INTRODUCTION
Successful management of fire-prone woody ecosystems is challenging and
requires knowledge of the spatial arrangement of the trees (Meyer et al.
2008, Das et al. 2008, Larson and Churchill 2012, Medens et al. 2019,
Furniss et al. 2019) and how the tree distribution patterns influence
the nature and consequences of subsequent fires (Turner 1989, Pacala and
Deutschmann 1995, Frelich et al. 1998, Stephens et al. 2008). Tree
aggregation, or clumping, is an especially common distribution pattern
(Armesto et al. 1986, Condit et al. 2000, Peterson 2020), particularly
in environments experiencing periodic fires, such as savannas (Rebertus
et al. 1989, Davis et al. 2005, Bacelar et al. 2014, Staver et al. 2019,
Tamjidi and Lutz 2020).
Savannas are notably dynamic ecosystems, in which intensity, frequency,
and patterns of fire determine whether the savanna is maintained or if
it begins to transition into a grassland or forest. Fire intensity in
savannas is related to the abundance of grasses, the primary fuel in
these environments (Frost and Robertson 1987, Holdo 2005, Archibald et
al. 2009, Hoffmann et al. 2012). Since grass biomass is lower under
savanna tree canopies (Scholes 2003, Peterson et al. 2007), the ability
of trees within the clumps to survive the fires will play a significant
role in determining the larger scale landscape dynamics.
Small-scale tree spatial patterns can be influenced by interactions
among neighbors, both negative and positive, the latter including
providing refuge from physical stress (Baumeister and Callaway 2006,
Tirado and Pugnaire 2015), reducing susceptibility to enemies (Pineda et
al. 2010), suppression of a strong competitor (Lutz et al. 2014), and
mycorrhizal sharing (Simard et al. 1997, Bennett et al. 2005, Das et al.
2008, Tamjidi and Lutz, 2020, Germain and Lutz 2021). In the case of
savannas, if positive neighbor interactions result in the increased
survival of individual trees, then they would also lead to the
persistence of the clumps of trees, which in turn would promote the
ongoing perpetuation of the savanna. However, the savanna will continue
to exist only if the positive neighbor interactions persist consistently
over time.
As emphasized by Brooker and Callaghan (1998), in cases where
disturbances are episodic, detecting positive interactions among
neighbors is only going to be possible through long-term studies. This
is especially true for studies trying to determine whether positive
interactions in response to disturbances remain consistent over time. To
conduct such research, a study would require a temporal scale long
enough for multiple disturbances to occur, along with a spatial scale
large enough to include many trees and tree neighborhoods. We are not
aware of any such study.
The primary purpose of this study was to determine the nature of
neighbor interactions among trees in an oak savanna experiencing
frequent fires, and to what extent these interactions are sustained over
time. We were able to do this using data obtained from a large grid-plot
study in which trees were censused annually for twenty-five years,
during which time the trees experienced between nine and eleven fires.
The long-term observations allowed us to test how local tree density
effects growth and survival of individuals in the face of repeated
fires, and, secondarily, whether certain small-scale abiotic factors are
important to a tree’s fate.