title: Biomass Increases Go under Cover: Woody Vegetation Dynamics in South African Rangelands.
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 1-3 m 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.
authors.affiliation: Restoration and Conservation Biology Research Group, School of Animal, Plant & Environmental Sciences, University of the Witwatersrand, Johannesburg, South Africa; Centre for African Ecology, School of Animal, Plant & Environmental Sciences, University of the Witwatersrand, Johannesburg, South Africa.
authors.forename: Penelope J
authors.initials: PJ
authors.lastname: Mograbi
authors.affiliation: Global Change and Sustainability Research Institute, University of the Witwatersrand, Johannesburg, South Africa.
authors.forename: Barend F N
authors.initials: BF
authors.lastname: Erasmus
authors.affiliation: Restoration and Conservation Biology Research Group, School of Animal, Plant & Environmental Sciences, University of the Witwatersrand, Johannesburg, South Africa.
authors.forename: E T F
authors.initials: ET
authors.lastname: Witkowski
authors.affiliation: Department of Global Ecology, Carnegie Institution for Science, Stanford, CA, United States of America.
authors.forename: Gregory P
authors.initials: GP
authors.lastname: Asner
authors.affiliation: Remote Sensing Research Unit, Council for Scientific and Industrial Research (CSIR)-Meraka Institute, Pretoria, South Africa; University of Pretoria, Department of Geography, Geomatics, and Meteorology, Pretoria, South Africa.
authors.forename: Konrad J
authors.initials: KJ
authors.lastname: Wessels
authors.affiliation: Ecosystems Earth Observations, Natural Resources & Environment, Council for Scientific and Industrial Research (CSIR), Pretoria, South Africa; University of Pretoria, Department of Geography, Geomatics, and Meteorology, Pretoria, South Africa.
authors.forename: Renaud
authors.initials: R
authors.lastname: Mathieu
authors.affiliation: Department of Global Ecology, Carnegie Institution for Science, Stanford, CA, United States of America.
authors.forename: David E
authors.initials: DE
authors.lastname: Knapp
authors.affiliation: Department of Global Ecology, Carnegie Institution for Science, Stanford, CA, United States of America.
authors.forename: Roberta E
authors.initials: RE
authors.lastname: Martin
authors.affiliation: Ecosystems Earth Observations, Natural Resources & Environment, Council for Scientific and Industrial Research (CSIR), Pretoria, South Africa.
authors.forename: Russell
authors.initials: R
authors.lastname: Main
mesh_terms: D018533:Biomass; D003247:Conservation of Natural Resources; D016468:Forestry; D065928:Forests; D006801:Humans; D013019:South Africa; D014197:Trees / Q000254:growth & development*; D014934:Wood