species whose stem densities are most reduced by logging. Tree regeneration is influenced by logging in several ways. Most importantly, the light environment on the forest floor is altered in selectively logged forests, which is likely to affect the composition of the tree sapling communities (Nicotra et al. 1999; Molino and Sabatier 2001). Soils are more disturbed in logged forests due to the use of heavy machinery for removing felled trees. This affects the regeneration of some 1
tree species (Guariguata and Dupuy 1997; Dickinson et al. 2000). Seed limitation may also affect the regeneration in logged forests (Gullison et al. 1996; Ghazoul and McLeish 2001), partly because the distance between suitable regeneration-sites and the nearest seed-producing trees are longer in logged than in unlogged forest. The microhabitat-changes imposed by logging affect regeneration differently in different guilds of trees, and regeneration remains affected at least until the forests have recovered the structure of unlogged forests. After one or two decades the logged 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 sufficient regeneration of economically valuable light demanding species in some forests (Mostacedo and Fredericksen 1999; Hall et al. 2003). The logging roads that are used for transportation of timber to the saw-mills are therefore likely to be important for the regeneration of many species due to the large-scale, long-lasting alterations of soil and light environments (Guariguata and Dupuy 1997; Buckley et al. 2003). The aim of this paper is to investigate whether the environmental conditions along abandoned logging roads favour regeneration of the tree species that are extracted for timber. Specifically we assess (1) how small-scale environmental variations 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 tree recruits. (2) We compare the density of timber tree recruits on abandoned logging roads and in adjacent selectively logged forest and analyse how the number of recruits changes with time after abandoning the road. Together these analyses indicate how logging roads affect the microenvironmental conditions in parts of the forest and how this affects timber tree recruitment. This can help us planning timber extraction so that it affects the forest environment in a way that improves timber tree regeneration, thereby making logging more sustainable. Table 1 Timber tree species found in the study and their frequency in the 5 × 5-m
plots. Plots were placed on logging road tracks (RT; 15 plots in total), roadsides (RS; 15 plots) and in unlogged forest (Old; 30 plots). Species
Family
No. of plots RT
Ampelocera ruizii Klotzsch Aspidosperma cylindrocarpon Müll. Arg. Cariniana estrellensis (Raddi) Kuntze Cariniana ianeirensis R. Knuth Cedrela fissilis Vell. Ficus boliviana C.C. Berg Gallesia integrifolia (Spreng.) Harms Hura crepitans L. Pouteria nemorosa Baehni Pseudolmedia laevis (Ruiz & Pav.) J.F. Macbr. Schizolobium amazonicum Huber ex Ducke Swietenia macrophylla King Sweetia fruticosa Spreng. Terminalia oblonga (Ruiz & Pav.) Steud.
Ulmaceae Apocynaceae Lecythidaceae Lecythidaceae Meliaceae Moraceae Phytolaccaceae Euphorbiaceae Sapotaceae Moraceae Fabaceae Meliaceae Fabaceae Combretaceae
2
RS 4 1 1 1 0 3 0 1 0 0 0 0 0 8
2 0 0 0 1 0 1 2 0 2 3 0 3 3
Old 6 0 0 0 0 0 0 13 6 14 0 0 0 1
Methods The study was conducted in a tropical humid forest in the 100,000 ha forestry concession of Agroindustria Forestal La Chonta, located 30 km east of Ascensio´n de Guarayos, Bolivia (15°47’ S, 62°55’ W). The area receives an annual rainfall of approximately 1500 mm and the mean annual temperature is 24.5 °C. The elevation is 250 m. The soils include oxisols, ultisols and inceptisols. During the June–October dry season wildfires occur, and the area has a long history of anthropogenic disturbances (at least 400 years; Paz 2003). The logging techniques used in La Chonta are 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 located several hundred metres from the place where the tree was felled. On the road the logs are either collected or loaded directly onto large trucks and transported to the sawmill. This results in a heavy and uniform compaction of the soil on the roads, which persists after the 2–3 year period where the road is used. The logging protocol in La Chonta ensures that at least 20% of the harvestable trees are retained as seed trees and that regenerating trees are damaged as little as possible. The protocol conforms to the standards of the Forest Stewardship Council (www.fsc.org) and the forest is therefore comparable to other forests where a low impact logging scheme is practiced. We studied the distribution of timber trees on abandoned logging roads using two different experimental designs, one for investigating the effect of fine-scale environmental variation, the other for comparing species abundances on road tracks with species abundances in the neighbouring selectively logged forest. Species are listed in Table 1. 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 of the plots were placed at random in a 450 × 300-m unlogged forest plot. The remaining 30 plots were distributed evenly among roads in three logging compartments 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 had similar stature and approximately the same species abundances in the three areas. The plots in each compartment were placed 30 m apart. Every second plot was placed on the road track and the following one was placed on the road side in areas that had been heavily disturbed when roads were constructed. Road track plots were placed at least 1 m from the roadside in areas with a level, homogeneous road surface. In each plot we identified all timber trees >20 cm high but with diameter at breast height (dbh) 3 plots in either unlogged forest or along the roads. In the second part of the study, we recorded the number of saplings (30–150 cm 3
high) and small poles (>150 cm high but 20 cm high but 150 cm high) was more common on roads than inside the forest, especially for Hura. Ficus, Schizolobium and the two species of Cariniana were not found in the unlogged forest at all. Other species appeared to regenerate more frequently in unlogged forest than along the roads. Pouteria was not found in the 30 road plots, whereas it was common in the unlogged reference plots. Similarly, Pseudolmedia was most abundant in unlogged forest and least abundant along the most recently abandoned logging roads. Most of the Pseudolmedia individuals in the forest plots were 1) more often on road 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 canopy openness, which was not significantly less than on the roadsides. The vegetation on the roadsides was much denser than on the roads. It was dominated by Urarea spp. and Heliconia spp. in some areas. Terminalia and Ampelocera were nearly exclusively found in road track plots and Ficus was never found in roadside plots (Table 1).
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Table 2 Effect of canopy openness and canopy height on presence of each species in
the 5 × 5-m plots. Tests were carried out using logistic regressions. Species
Canopy openness df χ2 P Ampelocera 5 8.84 0.12 Ficus 5 3.37 0.64 Hura 5 11.47 0.04 Pouteria 5 11.95 0.04 Pseudolmedia 5 13.4 0.02 Schizolobium 5 2.85 0.72 Terminalia 5 15.80
