Print PageIn arid environments, facilitation is expected to produce patterns of positive association between shrubs and other plants since shrub canopies and below ground architectures often buffer extreme abiotic conditions and improve soil water and nutrients, promoting recruitment, survival, and growth of beneficiary plants occurring under them. For this reason, many scientists consider positive spatial associations as potential indicators of facilitation while negative associations may indicate competition. Although the consequences of spatial associations on recruitment, survival, and growth are often considered, reproduction is rarely compared between plants occurring under shrubs and those occurring in shrub interspaces. Our objectives were to (1) describe spatial associations between shrubs and Penstemon palmeri over time and (2) relate associations to recruitment, survival, growth, and reproduction of P. palmeri in shrub vs. interspace microhabitats. These objectives were considered in three similar sites from 2008 to 2011.
Results/Conclusions
Initially, adult plants at all sites occurred under shrubs 93% to 138% more often than expected. Only one site had sufficient interspace plant quantities to reliably compare performance. At this site (1) growth was generally independent of spatial association, but interspace plants had 390% higher relative growth from June to mid-July 2009, (2) initial plant size and total capsule production was independent of spatial association (3) plants under shrubs had 34% higher survival (4) plants in interspaces were 39% more likely to attempt reproduction, (5) plant size and bud to capsule success were negatively related for plants in interspaces and positively related for plants under shrubs . In 2010, disproportionate recruitment of P. palmeri seedlings in interspaces produced negative spatial associations with shrubs at two sites; 63% and 47% fewer seedlings occurred under shrubs than expected. However, shrubs improved seedling survival by 34% to 38%, shifting associations to positive over time. Recruitment and survival at the remaining site was equivalent between microhabitats, producing neutral and temporally stable associations. Our results indicate that spatial associations between these plants shift temporally and affect life-history attributes differently; facilitating some (survival, reproductive efficiency) and negatively impacting others (recruitment, growth).
