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Regeneration of timber trees in a logged tropical forest in North Bolivia
Forest Ecology and Management 200 (2004) 39–48 www.elsevier.com/locate/foreco
Regeneration of timber trees in a logged tropical forest in North Bolivia H.M.P.J.B. (Jacaranda) van Rheenena,b,*, Rene G.A. Boota,b, Marinus J.A. Wergera, Miguel Ulloa Ulloac a
Department of Plant Ecology, Faculty of Biology, University of Utrecht, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands b Programa Manejo de Bosques de la Amazonia Boliviana (PROMAB), Casilla 107, Riberalta, Bolivia c Forestry Faculty of the Beni Technical University (CIF-UTB), Riberalta, Bolivia Received 12 November 2003; received in revised form 14 May 2004; accepted 8 June 2004
Abstract For sustainable forest management, it is important to know the response of timber species to the change in environment caused by logging. We performed a 2-year study on germination, survival and growth of four timber species, Cedrela odorata, Swietenia macrophylla, Hymenaea courbaril, and Cariniana micrantha, and one non-commercial species Tachigali vasquezii. We sowed seeds of these species in five microenvironments: log landing, gap-crown and gap-trunk, skidder trail and understory, in a tropical lowland moist rain forest in northern Bolivia. We related seed and seedling performance to light availability, soil compaction, and plant competition. Germination did not differ significantly between microenvironments but survival of germinated seeds for most species was significantly higher (P < 0.05) in the log landing (46–100%) than in the understory (0– 7%). After 2 years, the tallest plants were always found in the log landing (119–190 cm) and the smallest in the understory (12– 26 cm) caused by a higher relative height growth rate (RHGR) in the log landing (0.003–0.004 cm cm1 per day) compared to the understory (0.000–0.001 cm cm1 per day). During the first year RHGR was positively related to canopy openness for all species and negatively to the number of overtopping competitors for three species. During the second year also water infiltration explained observed variation to RHGR. These results show that abandoned log landings and logging gaps are suitable environments for the regeneration of timber species studied. This finding suggests that the removal of competitors in log landings and logging gaps combined with leaving seed trees near these microenvironments or sowing seeds, will improve regeneration of timber species in tropical forests. # 2004 Elsevier B.V. All rights reserved. Keywords: Bolivia; Regeneration; Competition; Light Availability; Soil compaction
* Corresponding author. Tel.: +31 30 2536876; fax: +31 30 2518366. E-mail address: [email protected] (H.M.P.J.B. (Jacaranda) van Rheenen).
1. Introduction Logging activities in tropical forests restructure the forest environment considerably by creating different
0378-1127/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2004.06.024
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microenvironments, such as log landings (large openings created in the forest where tree trunks are stored before transportation), logging roads, logging gaps and skidder trails. The main environmental conditions important for regeneration that differ between these microenvironments are light availability and soil disturbance (Fredericksen and Mostacedo, 2000). Furthermore, they differ in the presence of seedlings and saplings. Log landings are initially characterised by high light availability and high soil compaction and low abundance of seedlings (Gillman et al., 1985). Logging roads usually have intermediate light availability and high soil compaction, and seedlings are mostly absent. Skidder trails usually show little damage to the canopy and thus light availability is low and soil compaction intermediate while seedlings are few in number (Whitman et al., 1997). Gaps caused by tree felling have high light availability, relatively low soil compaction, and low initial seedling density (Fredericksen and Mostacedo, 2000). Knowledge of how species respond to a change in light availability, soil compaction and competition, is important for achieving sustainable forest management. Many studies have investigated the effects of each of these factors in isolation, but much less is known about the interacting effects of these factors. Studies in tropical forests have shown that light availability has a positive effect on seedling growth and survival (Zagt, 1997; Poorter, 1998; Van der Meer et al., 1998; Quevedo and Fredericksen, 2000; Rose, 2000; Pena, 2001) but soil compaction has been reported to delay regeneration (Reisinger et al., 1988; Malmer and Grip, 1990; Jusoff and Majid, 1992; Pinard et al., 1996; Guariguata and Dupuy, 1997; Whitman et al., 1997). Other studies have shown that disturbed soil results in an increase in the abundance and growth of tree seedlings (Dickensen et al., 2000; Fredericksen and Mostacedo, 2000; Fredericksen et al., 2000). Finally the absence of competitors has a positive effect on tree seedling and sapling growth (Kuusipalo et al., 1997; Quevedo and Fredericksen, 2000; Zagt, 1997; D’Oliviera, 2000; Rose, 2000). In the tropical forests of Bolivia, mandatory best management practices required under a new forestry law have created changes in the logging industry of this South American country. A shift has occurred from the harvesting of a few high-valued timber
species using conventional logging to the harvesting of many more lesser known timber tree species using low-impact logging practices. It is important to understand how timber species react to these new logging practices in order to evaluate their sustainability. For this study we chose four commonly harvested timber species (Cedrela odorata, Swietenia macrophylla, Hymenaea courbaril and Cariniana micrantha) and one non-commercial species (Tachigali vasquezii). The objective of this study was to answer the following questions. (1) Is there a difference in germination, survival, and growth of seedlings of timber species in the different microenvironments? (2) How do light availability, soil compaction, and competing vegetation affect regeneration? (3) Do logged microenvironments offer suitable regeneration sites for tree species? To answer these questions, we conducted an experiment in a logging concession in northern Bolivia focusing on five different microenvironments, including log landings, skidder trails, crown area within logging gaps, the trunk zone of logging gaps, and undisturbed forest understory.
