Shrub-grass interactions in the Colorado shortgrass steppe

Background/Question/Methods

Increases in the abundance of woody plants in systems with codominant woody and herbaceous life forms have been documented in many global systems and efforts by land managers to preserve biodiversity and ecosystem services in the face of changing climatic and disturbance regimes may depend on our understanding of the responses of shrubs and grasses to changes in abiotic, biotic, and­ edaphic factors. This study quantifies the effect of the presence or absence of shrubs on grass growth as well as the corresponding effects of grasses on shrub growth through neighbor-removal experiments performed on the shortgrass steppe (SGS) in northeastern Colorado. We also examine the relative contributions of woody, leaf, and flower biomass to the overall growth of shrubs to see if shrubs alter their allocation strategies in the presence or absence of neighbor interactions or in response to environmental factors (i.e. site location corresponding to differences in soil texture, community composition, and topography).  In so doing, we aim to provide a quantitative assessment of shrub-grass interactions from a system currently underrepresented in the growing body of literature evaluating competitive and facilitative trends of neighboring organisms in arid and semiarid ecosystems. 

Results/Conclusions  

      Our analyses revealed no significant changes in the relative growth rate of shrubs in the presence or absence of grass and likewise no effects of shrub removal on grass growth, suggesting that the balance between the competitive and facilitative mechanisms that enable coexistence between these life forms results in net-neutral effects. However, shrubs were found to allocate significantly less growth to the woody component in the presence of grass competition, implying that the presence or absence of neighbor interactions may indeed influence the growth strategy of plants in the SGS. Significant differences were found between sites for both grass production and overall shrub growth, suggesting an influence of environmental factors related to soil texture or topography in driving net primary productivity in this system. The observed increase in production on course-textured soils is consistent with the predictions of the inverse texture hypothesis. Previous analyses of woody-herbaceous interactions along environmental gradients suggest a facilitative effect of shrubs on grass production in arid regions such as the SGS. The net-neutral effect detected here may indicate that the biotic and edaphic properties unique to the SGS drive the observed response away from trends documented in climatically similar regions.