Ecophysiology of native and invasive deeply rooted forbs in northern mixed-grass prairie

Deeply rooted forbs maintain an ecological advantage over shallow-rooted graminoids and forbs in semi-arid grasslands by accessing deep water sources and sustaining growth late into the growing season. Although the northern mixed-grass prairie contains a diverse group of native forb species, it remains susceptible to invasion by deeply rooted nonnative perennial forbs, suggesting the presence of available resources deep in the soil profile. We compared leaf-level gas exchange, plant and soil water balance and rooting distribution in the deeply rooted native forb Artemisia frigida to that of the deeply rooted invasive forb Linaria dalmatica under field conditions at the USDA-ARS High Plains Grassland Research Station near Cheyenne, Wyoming. During the 2006 growing season, the invasive L. dalmatica had net photosynthetic rates that were as much as 6.5 times higher, maintained deeper maximal rooting depth and greater root mass density in deep soil layers, and exhibited more positive pre-dawn leaf water potentials, by up to 3.2 MPa, compared to the native A. frigida. Global changes affecting the availability of moisture in deep soil layers are likely to further promote susceptibility to invasion by L. dalmatica and other deeply rooted invasive forbs in semi-arid grassland ecosystems.