2. Methods 2.1. Description of study site The fieldwork was conducted in Verdum, a logging concession in the Department of Beni in northern Bolivia (108550 0500 S, 658400 5000 W). It is a lowland moist tropical forest with high plant diversity with average stem density of 544 ha1 and basal area of 26.0 m2 ha1 and a canopy height of approximately 30 m (Poorter, 1998). The average rainfall is 1700 mm per year, mainly occurring between November and March, while the dry season lasts from May to September. The soils are loamy and well drained with a pH in the topsoil of 4.9 and a high percentage of aluminium (DHV, 1993). The total area of the concession is 2511 ha, of which 500 ha were logged in 2000. Twenty-three species were logged, removing approximately four trees per hectare, roughly equivalent to 10 m3 ha1. Salas (2002) estimated that the total disturbed area was 28%, of which logging gaps made up the largest part (12.3%), followed by skidder trails (7.6%), logging trails (4.0%), secondary roads (2.1%), log landings (1.5%) and main roads (1.1%).
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Table 1 Tree species and their life history strategies Species
Family
Regeneration strategy
Adult stature
Fresh seed weight (g)a
Sowing month
Cedrela odorata Swietenia macrophylla Hymenaea courbaril Tachigali vasquezii Cariniana micrantha
Meliaceae Meliaceae Caesalpiniaceae Caesalpiniaceae Lecythidaceae
Long-lived pioneer Long-lived pioneer Shade tolerant Long-lived pioneer Shade tolerant
Canopy Canopy Emergent Canopy Emergent
0.05 0.38 3.18 0.39 0.12
November December October October November
a
(0.001) (0.070) (0.015) (0.002)
Mean fresh seed weight is presented with standard deviation in parentheses.
2.2. Experimental design The experiment was conducted in five different microenvironments: log landings, skidder trails, the crown area of logging gaps, the trunk area of logging gaps, and understory locations. The trunk area was directly next to where the trunk fell. Each microenvironment was replicated three times. First, abandoned log landing sites were selected and then the other microenvironments were chosen at minimum distances of 100 m from each log landing and from each other. The log landings had an average size of 1663 m2 (S.D. = 601) and the logging gaps 384 m2 (S.D. = 70). The microenvironments gap-trunk, understory and skidder trail had little advanced regeneration whereas the log landing and gap-crown had none. Two large plots were established in each microenvironment in which 30 seeds of each species were sown in 10 sub-plots, each covering 1 m2. These subplots were randomized within each plot. Chosen tree species differ in their regeneration strategies, adult stature and fresh seed weight as shown in Table 1. Seeds were sown shallowly, at approximately 0.5 cm depth, and covered with a very thin layer of litter to protect against predation, desiccation and rain damage. 2.3. Microenvironment characterization
(GLA), Simon Fraser University, British Colombia and the Institute of Ecosystem Studies, New York, 1999. Water infiltration in the soil was measured in each microenvironment in 2001 and 2002. The method measured the rate at which water infiltrated into the top soil (0–5 cm) within a cylinder (height: 40 cm; diameter: 30 cm). The number and height of non-experimental plants, referred to as ‘competitors’, were determined in each sub-plot during November of 2001 and 2002. If competitors were taller than experimental plants they were scored as ‘overtopping competitors’. 2.4. Plant measurements The following data were recorded for each plot starting in November of 2000, and measured monthly until March 2001 and then in November 2001 and November 2002: number of germinated seeds and surviving seedlings; and height of seedlings. The height data were used to determine relative height growth rate (RHGR) as height growth per unit plant height in cm cm1 per day.
Table 2 Canopy openness and water infiltration in five microenvironments in a Bolivian rain forest in 2001 and 2002 Microenvironment
The microenvironments were characterized in terms of light availability, measured indirectly as canopy openness, soil compaction and number of non-experimental plants. Hemispherical photos were taken in November 2001 with a Nikon Coolpix 880 digital camera to determine canopy openness. Photos were taken with an overcast sky and at a height of 1.5 m in the centre of each plot. The photos were analysed with imaging software: Gap Light Analyser
Log landing Gap-crown Gap-trunk Skidder trail Understory
Canopy openness (%)
Water infiltration (cm/min)
2001
2001
26.9 12.8 13.7 4.5 3.4
a
(1.95) (1.35)b (0.63)b (0.30)c (0.24)c
0.4 1.5 4.7 1.0 2.5
2002 b
(0.20) (0.00)ab (0.77)a (0.32)b (0.91)ab
0.2 5.0 5.5 0.4 4.1
(0.01)b (0.61)a (1.45)a (1.31)b (0.91)a
Differences between microenvironments are tested with a Scheffe test. Within each year, values followed by a different letter are significantly different (P < 0.05).
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Table 3 Density of competitors and overtopping competitors in five microenvironments in a Bolivian rain forest in 2001 and 2002 Microenvironment
Competitors (#/m2) 2001
Log landing Gap-crown Gap-trunk Skidder trail Understory
5.7 20.7 19.5 10.3 12.1
Overtopping competitors (#/m2) 2002
b
(0.71) (1.54)a (1.37)a (1.21)b (1.70)b
6.8 29.4 43.0 16.9 18.0
2001 c
(0.71) (3.41)a (9.55)a (2.96)b (1.19)ab
3.2 9.6 15.1 4.2 11.5
2002 c
(0.19) (0.41)b (0.70)a (0.26)c (0.84)b
2.8 13.3 10.8 6.7 12.4
(0.19)c (1.48)a (1.64)ab (0.69)bc (0.82)a
Differences between microenvironments are tested with a Scheffe test. Within each year, values followed by a different letter are significantly different (P < 0.05).
