Defoliation decreases competitive ability of resident plants, alters outcomes of interactions, and increases invader success

Background/Question/Methods

Biotic resistance induced by competition from resident plants is especially important in limiting the abundance of exotics invaders.  Grazing may decrease such resistance by reducing the competitive abilities of resident plants and altering outcomes of interactions between natives and invaders, and thereby increasing invader success.  We tested these predictions in Great Basin Artemisia tridentata ecosystems that are especially vulnerable to cheatgrass (Bromus tectorum) invasion.  We conducted a factorial experiment to test interactions between treatments in small un-invaded patches within extensively invaded communities.  Treatments included: (1) defoliation timing/frequency (control, spring, spring twice, spring/fall, fall), and (2) preference (native bunchgrasses, bunchgrasses and B. tectorum).  Treatments involved clipping plants to a 5cm stubble height. Since competition is predominately belowground for water and nutrients, competitive abilities of two native bunchgrass species, Achnatherum thurberianum (Thurber’s needlegrass) and Elymus elymoides (squirreltail), were assessed by measuring fine root biomass from root cores (0-15cm depth) at 5, 15, 30, and 45cm from bunchgrass bases and measuring fine root biomass growth in ingrowth cores.  B. tectorum collected from the study site was planted in transects between bunchgrasses at 2cm increments. Established plants were harvested prior to seed maturity and weighed.

Results/Conclusions

Preliminary results show all spring defoliations significantly reduced fine root biomass growth of both bunchgrass species (P<.0001).  In the control, there was a significant negative relationship between distance from the nearest bunchgrass and fine root biomass, and the model explained 66% and 57% of the variation in root biomass of A. thurberianum and E. elymoides, respectively. Except for the fall defoliation, defoliation treatments significantly reduced fine root biomass of both bunchgrass species at 5cm and 15cm and either significantly reduced the strength of (R²) or eliminated (non-significant) this negative relationship. All defoliation treatments significantly increased B. tectorum biomass (P<.0001) and the preference treatment was not significant (P=0.452). In the control, there was a significant positive relationship between distance from the nearest bunchgrass and B. tectorum biomass. All defoliation treatments either significantly reduced the strength of or eliminated this positive relationship.  Overall, these results demonstrate that bunchgrass competition reduces B. tectorum biomass and likely slows its rise to dominance.  Spring defoliations reduced the competitive ability of both bunchgrass species, increased the size of belowground gaps where resources are likely more available, and altered the outcome of interactions between bunchgrasses and B. tectorum.  Ultimately, these defoliation-mediated changes increased B. tectorum biomass.