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Woody Encroachment Across Tropical Savannas

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Woody Encroachment Across Tropical Savannas Woody encroachment across tropical savannas Nicola Stevens, Caroline Lehmann, Giselda Durigan, Brett Murphy Global increase in above ground woody biomass 1993 -2012 Liu et al 2015 Savannas and temperate forests are increasing in biomass La Nina conditions causing high rainfall is driving this sink in addition, the increase of atmospheric CO2 concentrations may contribute to the positive long-term trend in ABC Liu et al 2015 Encroachment is occurring in SA savannas 1937 Zululand 2010 • Studies indicate elevated CO2 is causing woody encroachment in SA savannas • Are these results applicable to savannas across the globe? • Is there evidence for a global increase in woody cover from prior to the satellite record? Collating woody cover change studies • Woody cover change from untransformed savannas • Cover measured at a minimum of two time points • ~GPS location – Environmental variables (from BIOCLIM) • Start year of study • Duration of the study • Type of woody cover change – Savanna thickening/clearing – Forest: savanna expansion/contraction • Encroaching species – N fixation – Life form • Landuse • Cause of encroachment Meta-analysis summary Total study sites 191 Africa 65 Australia 106 Brazil 20 Meta-analysis summary Total study sites 191 Africa 65 Savanna 60 Forest:savanna 5 Australia 106 Savanna 89 Forest:savanna 17 Brazil 20 Savanna 17 Forest:savanna 3 Encroachment is ubiquitous across savannas • Continent • Environmental variables • Starting woody cover • Rainfall, temperature, evaporation, • Duration of study frost • Year study started MAP, CONTINENT, START YEAR, START COVER, CONTINENT x START COVER 34% deviance Local site variables are important FIRE REGIME, HERBIVORY HISTORY, SOIL NUTRIENTS • Starting woody cover • Rainfall is not an – Higher the start cover = important predictor low encroachment Region is very important • Brazil: 8%/decade • Australia: 1%/decade • Africa: 3% /decade Annual rate of woody cover change cover woody of rate Annual Starting year of study Are savannas a useful “generalizable unit” to understand encroachment? Encroachment in Brazil • 8% /decade • Widespread land transformation • Fire suppression – Policy • Land fragmentation • Local landuse causes a regional change Why are African savannas more vulnerable to encroachment than Australian savannas? Commercial cattle ranches (F (1,26)=14.13, p<0.001). 1.1% decade 3.4% decade • Differential response to changing CO2 – Soil nutrient status – Fire regime – BIOLOGY of dominant plants Defining the regional context: Phenology Deciduous Evergreen Potential to extend growing season – Warmer temperatures – Improved WUE Defining the regional context: Nitrogen fixation Encroaching species and N fixation Africa:94% Australia: 0% Brazil: 10% N-fixation is enhanced under elevated CO2 - Increased fixation/nitrogenese activty - Less inhibition by drought (WUE improves) Defining the regional context: Allometry Elevated CO2 increased plant C gains - Canopy cover increase requires proportionally less C if you are an African species or a shrub - Implications for shrubby savannas Moncrieff et al 2014 Highlights and gaps • Overview of change beyond the satellite record – Encroachment is accelerating – Global phenomenon • Regional context needs to be explicit (biological and abiotic) – African savannas are more likely to be susceptible to elevated CO2 • Highlights gaps in our knowledge – Interaction between nutrient poor areas and encroachment – Miombo woodlands – Asian savannas • Global studies are good but individual species ecology can be as useful • Testable set of hypotheses • Observational studies help chart trajectory of change but can’t identify mechanisms of change • Importance of experimentation • Understand savanna tree responses to elevated [CO2]. Questions .
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