Fig. 1. The percentage germination, survival proportional to the number of germinated seeds (survival/germination) and survival proportional to the number of seeds sown (survival/seed) after 2 years for the indicated species in five microenvironments. Different letters within each category on the x-axis indicate significant differences (P < 0.05) between the microenvironments. The colored bars represent the different microenvironments; log landing (black bars), gap-crown (white bars), gap-trunk (dark gray bars), skidder trail (arched bars) and understory (light grey bars).
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2.5. Statistical analyses Differences in canopy openness, water infiltration, number of competitors and overtopping competitors between microenvironments were analysed with a two-way ANOVA using species and microenvironments as factors. The microenvironment was used as unit of replication. Data on canopy openness for the year 2001 and water infiltration and number of competitors from the year 2002 were ln-transformed to normalize the data. Comparison
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of means was conducted with the post-hoc Scheffe test. Data for percentage survival proportional to the number of germinated seeds and the percentage survival proportional to the number of sown seeds were square root transformed to normalize the data. To test for differences between the microenvironments these data and percentage germination were analysed using a two-way ANOVA with species and microenvironment as factors. To compare survival over time for each species in the different microenvironments a Cox
Fig. 2. Survival percentage of germinated seeds over time in five microenvironments for the different species as indicated. Different letters in each graph (A–E) indicate significant differences (P < 0.05) in survival (%) between microenvironments. The symbols represent the different microenvironments; log landing (*), gap-crown (~), gap-trunk (5), skidder trail (&) and understory (^).
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regression was used. For Cariniana, there were not enough individuals in all microenvironments, therefore the analysis was only conducted for the gapcrown and gap-trunk area. A repeated measurement analysis was used to determine the change in absolute height over time of the surviving seedlings in the different microenvironments. Only individuals that had survived until the end of the experiment were included in the latter analyses. The relative increase in plant height was determined using the relative height growth rate (cm cm1 per day), which was calculated by dividing the mean ln-height difference over the total period by the length of the time period of observations (Venus and Causton, 1979). The means were compared with Scheffe’s post-hoc test.
A multiple forward regression analysis was done for each species for both years to relate seedling growth to site characteristics. RHGR was the dependent variable and light availability, water infiltration, number of competitors and overtopping competitors were used as independent variables.
3. Results 3.1. Site characteristics Canopy openness ranged from 3.4% in the understory to 26.9% in the log landing 1 year after logging (Table 2). There were no differences between the
Fig. 3. Height growth rate over time for the different species in the five microenvironments. Different letters in each graph (A–E) indicate significant differences (P < 0.05) between the microenvironments. The symbols represent the different microenvironments; log landing (*), gap-crown (~), gap-trunk (5), skidder trail (&) and understory (^).
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two gap microenvironments, gap-crown and gaptrunk, but the openness of these microenvironments was significantly (P < 0.05) lower than in the log landing and higher than in the skidder trail and understory. Water infiltration was lowest in the log landing followed by the skidder trail and the other microenvironments (Table 2). All microenvironments except the log landing and skidder trail showed slight increases in infiltration from the first to the second year. For both years, the density of competitors was significantly (P < 0.05) lower in the log landing
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and skidder trail than in the gap microenvironments (Table 3). This trend was also true for overtopping competitors though not for the second year in the skidder trail. The gap-crown and gap-trunk did not significantly differ from each other in number of competitors/m2 and only in the first year was the number of overtopping competitors/m2 greater in the gap-trunk than in the gap-crown, 15.1 and 9.6/ m2, respectively. The density of competitors and overtopping competitors in the understory did not differ significantly from those in the gap microenvironments in the second year but differed significantly (P < 0.05) from those in the log landing.
Fig. 4. Mean relative height growth rates (RHGR) and standard errors for the different species in five microenvironments. Different letters in each graph (A–E) indicate significant differences (P < 0.05) between the microenvironments.
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3.2. Timber tree germination, survival and growth Overall, germination did not differ significantly between the microenvironments but survival of germinated seedlings did differ between sites (Fig. 1). The survival of these germinated seeds was significantly (P < 0.05) higher in the log landing than in the understory with the exception of Tachigali (Fig. 1D), which showed no difference in germination between the microenvironments. Percentage survival of germinated Cariniana seeds appeared to be much higher in log landings than in the other microenvironments, but was not significantly different because of the small sample size (n = 3). Cedrela, Swietenia and Tachigali survived in all microenvironments, although high mortality of Cedrela and Swietenia was observed in the understory. A temporal analysis of seedling survival shows that most of the mortality occurred during the first weeks after germination (Fig. 2). Tachigali had the highest survival in skidder trails and the lowest in log landings. Survival proportional to the number of seeds sown was significantly (P < 0.05) higher in the log landing than in the understory for all species except Tachigali, which had the highest survival (40.9%) in skidder trails followed by the understory (21.9%) as shown in Fig. 1. Two years after the initiation of the experiment, the tallest plants of most species were located in log
landings and the smallest were situated in the understory (Fig. 3). At the beginning of the study, there was little difference in plant height between the microenvironments and this difference only became apparent after 170 days for Cariniana, 200 days for Cedrela and Swietenia, and approximately 400 days for Hymenaea and Tachigali. The RHGR for all species was significantly (P < 0.05) higher in the log landing and lowest in the understory (Fig. 4). In addition, for all species with the exception of Swietenia, the RHGR was significantly (P < 0.05) higher in the gap microenvironment than in the understory. Finally RHGR in the skidder trail and understory microenvironment did not differ significantly for most species, except Cariniana and Tachigali which had higher RHGR in the skidder trail than in the understory. Multiple forward regression analysis show that, during the first year, canopy openness and overtopping competitors were the main variables that together explained the observed variation in RHGR (Table 4). One year after sowing, both canopy openness and overtopping competitors explained the observed variation in RHGR in Cedrela, Hymenaea and Tachigali, with r2 values of 0.7, 0.5 and 0.3, respectively; whereas for Cariniana and Swietenia, canopy openness explained 40% of RHGR. RHGR has a positive relationship with canopy openness and a negative relation-
Table 4 Effect of microenvironmental factors on relative height growth rate (RHGR) in five microenvironments in 2001 and 2002 Independent variables Year 2001 2001 2001 2001 2001 2002 2002 2002 2002 2002
Species Cedrela Swietenia Hymenaea Tachigali Cariniana Cedrela Swietenia Hymenaea Tachigali Cariniana
Na 156 133 91 438 30 156 132 90 441 29
Canopy openness
Water infiltration
Competitors
***
0.75 0.64*** 0.35* 0.53*** 0.62*** 0.76*** 0.70*** ***
0.67
Overtopping competitors ***
0.22
0.43* 0.11* 0.14** 0.20*** 0.34*** 0.14*** 1.15***
0.26***
***
0.26 0.79***
0.40*** 0.29***
r2 0.72 0.40 0.51 0.30 0.39 0.76 0.69 0.45 0.59 0.61
Shown are the results of the forward multiple regression with standardized coefficients for those variables that significantly contribute to the model. Regression models were checked for collinearity: tolerance values were always >0.4. a Number of observations. * P < 0.05. ** P < 0.005. *** P < 0.001.
