ROOT-SYSTEM RESPONSE OF A PERENNIAL BUNCHGRASS TO NEIGHBORHOOD-SCALE SOIL-WATER HETEROGENEITY

1. We evaluated the ability of the perennial bunchgrass Bouteloua grac ilis to exploit resources in central and outer portions of root system s. Morphological constraints were hypothesized to explain scales of st rong neighbour interaction and gap dynamics observed in shortgrass ste ppe. 2. Water or water plus nitrogen were supplied at horizontal dista nces greater or less than 10 cm from tillers in containers. 3. Growth was significantly less when water was supplied only to the outer root system than when water was supplied only to the central root system. R educed growth was associated with asymmetrical distribution of roots r elative to direction of tiller growth. N addition did not affect plant growth. 4. Although adventitious roots were mostly <10 cm from crowns , root length density and water use were similar in central portions o f root systems and at distances >10 cm from leading ends of tillers, w hich corresponded to distances >10cm from intact plants. 5. Given typi cal plant spacings in shortgrass steppe (<15 cm), established B. graci lis individuals are likely to compete strongly and pre-empt resources in most openings. 6. Results suggest that enhanced resource availabili ty and colonization in small gaps (approximate to 50 cm) in shortgrass communities do not reflect strong morphological constraints on the ou ter root system of B. gracilis plants.