Abiotic factors modify ponderosa pine regeneration outcomes after high-severity fire

Large high-severity burn patches are increasingly common in southwestern
US dry conifer forests. Seed-obligate conifers often fail to quickly
regenerate large patches because their seeds rarely travel the distances
required to reach the core patch area. Abiotic factors may further alter
the distance seeds can travel to regenerate a patch, which would change
expected post-fire regeneration patterns. We used the presence and density
of ponderosa pine regeneration as a proxy for seed dispersal to quantify
the effect of abiotic factors on seed dispersal into high-severity
patches. We established 45 transects in burn patches across the Gila
National Forest, NM, USA to measure regeneration density in areas that
varied by aspect, slope, and prevailing wind direction relative to intact
forest. We modeled the effect of abiotic features on regeneration presence
and density, comparing density estimates against a distance-only model to
assess differences in model performance and expected regeneration density.
We found the highest regeneration densities on north-facing aspects that
were near, downwind, and downslope of intact forest, which decreased in
density and likelihood as conditions for seed dispersal became less
favorable. Accounting for abiotic factors improved model performance and
increased regeneration density estimates compared to the distance-only
model. Our findings indicate that regeneration presence and density vary
as a function of the interaction between abiotic factors and distance to
the primary seed source, which is determined by patch characteristics.
Therefore, abiotic factors will have a smaller effect on regeneration
outcomes in large, simple patches, which have more area further from the
patch edge.