Regeneration of the East African timber tree Ocotea usambarensis in relation to historical logging
Highlights
► Regeneration in Ocotea usambarensis is low 10 years after logging cessation. ► Small trees are absent in formerly heavily logged areas. ► Root suckers are more abundant than seedlings. ► Vegetative regeneration and historical logging are negatively correlated. ► Enrichment plantings are recommended for forest recovery.
Introduction
Despite improved conservation strategies for many tropical forest ecosystems in the last decades, post-logging recovery of tropical timber tree species remains a matter of concern. Regeneration failure of commercial timber species has been reported from several tropical forests (e.g. Hall et al., 2003, Makana and Thomas, 2005). Many tree species require high light incidence at the seedling stage for survival and growth. However, canopy opening through logging does not necessarily lead to increased recruitment as local environmental conditions and population structure influence the number of safe sites for germination and establishment (Bazzaz, 1991, Makana and Thomas, 2005, Ouédraogo et al., 2011). Most tropical tree species are generally seed-limited as seeds only rarely reach potential regeneration sites (Muller-Landau, 2002). Vegetative regeneration by root suckers is an additional strategy in some species (Putz and Brokaw, 1989). Particularly in montane forests, logging may cause severe changes in the natural regeneration cycle (e.g. Bussmann, 2001). However, the assessment of tree recruitment after logging in the tropics mainly relies on short-term observation, and little is known about long-term effects of logging (Struhsaker, 1997) and the requirements of individual species in all stages of their development (Gómez-Pompa and Burley, 1991).
In Kenya, approximately 8% of the indigenous closed canopy forest area was deforested between 1990 and 2010 (FAO, 2010). According to Akotsi and Gachanja (2006), only 1.7% of the total country area is covered by closed canopy forest. For many years, exploitation of the forests and their wildlife was not regulated, which led to degradation and destruction on a large scale (Beentje, 1990). The forests of Mt. Kenya still cover an estimated area of 180,000 ha and thus represent the largest continuous forest block in Kenya (Beentje, 1991). The water catchment of Mt. Kenya forests is estimated to provide more than 40% of the country’s water needs (Akotsi and Gachanja, 2006).
The East African camphorwood (Ocotea usambarensis) used to be the dominant canopy tree species of moist East African montane forests below 2500 m (Lind and Morrison, 1974). Since the 1940s, it has been extensively exploited for its valuable timber (Nsolomo and Venn, 2000). Selective logging has impacted and destroyed an alarmingly high proportion of the O. usambarensis forest (Bussmann, 1996, Gathaara, 1999). This occurred not only at Mt. Kenya, but also at many other sites in Kenya (e.g. Aberdare Mountains, Taita Hills; Lambrechts et al., 2003, Aerts et al., 2011) and in Tanzania (e.g. Eastern Arc Mountains, Mt. Kilimanjaro; Hemp, 2006, Persha and Blomley, 2009). Similar to other tropical timber species, O. usambarensis is known to experience low sexual regeneration through seedlings (Newmark, 2002). However, vegetative regeneration through root suckers is common (Dale and Greenway, 1961), and in Tanzania this characteristic of O. usambarensis has been used to devise a management scheme for timber production (Willan, 1965). Bussmann, 1994, Bussmann, 2001, Bussmann and Beck, 1995 developed a scenario of the O. usambarensis regeneration cycle suggesting that, after heavy logging, the cycle is interrupted by the establishment of a secondary forest community dominated by the pioneer tree Macaranga kilimandscharica. However, this scenario has not yet been confirmed as the safe sites for germination and establishment of O. usambarensis have not been identified and the relative importance of vegetative regeneration through root suckers has not been assessed.
In 2000, the Kenya Wildlife Service took over the management responsibility of Mt. Kenya forest area as a whole. Since then, it has been managed as a National Reserve, which resulted in a 96% reduction in logging of O. usambarensis on Mt. Kenya due to improved patrolling and control (Vanleeuwe et al., 2003). In this study we examined the demography and current regeneration patterns of the O. usambarensis population at Mt. Kenya 10 years after logging suspension. In particular, we addressed the following questions: (1) Is the diameter distribution of O. usambarensis population linked to past logging intensity? (2) Are there significant differences between the extent of sexual and vegetative recruitment? (3) Are these regeneration strategies related to past logging rate and to other environmental conditions such as light incidence or altitude? This analysis allowed us to assess if the improved protection since 2000 alone has been sufficient for the recovery of O. usambarensis in areas affected by logging or if further silvicultural measures are necessary.
Section snippets
Study area
The study was carried out at the eastern slope of Mt. Kenya (Fig. 1) in the first quarter of 2010. Due to their biodiversity and important status as a source for water, timber, firewood, and wildlife, the Mt. Kenya forests have been declared a UNESCO World Heritage site (UNEP, 2005). They belong to the catchment areas of the two major rivers in the country, the Tana and the Ewaso Ngiro, which are crucial for many livelihoods (Gichuki, 1999) and for the production of hydroelectric power (Akotsi
Population structure
Overall, the O. usambarensis population was characterized by a high proportion of large trees with DBH >100 cm and few smaller trees (>1.3 m in height and DBH <75 cm) irrespective of logging (Fig. 3a–c). Trees of the diameter class <25 cm were found in only 4% of the plots.
Logging led to a mean removal of 16.76 m2 basal area and 12.37 stems ha−1 in all study plots. The mean diameter of logged tree stumps was 127 cm. Consequently, the number of adult O. usambarensis stems (DBH >25 cm) decreased marginally
Population structure
The overall diameter class distribution with a high percentage of larger trees along with few trees in lower diameter classes indicates generally low recruitment despite the fact that the species is considered shade-tolerant and may regenerate below canopy (Bussmann, 2001, Babaasa et al., 2004). Similar findings on the size distributions in populations of tropical tree species have been attributed to the irregular distribution of suitable regeneration sites in space and time (Poorter et al.,
Conclusions
Overall, our results show that natural recovery of the O. usambarensis population at Mt. Kenya is slow in heavily logged areas and these areas represent by far the largest part of the remaining forest. Even 10 years after logging suspension, neither seedlings nor root suckers have grown to bigger size classes in these areas. Sexual regeneration appears to require particular site conditions that are generally not readily met in this forest. As already observed by Bussmann, 1994, Bussmann, 2001,
Acknowledgments
Field research in Kenya was funded by Deutscher Akademischer Austauschdienst (DAAD) and Freunde der Technischen Universität Berlin. We thank Washington N. Njagi for his excellent assistance during field work. The National Council for Science and Technology and the Kenya Wildlife Service facilitated the study by granting research permission. We are grateful to Christian Lambrechts (United Nations Environmental Programme) for providing some crucial GIS data. Kelaine Vargas kindly checked our
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