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ship with the number of overtopping competitors. Two years after sowing, more variables became significantly (P < 0.005) important and the total variation explained increased slightly for most species, ranging from 0.45 to 0.76. Water infiltration became a significant (P < 0.005) explanatory variable for all species. In addition, canopy openness played a role for Cedrela, Swietenia and Tachigali while the number of overtopping competitors became an important variable for the species Cedrela, Hymenaea and Tachigali.
4. Discussion Generally, the species reacted similarly to the different microenvironments. Germination for most species did not differ significantly among the microenvironments but germination and the combined survival of seeds and seedlings were higher in the log landing than in the understory, with the exception of Tachigali. Furthermore, all species had taller plants in the log landing and the smallest in the understory. This difference in height between the microenvironments only became apparent at a minimum of 170–200 days for Cariniana, Cedrela and Swietenia and 400 days for Hymenaea and Tachigali. From these results, we can deduce that a regeneration study may require a minimum of six months to observe differences in height and adequately access initial regeneration success. In log landings, the high amount of light availability and the delay in establishment of competitors appeared to have been beneficial for sown trees. Nearly all of these competitors were small-seeded species, and germinating small seeds were probably strongly negatively affected by high light availability, high temperatures, and accumulation of water on the compact soils occurring in the log landing microenvironment. In a Malaysian tropical forest, Pinard et al. (1996) also found that unfavourable site conditions limited establishment of pioneers in skidder trails and log landings. Despite favourable conditions, experimental trees in gaps were not as tall as those in the log landing, as competitors quickly became established and competed for light and other resources with tree seedlings. Similar results were obtained by Fredericksen and Mostacedo (2000) in a tropical dry forest in eastern Bolivia where regeneration was initially higher in log landings and logging roads than in logging gaps,
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which competitors quickly colonized. In our results, skidder trails and understory microenvironments were equally suitable for germination as the gap microenvironments, but growth was inhibited by low light availability. Augspurger (1984) obtained similar results in a screened enclosure experiment on Barro Colorado Island where tree seedlings grew larger in light environments similar to that of small gaps, than in shaded environments that simulated forest understory conditions. Beech plants in Danish beech stands also showed increased height growth with increased canopy openness (Madsen and Larsen, 1997). Our results using multiple forward regression analysis also demonstrated the importance of light availability and overtopping competitors on the first year RHGR of all species. Light availability was positively related, whereas the number of overtopping competitors was negatively related to RHGR. In the second year, when trees were larger, other factors such as soil compaction and competing plant density started to play a role. For almost all species the more disturbed the soil the greater the RHGR. Fredericksen and Pariona (2002) also observed greater tree seedling height growth on more disturbed soil, which was scarified, than undisturbed soils within logging gaps in a tropical humid forest in eastern Bolivia. The results of this study suggest that some microenvironments created by logging offer suitable regeneration sites for all five species in this study. On the basis of tree seedling growth, the optimal microenvironment is the log landing followed by the gap microenvironments; whereas skidder trails and undisturbed forest understory microenvironments were less suitable. Despite the small area that these different microenvironments occupy in a logged forest, they may still play an important role in the regeneration of commercial timber species in the long-term. To obtain satisfactory regeneration in forests managed for timber and ensure sustainable harvesting, it is important that recently abandoned log landings and gap areas contain or receive sufficient seeds of the logged species and that these microenvironments are initially cleared of other seedlings in order to reduce competition with the regenerating timber species. This can be achieved when logging companies plan to leave seed trees close to log landings and gaps or by sowing seeds, but even this could be to little avail if competition with the advanced regeneration is not reduced.
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Acknowledgements We thank Comunidad 12 Octubre for allowing us to do the research in their forest. We are grateful to Luis Apaza, Rene Aramayo, Miguel Cuadiay, Nicolas Divico and Nazareno Martinez for their assistance in the fieldwork. We thank Betty Verduyn for analysing the fish eye photos and Oscar Llanque and Paul Westers for valuable discussions and statistical advice. We are grateful to Pieter Zuidema and Todd Fredericksen for critical reading of the manuscript. This research was part of the research program of the Programa de Manejo de Bosques de la Amazonia Boliviana (PROMAB).
