Potential mechanisms underlying the relationship between native forb abundance and invasion resistance in California grasslands

Biodiversity loss in the form of native species abundance declines can alter ecosystem functioning.  We conducted a microcosm experiment at Jasper Ridge Biological Preserve, California, to determine how abundance of native Hemizonia congesta ssp. luzulifolia may affect community resistance to invasion by yellow starthistle, Centaurea solstitalis.  We found that greater H. congesta abundance significantly reduced starthistle biomass.  The relationship between H. congesta abundance and invasion resistance was asymptotic, with invasion resistance saturating at higher H. congesta abundances.  To investigate possible mechanisms underlying this relationship, we measured soil moisture weekly at three depths throughout the growing season, % photosynthetically active radiation monthly at the soil surface, and plant-available ammonium and nitrate monthly using ion exchange resin bags.  Of these factors, only treatment effects on soil moisture availability could explain observed patterns of invasion resistance.  In uninvaded communities, soil moisture declined with increasing H. congesta abundance across all measurement periods.  In invaded communities, however, soil moisture at greater depths (30 and 45 cm) remained constant across H. congesta abundance levels throughout the experiment.  Deep soil moisture levels in invaded communities were similar to those found in uninivaded communities containing the highest abundances of H. congesta.  This suggests that the additional deep moisture available in communities with progressively less H. congesta was taken up by starthistle, possibly allowing it to produce greater biomass as a result.  Our results support the hypothesis that native species can effectively suppress invaders with strongly overlapping patterns of resource use in time and space.