Biological invasions can have severe consequences for community structure in disturbed, resource-poor areas, yet the mechanisms allowing invasion of resource-poor habitats remain largely unknown. A previous comparison of phylogenetically-paired native and non-native species in Hawaii found higher short-term resource use efficiency (RUE) in invasive plants but comparable long-term RUE. To investigate these mechanisms of invasion, we compared patterns of RUE and response to N addition in a pair of native and non-native species colonizing 27 yr old primary successional surfaces on Mt. St. Helens volcano in Washington State. Eurasian Hypochaeris radicata (hereafter Hypochaeris) and native Hieracium albiflorum (hereafter Hieracium) are both short-lived, rosette-forming herbaceous perennials in the Asteraceae (tribe Cichoriae) that are similar in size, habitat, phenology, and mode of propagation. We took measurements in 8 blocks of 4 treatment plots, each on the pyroclastic flow of St. Helen's.  Hypochaeris was removed from half the plots, and half received a nitrogen pulse in a factorial design. The photosynthetic rate of 16 focal plants of each species was measured in each block using a LICOR 6400, which measures CO2 gas exchange rate across the stomata.  

Results/Conclusions     

Non-native Hypochaeris was a major component of the focal community (25% of total above ground biomass), while Hieracium composed 3.7%. Instantaneous photosynthetic rates were equivalent in the two species, while Hypochaeris had higher stomatal conductance, and consequently a significantly lower instantaneous water use efficiency. Hypochaeris also had lower quantum yield (A/PAR, a measure of  photosynthetic rate per unit of incident light), lower root to shoot ratio, and a lower whole-plant C:N.     Photosynthetic rate increased in both species in response to N addition, indicating that photosynthetic rate is N-limited in this system. Root-to-shoot ratios declined significantly for Hieracium, but only marginally for Hypochaeris. The increase in Hypochaeris plot-level biomass (378%) in response to the nitrogen pulse was significantly greater than for Hieracium (166%), and other native species. These results suggest that invading Hypochaeris is less efficient than native Hieracium in the short term use of water and in the long term use of N for growth, but is more efficient in short-term growth responses to pulses of N, at least in the presence of adequate soil moisture.