Rates of woody encroachment in African savannas reflect water constraints and fire disturbance

Axelsson, Christoffer; Hanan, Niall P.

doi:10.1111/jbi.13221

Cited by 27 publications

(44 citation statements)

References 40 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…In these cases, further recruitment and growth may be increasingly inhibited due to stand-scale competition. This is consistent with observations that rates of shrub proliferation decrease with increases in shrub cover (e.g., Roques et al 2001, Fensham et al 2005, Axelsson and Hanan 2018 and reinforces that shrubshrub competition, although not always active for shrubs at low density, is an important factor structuring shrub communities at the Jornada, in addition to the woody-herbaceous interactions that are more typically invoked in arid ecosystems (Scholes andArcher 1997, Sankaran et al 2004).…”

Section: Discussionsupporting

confidence: 88%

“…, Fensham et al. , Axelsson and Hanan ) and reinforces that shrub–shrub competition, although not always active for shrubs at low density, is an important factor structuring shrub communities at the Jornada, in addition to the woody–herbaceous interactions that are more typically invoked in arid ecosystems (Scholes and Archer , Sankaran et al. ).…”

Section: Discussionmentioning

confidence: 73%

“…We used National Agriculture Imagery Program (NAIP) imagery acquired on 21 May 2011 to generate a shrub cover map of the Jornada Basin LTER. Given that year-to-year change in woody cover is generally small (Axelsson and Hanan 2018), 2011 estimates are considered to reasonably represent shrub cover across the Jornada during the recent 10-15 yr. National Agriculture Imagery Program images have a 1-m spatial resolution and four spectral bands (red, green, blue, and near-infrared). The May 2011 image date coincides with peak shrub canopy greenness which occurs before peak grass greenness in summer months , maximizing our capacity to identify shrub cover.…”

Section: Shrub Cover Patch Size and Patch Density From Aerial Photomentioning

confidence: 99%

“…, Good and Caylor , Lehmann et al. , Axelsson and Hanan , ) and temperate semi‐arid grasslands (Scholtz et al. ).…”

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Constraints on shrub cover and shrub–shrub competition in a U.S. southwest desert

Hanan

Browning

et al. 2019

Ecosphere

Self Cite

View full text Add to dashboard Cite

The cover of woody perennial plants (trees and shrubs) in arid ecosystems is at least partially constrained by water availability. However, the extent to which maximum canopy cover is limited by rainfall and the degree to which soil water holding capacity and topography impacts maximum shrub cover are not well understood. Similar to other deserts in the U.S. southwest, plant communities at the Jornada Basin Long‐Term Ecological Research site in the northern Chihuahuan Desert have experienced a long‐term state change from perennial grassland to shrubland dominated by woody plants. To better understand this transformation, and the environmental controls and constraints on shrub cover, we created a shrub cover map using high spatial resolution images and explored how maximum shrub cover varies with landform, water availability, and soil characteristics. Our results indicate that when clay content is below ~18%, the upper limit of shrub cover is positively correlated with plant available water as mediated by surface soil clay influence on water retention. At surface soil clay contents >18%, maximum shrub cover decreases, presumably because the amount of water percolating to depths preferentially used by deep‐rooted shrubs is diminished. In addition, the relationship between shrub cover and density suggests that self‐thinning occurs in denser stands in most landforms of the Jornada Basin, indicating that shrub–shrub competition interacts with soil properties to constrain maximum shrub cover in the northern Chihuahuan Desert.

show abstract

Section: Discussionsupporting

confidence: 88%

Section: Discussionmentioning

confidence: 73%

Section: Shrub Cover Patch Size and Patch Density From Aerial Photomentioning

confidence: 99%

“…, Good and Caylor , Lehmann et al. , Axelsson and Hanan , ) and temperate semi‐arid grasslands (Scholtz et al. ).…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Constraints on shrub cover and shrub–shrub competition in a U.S. southwest desert

Hanan

Browning

et al. 2019

Ecosphere

Self Cite

View full text Add to dashboard Cite

show abstract

“…This has important consequences for our ability to predict future tree cover trajectories in these systems if hydrological constraints limit the ability of trees to colonize currently treeless areas in the future. Woody encroachment has been documented across much of the savannah biome (Axelsson & Hanan, 2018;Venter, Cramer, & Hawkins, 2018), and much of this encroachment has been attributed to CO 2 fertilization and changes in fire regimes (Venter et al, 2018;Wigley, Bond, & Hoffman, 2010). A challenge for understanding woody encroachment trajectories is the lack of data on edaphic constraints and soil moisture dynamics.…”

Section: Discussionmentioning

confidence: 99%

Spatial transitions in tree cover are associated with soil hydrology, but not with grass biomass, fire frequency, or herbivore biomass in Serengeti savannahs

et al. 2019

View full text Add to dashboard Cite

Although there is a well‐known association between tree cover and soil texture in savannahs, the hydrological drivers of tree cover variation have not been systematically explored, particularly in parallel with factors such as fire, herbivory, and tree–grass interactions. The relationship between hydrological factors and tree cover is important for resolving the relative contribution of bottom‐up versus top‐down factors in structuring savannah vegetation. We quantified soil moisture dynamics across eight 1‐km transects spanning tree cover gradients from open to woody savannah in Serengeti National Park in Tanzania using soil moisture sensors coupled with dataloggers. We mapped tree cover at two spatial scales through supervised classification of high‐resolution satellite imagery. We simultaneously produced water retention curves in open and woody habitats within each transect to compare soil hydrological properties and to convert volumetric water content (θ) from dataloggers to plant‐available water over the course of an annual cycle. We also quantified grass biomass at 100 locations per transect, estimated fire frequency from MODIS satellite data, and quantified herbivore occupancy with paired camera traps situated in open and woody habitats within each transect. We found a positive relationship between tree cover and soil moisture drainage rate, and found that open habitats had more negative water potentials than woody habitats for a given value of θ. In contrast, we found no evidence for a consistent relationship between grass biomass or fire frequency and tree cover. We found evidence for higher browser occupancy in woody than open habitats, but no habitat effects on herbivores as a group (browsers plus grazers), suggesting that herbivory is unlikely to be the dominant factor explaining variation in tree cover. Synthesis. Our results suggest that variation in tree cover is partly driven by hydrological (edaphic) factors unrelated to fire, herbivory, tree–grass interactions or mean annual precipitation at these spatial scales in Serengeti. We contrast our findings with previous work attributing tree cover shifts in Serengeti to precipitation gradients.

show abstract

Soil water use sources and patterns in shrub encroachment in semiarid grasslands of Inner Mongolia

Liu

Zhuang

Lai

et al. 2021

Agricultural and Forest Meteorology

View full text Add to dashboard Cite

Rates of woody encroachment in African savannas reflect water constraints and fire disturbance

Cited by 27 publications

References 40 publications

Constraints on shrub cover and shrub–shrub competition in a U.S. southwest desert

Constraints on shrub cover and shrub–shrub competition in a U.S. southwest desert

Spatial transitions in tree cover are associated with soil hydrology, but not with grass biomass, fire frequency, or herbivore biomass in Serengeti savannahs

Soil water use sources and patterns in shrub encroachment in semiarid grasslands of Inner Mongolia

Contact Info

Product

Resources

About