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Regeneration of timber trees in a logged tropical forest in North Bolivia H.M.P.J.B. (Jacaranda) van Rheenena,b,*, Rene G.A. Boota,b, Marinus J.A. Wergera, Miguel Ulloa Ulloac a
Department of Plant Ecology, Faculty of Biology, University of Utrecht, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands b Programa Manejo de Bosques de la Amazonia Boliviana (PROMAB), Casilla 107, Riberalta, Bolivia c Forestry Faculty of the Beni Technical University (CIF-UTB), Riberalta, Bolivia Received 12 November 2003; received in revised form 14 May 2004; accepted 8 June 2004
Abstract For sustainable forest management, it is important to know the response of timber species to the change in environment caused by logging. We performed a 2-year study on germination, survival and growth of four timber species, Cedrela odorata, Swietenia macrophylla, Hymenaea courbaril, and Cariniana micrantha, and one non-commercial species Tachigali vasquezii. We sowed seeds of these species in five microenvironments: log landing, gap-crown and gap-trunk, skidder trail and understory, in a tropical lowland moist rain forest in northern Bolivia. We related seed and seedling performance to light availability, soil compaction, and plant competition. Germination did not differ significantly between microenvironments but survival of germinated seeds for most species was significantly higher (P < 0.05) in the log landing (46–100%) than in the understory (0– 7%). After 2 years, the tallest plants were always found in the log landing (119–190 cm) and the smallest in the understory (12– 26 cm) caused by a higher relative height growth rate (RHGR) in the log landing (0.003–0.004 cm cm1 per day) compared to the understory (0.000–0.001 cm cm1 per day). During the first year RHGR was positively related to canopy openness for all species and negatively to the number of overtopping competitors for three species. During the second year also water infiltration explained observed variation to RHGR. These results show that abandoned log landings and logging gaps are suitable environments for the regeneration of timber species studied. This finding suggests that the removal of competitors in log landings and logging gaps combined with leaving seed trees near these microenvironments or sowing seeds, will improve regeneration of timber species in tropical forests. # 2004 Elsevier B.V. All rights reserved. Keywords: Bolivia; Regeneration; Competition; Light Availability; Soil compaction
* Corresponding author. Tel.: +31 30 2536876; fax: +31 30 2518366. E-mail address: [email protected] (H.M.P.J.B. (Jacaranda) van Rheenen).
1. Introduction Logging activities in tropical forests restructure the forest environment considerably by creating different
0378-1127/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2004.06.024
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microenvironments, such as log landings (large openings created in the forest where tree trunks are stored before transportation), logging roads, logging gaps and skidder trails. The main environmental conditions important for regeneration that differ between these microenvironments are light availability and soil disturbance (Fredericksen and Mostacedo, 2000). Furthermore, they differ in the presence of seedlings and saplings. Log landings are initially characterised by high light availability and high soil compaction and low abundance of seedlings (Gillman et al., 1985). Logging roads usually have intermediate light availability and high soil compaction, and seedlings are mostly absent. Skidder trails usually show little damage to the canopy and thus light availability is low and soil compaction intermediate while seedlings are few in number (Whitman et al., 1997). Gaps caused by tree felling have high light availability, relatively low soil compaction, and low initial seedling density (Fredericksen and Mostacedo, 2000). Knowledge of how species respond to a change in light availability, soil compaction and competition, is important for achieving sustainable forest management. Many studies have investigated the effects of each of these factors in isolation, but much less is known about the interacting effects of these factors. Studies in tropical forests have shown that light availability has a positive effect on seedling growth and survival (Zagt, 1997; Poorter, 1998; Van der Meer et al., 1998; Quevedo and Fredericksen, 2000; Rose, 2000; Pena, 2001) but soil compaction has been reported to delay regeneration (Reisinger et al., 1988; Malmer and Grip, 1990; Jusoff and Majid, 1992; Pinard et al., 1996; Guariguata and Dupuy, 1997; Whitman et al., 1997). Other studies have shown that disturbed soil results in an increase in the abundance and growth of tree seedlings (Dickensen et al., 2000; Fredericksen and Mostacedo, 2000; Fredericksen et al., 2000). Finally the absence of competitors has a positive effect on tree seedling and sapling growth (Kuusipalo et al., 1997; Quevedo and Fredericksen, 2000; Zagt, 1997; D’Oliviera, 2000; Rose, 2000). In the tropical forests of Bolivia, mandatory best management practices required under a new forestry law have created changes in the logging industry of this South American country. A shift has occurred from the harvesting of a few high-valued timber
species using conventional logging to the harvesting of many more lesser known timber tree species using low-impact logging practices. It is important to understand how timber species react to these new logging practices in order to evaluate their sustainability. For this study we chose four commonly harvested timber species (Cedrela odorata, Swietenia macrophylla, Hymenaea courbaril and Cariniana micrantha) and one non-commercial species (Tachigali vasquezii). The objective of this study was to answer the following questions. (1) Is there a difference in germination, survival, and growth of seedlings of timber species in the different microenvironments? (2) How do light availability, soil compaction, and competing vegetation affect regeneration? (3) Do logged microenvironments offer suitable regeneration sites for tree species? To answer these questions, we conducted an experiment in a logging concession in northern Bolivia focusing on five different microenvironments, including log landings, skidder trails, crown area within logging gaps, the trunk zone of logging gaps, and undisturbed forest understory.
