More Like this

1. Savannas are not old fields: Functional trajectories of forest expansion in a fire‐suppressed Brazilian savanna are driven by habitat generalists

   https://doi.org/10.1111/1365-2435.13818  

   Flake, Samuel W. ; Abreu, Rodolfo C. R. ; Durigan, Giselda ; Hoffmann, William A. ( May 2021 , Functional Ecology)

   Under fire suppression, many tropical savannas transform into forests. Forest expansion entails changes in environmental variables and plant community structure. We hypothesized that forest expansion into savanna results in a shift in community‐weighted mean functional traits from stress tolerance to competitiveness, with generalist species having trait values intermediate between those of specialists of savanna and forest habitats.

   We studied 30 plots distributed over three savanna–forest boundaries undergoing forest expansion in the Brazilian Cerrado, capturing a gradient from open savanna to recently formed forest. We measured functional traits of 116 woody species of savanna specialist, generalist and forest specialist functional groups and quantified changes in species composition and mean traits across the basal area gradient.

   We identified two main axes of species traits. The first separated forest and generalist species from savanna specialists, with the latter possessing traits associated with resistance to disturbance and stress— such as thick leaves, thick bark, slower height growth and lower shade tolerance. Our second trait axis separated shrubs and understorey trees from pioneer species. Generalist species’ traits did not differ substantially from forest species, nor did they tend to have a typical pioneer strategy.

   Community‐weighted trait means changed linearly with forest development. There was a steady increase in traits associated with competitive dominance rather than stress tolerance and fire resistance, indicating a wholesale shift in the selective environment. Several of these patterns—for example, increasing height and decreasing light requirements—are common in old‐field succession. In contrast to old‐field succession, we found that SLA increased, leaf thickness decreased and wood density stayed constant.

   The assembly of forests appears to be shaped by environmental filters that contribute to a functional trajectory distinct from most other studied ecosystems. Our results highlight the importance of the functional composition of the early community and of the early colonizers of the open environment. Differences between savanna and forest specialists reflect the selective effects of the contrasting environments, while the traits of generalists—and their interaction with environmental filters—drive the dynamics of forest expansion.

   A freePlain Language Summarycan be found within the Supporting Information of this article.

    

   more » « less
2. Facilitation by isolated trees triggers woody encroachment and a biome shift at the savanna–forest transition

   https://doi.org/10.1111/1365-2664.13994  

   Abreu, Rodolfo C. R. ; Durigan, Giselda ; Melo, Antônio C. G. ; Pilon, Natashi A. L. ; Hoffmann, William A. ( August 2021 , Journal of Applied Ecology)

   Woody encroachment into grassy biomes is a global phenomenon, often resulting in a nearly complete turnover of species, with savanna specialists being replaced by forest‐adapted species. Understanding the mechanisms involved in this change is important for devising strategies for managing savannas.

   We examined how isolated trees favour woody encroachment and species turnover by overcoming dispersal limitation and environmental filtering. In a savanna released from fire in south‐eastern Brazil (Cerrado), we sampled woody plants establishing under 40 tree canopies and in paired treeless plots. These trees comprised eight species selected for habitat preference (savanna or forest) and dispersal syndrome (bird dispersed or not). We recorded dimensions of each tree, dispersal syndrome and habitat preference of recruits, and quantified the physical environment within each plot, aiming at a mechanistic understanding of woody encroachment.

   We found clear evidence that isolated trees cause nucleation and drive changes in functional composition of savanna. Effectiveness as nucleator differed among species, but was unrelated to their functional guilds (habitat preference or dispersal syndrome). The density of saplings in nuclei was partially explained by soil moisture (+), daily temperature amplitude (−) and sum of bases (−).

   Our results indicate that isolated trees act first as perches, strongly favouring bird‐dispersed species. They then act as nurse trees, considerably changing the environment in favour of forest‐adapted recruits. In the long term, as the nuclei expand and merge, savanna specialists tend to disappear and the savanna turns into a low‐diversity forest.

   Synthesis and applications. Fire suppression has allowed the nucleation process and consequently the woody encroachment and fast replacement of savanna specialists by forest species in the Cerrado. By elucidating the mechanisms behind woody encroachment, we recommend using prescribed fires to burn forest seedlings and to reduce tree canopy size wherever the management goal is to maintain the typical savanna structure and composition.

    

   more » « less
3. Shade alters savanna grass layer structure and function along a gradient of canopy cover

   https://doi.org/10.1111/jvs.12959  

   Pilon, Natashi A. L. ; Durigan, Giselda ; Rickenback, Jess ; Pennington, R. Toby ; Dexter, Kyle G. ; Hoffmann, William A. ; Abreu, Rodolfo C. R. ; Lehmann, Caroline E. R. ; Pugnaire, ed., Francisco ( October 2020 , Journal of Vegetation Science)

   In savannas, a grass‐dominated ground layer is key to ecosystem function via grass–fire feedbacks that maintain open ecosystems. With woody encroachment, tree density increases, thereby decreasing light in the ground layer and potentially altering ecosystem function. We investigated how light availability can filter individual grass species distributions and whether different functional traits are associated with response to a shade gradient in a landscape experiencing woody encroachment.

   Savanna–forest mosaic in the Cerrado domain, southeastern Brazil.

