Summary
We found the ability of a tree to survive fires is greatly increased when it has neighbors, either conspecific or heterospecific. Particularly noteworthy is that we documented facilitated survival repeatedly over time. As shown in Figs. 2 and 3, neighbor facilitation increased survival rates during each of the five 5-year intervals. This persistent neighbor facilitation is likely due to the reduction of both herbaceous and woody fuel in denser sites, and possibly by mycorrhizal sharing among nearby trees. Our findings support the idea that facilitation among plants occurs more commonly in severe and highly disturbed environments (Walker and Chapin 1987, Brooker and Callaghan 1998), in accordance with the stress gradient hypothesis (Bertness and Callaway 1994). The fact that survival was enhanced by neighbor facilitation, while growth was often reduced by negative neighbor interactions, is consistent with the findings from a meta-analysis of the stress-gradient hypothesis by Adams et al. (2021), which showed that increasing stress tends to result in an increase in positive ecological interactions in terms of survival but an increase in negative interactions with respect to growth.
Based on a review of fifty studies of spatial pattern in fire-frequent forests, Larson and Churchill (2012) concluded that aggregated tree regeneration is the primary cause for the development and maintenance of tree clumps in these environments. In the study reported here, due to persistent neighbor facilitation, trees survived best growing in groups. Since the frequent fires prevented any significant tree regeneration in the study area, either through sprouting or seedling recruits (Davis 2021), the study showed that aggregated tree survival by itself can also be an important driver of post-fire tree clumping. The results reported here, along with those by Lutz et al. (2014) and Larson et al. (2015), emphasize the importance of documenting the spatial patterns of tree survival following fires, in addition to those of regeneration, to gain a fuller understanding of the processes creating the observed spatial patterns of savannas, woodlands, and forests. Given the number of recurrent fires during the study, the results should especially inform efforts to restore savannas through the implementation of frequent controlled burns.
The neighbor facilitation documented in this study may explain aspects of the evolution of Quercus . Compared to most North American hardwood trees, oaks are well adapted to survive periodic fires (Abrams 1992). Adaptations include thick bark, ability to resprout following top kill, and, in the white oak group, the ability to compartmentalize fire wounds (Brose et al. 2014). This study showed how being near to other oaks can also reduce fire mortality.
Explanations for the value of seed dispersal usually have emphasized only the benefits of dispersing away from the parent, i.e., avoiding disproportionate seed and seedling mortality near the parent, being able to colonize suitable sites unpredictable in space and time, and, through directed dispersal, being able to disperse to particular types of sites suitable for establishment and growth (Howe and Smallwood 1982, Wenny 2001). However, emphasizing that seeds dispersed close to the parent likely will encounter favorable environmental conditions, Snyder (2011) concluded that unless the environment is spatially and temporally highly unpredictable, short-distance dispersal is likely to be evolutionarily stable. The positive neighbor interactions reported here illustrate another value of not dispersing far from the parent. Thus, it is possible that neighbor facilitation may have played a role in the evolution of short-distance dispersing fruits, such as acorns.