The rhythm of savanna patch dynamics

Abstract

Summary1. Patch dynamics is a new, potentially unifying mechanism for the explanation of tree-grass coexistence in savannas. In this scale-explicit paradigm, savannas consist of patches in which a cyclical succession between woody and grassy dominance proceeds spatially asynchronously. The growing ecological and economic problem of shrub encroachment is a natural transient phase in this cycle.2. An important step towards understanding patterns at the landscape scale is achieved by investigating mechanisms at a smaller scale. We developed the spatially explicit individual-based simulation model SATCHMO to test the null hypothesis that cyclical succession cannot emerge from a realistic patch scale simulation model of the population dynamics of savanna woody species. 3. We calculated the partial temporal autocorrelation coefficient for 100 simulated time series of shrub cover over 500 years for time lags of up to 200 years to establish the existence and duration of successional cycles. We found a significant positive autocorrelation indicating the existence of cycles with a typical duration of about 33 years. 4. The shrub size frequency distributions over the course of a cycle showed shifts from dominance of small shrub sizes towards larger sizes during the increasing phase of a cycle and the reverse in the declining phase. This supports the three phase explanation as follows: (i) an initial phase when spatially and temporally overlapping favourable conditions lead to mass recruitment of shrubs; (ii) a build-up phase when the shrub cohort grows; and (iii) a break-down phase when increased competition due to crowding and unfavourable conditions lead to the break-down of the shrub cohort. The frequency distribution of shrub age at death over 10 simulations was also in agreement with this explanation. 5. We investigated the relationship between shrub cover, annual precipitation and time-lagged shrub cover to identify the driver of the cyclical successions. More than 90% of the variation in shrub cover was explained by shrub cover of the previous year, precipitation, and their interaction. 6. With the demonstration of precipitation-driven cyclical succession at the patch scale, we show that the mechanistic, temporal component of patch dynamics can be used to explain tree-grass coexistence in semi-arid savannas.

What are the effects of substrate and grass removal on recruitment of Acacia mellifera seedlings in a semi-arid environment?

“…We have long been interested in the factors that cause recruitment of one of southern Africa's most serious encroaching species, Acacia mellifera (Ward 2005;Kraaij and Ward 2006;Wiegand et al 2006;Britz and Ward 2007;Meyer et al 2007a). We have observed that encroachment by A. mellifera is more common on rocky than on sandy substrates, although the rate of increase in encroachment is greater on sand (Britz and Ward 2007).…”

Persistent Effects of Chemicals Used to Control Shrub Densification in Semi-Arid Savanna

“…African Senegalia and Vachellia seeds require a critical level of soil moisture for 10-14 days before emergence, while the minimum water requirements for the first two weeks of establishment are c. 3 mm of rainfall every second day (Wilson & Witkowski, 1998) or >5 mm per week during the establishment phase (Joubert et al, 2008). Another good rainfall season or two are required for sapling survival (Meyer, Wiegand, Ward & Moustakas, 2007;Joubert et al, 2008). Multiple consecutive years of good rains may thus be necessary for successful recruitment of S. mellifera -something which may only happen a few times per century (Joubert et al, 2008).…”

Shade, nutrients, and grass competition are important for tree sapling establishment in a humid savanna

“…Therefore, frequent rainfall is more important than overall mean annual precipitation in estimating tree sapling establishment (Hoffmann 1996, Wilson and Witkowski 1998, Kraaij and Ward 2006. In their patch dynamics model, Meyer et al (2007) have also explained the importance of recurring rainfall events for trees to establish.…”