The Nebraska Sandhills are the largest stabilized sand dune formation in the Western Hemisphere (50,000 km2, Bleed & Flowerday 1998). Although the Sandhills are dominated by grasses and managed mainly as rangelands for livestock production, the area has been witnessing changes in vegetation cover, including a significant increase in woody species expansion (Fuhlendorf et al. 2008; McKinley & Blair 2008), especially eastern red cedar (Juniperus virginiana L.) and ponderosa pine (Pinus ponderosa P.&C. Lawson). P. ponderosa is a major forest type in the western USA (Sala et al. 2005), and has expanded into adjacent grasslands from historical grassland–woodland ecotones (Steinauer & Bragg 1987; Shinneman & Baker 1997). J. virginiana is the most widely distributed Juniperus species in the continental USA, and can be found in every state east of the 100th meridian (McKinley & Blair 2008). It is spreading and increasing in abundance and dominance throughout the Nebraska Sandhills (Schmidt & Stubbendieck 1993) and the Great Plains (McKinley & Blair 2008; Willson et al. 2008). J. virginiana was characterized by Schmidt & Stubbendieck (1993) as ‘the most rapidly expanding woody species on rangelands in the Great Plains,’ invading more than 20,000 ha of grasslands in western Nebraska in the last 10 years (US Forest Service), affecting approximately 7 million ha of grasslands in its western distribution of the eastern Great Plains (McKinley et al. 2008), and resulting in complete conversion to closed-canopy forests in as little as 40 years (Hoch 2000). Altered fire regimes (Bond et al. 2005; McKinley & Blair 2008), climate change (Bradley & Fleishman 2008), atmospheric nitrogen deposition, and changing land-use practices are implicated in the increased success of woody species over native grasses in semiarid grasslands in the USA and elsewhere (Scholes & Archer 1997; McCarron & Knapp 2001; Briggs et al. 2002; Fuhlendorf et al. 2008).
Soil moisture deficit has been reported to be one of the key factors limiting plant growth and ecosystem productivity worldwide (Chaves et al. 2003; Duursma et al. 2008), and is a key determinant of vegetation type, including relative abundance of grasses and woody species in semiarid grasslands like the Nebraska Sandhills (Huxman et al. 2005; Darrouzet-Nardi et al. 2006; Eggemeyer et al. 2006, 2009). A field study that examined ecophysiological and growth traits of mature P. ponderosa and J. virginiana trees and coexistent dominant grasses, Eggemeyer et al. (2006, 2009) reported that trees avoided and recovered from summer drought due in part to their deep and plastic root systems, while grasses senesced. P. ponderosa trees are also reported to have high stomatal control to avoid xylem cavitation induced by water stress (Law et al. 2001; Martínez-Vilalta & Piñol 2004; Sala et al. 2005; Eggemeyer et al. 2006), and J. virginiana trees possess inherently high water use efficiency and an ability to maintain stomatal opening at low water potentials and are, therefore, well-adapted to drought conditions (Eggemeyer et al. 2006; Willson et al. 2008).
Differences in drought resistance strategies between mature P. ponderosa and J. virginiana trees in the semiarid grasslands of the Nebraska Sandhills have been investigated (i.e., Eggemeyer et al. 2006, 2009) and, while there is a large body of literature on the ecophysiology of P. ponderosa and to a lesser extent on J. virginiana trees, it remains unclear how individuals at the seedling stage survive in areas where several years of summer drought are not uncommon (Van Auken & McKinley 2008). Seedling growth and survival is governed by dynamic interaction of biotic and abiotic factors, with water availability being a key factor in semiarid grasslands (Van Auken & McKinley 2008). Age-specific differences in plant responses to drought have been reported (e.g., Richardson 2000; Domec et al. 2004) and, given the recent history of invasiveness in these two species, understanding their resistance mechanisms to drought at different stages of development may provide important insights into their new-found encroachment into this region. This study evaluated the ecophysiological responses of J. virginiana and P. ponderosa seedlings to gradual soil water depletion – the most common situation in natural ecosystems (Bogeat-Triboulot et al. 2007) – in a controlled environment. Our specific objectives were to examine the effects of gradual soil water depletion on photosynthesis (A), photosynthetic capacity (Vcmax and Jmax), stomatal limitation to photosynthesis (Ls), PSII maximum efficiency (Fv/Fm), and water relations in P. ponderosa and J. virginiana seedlings. Recurrent measurements of these parameters provide a means to better understand species strategies to cope with water deficit in the seedling stage, and to explain species patterns of expansion in semiarid grasslands.