2. Methods 2.1. Description of study site The fieldwork was conducted in Verdum, a logging concession in the Department of Beni in northern Bolivia (108550 0500 S, 658400 5000 W). It is a lowland moist tropical forest with high plant diversity with average stem density of 544 ha1 and basal area of 26.0 m2 ha1 and a canopy height of approximately 30 m (Poorter, 1998). The average rainfall is 1700 mm per year, mainly occurring between November and March, while the dry season lasts from May to September. The soils are loamy and well drained with a pH in the topsoil of 4.9 and a high percentage of aluminium (DHV, 1993). The total area of the concession is 2511 ha, of which 500 ha were logged in 2000. Twenty-three species were logged, removing approximately four trees per hectare, roughly equivalent to 10 m3 ha1. Salas (2002) estimated that the total disturbed area was 28%, of which logging gaps made up the largest part (12.3%), followed by skidder trails (7.6%), logging trails (4.0%), secondary roads (2.1%), log landings (1.5%) and main roads (1.1%).
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Table 1 Tree species and their life history strategies Species
Family
Regeneration strategy
Adult stature
Fresh seed weight (g)a
Sowing month
Cedrela odorata Swietenia macrophylla Hymenaea courbaril Tachigali vasquezii Cariniana micrantha
Meliaceae Meliaceae Caesalpiniaceae Caesalpiniaceae Lecythidaceae
Long-lived pioneer Long-lived pioneer Shade tolerant Long-lived pioneer Shade tolerant
Canopy Canopy Emergent Canopy Emergent
0.05 0.38 3.18 0.39 0.12
November December October October November
a
(0.001) (0.070) (0.015) (0.002)
Mean fresh seed weight is presented with standard deviation in parentheses.
2.2. Experimental design The experiment was conducted in five different microenvironments: log landings, skidder trails, the crown area of logging gaps, the trunk area of logging gaps, and understory locations. The trunk area was directly next to where the trunk fell. Each microenvironment was replicated three times. First, abandoned log landing sites were selected and then the other microenvironments were chosen at minimum distances of 100 m from each log landing and from each other. The log landings had an average size of 1663 m2 (S.D. = 601) and the logging gaps 384 m2 (S.D. = 70). The microenvironments gap-trunk, understory and skidder trail had little advanced regeneration whereas the log landing and gap-crown had none. Two large plots were established in each microenvironment in which 30 seeds of each species were sown in 10 sub-plots, each covering 1 m2. These subplots were randomized within each plot. Chosen tree species differ in their regeneration strategies, adult stature and fresh seed weight as shown in Table 1. Seeds were sown shallowly, at approximately 0.5 cm depth, and covered with a very thin layer of litter to protect against predation, desiccation and rain damage. 2.3. Microenvironment characterization
(GLA), Simon Fraser University, British Colombia and the Institute of Ecosystem Studies, New York, 1999. Water infiltration in the soil was measured in each microenvironment in 2001 and 2002. The method measured the rate at which water infiltrated into the top soil (0–5 cm) within a cylinder (height: 40 cm; diameter: 30 cm). The number and height of non-experimental plants, referred to as ‘competitors’, were determined in each sub-plot during November of 2001 and 2002. If competitors were taller than experimental plants they were scored as ‘overtopping competitors’. 2.4. Plant measurements The following data were recorded for each plot starting in November of 2000, and measured monthly until March 2001 and then in November 2001 and November 2002: number of germinated seeds and surviving seedlings; and height of seedlings. The height data were used to determine relative height growth rate (RHGR) as height growth per unit plant height in cm cm1 per day.
Table 2 Canopy openness and water infiltration in five microenvironments in a Bolivian rain forest in 2001 and 2002 Microenvironment
The microenvironments were characterized in terms of light availability, measured indirectly as canopy openness, soil compaction and number of non-experimental plants. Hemispherical photos were taken in November 2001 with a Nikon Coolpix 880 digital camera to determine canopy openness. Photos were taken with an overcast sky and at a height of 1.5 m in the centre of each plot. The photos were analysed with imaging software: Gap Light Analyser
Log landing Gap-crown Gap-trunk Skidder trail Understory
Canopy openness (%)
Water infiltration (cm/min)
2001
2001
26.9 12.8 13.7 4.5 3.4
a
(1.95) (1.35)b (0.63)b (0.30)c (0.24)c
0.4 1.5 4.7 1.0 2.5
2002 b
(0.20) (0.00)ab (0.77)a (0.32)b (0.91)ab
0.2 5.0 5.5 0.4 4.1
(0.01)b (0.61)a (1.45)a (1.31)b (0.91)a
Differences between microenvironments are tested with a Scheffe test. Within each year, values followed by a different letter are significantly different (P < 0.05).
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Table 3 Density of competitors and overtopping competitors in five microenvironments in a Bolivian rain forest in 2001 and 2002 Microenvironment
Competitors (#/m2) 2001
Log landing Gap-crown Gap-trunk Skidder trail Understory
5.7 20.7 19.5 10.3 12.1
Overtopping competitors (#/m2) 2002
b
(0.71) (1.54)a (1.37)a (1.21)b (1.70)b
6.8 29.4 43.0 16.9 18.0
2001 c
(0.71) (3.41)a (9.55)a (2.96)b (1.19)ab
3.2 9.6 15.1 4.2 11.5
2002 c
(0.19) (0.41)b (0.70)a (0.26)c (0.84)b
2.8 13.3 10.8 6.7 12.4
(0.19)c (1.48)a (1.64)ab (0.69)bc (0.82)a
Differences between microenvironments are tested with a Scheffe test. Within each year, values followed by a different letter are significantly different (P < 0.05).
