Historical patterns of oak population expansion in the Chautauqua Hills, Kansas
Rates of tree population expansion have increased in many North American landscapes that were mosaics of grasslands, savannas and woodlands historically. Consequences of woodland expansion include reduced economic return from grazing and changes in native biodiversity, but causes of woodland expansion are poorly understood. We address historical timing of blackjack oak (Quercus marilandica) and post oak (Quercus stellata) population expansion in tree–grass ecosystems, roles of climate and land use in driving this expansion, and future stability of these woodlands.
The Cross Timbers ecosystem in Kansas, USA.
Using increment cores, we quantified blackjack oak and post oak age structures in four woodlands on sites that were not wooded in the 1860s. We compared timing of oak regeneration with climate fluctuations (using the Palmer drought severity index) and land-use history. We quantified tree species composition within 5-m radii of sampled oaks.
Recruitment of both oak species increased between 1925 and 1945. Modal age classes recruited from 1935 to 1960. Recruitment was associated with dry intervals at the two sites with north- or east-facing aspects. This association was driven by blackjack oak recruitment in dry intervals. Woodlands on the sites with south- and west-facing aspects contained only oaks, whereas those on the sites with north- and east-facing aspects contained saplings of fire-intolerant, shade-tolerant tree species.
Our results contribute to growing evidence for woodland expansion in the region during dry climate intervals. The association between drought and recruitment was influenced by slope aspect and was more pronounced in the less fire-tolerant oak species. Although woodland expansion coincided with regional increases in fire frequency, drought and greater use of prescribed burning are likely to have reduced fire intensities by reducing fuel loads. These oak woodlands, which have developed during the 20th century, appear to form stable communities on xeric slopes but to be undergoing succession towards a mesophytic tree community on mesic slopes.