When resources are limited, intraspecific competition may be particularly strong as individuals are likely to use a resource in the same way. Thus, tree seedlings with a “near neighbor” – from seed deposited in the same microsite – are predicted to exhibit evidence of competition such as reduced growth and/or survival. To test the assumption that the near neighbors are competing, we examined several cohorts of valley oak (Quercus lobata) seedlings established from acorns planted at the University of California Sedgwick Reserve. Pairs of seedlings emerged within one location where two acorns were planted within centimeters of each other. In one cohort planted in 2003 we conducted removal experiments in which pairs of established, 8-year-old seedlings were randomly assigned to one of two treatments: control (both seedlings remain intact) and removal (one seedling in the pair is removed and the target remains). We compared growth for a nine-year period following neighbor removal, and measured relative access to water via pre-dawn xylem pressure potentials in early fall. In a second cohort planted in 1998, we assessed relationships between presence/absence of neighbors and individual performance. We hypothesized that seedlings that lacked a neighbor would have higher growth and survival within each one-year interval.
Results/Conclusions
In our removal experiment we found that, contrary to expectations, growth (height and relative growth rates) and access to water (PDXPPs) were not significantly different between control and removals for any year or over the entire time period post removal as assessed with repeated measures ANOVA. In addition, we found no relationship between presence of a neighbor and growth or survival in our second cohort, monitored from 1998 to 2009. Both our observational and experimental findings indicate that near neighbors do not have negative impacts on performance for early life stages in Q. lobata.