Distinct root and shoot responses to simulated mowing and fertility in a grassland invasion

Background/Question/Methods

Grassland root responses to grazing and fertility are poorly known relative to shoot responses. Roots are important because 90% of net primary productivity in semi-arid grasslands occurs belowground. Further, understanding roots may aid the management of invasive grassland species such as Agropyron cristatum,if belowground processes help explain invasion success. A fundamental question is whether management influences grassland invasion. Trait-based models predict that invaders thrive by superior rates of growth or reproduction, regardless of management. Context-based models predict that grazing and nutrients can either open invasion windows or, in some cases, promote resistance to invasion by favoring native grasses. To test these alternatives, we measured native and invaded grassland responses to five years of mowing, at two nitrogen (N) levels. Minirhizotrons allowed continuous, non-destructive fine root measurements in 20 plots each in invaded grassland and native grassland in NE Montana.

Results/Conclusions

Our work supported the trait-based model. Root length was significantly greater in invaded (363 ± 200 [SD] m x m^-2 image) than native grassland (168 ± 128 m × m^-2 image). In contrast, root length did not vary significantly with mowing, N addition, or the interaction between these treatments. No significant interaction between treatment and vegetation type occurred for root length.

     Shoot mass was significantly reduced by mowing and was significantly increased by N addition. Shoot mass was significantly greater for invaded (499 ± 310 g × m^-2) than native (373 ± 182 g × m^-2) grassland. A significant vegetation type*mowing interaction occurred for shoot mass because mowing reduced A. cristatum shoot mass more than it did native shoot mass.

     In conclusion, shoot mass of the invader A. cristatum was reduced by mowing more than that of native grasses, but root length in both vegetation types was unaffected. Given that most production and competition in grasslands occurs below ground, this suggests that invasion success by A. cristatum, a species now abundant on over ~ 10 million ha of the northern Great Plains, owes its success to its traits and may be difficult to control by mowing or grazing.