Submitted to: Oecologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/11/2010
Publication Date: 9/2/2010
Citation: Hamerlynck, E.P., Scott, R.L., Moran, M.S., Huxman, T.E. 2010. Inter - and under- canopy soil water, leaf-level and whole-plant gas exchange of a semiarid perennial C4 grass. Oecologia. 165: 17-29. Interpretive Summary: Interception and evaporation from canopies of large trees in savannas can potentially limit the amount of water infiltrating into under-canopy soils, while at the same time, lower temperatures and reduced solar radiation under canopies could limit soil evaporation losses, facilitating wetter subcanopy soils. To better understand the net effect of tree cover on soil moisture and plant performance dynamics, we measured soil water content at two depths (5 and 25 cm), and leaf-level and whole-plant water use of the perennial grass, bush muhly, growing under and between mesquite canopies at the Santa Rita Experimental Range. Overall, we found that drier soil conditions prevailed under mesquite trees, and these limited whole-plant photosynthesis, primarily by limiting the total amount of photosynthetically active area, not lower leaf-level gas exchange rates. Thus, bush muhly may dominate understory locations due to greater drought tolerance, not from direct benefits from mesquite cover.
Technical Abstract: It is not clear if trees in water-limited savanna ecosystems exert positive or negative effects on soil moisture that could affect inter-canopy and sub-canopy plant performance. To address this, we examined soil volumetric water content (q) and leaf-level and whole-plant water and carbon gas exchange of bush muhly (Muhlenbergia porteri) growing under and between mesquite (Prosopis velutina) canopies through the 2008 monsoon rainy season in a savanna ecosystem in SE Arizona, USA. In bare soils, under-canopy q across 25 cm profiles was consistently 1.0 – 2.5% lower than in intercanopy locations, but it was identical beneath grasses in either location. Across shallower depths (5 cm), q was occasionally higher in the understory. Leaf level photosynthesis (Anet) and stomatal conductance to water vapor (gs) were lower in understory bush muhly, especially when q was lower later in the growing season. Net ecosystem carbon exchange (NEE) was positive in understory bush muhly and consistently negative in open intercanopy, mainly due to higher gross ecosystem photosynthesis (GEP) in the open compared to levels in the sub-canopy. GEP differences were proportionally much greater than leaf-level Anet, and leaf-level scaled GEP was identical between locations, suggesting higher, plot-level GEP was due to greater total green leaf area in the open. ET and soil E was identical between open and understory positions. We concluded that mesquite understory habitats in this savanna ecosystem are likely to be more water-limited than intercanopy locations, and bush muhly’s persistence and dominance under mesquite might reflect high drought tolerance rather than any soil moisture benefits provided by the mesquite canopy.