Colonial logging and recent subsistence harvesting affect the composition and physiognomy of a podocarp dominated Afrotemperate forest

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Abstract

Indigenous forests are declining throughout southern Africa, partly because of exogenous disturbances such as harvesting of forest resources. We evaluate the influence of colonial logging and present-day harvesting of pole-size trees (2–15 cm dbh), used for construction and fuelwood, on the structure and composition of iGxalingenwa forest in KwaZulu-Natal, South Africa. This forest has a distinctive physiognomy, with a thicket-like understorey and an emergent layer of primarily coniferous Podocarpus spp. These emergent trees represent individuals that were too small to be harvested during the colonial period. We argue that the dense angiosperm-dominated understorey arose in gaps created by logging, which allowed greater light levels at the forest floor. The understorey has since been intensively harvested (36% of available stems) for pole-sized stems, including canopy species, effectively suppressing the advanced regeneration as well as the replacement of the senescing canopy and emergent tree populations. The harvesting intensity of pole-sized stems is related to the species, size, and availability of trees. Few straight pole-sized stems are available among the understorey species and typically straight-stemmed canopy species are opportunistically harvested. Examination of community composition further suggests that, without management intervention, this forest will have a reduced stature, a thicket-like physiognomy, and be less diverse and less dominated by conifers. To rescue iGxalingenwa forest from this successional path requires a reduction in harvesting pressure on seedlings and saplings of species that normally dominate the canopy layer, many of which (Podocarpus falcatus, Ptaeroxylon obliquum and Calodendrum capense) appear to have declining populations. The effects of commercial logging a century ago still persist and are being compounded by subsistence harvesting with significant changes to forest physiognomy and composition and the possibility of arrested succession.

Introduction

It is essential to understand the history of natural and anthropogenic disturbances to forests to interpret their current ecology and ensure sustainable management (Bowman, 2001, Coomes et al., 2003, Echeverria et al., 2006, Lawes et al., 2006). Natural disturbances and the use of forests by people have dramatically altered southern African forests, resulting in fragmentation (Lawes et al., 2004a), changes in important ecosystem functions (Kotze and Lawes, 2007), and modification in forest structure and composition (Lawes et al., 2006). Here we examine the effects of extractive use of timber species over 150 years on an Afrotemperate forest.

Research on the threats to indigenous African forests primarily focuses on the present-day harvesting of large trees by local inhabitants. Even though the collection of pole-sized stems constitutes the most immediate risk to wooded areas (Hall and Rodgers, 1986, Vermeulen, 1996, Luoga et al., 2000, Luoga et al., 2002), their use by subsistence harvesters has received little attention because these poles are not commercially viable. However, pole-sized stems (2–15 cm dbh) are a critical source of timber and non-timber products that support the livelihoods of rural communities (Obiri et al., 2002, Boudreau et al., 2005). The understorey from which these poles are collected is an integral part of the forest ecosystem (Newbery et al., 1999, LaFrankie et al., 2006). While some studies have found that small understorey gaps created by harvesting of pole-sized stems do not have adverse effects on tree diversity in indigenous forest (Boudreau and Lawes, 2005), others suggest that selective removal of pole-sized stems could lead to the local extinction of certain species (Obiri et al., 2002, Boudreau et al., 2005).

Forests throughout Africa also have a history of logging by colonial settlers (King, 1941, McCracken, 1986, McCracken, 1987, Struhsaker, 1997). The long-term effects of the removal of large trees on forest community composition are not fully understood, although it is generally acknowledged that few, if any, forests have not been affected by anthropogenic disturbances. A primary ecological impact of logging is a reduction in canopy closure and subsequent increase in the amount of light reaching the forest floor (Sekercioglu, 2002). This can ultimately lead to a change in the species composition of the regenerating forest (Brokaw and Scheiner, 1989).

