THE EFFECTS OF NEIGHBORS ON THE GROWTH AND SURVIVAL OF SHRUB SEEDLINGS FOLLOWING FIRE

Where plant species vie for limited resources, disturbances might prec lude competition by releasing a flush of nutrients, or by reducing bio mass and thereby diminishing the consumption of resources. However, if new seedlings colonize in clumps, they may still deplete resources wi thin the local aggregations, which may then reduce their growth and su rvivorship. We investigated competition among seedlings in a burned ar ea by examining the relationship between the performance of newly esta blished shrub seedlings of Ceanothus impressus and (1) the proximity a nd (2) the identity of their near neighbors. We also investigated the relationship between neighbor proximity and the availability of water. Both survivorship and growth of C. impressus were positively associat ed with increasing distance to near neighbors, in a manner consistent with resource competition. The availability of water (as determined by pre-dawn xylem pressure potentials) tended to be greater when neighbo rs were farther away, providing evidence that water was a resource for which plants were competing. This conclusion is reinforced by the fin ding that the effects of neighbors were stronger in drought years, sug gesting that yearly variation in the availability of an important reso urce (water) can affect the strength of competitive interactions. This suggests that after disturbances, when some resources are apparently abundant on a large scale, competition may be important in determining the small scale patterns of seedling growth and survival.