Survival
This is the first study to
document consistent neighbor facilitation among trees experiencing
repeated stressors over an extended time period. Trees of all sizes in
close proximity with neighbors were considerably more likely to survive
fire throughout the study. Our findings support the literature
documenting positive neighbor effects among plants in stressful and
highly disturbed environments (Bertness and Callaway 1994, Brooker et
al. 2008, Guo et al. 2021).
We believe three factors contributed to this neighbor facilitation in
terms of survival. The first involves the reduction of herbaceous fuel
under tree canopies. The decline in fire frequency and intensity, and
the associated decline in tree mortality that occurs when savannas
transition to forests, is believed to be due to the reduced biomass of
grasses, which are the primary fuel for surface fires in these
environments (Frost and Robertson
1987, Holdo 2005, Archibald et al. 2009, Hoffmann et al. 2012). Rebertus
and Burns (1997) found that tree survival in Quercus savannas was
inversely related to grass cover. At CCESR, grass abundance is strongly
negatively associated with tree canopy cover in the oak savanna/woodland
habitats (Peterson et al. 2007). Thus, more neighboring trees means less
herbaceous fuel and hence less fire risk.
The second factor involves the reduction of woody fuel. While both oak
species contributed to neighbor facilitation involving survival, the
survival benefit of having Q. macrocarpa as neighbors exceeded
the benefit of having neighboring Q. ellipsoidalis for both
species (Figs. 2 and 3). We think the difference in fire-susceptibility
between the two oak species may account for this difference. Q.
macrocarpa is considered among the most fire-resistant oaks,
particularly as compared to Q. ellipsoidallis (Lorimer 1985,
Frelich et al. 2015), illustrated by the much higher survival rate
(260% higher) of Q. macrocarpa during the study (Davis 2021).
This means that Q. ellipsoidalis is more likely to serve as fuel,
threatening the survival of their neighbors. Conversely, the more
fire-resistant Q. macrocarpa is likely to impede the spread of
fire, reducing the threat of fire-induced mortality among its neighbors.
Belote et al. (2015) found the fire-resistant Larix occidentalisincreased survival rates among nearby trees in a similar way.
The third factor involves mycorrhizae symbioses. A common explanation
for neighbor facilitation among plants involves mycorrhizal sharing
(Francis and Read 1984, Hunter and Aarrsen 1988, Simard et al. 1997,
Teste and Simard 2008, Das et al. 2008, van der Heijden and Horton 2009,
Bennett et al. 2017, Germain and Lutz 2021). Q. macrocarpa andQ. ellipsoidalis are both ectomycorrhizal trees (Avis et al.
2003) and red and white oaks are known to share mycorrhizae (Walker
2003, Avis et al. 2008). Thus, we believe that mycorrhizal sharing among
both conspecifics and heterospecifics may also be contributing to the
survival-related neighbor facilitation documented in this study. Other
possible explanations for the increased likelihood of trees with
neighbors surviving fire involve the reduced wind speeds and higher
moisture levels associated with denser sites (Hoffmann et al. 2012,
Belote et al. 2015).
It is possible an analysis could identify a positive neighbor effect
among trees when one does not actually exist. For example, a false
positive could result if surviving trees tend to be clumped in desirable
microhabitats (Guo et al. 2021). However, while we did not examine all
factors that could contribute to a favorable microhabitat, our analyses
did include elevation (distance to water table) and soil nitrogen
levels, both of which may be expected to influence tree survival.
Although neighbor facilitation with respect to survival persisted
throughout the study, remaining strong in most cases, the strength of
this relationship slightly declined over time for most size classes
among both species (Figs. 2 and 3). The documented decline in neighbor
facilitation over time might be due to the substantial reduction in the
number of trees during the study. Only about one-third of the trees that
were alive at the start of the study were still alive in 2020 (Davis
2021). This means that the number of neighbors for most trees would have
declined substantially during this time. As the difference in the
magnitude of high and low neighbor measurements declines, the ability of
the models to discern positive neighbor relationships would be expected
to decline as well.
A second possible explanation for the decline in the strength of
neighbor facilitation involves the arrival of oak wilt (Bretziella
fagacearum ) in east-central Minnesota several decades ago (Gibbs and
French 1980), and which is thought to have spread rapidly at CCESR since
2010 (Sapes et al. 2021, Pellegrini et al. 2021). The fungus is spread
primarily through root grafting among neighboring trees and by sap
beetles (Nitidulidae) (Kuntz and Riker 1956, Jagemann et al. 2018).
Thus, while underground root and mycorrhizal connections may reduce the
likelihood of fire-induced mortality for the two oak species, this same
network could facilitate the transmittance of oak wilt.
It is possible, then, that oak wilt might have reduced the neighbor
benefits over time. However, in a separate study at CCESR, Pellegrini et
al. (2021) found that while oak wilt dramatically increased tree
mortality in unburned areas, where trees are densely packed and which
are dominated by Q. ellipsoidalis , it had little effect in
frequently burned sites. Also, since species in the red oak group lack
tyloses, they are not as able to effectively compartmentalize scarring
injuries as are white oaks, thereby making them considerably more
susceptible than white oaks to subsequent fungal infection (Gibbs and
French 1980, Juzwik et al. 2011, Dey and Schweitzer 2018). However,
while Q. macrocarpa is much less vulnerable to oak wilt thanQ. ellipsoidalis , it exhibited a similar decline in neighbor
facilitation. Thus, it seems unlikely oak wilt contributed much, if
anything, to the decline in the strength of neighbor facilitation during
the study.