Compositional decoupling of savanna canopy and understory tree communities in Serengeti

Abstract

Questions What is the pattern of compositional similarity for woody plants across the demographic bottleneck (i.e. canopy trees >2 m vs understorey trees <2 m) commonly observed in savannas? Does compositional similarity between woody plants in the canopy and understorey change across environmental gradients, and if so, which resource or disturbance factors explain the pattern? Finally, does disturbance history, i.e. whether individuals are newly established seedlings or top‐killed resprouts, help to explain the compositional similarity between canopy and understorey woody plants across sites? Location The Serengeti ecosystem, Tanzania, East Africa. Methods We compared the observed similarity to that expected from a simulated null model, and investigated the association between compositional similarity and soil bulk density, mean annual rainfall, fire return interval and elephant damage using a model selection approach. Moreover, we asked if disturbance history, i.e. whether individuals were newly established seedlings or top‐killed resprouts, helped explain compositional similarity between the canopy and understorey. Results The composition of canopy (>2 m) and understorey (<2 m) trees was surprisingly distinct: Acacia robusta (37.9%) and Acacia tortilis (23.7%) dominated the canopy, while Ormocarpum trichocarpum (43.0%) and Dichrostachys cinerea (19.6%) dominated the understorey. Compositional similarity between canopy and understorey woody plants generally decreased with soil bulk density, while for top‐killed individuals there was support for an increase in compositional similarity with fire return interval. Conclusions Our results suggest that the similarity between the canopy and recruitment communities is primarily associated with soil factors. Our results also suggest that long fire return intervals (i.e. infrequent fires) lead to increased similarity between the composition of the canopy and the recruitment pool, suggesting that disturbance promotes heterogeneity in the community across demographic bottlenecks. Despite the prominent role that megaherbivores and disturbance play in driving structural changes in savannas, our results suggest that previously unreported but pervasive species decoupling across demographic bottlenecks is primarily driven by resource availability or other bottom‐up constraints.

Elephant damage, not fire or rainfall, explains mortality of overstorey trees in Serengeti

“…Ormocarpum tricarpum incurred the highest rates of top‐killing (75% of all individuals over the study period) but also had relatively high resprouting rates (33%), which likely helps explain its relative dominance in the understorey (Anderson et al . ). Both Commiphora spp.…”

“…Indeed, reductions in the size of Serengeti trees have already been detected (Holdo, Anderson & Morrison ), and understorey tree composition is currently strongly decoupled from the overstorey community (Anderson et al . ). Large differences in palatability between tree species mean that these changes will likely have important feedbacks on the browser community (Moe et al .…”

“…These broad vegetation descriptions belie the considerable dynamics in Serengeti tree communities observed over time (Holdo, Anderson & Morrison ; Anderson et al . ). During the 1960s and 1970s, tree populations declined rapidly in central (‘Seronera’) portions of the park (Croze ; Pellew ; Ruess & Halter ).…”

Grass competition overwhelms effects of herbivores and precipitation on early tree establishment in Serengeti

“…During the late 1970s and early 1980s, declining elephant densities and a reduction in dry season fire frequency coincided with rapid ecosystem‐wide increases in tree recruitment (Sinclair et al., ). Two tree species— A. tortilis and A. robusta —presently dominate the Serengeti woodland overstory, account for 60.1% of all individual trees in long‐term woodland plots (Anderson, Morrison, Rugemalila, & Holdo, ; Rugemalila et al., ). Although the ranges of these species overlap in central Serengeti, A. tortilis is more strongly associated with drier sites, while A. robusta dominates in higher rainfall sites.…”

“…We established 38 20 m × 50 m (0.1‐ha) plots across the Serengeti rainfall gradient (Anderson et al., ; Holdo et al., ), grouped into 10 sites (variable SITE ) with four plots per site, except at our driest site (Ngorongoro Conservation Area) which had only two plots. At each site, half of the plots were randomly assigned to the seed germination–early seedling survival experiment and half of the plots were assigned to the juvenile seedling experiment.…”