Abstract
Vegetation in grasslands is changing at an unprecedented rate. In the Nebraska Sandhills, this shift is attributed in part to encroachment of the woody species Juniperus virginiana. We investigated changes in resource availability and their feedback on seasonal trends in photosynthetic characteristics of J. virginiana trees scattered in open grasslands vs. a dense 57-year-old stand. Dense stand exhibited lower volumetric soil water content, NH4+, NO3–, and δ13C, as well as foliage δ13C, δ15N, and N content, compared to grasslands. Water potential was higher in trees in grasslands compared to dense stand. J. virginiana in dense stand exhibited similar trends to trees in grasslands for net photosynthetic rate (PN), stomatal conductance, transpiration, maximum photochemical efficiency of PSII, maximum carboxylation velocity, and maximum rate of electron transport. PN peaked early summer and declined in the fall, with trees in open grasslands lagging behind those in dense stand. Plasticity of this species may place it at a competitive advantage in the Sandhills, further altering grasslands vegetation and ecosystem processes.
Highlights
Vegetation cover in grasslands and savannas and the resulting ecosystem processes are changing at an unprecedented rate across the globe (Archer 1994, Wilcox 2010, Twidwell et al 2013, Ganguli et al 2016)
McKinley and Blair (2008) reported an increase in total ecosystem N and C pools in Juniperus virginiana forests compared to grasslands, likely because the absence of fire in J. virginiana stands allowed substantial accrual of C and N in aboveground biomass and litter layers
Sites characteristics: The study was conducted at the Nebraska National Forest (NNF), Halsey, Nebraska, USA (825 m a. s. l., 41°51'45"N, 100°22'06"W), between 2006 [starting day of year (DOY) 262] and 2007 over a 12-month period
Summary
Vegetation cover in grasslands and savannas and the resulting ecosystem processes are changing at an unprecedented rate across the globe (Archer 1994, Wilcox 2010, Twidwell et al 2013, Ganguli et al 2016) This phenomenon has been attributed to climate variability and change, increased carbon dioxide concentrations in the atmosphere, and changes in disturbance fire regimes and anthropogenic management (Wilcox 2010, Diez et al 2012). Woody species encroachment has been found to modify water distribution in soils through shifts in transpiration rates, more precipitation interception, and less water infiltration, potentially resulting in drier soils (Liao et al 2008, Rout and Callaway 2009, Boutton and Liao 2010, Awada et al 2013) These changes have negative impacts on grasses and resilience of the these ecosystems (Diez et al 2012). Billings (2006) found that plant litterfall from forested sites (Ulmus sp. and Quercus sp.) had greater N concentration than that in smooth brome (Bromus inermis Leyss) grass sites
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