We survey the structure and composition of the tree community of the iGxalingenwa forest in South Africa and identify the influence of logging of large trees (>40 cm dbh) during the colonial era (1850–1910) and the present-day harvest of mostly pole-sized stems by local communities, on contemporary forest community structure. Although remotely located, this montane Afrotemperate forest has a long history of anthropogenic disturbance. During the 1880s to the early 1900s, Podocarpus spp. were the primary trees logged from the forest, although there was also selected harvesting of Vepris lanceolata, Ocotea bullata, Calodendrum capense, and Ptaeroxylon obliquum (Fourcade, 1889, Hutchins, 1905, King, 1941).

In recent decades, there has been heavy use of forest products by local inhabitants, particularly of fuelwood, building and fencing material, and plant materials used in traditional medicines (muthi) (Nomtshongwana, 1999, Robertson and Lawes, 2005). This Afrotemperate forest also has high biodiversity value and is a critical habitat for the endangered Cape parrot (Poicephalus robustus) (Wirminghaus et al., 1999, Downs, 2005). Reconciling these potentially conflicting needs is central to ensuring the survival of this forest and requires a comprehensive understanding of past exogenous disturbance to the forest, the natural structure and regeneration dynamics of the forest, and the factors controlling present-day harvesting.

In this study we determine the regeneration patterns of the common understorey, canopy and emergent species at iGxalingenwa. We evaluate the level of use of pole-sized trees by local harvesters and identify the factors influencing selection of particular stems for harvest. Our main objective is to contrast the impacts of colonial logging and recent subsistence harvesting of trees on forest composition and physiognomy. In doing so, we demonstrate the relative importance of these events to the long-term structure and survival of this representative Afrotemperate forest and other forests that have endured colonial logging followed by subsistence harvesting in recent times.

Section snippets

Study site

Surveys were conducted in iGxalingenwa forest (30.01°S, 29.63°E), approximately 25 km from the town of Creighton in southern KwaZulu-Natal, South Africa (Fig. 1). This Afrotemperate mistbelt forest (673 ha) occupies a south-facing slope between 1300 and 1600 m above sea level. The region receives a mean annual rainfall of 1060 mm. Winters (June through August) are dry (median monthly rainfall = 3 mm, with short periods of snowfall every 3–4 years) and summers (December through February) are wet

Forest community structure

We measured 802 trees in the Whittaker plots, representing 38 species. The most common emergent, canopy and subcanopy tree species were P. henkelii (16%), Celtis africana (13%), and X. monospora (11%), respectively, while the most common understorey tree species was E. zuluensis (16%). At the sapling level, 389 individuals were measured from 34 species. The most common sapling species were Clausena anisata (14%), E. zuluensis (11%), D. whyteana (9%), and Celtis africana (9%). For seedlings, we

Discussion

The structure and community composition of iGxalingenwa forest has been significantly affected by logging activity a century ago and by the harvesting of pole-sized stems from the understorey in recent times. The legacy of colonial logging persists through the occupation of the emergent stratum by a cohort of Podocarpus spp. trees and by the presence of a dense angiosperm-dominated understorey stratum. Together, the dense understorey and the harvest of pole-sized stems have suppressed the

Conservation implications and management recommendations

In the absence of advanced regeneration to replace them, removal of canopy and emergent trees will perpetuate the thicket-like understorey and significantly alter the composition and vertical structure of the tree community (Lawes et al., 2006). The careful conservation of iGxalingenwa forest depends on our understanding of the consequences of disturbance in the distant past to forest structure and development, the dynamics of species responses to disturbance, and on the management of current

Acknowledgements

We thank Stefanie Pollock, John Robertson and Harriet Eeley for assistance in the field, and Pete and Bev Everett for providing accommodation. This study was made possible by financial support from the National Research Foundation of South Africa (Conservation and Management of Ecosystems and Biodiversity) under grant number 2069339. Any opinion, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NRF.

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    Present address: Département de biologie, Local 3047B, Pavillon Alexandre-Vachon, Université Laval, Sainte-Foy, Québec G1K 7P4, Canada.

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