Functional characteristics of seedlings of native and exotic grasses of the Chihuahua Desert in response to contrasting levels of soil humidity

Background/Question/Methods The invasion of African grasses have severely altered the composition and functioning of semiarid grasslands in Central Mexico, where Bouteloua gracilis is the dominant native grass species. Eragrostis lehmanniana is one of the most abundant exotic species in these ecosystems, while Melinis repens has only recently been observed in these semiarid grasslands. The underlying mechanisms of success of exotic species are usually associated with high seed numbers and effective resource use; however they have rarely been explored at seedling stage. In this study, we examined functional characteristics (growth, biomass allocation, morphology, and physiology) of seedlings of three grass species: B. gracilis, the native key species of the grassland ecosystem, and two exotic grass species with contrasting life history traits: Eragrostis lehmanniana, a successful perennial species and Melinis repens, a less successful biennial species, under two contrasting conditions of soil humidity (80% and 50% of field capacity; FC). We performed a 4-wk potted experiment with a 3 x 2 factorial design under greenhouse conditions. We harvested seedlings at 15 and 30 days after treatment application.

Results/Conclusions The three species produced greater total, shoot and root biomass under 80% FC than under 50% FC (species-humidity-time interaction, P>0.48) in both harvests. However, E. lehmanniana exhibited overall three times as much total biomass, twice as much root length and leaf area compared to the other species, which were similar in their responses (species main effect; P<0.0001). After 30 days at 50% FC, E. lehmanniana presented the highest leaf water potential (P =0.02) and specific root length compared to the other species (species-humidity-time interaction, P<0.03). After 30 days of 50% FC, M. repens had a higher root-to-shoot ratio and specific leaf area than B. gracilis (species main effect; P<0.0001).  Hence, the enhanced growth responses exhibited by E. lehmanniana indicate clear advantages in resource acquisition through distinct morphological and physiological characteristics at the seedling stage. Melinis repens, on the other hand exhibited overall similar growth responses (measured in biomass) as the native species, however its potential advantages in resource acquisition may be related to efficient biomass allocation to roots and leaf area. This work provides first insights into key functional attributes conferring success at an early seedling stage in the process of habitat invasion in arid zones. It also permits identifying growth characteristics of currently less successful non-native species which may allow these species outcompete native species considering future scenarios of climate change.