Abstract
Changes in vegetation structure and composition, particularly due to the invasion of exotic species, are predicted to influence biosphere-atmosphere exchanges of mass and energy. Invasion of Cynara cardunculus (cardoon or artichoke thistle), a perennial, non-native thistle in coastal California grasslands presently dominated by non-native annual grasses, may alter rates of ecosystem CO2 exchange and evapotranspiration (ET). During spring and summer 2006, we compared midday maximum net ecosystem CO2 exchange (NEE) and ET among adjacent grassland plots where Cynara was present and where it was absent. Measurements of NEE supported the prediction that deeply-rooted Cynara increase midday ecosystem C-assimilation. Cynara-mediated shifts in NEE were associated with increases in ecosystem photosynthesis rather than changes in ecosystem respiration. Furthermore, the presence of Cynara was associated with increased ET during the growing season. An increase in aboveground live biomass (a proxy for leaf area) associated with Cynara invasion may underlie shifts in ecosystem CO2 and water vapor exchange. Following mid-growing season sampling during April, we removed Cynara from half of the Cynara-containing plots with spot applications of herbicide. Three weeks later, midday fluxes in removal plots were indistinguishable from those in plots where Cynara was never present suggesting a lack of biogeochemical legacy effects. Similar to woody-encroachment in some semi-arid ecosystems, Cynara invasion increases midday ecosystem CO2 assimilation and evapotranspiration rates and has the potential to increase C-storage in California coastal grasslands.
Highlights
Human-mediated introductions of non-native plants are a ubiquitous global change factor affecting the composition of plant communities and the function of terrestrial ecosystems (Vitousek et al 1996)
The introduction and spread of non-native plants may increase the frequency of disturbance (D’Antonio and Vitousek 1992), alter soil nutrient availability (Ehrenfeld 2003), influence soil biota (Callaway et al 2004) and modify the population structure of native species (Litton et al 2006). These impacts suggest that shifts in plant community structure and composition associated with the spread of non-native plants may interact with other human-mediated global change factors to further impact patterns and rates of ecosystem processes (Smith et al 2000; Zavaleta et al 2001; Geiger and McPherson 2005)
Because we focused on relative differences among experimental treatments, we did not assess the potential influence of leaks or pressure effects on estimates of ecosystem CO2 and water vapor exchange (Saleska et al 1999)
Summary
Human-mediated introductions of non-native plants are a ubiquitous global change factor affecting the composition of plant communities and the function of terrestrial ecosystems (Vitousek et al 1996). The introduction and spread of non-native plants may increase the frequency of disturbance (D’Antonio and Vitousek 1992), alter soil nutrient availability (Ehrenfeld 2003), influence soil biota (Callaway et al 2004) and modify the population structure of native species (Litton et al 2006) These impacts suggest that shifts in plant community structure and composition associated with the spread of non-native plants may interact with other human-mediated global change factors to further impact patterns and rates of ecosystem processes (Smith et al 2000; Zavaleta et al 2001; Geiger and McPherson 2005). Likewise, increased active rooting depth (Enloe et al 2004) and increased water use efficiency (McDowell 2002) are traits associated with successful plant invaders that may potentially affect patterns of ecosystem water cycling
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