Biomass Increases Go under Cover: Woody Vegetation Dynamics in South African Rangelands.

Penelope J Mograbi,David E Knapp,Gregory P Asner,Russell Main,Renaud Mathieu,Roberta E Martin,Barend F N Erasmus,E T F Witkowski,Konrad J Wessels,Bazartseren Boldgiv

doi:10.1371/journal.pone.0127093

Abstract

Woody biomass dynamics are an expression of ecosystem function, yet biomass estimates do not provide information on the spatial distribution of woody vegetation within the vertical vegetation subcanopy. We demonstrate the ability of airborne light detection and ranging (LiDAR) to measure aboveground biomass and subcanopy structure, as an explanatory tool to unravel vegetation dynamics in structurally heterogeneous landscapes. We sampled three communal rangelands in Bushbuckridge, South Africa, utilised by rural communities for fuelwood harvesting. Woody biomass estimates ranged between 9 Mg ha-1 on gabbro geology sites to 27 Mg ha-1 on granitic geology sites. Despite predictions of woodland depletion due to unsustainable fuelwood extraction in previous studies, biomass in all the communal rangelands increased between 2008 and 2012. Annual biomass productivity estimates (10–14% p.a.) were higher than previous estimates of 4% and likely a significant contributor to the previous underestimations of modelled biomass supply. We show that biomass increases are attributable to growth of vegetation <5 m in height, and that, in the high wood extraction rangeland, 79% of the changes in the vertical vegetation subcanopy are gains in the 1-3m height class. The higher the wood extraction pressure on the rangelands, the greater the biomass increases in the low height classes within the subcanopy, likely a strong resprouting response to intensive harvesting. Yet, fuelwood shortages are still occurring, as evidenced by the losses in the tall tree height class in the high extraction rangeland. Loss of large trees and gain in subcanopy shrubs could result in a structurally simple landscape with reduced functional capacity. This research demonstrates that intensive harvesting can, paradoxically, increase biomass and this has implications for the sustainability of ecosystem service provision. The structural implications of biomass increases in communal rangelands could be misinterpreted as woodland recovery in the absence of three-dimensional, subcanopy information.

Highlights

Woody biomass is a fundamental expression of terrestrial ecosystem functioning, and can be used for the quantification of ecosystem services, such as fuelwood and carbon sequestration
The aim of this research is to utilize the power of airborne Light detection and ranging (LiDAR) to assess changes in aboveground biomass and subcanopy structure, as a unique window into unravelling vegetation dynamics in structurally heterogeneous landscapes
A strong relationship existed between the field allometry and LiDAR metrics, the highly heterogeneous rangeland resulted in high root mean square error (RMSE) values in both high and low use sites on granitic substrates (18.6 and 19.1 Mg ha-1, respectively) (Table 1)

Summary

Introduction

Woody biomass is a fundamental expression of terrestrial ecosystem functioning, (e.g. primary productivity, land-atmosphere gas exchange and nutrient regulation), and can be used for the quantification of ecosystem services, such as fuelwood and carbon sequestration. As complex tree-grass ecosystems, are structurally heterogeneous and are best described by three-dimensional metrics [11]. Savannas are ideal for examining the biomass dynamics in structurally complex vegetation. A major driver in savanna ecosystem structure and function is the influence of people on the landscape [15,23], through natural resource use, such as fuelwood harvesting [24]. The contributions of anthropogenic changes to savanna biomass dynamics are poorly understood

Objectives

Methods

Results

Discussion

Conclusion