Fig. 1. The percentage germination, survival proportional to the number of germinated seeds (survival/germination) and survival proportional to the number of seeds sown (survival/seed) after 2 years for the indicated species in five microenvironments. Different letters within each category on the x-axis indicate significant differences (P < 0.05) between the microenvironments. The colored bars represent the different microenvironments; log landing (black bars), gap-crown (white bars), gap-trunk (dark gray bars), skidder trail (arched bars) and understory (light grey bars).
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2.5. Statistical analyses Differences in canopy openness, water infiltration, number of competitors and overtopping competitors between microenvironments were analysed with a two-way ANOVA using species and microenvironments as factors. The microenvironment was used as unit of replication. Data on canopy openness for the year 2001 and water infiltration and number of competitors from the year 2002 were ln-transformed to normalize the data. Comparison
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of means was conducted with the post-hoc Scheffe test. Data for percentage survival proportional to the number of germinated seeds and the percentage survival proportional to the number of sown seeds were square root transformed to normalize the data. To test for differences between the microenvironments these data and percentage germination were analysed using a two-way ANOVA with species and microenvironment as factors. To compare survival over time for each species in the different microenvironments a Cox
Fig. 2. Survival percentage of germinated seeds over time in five microenvironments for the different species as indicated. Different letters in each graph (A–E) indicate significant differences (P < 0.05) in survival (%) between microenvironments. The symbols represent the different microenvironments; log landing (*), gap-crown (~), gap-trunk (5), skidder trail (&) and understory (^).
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regression was used. For Cariniana, there were not enough individuals in all microenvironments, therefore the analysis was only conducted for the gapcrown and gap-trunk area. A repeated measurement analysis was used to determine the change in absolute height over time of the surviving seedlings in the different microenvironments. Only individuals that had survived until the end of the experiment were included in the latter analyses. The relative increase in plant height was determined using the relative height growth rate (cm cm1 per day), which was calculated by dividing the mean ln-height difference over the total period by the length of the time period of observations (Venus and Causton, 1979). The means were compared with Scheffe’s post-hoc test.
A multiple forward regression analysis was done for each species for both years to relate seedling growth to site characteristics. RHGR was the dependent variable and light availability, water infiltration, number of competitors and overtopping competitors were used as independent variables.
3. Results 3.1. Site characteristics Canopy openness ranged from 3.4% in the understory to 26.9% in the log landing 1 year after logging (Table 2). There were no differences between the
Fig. 3. Height growth rate over time for the different species in the five microenvironments. Different letters in each graph (A–E) indicate significant differences (P < 0.05) between the microenvironments. The symbols represent the different microenvironments; log landing (*), gap-crown (~), gap-trunk (5), skidder trail (&) and understory (^).
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two gap microenvironments, gap-crown and gaptrunk, but the openness of these microenvironments was significantly (P < 0.05) lower than in the log landing and higher than in the skidder trail and understory. Water infiltration was lowest in the log landing followed by the skidder trail and the other microenvironments (Table 2). All microenvironments except the log landing and skidder trail showed slight increases in infiltration from the first to the second year. For both years, the density of competitors was significantly (P < 0.05) lower in the log landing
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and skidder trail than in the gap microenvironments (Table 3). This trend was also true for overtopping competitors though not for the second year in the skidder trail. The gap-crown and gap-trunk did not significantly differ from each other in number of competitors/m2 and only in the first year was the number of overtopping competitors/m2 greater in the gap-trunk than in the gap-crown, 15.1 and 9.6/ m2, respectively. The density of competitors and overtopping competitors in the understory did not differ significantly from those in the gap microenvironments in the second year but differed significantly (P < 0.05) from those in the log landing.
Fig. 4. Mean relative height growth rates (RHGR) and standard errors for the different species in five microenvironments. Different letters in each graph (A–E) indicate significant differences (P < 0.05) between the microenvironments.
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3.2. Timber tree germination, survival and growth Overall, germination did not differ significantly between the microenvironments but survival of germinated seedlings did differ between sites (Fig. 1). The survival of these germinated seeds was significantly (P < 0.05) higher in the log landing than in the understory with the exception of Tachigali (Fig. 1D), which showed no difference in germination between the microenvironments. Percentage survival of germinated Cariniana seeds appeared to be much higher in log landings than in the other microenvironments, but was not significantly different because of the small sample size (n = 3). Cedrela, Swietenia and Tachigali survived in all microenvironments, although high mortality of Cedrela and Swietenia was observed in the understory. A temporal analysis of seedling survival shows that most of the mortality occurred during the first weeks after germination (Fig. 2). Tachigali had the highest survival in skidder trails and the lowest in log landings. Survival proportional to the number of seeds sown was significantly (P < 0.05) higher in the log landing than in the understory for all species except Tachigali, which had the highest survival (40.9%) in skidder trails followed by the understory (21.9%) as shown in Fig. 1. Two years after the initiation of the experiment, the tallest plants of most species were located in log
landings and the smallest were situated in the understory (Fig. 3). At the beginning of the study, there was little difference in plant height between the microenvironments and this difference only became apparent after 170 days for Cariniana, 200 days for Cedrela and Swietenia, and approximately 400 days for Hymenaea and Tachigali. The RHGR for all species was significantly (P < 0.05) higher in the log landing and lowest in the understory (Fig. 4). In addition, for all species with the exception of Swietenia, the RHGR was significantly (P < 0.05) higher in the gap microenvironment than in the understory. Finally RHGR in the skidder trail and understory microenvironment did not differ significantly for most species, except Cariniana and Tachigali which had higher RHGR in the skidder trail than in the understory. Multiple forward regression analysis show that, during the first year, canopy openness and overtopping competitors were the main variables that together explained the observed variation in RHGR (Table 4). One year after sowing, both canopy openness and overtopping competitors explained the observed variation in RHGR in Cedrela, Hymenaea and Tachigali, with r2 values of 0.7, 0.5 and 0.3, respectively; whereas for Cariniana and Swietenia, canopy openness explained 40% of RHGR. RHGR has a positive relationship with canopy openness and a negative relation-
Table 4 Effect of microenvironmental factors on relative height growth rate (RHGR) in five microenvironments in 2001 and 2002 Independent variables Year 2001 2001 2001 2001 2001 2002 2002 2002 2002 2002
Species Cedrela Swietenia Hymenaea Tachigali Cariniana Cedrela Swietenia Hymenaea Tachigali Cariniana
Na 156 133 91 438 30 156 132 90 441 29
Canopy openness
Water infiltration
Competitors
***
0.75 0.64*** 0.35* 0.53*** 0.62*** 0.76*** 0.70*** ***
0.67
Overtopping competitors ***
0.22
0.43* 0.11* 0.14** 0.20*** 0.34*** 0.14*** 1.15***
0.26***
***
0.26 0.79***
0.40*** 0.29***
r2 0.72 0.40 0.51 0.30 0.39 0.76 0.69 0.45 0.59 0.61
Shown are the results of the forward multiple regression with standardized coefficients for those variables that significantly contribute to the model. Regression models were checked for collinearity: tolerance values were always >0.4. a Number of observations. * P < 0.05. ** P < 0.005. *** P < 0.001.