   Along an encroachment gradient of increasing tree leaf area index (LAI) and shade, we determined how changing light availability alters grass diversity and ground layer structure relative to grass cover and grass functional traits (photosynthetic pathway, underground storage organs, bud protection and traits related to grass shape, size and leaf dimensions).

   Increasing shade led to a decrease in grass cover and grass species richness, and also compositional and functional changes. We found that where tree LAI reached 1, grass cover was reduced by 50% and species richness by 30%. While C₄grass species abundances decreased with increasing shade, the opposite pattern was true for C₃grasses. There were only small differences in light preferences among C₄subtypes, with phosphoenolpyruvate carboxykinase (PCK) species tolerating slightly more shaded conditions. Persistence of some C₄species under more shaded conditions was possible, likely due to an ability to store starch reserves via underground storage organs.

   Woody encroachment changes diversity and structure of the grassy layer that is critical to the functioning of savanna ecosystems, highlighting the dependence of the diverse grass layer on open and sunny conditions. Our results suggest a threshold of tree cover close to LAI ≈ 1 as being critical to cerrado grassy layer conservation.

    

   more » « less
4. The past, present, and future of herbivore impacts on savanna vegetation

   https://doi.org/10.1111/1365-2745.13685  

   Staver, A. Carla ; Abraham, Joel O. ; Hempson, Gareth P. ; Karp, Allison T. ; Faith, J. Tyler ( June 2021 , Journal of Ecology)

   Herbivory is a key process structuring vegetation in savannas, especially in Africa where large mammal herbivore communities remain intact. Exclusion experiments consistently show that herbivores impact savanna vegetation, but effect size variation has resisted explanation, limiting our understanding of the past, present and future roles of herbivory in savanna ecosystems.

   Synthesis of vegetation responses to herbivore exclusion shows that herbivory decreased grass abundance by 57.0% and tree abundance by 30.6% across African savannas.

   The magnitude of herbivore exclusion effects scaled with herbivore abundance: more grazing herbivores resulted in larger grass responses and more browsing herbivores in larger tree responses. However, existing experiments are concentrated in semi‐arid savannas (400–800‐mm rainfall) and soils data are mostly lacking, which makes disentangling environmental constraints a challenge and priority for future research.

   Observed herbivore impacts were ~2.1× larger than existing estimates modelled based on consumption. Wildlife metabolic rates may be higher than are usually used for estimating consumption, which offers one clear avenue for reconciling estimated herbivore consumption with observed herbivore impacts. Plant‐soil feedbacks, plant community composition, and the phenological or demographic timing of herbivory may also influence vegetation productivity, thereby magnifying herbivore impacts.

   Because herbivore abundance so closely predicts vegetation impact, changes in herbivore abundance through time are likely predictive of the past and future of their impacts. Grazer diversity in Africa has declined from its peak 1 million years ago and wild grazer abundance has declined historically, suggesting that grazing likely had larger impacts in the past than it does today.

   Current wildlife impacts are dominated by small‐bodied mixed feeders, which will likely continue into the future, but the magnitude of top‐down control may also depend on changing climate, fire and atmospheric CO₂.

   Synthesis. Herbivore biomass determines the magnitude of their impacts on savanna vegetation, with effect sizes based on direct observation that outstrip existing modelled estimates across African savannas. Findings suggest substantial ecosystem impacts of herbivory and allow us to generate evidence‐based hypotheses of the past and future impacts of herbivores on savanna vegetation.

    

   more » « less
5. The mechanisms affecting seedling establishment in restored savanna understories are seasonally dependent

   https://doi.org/10.1111/1365-2664.13354  

   Sorenson, Quinn M. ; Damschen, Ellen I. ; Lee, ed., Tien Ming ( March 2019 , Journal of Applied Ecology)

   Agricultural land is being increasingly abandoned on a global scale, with over 200 million hectares recovering from agricultural use. Plant community regeneration differs greatly in structure and composition after agricultural impacts, yet the mechanisms underpinning these dramatic changes are poorly understood. It is critical to determine the relative importance of abiotic and biotic factors that limit plant establishment and success during the recovery process. In particular, below‐ground competition for resources in soils affected by former agricultural uses may play an important role in limiting plant establishment. Yet, below‐ground competition is generally studied less than above‐ground, especially in the context of land‐use history.

   We compare plant establishment with and without below‐ground competition in the context of a large‐scale experiment manipulating land‐use histories (i.e. with and without a history of agriculture) and restoration of historical vegetation structure (i.e. thinned and unthinned canopy trees) and determine how life stage and the local environment (e.g. soil water holding capacity, vegetation cover) influence this relationship.

   For three of our four target species, below‐ground competition strongly limited establishment success, but did not interact with land‐use history and canopy thinning directly. Instead, land‐use history and canopy thinning interacted to affect establishment during germination and survival in spring, while below‐ground competition limited growth during the summer. The strength of below‐ground competition was affected by local resources, but the directionality of this relationship depended on agricultural history and canopy thinning.

   Synthesis and applications. Because adding seeds increased establishment in all cases, we recommend confronting land‐use legacies by overcoming dispersal limitation with seed additions (even in degraded sites) and ensuring that below‐ground structures are managed during restoration, especially in summer. In addition, managers should consider how the relationship between local resources and below‐ground competition at individual sites might depend on land‐use history or canopy thinning.

    

   more » « less