Browsing is a strong filter for savanna tree seedlings in their first growing season

Abstract

1. Newly germinated seedlings are vulnerable to biomass removal but usually have at least 6 months to grow before they are exposed to dry-season fires, a major disturbance in savannas. In contrast, plants are exposed to browsers from the time they germinate, making browsing potentially a very powerful bottleneck for establishing seedlings.2. Here we assess the resilience of seedlings of 10 savanna tree species to topkill during the first 6 months of growth. Newly germinated seeds from four dominant African genera from across the rainfall gradient were planted in a common garden experiment at the Wits Rural Facility and clipped at 1 cm when they were ~2, 3, 4 and 5 months old. Survival, growth and key plant traits were monitored for the following 2.5 years.3. Seedlings from environments with high herbivory pressure survived topkill at a younger age than those from low-herbivore environments, and more palatable genera had higher herbivore tolerance. Most individuals that survived were able to recover lost biomass within 12 months, but the clipping treatment affected root mass fraction and branching patterns. 4. Synthesis. The impact of early browsing as a demographic bottleneck can be predicted by integrating information on the probability of being browsed and the probability of surviving a browse event. Establishment limitation through early browsing is an under-recognized constraint on savanna tree species distributions.

Drought and fire determine juvenile and adult woody diversity and dominance in a semi‐arid African savanna

“…These newly recruited species were Ehretia amoena (Boraginaceae), Commiphora glandulosa (Burseraceae), Gymnosporia senegalensis (Celastraceae), Euclea natalensis (Ebenaceae) , Grewia bicolor (Malvaceae), Ximenia caffra (Olacaceae), and Flueggea virosa (Phyllanthaceae)—a high diversity of families. While an increase in species post‐drought may lead to changes in succession and species composition, such gain of species can be ephemeral if resources are too scarce, competition with grasses is too high, or fires and herbivory remove juvenile plants (Archibald et al, 2021, Kraaij and Ward, 2006). While some individuals may have been suppressed by intense grazing and browsing before the drought and were therefore not visible (e.g., Ormocarpum trichocarpum ), others are likely to be recruits which have either seedbanks that respond to drought by germination with post‐drought rainfall (Figure S4c, Figure S4d) (Joubert et al, 2013, Tessema et al, 2017), or colonize post‐drought via a diverse range of dispersal, fire‐resistance, physiological, and anatomical functional traits.…”

CO2‐fertilisation enhances resilience to browsing in the recruitment phase of an encroaching savanna tree

“…This pressure has likely resulted in the evolution of tolerance of herbivory, as found here with individuals rapidly reaching survival size thresholds. Indeed, Archibald et al (2021) found that savanna tree species from drier systems (like V. karroo ) were tolerant of herbivory at younger ages than those from higher rainfall systems. This higher early survival was associated with lower rainfall particularly in palatable species, suggesting that herbivory, and not just aridity, is a selective force on this trait (Archibald et al, 2021).…”

“…Plant size thresholds of disturbance tolerance have been previously described for savanna tree species (Archibald et al, 2021), and so we hypothesise that CO 2 -stimulated growth increases the rate at which V. karroo seedlings reach a size threshold that allows them to survive herbivory. We hypothesise that increasing [CO 2 ] promotes belowground growth which aids recovery from browsing events, and decreases the likelihood of herbivory through reduced palatability and increased plant defences.…”

“…In contrast, seedlings can neither escape nor tolerate herbivory, and so herbivory during this life stage is a significant cause of mortality (De Steven, 1991). As herbivory occurs from germination (in contrast to dry‐season fire exposure which occurs usually from the age of 6 months; Archibald et al, 2021), trees are most vulnerable to the fatal effects of herbivory during the first few months of their life cycle.…”

“…As herbivory occurs from germination (in contrast to dry-season fire exposure which occurs usually from the age of 6 months; Archibald et al, 2021), trees are most vulnerable to the fatal effects of herbivory during the first few months of their life cycle.…”