Abstract
Key message
Variation in tree biomass among African savanna species of equal size is driven by a wide inter-specific variation in wood specific gravity.
Abstract
Tree form and taper is a fundamental component of tree structure and has been used for over a century in forestry to estimate timber yields and in ecological theories of scaling laws. Here, we investigate variation in form factor in the context of biomass in African savannas. Biomass is a fundamental metric of vegetation state, yet in African savannas it remains unclear whether variation in form factor F (taper) or wood specific gravity (G) is a more dominant driver of biomass differences between tree species of equal stem diameter and height. Improving our knowledge of vertical mass distribution in savanna trees provides insight into differences in life strategies, such as tradeoffs between production, disturbance avoidance, and water storage. Here, we destructively harvested 782 stems in a savanna woodland near Kruger National Park, South Africa, and measured whole tree wet mass, wood specific gravity, water content, and form factor. We found that three of four dominant species can vary in mass by over twofold, yet inter-specific variation in taper was low and taper did not vary significantly between common species (P > 0.05) (species-mean form factors ranged from F = 0.57 to 0.77, where cone F = \(0.\bar{3}\), quadratic paraboloid F = 0.5, cylinder F = 1.0). Comparison of a general biomass allometry model to species-specific models supported the conclusion that the large difference in biomass between species of the same size was explained almost entirely (R 2 = 0.97) by including species-mean G with D and H in a general allometric equation, where F was constant. Our results suggest that inter-specific variation in wood density, not form factor, is the primary driver of biomass differences between species of the same size. We also determined that a simple analytical volume-filling model accurately relates wood specific gravity of these species to their water and gas content (R 2 = 0.68). These results indicate which species use a wide spectrum of water storage strategies in savanna woodlands, adhering to a trade-off between the benefits of denser wood or increased water storage.
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Acknowledgments
We thank the Palabora Mining Company, the South African Environmental Observation Network (SAEON) and SANParks staff for their outstanding logistical and scientific support. The authors thank all of the workers who contributed to massive undertaking of the harvest. Field work was made possible by the Andrew Mellon Foundation, SAEON, the Teresa and H. John Heinz III Foundation, Grantham Foundation for the Protection of the Environment, Stanford University, and the endowment of the Carnegie Institution for Science.
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Communicated by T. Kajimoto.
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Colgan, M.S., Swemmer, T. & Asner, G.P. Structural relationships between form factor, wood density, and biomass in African savanna woodlands. Trees 28, 91–102 (2014). https://doi.org/10.1007/s00468-013-0932-7
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DOI: https://doi.org/10.1007/s00468-013-0932-7