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Citation. Peterson, D.W.; Reich, P.B.; Wrage, K.J. 2007. Plant functional group responses to fire frequency and tree canopy cover gradients in oak savannas and woodlands. JOURNAL OF VEGETATION SCIENCE 18:3-12.


Questions: How do fire frequency, tree canopy cover, and their interactions influence cover of grasses, forbs and understorey woody plants in oak savannas and woodlands?

Location: Minnesota, USA.

Methods: We measured plant functional group cover and tree canopy cover on permanent plots within a long-term prescribed fire frequency experiment and used hierarchical linear modeling to assess plant functional group responses to fire frequency and tree canopy cover.

Results: Understorey woody plant cover was highest in unburned woodlands and was negatively correlated with fire frequency. C4-grass cover was positively correlated with fire frequency and negatively correlated with tree canopy cover. C3-grass cover was highest at 40% tree canopy cover on un-burned sites and at 60% tree canopy cover on frequently burned sites. Total forb cover was maximized at fire frequencies of 4-7 fires per decade, but
was not significantly influenced by tree canopy cover. Cover of N-fixing forbs was highest in shaded areas, particularly on frequently burned sites, while combined cover of all other forbs was negatively correlated with tree canopy cover.

Conclusions: The relative influences of fire frequency and tree canopy cover on understorey plant functional group cover vary among plant functional groups, but both play a significant role in structuring savanna and woodland understorey vegetation. When restoring degraded savannas, direct manipulation of overstorey tree canopy cover should be considered to rapidly reduce shading from fire-resistant overstorey trees. Prescribed fires can then be used to suppress understorey woody plants and promote establishment of light-demanding grasses and forbs.


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