Increased tree densities in South African savannas: >50 years of data suggests CO₂ as a driver

Abstract

For the past century, woody plants have increased in grasslands and savannas worldwide. Woody encroachment may significantly alter ecosystem functioning including fire regimes, herbivore carrying capacity, biodiversity and carbon storage capacity. Traditionally, increases in woody cover and density have been ascribed to changes in the disturbance regime (fire and herbivores) or rainfall. Increased atmospheric CO 2 concentrations may also contribute, by increasing growth rates of trees relative to grasses. This hypothesis is still heavily debated because usually potential CO 2 effects are confounded by changes in land use (disturbance regime). Here we analyse changes in woody density in fire experiments at three sites in South African savannas where the disturbance regime (fire and herbivores) was kept constant for 30 and 50 years. If global drivers had significant effects on woody plants, we would expect significant increases in tree densities and biomass over time under the constant disturbance regime. Woody density remained constant in a semiarid savanna but tripled in a mesic savanna between the 1970s and 1990s. At the third site, a semiarid savanna near the southern limits of the biome, tree density doubled from the mid 1990s to 2010. Interpretation of the causes is confounded by population recovery after clearing, but aerial photograph analysis on adjacent non-cleared areas showed an accompanying 48% increase in woody cover. Increased CO 2 concentrations are consistent with increased woody density while other global drivers (rainfall) remained constant over the duration of the experiments. The absence of a response in one semiarid savanna could be explained by a smaller carbon sink capacity of the dominant species, which would therefore benefit less from increased CO 2 . Understanding how savannas and grasslands respond to increased CO 2 and identifying the causes of woody encroachment are essential for the successful management of these systems.

Climate, CO<sub>2</sub> and human population impacts on global wildfire emissions

“…However, we also find a negative impact of rising CO 2 on wildfire emissions for all tropical savannah ecosystems, which outweighs the positive impact through increasing fuel load and is caused by an increase in the dominance of woody biomes at the expense of grass vegetation. This phenomenon of shrub encroachment, or woody thickening, in tropical savannahs has been repeatedly observed in field studies (Wigley et al, 2010;Bond and Midgley, 2012) and frequently attributed to CO 2 enrichment of the atmosphere (Morgan et al, 2007;Buitenwerf et al, 2012). This link is less observed for arid savannahs (Bond and Midgley, 2012), consistent with the finding here that in the most arid regions, no decrease in the grass fraction is predicted.…”

“…The main future drivers of changing wildfire have potentially opposing effects on emissions -temperature (increasing), CO 2 via productivity (increasing), CO 2 via woody thickening (Wigley et al, 2010;Buitenwerf et al, 2012;decreasing) and human population density (decreasing emissions). Sociodemographic change, interacting with other economic and technological factors, may also lead to climate change -e.g.…”

Human impacts drive a global topographic signature in tree cover

“…These contrasting trends suggest a global reduction of forest cover that was offset by increasing densities of trees within remaining areas. This inference is consistent with recent increases of woody cover in savannahs, which may be attributable to atmospheric CO 2 increases 28 and with agricultural abandonment in regions such as eastern Europe, the former Soviet Union 9 and parts of Latin America 29 . In addition, focused afforestation programmes, notably in China, are leading to large increases in tree cover in some regions 8 .…”

Woody Vegetation Die off and Regeneration in Response to Rainfall Variability in the West African Sahel

“…However, as woody encroachment is not restricted to Sahelian drylands but has been observed in savannas all over Africa and globally [29], drought recovery may not be the only reason for this phenomenon. Studies located in South Africa's savannas suggest an increasing atmospheric CO2 level as an additional driver beside precipitation [31,32]. However, even though CO2 may be an important driver of woody encroachment in the long term [31], our results show that short and medium term rainfall dynamics control growth and mortality of Sahelian woody vegetation.…”