Intra‐annual variability and environmental controls over transpiration in a 58‐year‐old even‐aged stand of invasive woodyJuniperus virginianaL. in the Nebraska Sandhills, USA

Abstract

We investigated the intra‐annual variability and environmental controls over transpiration (E) in a planted, even‐aged (58 years; 537 trees ha−1), experimental forest of invasive native Juniperus virginiana in the Nebraska Sandhills, with three canopy classes (dominant, co‐dominant, and suppressed) by using sap flux techniques, in a year where drought was absent (2008, 34% above average precipitation). Daily E was closely linked to growing‐season length and variability in the environment. Minimum and average daily air temperatures, photosynthetically active radiation, and precipitation explained the majority of the intra‐annual daily variability in E. Vapour pressure deficit was a significant factor in spring and summer, shallow volumetric soil water content (VSWC 0·2 m) was important during summer particularly June, and deep VSWC (0·6 m) was a significant factor in January and August. E was highest in the dominant trees and contributed to the majority (~77%) of stand transpiration (Ec) on site because of their larger canopy size, greater tree density, more leaf area, and accessibility to water resources compared with the co‐dominant and suppressed tree canopies, which contributed to 16% and 7%, respectively. Ec averaged ~413 mm year−1, corresponding to ~24% of potential evapotranspiration. Soils were significantly drier in the J. virginiana stand than in adjacent C4‐dominated grasslands, which could be due to the longer growing season over which physiological activity extends in J. virginiana compared with C4‐dominated grasslands in the region and precipitation interception by the canopy and forest floor, which evaporates before reaching the soil. Copyright © 2012 John Wiley & Sons, Ltd.

Ecophysiological responses of native invasive woody Juniperus virginiana L. to resource availability and stand characteristics in the semiarid grasslands of the Nebraska Sandhills

“…Over the past century, grasslands and savannas worldwide have experienced directional and sometimes persistent ---shifts from grass-dominated communities to open and, in some instances, closed-canopy woodlands. This trend has accelerated in recent decades (Wilcox 2010, Twidwell et al 2013) modifying and in some cases threatening key ecosystem services (Norris et al 2001, Throop et al 2012, including biodiversity (Pierce and Reich 2010), species distribution and composition (Dobson et al 1997, Chapin et al 1998, Norris et al 2007, ecosystem NPP (Wilcox 2010), herbaceous (Ganguli et al 2008) and livestock production (Throop et al 2012), nutrient cycling (Archer 2010, Pierce andReich 2010), and system ecohydrology (Eggemeyer et al 2009, Awada et al 2013. There is much to be learned about the implications of shifts from open grasslands and savannas to closed-canopy woodlands on water and biogeochemical cycles (Wilcox 2010, Starks et al 2014, and the feedback of these cycle and resource availability changes on plant ecophysiological performance.…”

“…Conversion of C4-dominated grasslands with relatively high photosynthetic capacity, high nitrogen-and water-use efficiencies and short growing season to C 3 woodlands with lower photosynthetic rates, longer growing season, and lower nitrogen-and water-use efficiencies in low to mid-latitudinal regions of the Great Plains of the continental U.S. (Eggemeyer et al 2006, Eggemeyer et al 2009, Awada et al 2013) has implications on NPP, biogeochemical and ecohydrological cycles (McKinley andBlair 2008, Wilcox 2010), and fire regime (D'Antonio and Vitousek 1992, Brooks et al 2004). With respect to ecohydrology, 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.…”

“…With respect to ecohydrology, 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).…”

“…Limited studies have addressed the ecophysiology of J. virginiana in the Nebraska Sandhills (e.g., Eggemeyer et al 2006, Eggemeyer et al 2009, Awada et al 2013, and these studies did not investigate the impact of density or changes in plant-soil processes and resource availability associated with encroachment. The Sandhills offer a unique ecosystem due to their economic (i.e., livestock production) and ecological importance, and the potential implications of this encroachment on regional ecohydrology.…”

“…The Nebraska Sandhills are a major recharge zone for the High Plains Ogallala Aquifer [36 and 65% of the aquifer area and water, respectively, are located under Nebraska (Bleed and Flowerday 1998)]. Thus, woody species encroachment has the potential to significantly alter groundwater recharge (Awada et al 2013), which is estimated to be 6 to 14% of annual precipitation in the Sandhills (Szilagyi et al 2005), as well as soil C and N pools and carbon storage (Mellor et al 2013). Additionally, the Sandhills occupy the drier region of the northern Great Plains where several years of consecutive drought are not uncommon (Awada et al 2013).…”