Abstract
1 The species composition of fragmented semi-natural grasslands may change over time due to stochastic local extinction and colonization events, successional change and/or as a response to changing management or abiotic conditions. The resistance of vegetation to change may be mediated through the effects of topography (slope and aspect) on soils and microclimate. 2 To assess long-term vegetation change in British chalk grasslands, 92 plots first surveyed by F. H. Perring in 1952–53, and distributed across four climatic regions, were re-surveyed during 2001–03. Changes in vegetation since the original survey were assessed by comparing local colonization and extinction rates at the plot scale, and changes in species frequency at the subplot scale. Vegetation change was quantified using indirect ordination (Detrended Correspondence Analysis; DCA) and Ellenberg indicator values. 3 Across all four regions, there was a significant decrease in species number and a marked decline in stress-tolerant species typical of species-rich calcareous grasslands, both in terms of decreased plot occupancy and decreased frequency within occupied plots. More competitive species typical of mesotrophic grasslands had colonized plots they had not previously occupied, but had not increased significantly in frequency within occupied plots. 4 A significant increase in Ellenberg fertility values, which was highly correlated with the first DCA axis, was found across all regions. The magnitude of change of fertility and moisture values was found to decrease with angle of slope and with a topographic solar radiation index derived from slope and aspect. 5 The observed shift from calcareous grassland towards more mesotrophic grassland communities is consistent with the predicted effects of both habitat fragmentation and nutrient enrichment. It is hypothesized that chalk grassland swards on steeply sloping ground are more resistant to invasion by competitive grass species than those on flatter sites due to phosphorus limitation in shallow minerogenic rendzina soils, and that those with a southerly aspect are more resistant due to increased magnitude and frequency of drought events.
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Claims 10
Claim · model-written summary
Species number significantly decreased across all four regions.
Verbatim
e0“Across all four regions, there was a significant decrease in species number”
t0“British chalk grasslands”
Claim · model-written summary
Stress-tolerant species declined in plot occupancy and within-plot frequency.
Verbatim
e0“a marked decline in stress-tolerant species typical of species-rich calcareous grasslands, both in terms of decreased plot occupancy and decreased frequency within occupied plots.”
Claim · model-written summary
Competitive mesotrophic grassland species colonized previously unoccupied plots.
Not reportedDirection
Verbatim
e0“More competitive species typical of mesotrophic grasslands had colonized plots they had not previously occupied”
Claim · model-written summary
Competitive species did not significantly increase in frequency within occupied plots.
Verbatim
e0“More competitive species typical of mesotrophic grasslands had colonized plots they had not previously occupied, but had not increased significantly in frequency within occupied plots.”
Claim · model-written summary
Ellenberg fertility values significantly increased across all regions.
Verbatim
e0“A significant increase in Ellenberg fertility values, which was highly correlated with the first DCA axis, was found across all regions.”
t0“British chalk grasslands”
Claim · model-written summary
Ellenberg fertility values were highly correlated with the first DCA axis.
Not reportedDirection
Verbatim
e0“Ellenberg fertility values, which was highly correlated with the first DCA axis”
t0“British chalk grasslands”
Claim · model-written summary
Changes in fertility and moisture values decreased with slope angle and solar radiation index.
Verbatim
e0“The magnitude of change of fertility and moisture values was found to decrease with angle of slope and with a topographic solar radiation index derived from slope and aspect.”
t0“British chalk grasslands”
Claim · model-written summary
The shift toward mesotrophic communities is consistent with fragmentation and nutrient enrichment effects.
Verbatim
e0“The observed shift from calcareous grassland towards more mesotrophic grassland communities is consistent with the predicted effects of both habitat fragmentation and nutrient enrichment.”
Claim · model-written summary
Steep chalk grasslands are hypothesized to resist competitive grass invasion through phosphorus limitation.
Verbatim
e0“It is hypothesized that chalk grassland swards on steeply sloping ground are more resistant to invasion by competitive grass species than those on flatter sites due to phosphorus limitation in shallow minerogenic rendzina soils”
Claim · model-written summary
Southerly chalk grasslands are hypothesized to resist invasion through more intense and frequent drought.
Verbatim
e0“and that those with a southerly aspect are more resistant due to increased magnitude and frequency of drought events.”
e1“chalk grassland swards on steeply sloping ground are more resistant to invasion by competitive grass species”