Population structures in mixed pine-oak forests demonstrate varying shade-tolerances with elevation in the Sierra Norte, Oaxaca, Mexico

Pine and oak are among the world’s most widespread and important tree genera both ecologically and economically, but pine-oak forests in subtropical climates are understudied compared to their temperate counterparts. In Mexico, the successional role of oaks has been subject to continued debate, in part due to the high diversity of species there. In highly diverse pine-oak forests in Oaxaca, understanding regeneration patterns is critical to forest management that conserves biodiversity while generating sustainable sources of timber and fuelwood. Here we analyze population structures and regeneration using cluster analysis and pair-wise comparisons to infer relative shade-tolerances and successional roles across an elevation gradient. We present detailed assessments of seven pine, seven oak, and six associated hardwood species across three vegetation types. We find that pine generally behaves as a light-demanding pioneer and is largely failing to regenerate under current canopy conditions. Oak generally behaves as a more shade-tolerant mid to late successional species, but there appears to be a range in shade-tolerance characteristics within both genera. Notably, these patterns appeared to be linked to vegetation type and elevation, with both genera in more mesic communities demonstrating higher shade-tolerance levels compared to less productive areas. We highlight how oak species likely play different successional roles across different sites. Finally, our results also show impacts of human disturbance and land-use, as non-harvested oak species appear to be increasing in density while preferred firewood species show diameter distributions restricted to small size-classes due to intensive harvest. • Oak species in Oaxaca show varying shade-tolerance levels across elevation. • Pine and some oaks are largely failing to regenerate under closed canopies. • Pine-oak successional roles may shift across vegetation types and site productivity. • Human disturbance impacts oak population structures through harvest.