Eamus, D.

Palmer, Anthony R.

English

University of Kentucky

University of Kentucky UKnowledge International Grassland Congress Proceedings 21st International Grassland Congress / 8th International Rangeland Congress A Conceptual Model to Explain Increasing Woody Biomass in Arid and Semi‐Arid Regions Anthony R. Palmer Agricultural Research Council, South Africa D. Eamus University of Technology Sydney, Australia Follow this and additional works at: https://uknowledge.uky.edu/igc  Part of the Plant Sciences Commons, and the Soil Science Commons This document is available at https://uknowledge.uky.edu/igc/21/8-1/29 The 21st International Grassland Congress / 8th International Rangeland Congress took place in Hohhot, China from June 29 through July 5, 2008. Proceedings edited by Organizing Committee of 2008 IGC/IRC Conference Published by Guangdong People's Publishing House This Event is brought to you for free and open access by the Plant and Soil Sciences at UKnowledge. It has been accepted for inclusion in International Grassland Congress Proceedings by an authorized administrator of UKnowledge. For more information, please contact UKnowledge@lsv.uky.edu. 　 Multifunctional Grasslands in a Changing World 　 Volume Ⅰ 　 瞯 ]897　 瞯Grasslands/Rangelands Resources and Ecology ——— Climate Change and Impact on Grasslands/RangelandsA conceptual model to explain increasing woody biomass in arid and semi‐arid regionsA .R . Palmer 1 ,2 and D . Eamus 21 A RC‐L ivestock Business Div ision , PO Box 101 , G rahamstown , South A f rica and 2 IW ERM , University o f TechnologySydney , PO Box 123 , Broadway , NSW , 2007 A ustralia . E‐mail : A nthony .p almer＠ uts .edu .auKey words : leaf area index , woody encroachment , climate change , evaporationIntroduction An increase in woody biomass in semi‐arid and arid rangelands has been reported from Africa , Australia theAmericas . This trend impacts negatively on the ability of rangelands to support livestock . The trend has been attributed tochanges in １) the fire regimes , ２ ) the type and amount of herbivory , ３ ) the timing and intensity of climatic factors ( e .g .drought ) and ４) increased atmospheric carbon dioxide concentrations ( [ CO２ ] a ) . [ CO２ ] a are increasing and simultaneously , panevaporation rates have been declining and the density of woody biomass has been increasing in arid and semi‐arid regions . Therate of CO２ fixation by leaves increases as the supply of [ CO２ ] a to chloroplasts increases . The increase in biomass production isgenerally larger under xeric than mesic conditions . An increase in photosynthesis and the observed decline in stomatalconductance explains increased water‐use efficiency , which , in conjunction with decreased pan evaporation rates , is equivalentto an increased availability of water . We propose that woody thickening could be attributable to the enhanced soil and plantwater status . We highlight observations of increased tree water‐use‐efficiency , reduced global run‐off and increased soilmoisture as evidence supporting the mechanism .Materials and methods We explored long‐term climate data sets for southern Africa to determine whether site wetness has beenincreasing . This included analysing records from evaporation pans , rainfall and run‐off at these sites . We review evidence ofdecreased stomatal conductance and resulting increased �climate wetness index" , and test the three predictions detailed abovethat increased tree water‐use‐efficiency , reduced global run‐off and enhanced plant water status occur in response to CO２enrichment . We explored trends in the leaf area index for areas with a known history of woody biomass increase using theMODIS LAI .Results Pan evaporation rates have decreased for arid and semi‐arid regions of southern Africa and Australia . There is nodiscernable trend in annual precipitation . The MODIS LAI data confirmed that leaf area index has increased in rangelandsexperiencing woody encroachment in Australia , South Africa and the USA . Vapour pressure deficit ( VPD) has decreased forwater‐limiting ecosystems of Africa , Australia and the Indian sub‐continent ( Nemani et al . , ２００３) . There is evidence of globalsoil moisture increasing ( Robock et al . , ２００５ ) , w ith a positive soil moisture trend from Jornada LTER . Elevated moisturelevels across land‐use gradients have been documented in the southern Kalahari .Conclusions If CO２ enrichment is reducing stomatal conductance and enhancing soil moisture stores , we predict a more positiveplant water status will be observed under CO２ enriched conditions . As pan evaporation rates have declined , the availability ofsoil moisture has increased , effectively equivalent to increased rainfall . This , coupled to the increase in N deposition , hasincreased canopy LAI and hence CO２ uptake and has resulted in an increased ecosystem‐scale woody thickening . Super‐imposedon this is the decrease in stomatal conductance resulting from increased atmospheric [ CO２ ] a . The model proposed here haswide‐ranging ramifications to policies on afforestation , woody weed control and carbon sequestration .ReferencesNemani , R .R . , Keeling , C .D . , Hashimoto , H . , Jolly , W .M . , Piper , S .C . , Tucker , C .J . , Myneni , R .B . , Running , S .W . , ２００３ . Climate‐driven increases in global terrestrial net primary production from １９８２ to １９９９ . Science ３００ , １５６０‐１５６３ .Robock , A . , Mu , M .Q . , Vinnikov , K . , T rofimova , I .V . , Adamenko , T .I . , ２００５ . Forty‐five years of observed soilmoisture in the Ukraine : No summer desiccation ( yet) . Geophys . Res . Lett . ３２ , L０３４０１ .

A Conceptual Model to Explain Increasing Woody Biomass in Arid and Semi‐Arid Regions

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A Conceptual Model to Explain Increasing Woody Biomass in Arid and Semi‐Arid Regions

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