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1. Herbivore regulation of savanna vegetation: Structural complexity, diversity, and the complexity–diversity relationship

   https://doi.org/10.1002/ecm.1624  

   Coverdale, Tyler C; Boucher, Peter B; Singh, Jenia; Palmer, Todd M; Goheen, Jacob R; Pringle, Robert M; Davies, Andrew B (November 2024, Ecological Monographs)

   Abstract Large mammalian herbivores exert strong top‐down control on plants, which in turn influence most ecological processes. Accordingly, the decline, displacement, or extinction of wild large herbivores in African savannas is expected to alter the physical structure of vegetation, the diversity of plant communities, and downstream ecosystem functions. However, herbivore impacts on vegetation comprise both direct and indirect effects and often depend on herbivore body size and plant type. Understanding how herbivores affect savanna vegetation requires disaggregating the effects of different herbivores and the responses of different plants, as well as accounting for both the structural complexity and composition of plant assemblages. We combined high‐resolution Light Detection and Ranging (LiDAR) with field measurements from size‐selective herbivore exclosures in Kenya to determine how herbivores affect the diversity and physical structure of vegetation, how these impacts vary with body size and plant type, and whether there are predictable associations between plant diversity and structural complexity. Herbivores generally reduced the diversity and abundance of both overstory and understory plants, though the magnitude of these impacts varied substantially as a function of body size and plant type: only megaherbivores (elephants and giraffes) affected tree cover, whereas medium‐ and small‐bodied herbivores had stronger effects on herbaceous diversity and abundance. We also found evidence that herbivores altered the strength and direction of interactions between trees and herbaceous plants, with signatures of facilitation in the presence of herbivores and of competition in their absence. While megaherbivores uniquely affected tree structure, medium‐ and small‐bodied species had stronger (and complementary) effects on metrics of herbaceous vegetation structure. Plant structural responses to herbivore exclusion were species‐specific: of five dominant tree species, just three exhibited significant individual morphological variation across exclosure treatments, and the size class of herbivores responsible for these effects varied across species. Irrespective of exclosure treatment, more species‐rich plant communities were more structurally complex. We conclude that the diversity and architecture of savanna vegetation depend on consumptive and nonconsumptive plant–herbivore interactions; the roles of herbivore diversity, body size, and plant traits in mediating those interactions; and a positive feedback between plant diversity and structural complexity. 

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2. Pointframe measurements of vegetation species and height from Small Mammal Experiment Summer 2018 and 2019 from Nome, Alaska

   https://doi.org/10.18739/a27s7ht97  

   Boelman, Natalie; Griffin, Kevin; Gough, Laura; McLaren, Jennie; Rastetter, Edward; Rowe, Rebecca (January 2024, NSF Arctic Data Center)

   We conducted a field experiment in which small mammal density was manipulated at three replicate sites near Nome, Alaska. The motivation for collecting this pointframe dataset was to understand and quantify the impact of the most abundant herbivores, small rodents (voles and lemmings) on tussock tundra vegetation community composition and canopy structure in the Arctic. In July of 2018, at each site we set up an 8 meter (m) x 8 m fenced exclosure with a 0.635 centimeter (cm) square mesh to prevent herbivore grazing and increase vegetation cover (Exclosure treatment (EX)), set up a 20 m x 20 m enclosure that was stocked with up to 4 voles from spring snowmelt through September in 2018 and 2019 (Press treatment (PR)), and identified an 8 m x 8 m unfenced control plot marked at each corner with pin flags at each site (Control (CT)). Within each treatment at each site, we collected vegetation species and height data from pointframe measurements (62 points, 80cm across with a 0.5m^2 footprint) at three distinct locations during the period of peak leaf out (~July) in 2019 

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3. Wild herbivores and cattle have differing effects on postfire herbaceous vegetation recovery in an A frican savanna

   https://doi.org/10.1002/eap.2975  

   Masudi, Sherril P; Odadi, Wilfred O; Kimuyu, Duncan M; Gachuiri, Charles K; Sensenig, Ryan L; Young, Truman P (May 2024, Ecological Applications)

