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.