Inter- and under-canopy soil water, leaf-level and whole-plant gas exchange dynamics of a semi-arid perennial C₄ grass

• Published in: Oecologia 2011, Vol.165 (1), p.17-29
• Main Authors: Hamerlynck, Erik P, Scott, Russell L, Susan Moran, M, Schwander, Andrea M, Connor, Erin, Huxman, Travis E
• Format: Article
• Language: English
• Subjects: Abiotic factors; Animal and plant ecology; Animal, plant and microbial ecology; Animals; Biological and medical sciences; Biomedical and Life Sciences; Bushes; Canopies; Carbon; Conductance; Demecology; Deserts; Dominance; Drought resistance; Ecology; Environment; Fundamental and applied biological sciences. Psychology; Gas exchange; General aspects; Grasses; Hydrology/Water Resources; Leaves; Life Sciences; Moisture content; Monsoons; Muhlenbergia porteri; Photosynthesis; PHYSIOLOGICAL ECOLOGY; Physiological ecology - Original Paper; Plant Sciences; Plants; Plants and fungi; Poaceae - anatomy & histology; Poaceae - metabolism; Prosopis velutina; Rainy seasons; Respiration; Savanna soils; Savannahs; Seasons; Soil; Soil ecology; Soil moisture; Soil water; Stomata; Stomatal conductance; Transpiration; Trees; Understory; Vegetation canopies; Volumetric soil water; Water - metabolism
• ISSN: 0029-8549; 1432-1939
• DOI: 10.1007/s00442-010-1757-3

It is not clear if tree canopies in savanna ecosystems exert positive or negative effects on soil moisture, and how these might affect understory plant carbon balance. To address this, we quantified rooting-zone volumetric soil moisture (θ₂₅ cm), plant size, leaf-level and whole-plant gas exchange of the bunchgrass, bush muhly (Muhlenbergia porteri), growing under and between mesquite (Prosopis velutina) in a southwestern US savanna. Across two contrasting monsoon seasons, bare soil θ₂₅ cm was 1.0-2.5% lower in understory than in the intercanopy, and was consistently higher than in soils under grasses, where θ₂₅ cm was similar between locations. Understory plants had smaller canopy areas and volumes with larger basal diameters than intercanopy plants. During an above-average monsoon, intercanopy and understory plants had similar seasonal light-saturated leaf-level photosynthesis (A net₋sat), stomatal conductance (g s₋sat), and whole-plant aboveground respiration (R auto), but with higher whole-plant photosynthesis (GEPplant) and transpiration (T plant) in intercanopy plants. During a below-average monsoon, intercanopy plants had higher diurnally integrated GEPplant, R auto, and T plant. These findings showed little evidence of strong, direct positive canopy effects to soil moisture and attendant plant performance. Rather, it seems understory conditions foster competitive dominance by drought-tolerant species, and that positive and negative canopy effects on soil moisture and community and ecosystem processes depends on a suite of interacting biotic and abiotic factors.