Growing season ecosystem and leaf-level gas exchange of an exotic and native semiarid bunchgrass

Abstract

The South African grass, Lehmann lovegrass (Eragrostis lehmanniana), may alter ecosystem processes across extensive semiarid grasslands and savannahs of western North America. We compared volumetric soil moisture (theta), total and green tissue leaf area index (LAI), ecosystem (i.e. whole-plant and soil), and leaf-level gas exchange of Lehmann lovegrass and the native bush muhly (Muhlenbergia porteri) over the 2008 monsoon season in a semiarid savanna in southern Arizona, USA, to see if these were consistent with high productivity associated with lovegrass invasive success. theta across 0-5 and 0-25 cm was higher while evapotranspiration (ET) was similar between lovegrass and bush muhly plots, except shortly after rainfall, when ET was 32-81% higher in lovegrass plots. Lehmann lovegrass had lower, quickly developing LAI with greater leaf proportions than bush muhly. When early season theta was high, net ecosystem CO(2) exchange (NEE) was similar, but as storm frequency and theta declined, NEE was more negative in lovegrass (-0.69 to -3.00 micromol m(-2) s(-1)) than bush muhly (+1.75 to -1.55 micromol m(-2) s(-1)). Ecosystem respiration (R (eco)) responded quickly to monsoon onset and late-season rains, and was lower in lovegrass (2.44-3.74 micromol m(-2) s(-1)) than bush muhly (3.60-5.3 micromol m(-2) s(-1)) across the season. Gross ecosystem photosynthesis (GEP) was greater in Lehmann lovegrass, concurrent with higher leaf-level photosynthesis and stomatal conductance. We conclude that canopy structure facilitates higher theta under Lehmann lovegrass, reducing phenological constraints and stomatal limitations to whole-plant carbon uptake through the short summer monsoon growing season.

Carbon dioxide exchange in a semidesert grassland through drought‐induced vegetation change

“…Short‐term field experiments show lovegrass‐dominated ecosystems are less effective at using precipitation pulses than native species due to higher ecosystem respiratory fluxes ( R eco ) and more rapid declines in net ecosystem CO 2 exchange (NEE) following precipitation pulses [ Huxman et al , 2004b; Potts et al , 2006]. However, this may apply only in comparison to certain native species, as a seasonal comparison with another native bunchgrass showed lovegrass‐dominated plots accumulate more carbon over the summer growing season [ Hamerlynck et al , 2010]. In addition, the transition to lovegrass dominance can increase the bare soil evaporation contributions to total evapotranspiration [ Moran et al , 2009].…”

Exploiting Differential Vegetation Phenology for Satellite-Based Mapping of Semiarid Grass Vegetation in the Southwestern United States and Northern Mexico

“…For an individual vegetation type, this response has multiple expressions, including unique physiognomy, physiology and phenology [76,77] and variation in spatial distribution and surface cover. The phenology exhibited by the perennial C 4 grasses closely follows the seasonal precipitation pattern dictated by the North American Monsoon [60,62,68,72,73,78].…”

Soil moisture and ecosystem function responses of desert grassland varying in vegetative cover to a saturating precipitation pulse

“…Thus, the increases in high-cover GEP (Figure 4) likely reflect increasing proportions of photosynthetically active tissue, even when total canopy LAI (as quantified by the LAI-2000) in the plots did not change substantially across this period (data not shown). This would explain why GEP lai did not differ between plots, and in fact we may have gotten distinctly different GEP lai results if we had been able to scale GEP to green, not total, canopy leaf area (Suyker and Verma, 2001;Hamerlynck et al, 2010).…”

“…As in Huxman et al (2004) and Potts et al (2006), A net achieved peak levels well before NEE or GEP (Figures 3 and 4). Early in the monsoon season, native grass A net development generally tracks leaf area growth, whereas the exotic lovegrass maintains green leaves that rapidly uncurl and green up following the first rainfall (Huxman et al, 2004;Ignace et al, 2007;Hamerlynck et al, 2010). Native grasses composed ca.…”

“…Leaf-level and ecosystem exchange measurements were made during the peak period for diurnal physiological activity throughout the summer growing season , and ecosystem-level fluxes at this time are highly correlated with diurnally integrated measures (Hamerlynck et al, 2010). Measurements were made on individual leaf blades of two Lehmann's lovegrass plants and any cooccurring native bunch grass, usually low woolly grass (Dasyochloa pulchella) or black grama (Bouteloua eriopoda), and each was averaged per gas-exchange plot.…”