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.