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ship with the number of overtopping competitors. Two years after sowing, more variables became significantly (P < 0.005) important and the total variation explained increased slightly for most species, ranging from 0.45 to 0.76. Water infiltration became a significant (P < 0.005) explanatory variable for all species. In addition, canopy openness played a role for Cedrela, Swietenia and Tachigali while the number of overtopping competitors became an important variable for the species Cedrela, Hymenaea and Tachigali.
4. Discussion Generally, the species reacted similarly to the different microenvironments. Germination for most species did not differ significantly among the microenvironments but germination and the combined survival of seeds and seedlings were higher in the log landing than in the understory, with the exception of Tachigali. Furthermore, all species had taller plants in the log landing and the smallest in the understory. This difference in height between the microenvironments only became apparent at a minimum of 170–200 days for Cariniana, Cedrela and Swietenia and 400 days for Hymenaea and Tachigali. From these results, we can deduce that a regeneration study may require a minimum of six months to observe differences in height and adequately access initial regeneration success. In log landings, the high amount of light availability and the delay in establishment of competitors appeared to have been beneficial for sown trees. Nearly all of these competitors were small-seeded species, and germinating small seeds were probably strongly negatively affected by high light availability, high temperatures, and accumulation of water on the compact soils occurring in the log landing microenvironment. In a Malaysian tropical forest, Pinard et al. (1996) also found that unfavourable site conditions limited establishment of pioneers in skidder trails and log landings. Despite favourable conditions, experimental trees in gaps were not as tall as those in the log landing, as competitors quickly became established and competed for light and other resources with tree seedlings. Similar results were obtained by Fredericksen and Mostacedo (2000) in a tropical dry forest in eastern Bolivia where regeneration was initially higher in log landings and logging roads than in logging gaps,
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which competitors quickly colonized. In our results, skidder trails and understory microenvironments were equally suitable for germination as the gap microenvironments, but growth was inhibited by low light availability. Augspurger (1984) obtained similar results in a screened enclosure experiment on Barro Colorado Island where tree seedlings grew larger in light environments similar to that of small gaps, than in shaded environments that simulated forest understory conditions. Beech plants in Danish beech stands also showed increased height growth with increased canopy openness (Madsen and Larsen, 1997). Our results using multiple forward regression analysis also demonstrated the importance of light availability and overtopping competitors on the first year RHGR of all species. Light availability was positively related, whereas the number of overtopping competitors was negatively related to RHGR. In the second year, when trees were larger, other factors such as soil compaction and competing plant density started to play a role. For almost all species the more disturbed the soil the greater the RHGR. Fredericksen and Pariona (2002) also observed greater tree seedling height growth on more disturbed soil, which was scarified, than undisturbed soils within logging gaps in a tropical humid forest in eastern Bolivia. The results of this study suggest that some microenvironments created by logging offer suitable regeneration sites for all five species in this study. On the basis of tree seedling growth, the optimal microenvironment is the log landing followed by the gap microenvironments; whereas skidder trails and undisturbed forest understory microenvironments were less suitable. Despite the small area that these different microenvironments occupy in a logged forest, they may still play an important role in the regeneration of commercial timber species in the long-term. To obtain satisfactory regeneration in forests managed for timber and ensure sustainable harvesting, it is important that recently abandoned log landings and gap areas contain or receive sufficient seeds of the logged species and that these microenvironments are initially cleared of other seedlings in order to reduce competition with the regenerating timber species. This can be achieved when logging companies plan to leave seed trees close to log landings and gaps or by sowing seeds, but even this could be to little avail if competition with the advanced regeneration is not reduced.
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Acknowledgements We thank Comunidad 12 Octubre for allowing us to do the research in their forest. We are grateful to Luis Apaza, Rene Aramayo, Miguel Cuadiay, Nicolas Divico and Nazareno Martinez for their assistance in the fieldwork. We thank Betty Verduyn for analysing the fish eye photos and Oscar Llanque and Paul Westers for valuable discussions and statistical advice. We are grateful to Pieter Zuidema and Todd Fredericksen for critical reading of the manuscript. This research was part of the research program of the Programa de Manejo de Bosques de la Amazonia Boliviana (PROMAB).
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