Increases in Desert Shrub Productivity under Elevated Carbon Dioxide Vary with Water Availability

• Published in: Ecosystems (New York) 2006-04, Vol.9 (3), p.374-385
• Main Authors: Housman, David C., Naumburg, Elke, Huxman, Travis E., Charlet, Therese N., Nowak, Robert S., Smith, Stanley D.
• Format: Article
• Language: English
• Subjects: Ambrosia; Ambrosia durnosa; Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Canopies; Carbon dioxide; Deserts; Drought; Dry season; Fundamental and applied biological sciences. Psychology; General aspects; Krameria erecta; Larrea tridentata; Leaf area; Leaves; Photosynthesis; Plant ecology; Plants; Productivity; Rain; Rainfall; Seasons; Shrubs; Synecology; Terrestrial ecosystems; Vegetation; Water availability; Water relations; Water supply; Water use
• ISSN: 1432-9840; 1435-0629
• DOI: 10.1007/s10021-005-0124-4

Productivity of aridland plants is predicted to increase substantially with rising atmospheric carbon dioxide (CO₂) concentrations due to enhancement in plant water-use efficiency (WUE). However, to date, there are few detailed analyses of how intact desert vegetation responds to elevated CO₂. From 1998 to 2001, we examined aboveground production, photosynthesis, and water relations within three species exposed to ambient (around 38 Pa) or elevated (55 Pa) CO₂ concentrations at the Nevada Desert Free-Air CO₂ Enrichment (FACE) Facility in southern Nevada, USA. The functional types sampled-evergreen (Larrea tridentata), drought-deciduous (Ambrosia dumosa), and winter-deciduous shrubs (Krameria erecta)-represent potentially different responses to elevated CO₂ in this ecosystem. We found elevated CO₂ significantly increased aboveground production in all three species during an anomalously wet year (1998), with relative production ratios (elevated:ambient CO₂) ranging from 1.59 (Krameria) to 2.31 (Larrea). In three below-average rainfall years (1999-2001), growth was much reduced in all species, with only Ambrosia in 2001 having significantly higher production under elevated CO₂. Integrated photosynthesis (mol CO₂ m⁻² y⁻¹) in the three species was 1.26-2.03-fold higher under elevated CO₂ in the wet year (1998) and 1.32-1.43-fold higher after the third year of reduced rainfall (2001). Instantaneous WUE was also higher in shrubs grown under elevated CO₂. The timing of peak canopy development did not change under elevated CO₂; for example, there was no observed extension of leaf longevity into the dry season in the deciduous species. Similarly, seasonal patterns in CO₂ assimilation did not change, except for Larrea. Therefore, phenological and physiological patterns that characterize Mojave Desert perennials-early-season lags in canopy development behind peak photosynthetic capacity, coupled with reductions in late-season photosynthetic capacity prior to reductions in leaf area-were not significantly affected by elevated CO₂. Together, these findings suggest that elevated CO₂ can enhance the productivity of Mojave Desert shrubs, but this effect is most pronounced during years with abundant rainfall when soil resources are most available.