Mograbi Penelope J, Erasmus Barend F N, Witkowski E T F, Asner Gregory P, Wessels Konrad J, Mathieu Renaud, Knapp David E, Martin Roberta E, Main Russell
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.
Global Change and Sustainability Research Institute, University of the Witwatersrand, Johannesburg, South Africa.
PLoS One. 2015 May 13;10(5):e0127093. doi: 10.1371/journal.pone.0127093. eCollection 2015.
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.
木质生物量动态是生态系统功能的一种表现形式，然而生物量估计并不能提供关于垂直植被亚冠层内木本植被空间分布的信息。我们展示了机载激光雷达测量地上生物量和亚冠层结构的能力，将其作为一种解释工具，以揭示结构异质景观中的植被动态。我们在南非布什布克里奇的三个公共牧场进行了采样，这些牧场被农村社区用于薪柴采集。木质生物量估计值在辉长岩地质站点为9 Mg/公顷，到花岗岩地质站点为27 Mg/公顷之间。尽管之前的研究预测由于不可持续的薪柴采伐会导致林地枯竭，但在2008年至2012年期间，所有公共牧场的生物量都有所增加。年生物量生产力估计值（每年10 - 14%）高于之前4%的估计值，这可能是之前对模拟生物量供应估计不足的一个重要原因。我们表明生物量增加归因于高度小于5米的植被生长，并且在高木材采伐的牧场中，垂直植被亚冠层中79%的变化是1 - 3米高度类别的增加。牧场的木材采伐压力越大，亚冠层内低高度类别的生物量增加就越大，这可能是对密集采伐的强烈萌蘖反应。然而，薪柴短缺仍然存在，高采伐牧场中高大树木高度类别的损失就证明了这一点。大树的损失和亚冠层灌木的增加可能导致景观结构简单，功能能力降低。这项研究表明，矛盾的是，密集采伐可以增加生物量，这对生态系统服务提供的可持续性具有影响。在缺乏三维亚冠层信息的情况下，公共牧场生物量增加的结构影响可能会被误解为林地恢复。
