AUTHOR=Ens Emilie , Hutley Lindsay B. , Rossiter-Rachor Natalie A. , Douglas Michael M. , Setterfield Samantha A. 

TITLE=Resource-use efficiency explains grassy weed invasion in a low-resource savanna in north Australia

JOURNAL=Frontiers in Plant Science

VOLUME=6

YEAR=2015

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2015.00560

DOI=10.3389/fpls.2015.00560

ISSN=1664-462X

ABSTRACT=<p>Comparative studies of plant resource use and ecophysiological traits of invasive and native resident plant species can elucidate mechanisms of invasion success and ecosystem impacts. In the seasonal tropics of north Australia, the alien C<sub>4</sub> perennial grass <italic>Andropogon gayanus</italic> (gamba grass) has transformed diverse, mixed tree-grass savanna ecosystems into dense monocultures. To better understand the mechanisms of invasion, we compared resource acquisition and usage efficiency using leaf-scale ecophysiological and stand-scale growth traits of <italic>A. gayanus</italic> with a co-habiting native C<sub>4</sub> perennial grass <italic>Alloteropsis semialata</italic>. Under wet season conditions, <italic>A. gayanus</italic> had higher rates of stomatal conductance, assimilation, and water use, plus a longer daily assimilation period than the native species <italic>A. semialata.</italic> Growing season length was also ~2 months longer for the invader. Wet season measures of leaf scale water use efficiency (WUE) and light use efficiency (LUE) did not differ between the two species, although photosynthetic nitrogen use efficiency (PNUE) was significantly higher in <italic>A. gayanus</italic>. By May (dry season) the drought avoiding native species <italic>A. semialata</italic> had senesced. In contrast, rates of <italic>A. gayanus</italic> gas exchange was maintained into the dry season, albeit at lower rates that the wet season, but at higher WUE and PNUE, evidence of significant physiological plasticity. High PNUE and leaf <sup>15</sup>N isotope values suggested that <italic>A. gayanus</italic> was also capable of preferential uptake of soil ammonium, with utilization occurring into the dry season. High PNUE and fire tolerance in an <italic>N</italic>-limited and highly flammable ecosystem confers a significant competitive advantage over native grass species and a broader niche width. As a result <italic>A. gayanus</italic> is rapidly spreading across north Australia with significant consequences for biodiversity and carbon and retention.</p>