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

Mograbi PJ, Erasmus BF, Witkowski ET, Asner GP, Wessels KJ, Mathieu R, Knapp DE, Martin RE, Main R - PLoS ONE (2015)

Bottom Line: 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.

View Article: PubMed Central - PubMed

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.

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.

No MeSH data available.


Related in: MedlinePlus

Height-specific biomass change as a function of relative height change per grid cell.Height categories are a) 1–3 m, b) 3–5 m and c) 5–10 m for rangelands of high, intermediate and low wood extraction pressure. There were no data for the 5–10 m height class in the high wood extraction rangeland and the >10 m height class for all rangelands as there were no grid cells with an average height over 10 m. Grid cell size: 25 m x 25 m.
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pone.0127093.g002: Height-specific biomass change as a function of relative height change per grid cell.Height categories are a) 1–3 m, b) 3–5 m and c) 5–10 m for rangelands of high, intermediate and low wood extraction pressure. There were no data for the 5–10 m height class in the high wood extraction rangeland and the >10 m height class for all rangelands as there were no grid cells with an average height over 10 m. Grid cell size: 25 m x 25 m.

Mentions: Variability increased with increased biomass, particularly in the high and low extraction pressure sites (Table 2). Represented as a rate of biomass change, the mean annual woody biomass productivity (± 95% spatial confidence interval) translates to 14 ± 1.39% p.a, 12 ± 0.08% p.a. and 11 ± 0.00% p.a for the high, intermediate and low wood extraction sites, respectively. These increases were despite ongoing wood harvesting in these rangelands. Relative to the starting biomass, all mean increases were greater than 50% (Table 2). Extreme biomass increases were related to large changes in relative height (Fig 2) and relative canopy cover (e.g. >50% increase in canopy cover results in biomass increases of >20 Mg ha-1, Fig 3). However, the extreme biomass changes (i,e. >40 Mg ha-1) predominantly occurred in the 1–3 m height class (Fig 2A and Fig 3A). Biomass increases of >40 Mg ha-1 did not occur in height classes >5 m (Fig 2C and Fig 3C). The largest increases in biomass occur in the high wood extraction site when compared with the same increases in relative height (Fig 2A and 2B) and canopy cover (Fig 3A and 3B) in the other rangelands. There are no data for the high extraction site for the 5-10m height class as there are no grid cells with an average height >5 m in this rangeland (Fig 2C and Fig 3C).


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

Mograbi PJ, Erasmus BF, Witkowski ET, Asner GP, Wessels KJ, Mathieu R, Knapp DE, Martin RE, Main R - PLoS ONE (2015)

Height-specific biomass change as a function of relative height change per grid cell.Height categories are a) 1–3 m, b) 3–5 m and c) 5–10 m for rangelands of high, intermediate and low wood extraction pressure. There were no data for the 5–10 m height class in the high wood extraction rangeland and the >10 m height class for all rangelands as there were no grid cells with an average height over 10 m. Grid cell size: 25 m x 25 m.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4430526&req=5

pone.0127093.g002: Height-specific biomass change as a function of relative height change per grid cell.Height categories are a) 1–3 m, b) 3–5 m and c) 5–10 m for rangelands of high, intermediate and low wood extraction pressure. There were no data for the 5–10 m height class in the high wood extraction rangeland and the >10 m height class for all rangelands as there were no grid cells with an average height over 10 m. Grid cell size: 25 m x 25 m.
Mentions: Variability increased with increased biomass, particularly in the high and low extraction pressure sites (Table 2). Represented as a rate of biomass change, the mean annual woody biomass productivity (± 95% spatial confidence interval) translates to 14 ± 1.39% p.a, 12 ± 0.08% p.a. and 11 ± 0.00% p.a for the high, intermediate and low wood extraction sites, respectively. These increases were despite ongoing wood harvesting in these rangelands. Relative to the starting biomass, all mean increases were greater than 50% (Table 2). Extreme biomass increases were related to large changes in relative height (Fig 2) and relative canopy cover (e.g. >50% increase in canopy cover results in biomass increases of >20 Mg ha-1, Fig 3). However, the extreme biomass changes (i,e. >40 Mg ha-1) predominantly occurred in the 1–3 m height class (Fig 2A and Fig 3A). Biomass increases of >40 Mg ha-1 did not occur in height classes >5 m (Fig 2C and Fig 3C). The largest increases in biomass occur in the high wood extraction site when compared with the same increases in relative height (Fig 2A and 2B) and canopy cover (Fig 3A and 3B) in the other rangelands. There are no data for the high extraction site for the 5-10m height class as there are no grid cells with an average height >5 m in this rangeland (Fig 2C and Fig 3C).

Bottom Line: 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.

View Article: PubMed Central - PubMed

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.

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.

No MeSH data available.


Related in: MedlinePlus