The ghosts of trees past: savanna trees create enduring legacies in plant species composition

Abstract

Isolated trees in savannas worldwide are known to modify their local environment and interact directly with neighboring plants. Less is known about how related tree species differ in their impacts on surrounding communities, how the effects of trees vary between years, and how composition might change following loss of the tree. To address these knowledge gaps, we explored the following questions: How do savanna trees influence the surrounding composition of herbaceous plants? Is the influence of trees consistent across different species and years? How does this change following the death of the tree? We surveyed herbaceous species composition and environmental attributes surrounding living and dead evergreen and deciduous Quercus trees in California (USA) savannas across several years that differed in their total precipitation. Oak trees of all species created distinct, homogenous understory communities dominated by exotic grasses across several sites. The composition of the low-diversity understory communities showed less interannual variation than open grassland, despite a two-fold difference in precipitation between the driest and wettest year. Vegetation composition was correlated with variation in soil properties, which were strongly affected by trees. Oaks also influenced the communities beyond the edge of the crown, but this depended on site and oak species. Low-diversity understory communities persisted up to 43 years following the death of the tree. A gradual decline in the effect of trees on the physical, environment following death did not result in vegetation becoming more similar to open grassland over time. The presence of long-lasting legacies of past tree crowns highlights the difficulty of assigning control of the current distribution of herbaceous species in grassland to their contemporary environment.

Demography of evergreen and deciduous oaks in a mixed oak savanna: insights from a long‐term experiment

“…The faster growth at site 3 may be related to soil differences in the untilled site (3) vs. historically tilled sites (1, 2), which support different and less diverse herbaceous plant communities than site 3 (Stahlheber et al. ).…”

Savannah trees buffer herbaceous plant biomass against wild and domestic herbivores

“…Supporting a growing consensus of studies, we found taller trees maintain greater herbaceous biomass in the understory (Blaser et al, ; Moustakas et al, ; Treydte et al, ), and our results show that this maintenance of biomass also occurs as an indirect effect, mediated through herbaceous species composition. Moreover, in our nutrient‐rich tropical savannahs, plant species turnover due to varying herbivory pressure is likely to be higher than in non‐tropical savannahs where understory specialists can persist beside dead tree stumps for several decades (Stahlheber et al, ). The cover and distribution of understory species, such as those identified in this study, are therefore important indicators of the resilience of the African tropical savannahs under increasing herbivore pressure.…”

Past tree influence and prescribed fire exert strong controls on reassembly of mountain grasslands after tree removal

“…) had dissipated after trees were removed, or that colonists were insensitive to this variation—a sharp contrast to the persistence of soil legacies in other tree‐invaded grasslands (Stahlheber et al. ). It is possible that species composition will diverge among subplots in the future if subsequent colonists (e.g., meadow specialists) have narrower or differing resource requirements (Lepš and Rejmánek , Matthews and Spyreas ) or if their seed sources are spatially segregated and dispersal is poor (Chase ).…”

“…), and preemption of space or resources by surviving, non‐target species (Stahlheber et al. ). How rapidly these changes accrue, how long they persist after woody plants are removed, and whether they impede grassland reassembly, are not well understood (Stahlheber et al.…”

“…How rapidly these changes accrue, how long they persist after woody plants are removed, and whether they impede grassland reassembly, are not well understood (Stahlheber et al. ). We explore these questions by comparing rates of community reassembly following tree removal from experimental plots comprising a fine‐scale mosaic of encroachment states: remnant meadow openings and areas of recent invasion (minimally altered by trees) to century‐old forests (largely lacking in meadow species; Halpern et al.…”