@article {KNZ001606,
	title = {Evidence of physiological decoupling from grassland ecosystem drivers by an encroaching woody shrub},
	journal = {PLoS ONE},
	volume = {8},
	year = {2013},
	pages = {81630 -},
	abstract = {<p>Shrub encroachment of grasslands is a transformative ecological process by which native woody species increase in cover and frequency and replace the herbaceous community. Mechanisms of encroachment are typically assessed using temporal data or experimental manipulations, with few large spatial assessments of shrub physiology. In a mesic grassland in North America, we measured inter- and intra-annual variability in leaf δ13C in Cornus drummondii across a grassland landscape with varying fire frequency, presence of large grazers and topographic variability. This assessment of changes in individual shrub physiology is the largest spatial and temporal assessment recorded to date. Despite a doubling of annual rainfall (in 2008 versus 2011), leaf δ13C was statistically similar among and within years from 2008-11 (range of \&minus;28 to \&minus;27\&permil;). A topography*grazing interaction was present, with higher leaf δ13C in locations that typically have more bare soil and higher sensible heat in the growing season (upland topographic positions and grazed grasslands). Leaf δ13C from slopes varied among grazing contrasts, with upland and slope leaf δ13C more similar in ungrazed locations, while slopes and lowlands were more similar in grazed locations. In 2011, canopy greenness (normalized difference vegetation index \&ndash; NDVI) was assessed at the centroid of individual shrubs using high-resolution hyperspectral imagery. Canopy greenness was highest mid-summer, likely reflecting temporal periods when C assimilation rates were highest. Similar to patterns seen in leaf δ13C, NDVI was highest in locations that typically experience lowest sensible heat (lowlands and ungrazed). The ability of Cornus drummondii to decouple leaf physiological responses from climate variability and fire frequency is a likely contributor to the increase in cover and frequency of this shrub species in mesic grassland and may be generalizable to other grasslands undergoing woody encroachment.</p>},
	keywords = {LTER-KNZ, Ecosystems, Grasslands, Grazing, Leaves, Plant physiology, Seasons, Shrubs, Water resources},
	doi = {10.1371/journal.pone.0081630},
	url = {https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0081630},
	author = {Jesse B. Nippert and Ocheltree, T.W. and Orozco, G.L. and Z. Ratajczak and Ling, B. and Skibbe, A.M.}
}
@article {KNZ00830,
	title = {Long-term patterns of shrub expansion in a C4-dominated grassland: fire frequency and the dynamics of shrub cover and abundance},
	journal = {American Journal of Botany},
	volume = {90},
	year = {2003},
	pages = {423 -428},
	abstract = {Worldwide, grassland ecosystems have experienced a major shift in growth-form dominance as woody plant species have expanded and replaced native grasses. In the C4-dominated grasslands of central North America, a reduction in fire frequency is the most cited cause of this shift in growth forms as fire both enhances grass productivity and constrains the establishment and expansion of native woody vegetation. Using an 18-yr plant species composition data set, we quantified patterns of change in shrub cover, frequency, and species richness associated with three distinct fire regimes. During the study period (1983{\textendash}2000), shrub cover increased most dramatically in sites in which the frequency of fire was once every 4 yr (intermediate frequency; 28.6\%) followed by sites in which fire occurred only once during the 18-yr period (low frequency; 23.7\%). Annual fire effectively prevented the recruitment of new woody species, but even with this high fire frequency, shrub cover increased slightly (3.7\%). Comparatively, shrub species richness increased by three and six, respectively, in the intermediate- and low-frequency fire sites. These data indicate that within this grassland, periods without fire are necessary for recruitment of both new individuals and additional shrub species; however, once established, shrub cover will increase regardless of fire frequency and even annual fire will not reduce shrub abundance.},
	keywords = {LTER-KNZ, C4 grassland, Fire frequency, growth form substitution, Shrubs, woody encroachment},
	doi = {10.3732/ajb.90.3.423},
	author = {Heisler, J.L. and J. M. Briggs and Alan K. Knapp}
}