   Abstract Fire and herbivory have profound effects on vegetation in savanna ecosystems, but little is known about how different herbivore groups influence vegetation dynamics after fire. We assessed the separate and combined effects of herbivory by cattle and wild meso‐ and megaherbivores on postfire herbaceous vegetation cover, species richness, and species turnover in a savanna ecosystem in central Kenya. We measured these vegetation attributes for five sampling periods (from 2013 to 2017) in prescribed burns and unburned areas located within a series of replicated long‐term herbivore exclosures that allow six different combinations of cattle and wild meso‐ and megaherbivores (elephants and giraffes). Vegetation cover (grasses, mainly) and species richness were initially reduced by burning but recovered by 15–27 months after fire, suggesting strong resilience to infrequent fire. However, the rates of recovery differed in plots accessible by different wild and domestic herbivore guilds. Wildlife (but not cattle) delayed postfire recovery of grasses, and the absence of wildlife (with or without cattle) delayed recovery of forbs. Herbivory by only cattle increased grass species richness in burned relative to unburned areas. Herbivory by cattle (with or without wildlife), however, reduced forb species richness in burned relative to unburned areas. Herbivory by wild ungulates (but not cattle) increased herbaceous species turnover in burned relative to unburned areas. Megaherbivores had negligible modifying effects on these results. This study demonstrates that savanna ecosystems are remarkably resilient to infrequent fires, but postfire grazing by cattle and wild mesoherbivores exerts different effects on recovery trajectories of herbaceous vegetation. 

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4. Shifts in light availability driven by dieback across a marsh‐forest gradient

   https://doi.org/10.1002/ecs2.70021  

   Nordio, Giovanna; Gedan, Keryn; Fagherazzi, Sergio (October 2024, Ecosphere)

   Abstract Ecological zonation in coastal forests is driven by sea level rise and storm‐surge events. Mature trees that can survive moderately saline conditions show signs of stress when soil salinity increases above its tolerance levels. As leaf burn, foliar damage, and defoliation reduce tree canopy cover, light gaps form within the crown. At the forest‐marsh edge, canopy cover loss is most severe; trunks of dead trees without canopies form “ghost forests.” Canopy thinning and light from the edge alter conditions for understory vegetation, promoting the growth of shrubs and facilitating establishment and spread of invasive species that were previously limited by light competition. In this research, we present an analysis of illuminance and temperature in a coastal forest transitioning to a salt marsh. Light sensors above the ground surface were used to measure light attenuation of trees and understory vegetation and to observe the effect of reduced canopies at the forest‐marsh edge. Farther from the marsh, where salinity is lower and trees are healthy, dense canopies attenuate light. We estimate that during the growing season, tree canopies intercept 50% of illuminance on average. Closer to the marsh, canopy thinning, and tree death allow greater light penetration from above, as well as from the adjacent marsh. These illuminance values are further increased by light penetration from the forest‐marsh edge (edge effect). Here, higher illuminance may permitPhragmites australisexpansion. At intermediate locations, trees intercept between 32% and 49% of light and the understory shrubMorella ceriferaintercepts a further 45% of penetrating light based on comparisons of illuminance above and below shrub canopies. Light penetration from the edge can also be felt. The presence ofM. ceriferareduces the air temperature close to the soil surface, creating a cooler summer microclimate. The tree health state is reflected in the canopy size. The canopy patterns and the edge effect are responsible for light availability distribution along forest‐marsh gradients, consequently affecting the understory vegetation biomass. We conclude that during forest retreat driven by sea level rise, tree dieback increases light availability favoring the temporary encroachment ofPh. australisandM. ceriferain the understory. 

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5. Pointframe measurements of vegetation species and height from Small Mammal Experiment Summer 2018 and 2019 near Toolik Lake, Alaska

   https://doi.org/10.18739/a2cf9j82n  

   Boelman, Natalie; Griffin, Kevin; Gough, Laura; McLaren, Jennie; Rastetter, Edward; Rowe, Rebecca (January 2024, NSF Arctic Data Center)

   We conducted a field experiment in which small mammal density was manipulated at three replicate sites near Toolik Lake, Alaska. The motivation for collecting this pointframe dataset was to understand and quantify the impact of the most abundant herbivores, small rodents (voles and lemmings) on tussock tundra vegetation community composition and canopy structure in the Arctic. In July of 2018, at each site we set up an 8 meter (m) x 8 m fenced exclosure with a 0.635 centimeter (cm) square mesh to prevent herbivore grazing and increase vegetation cover (Exclosure treatment (EX)), set up a 20 m x 20 m enclosure that was stocked with up to 4 voles from spring snowmelt through September in 2018 and 2019 (Press treatment (PR)), and identified an 8 m x 8 m unfenced control plot marked at each corner with pin flags at each site (Control (CT)). In 2019, at each site we set up a 20 m x 20 m enclosure that was stocked with up to 4 voles from spring snowmelt through September in 2019 only, after which voles were removed and the fences served to exclude voles from 2020-2023 (Pulse treatment (PU)). Within each treatment at each site, we collected vegetation species and height data from pointframe measurements (62 points, 80cm across with a 0.5m^2 footprint) at three distinct locations during the period of peak leaf out (~July) in 2018 and 2019. 

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