Masting is shaped by tree-level attributes and stand structure, more than climate, in a Rocky Mountain conifer species

Many tree species mast, meaning seed production is highly variable from
year to year and synchronous within a stand, but this phenomenon remains
poorly understood. To better understand how a changing climate, altered
disturbance regimes, or novel management strategies might affect future
seed production, we quantified the joint influence of both biotic (tree
size, age, and neighborhood competition) and abiotic factors (climate and
weather) on seed production in a widespread conifer species, Rocky
Mountain ponderosa pine (Pinus ponderosa var. scopulorum). We
reconstructed individual-level annual cone production across a large
portion of this species' range using the cone abscission scar method,
and mixed models were used to test hypotheses related to the causes and
drivers of masting in this species. Our results suggest that masting in
ponderosa pine is a process shaped at the individual-level, and this leads
to high, local-scale variation in annual cone production. The effects of
weather were strongest at climatically marginal sites, but overall, the
joint effects of weather and climate only weakly described
individual-level patterns of annual cone production in ponderosa pine (R2m
= 1.6%, R2c = 30.1%). Rather, we found that masting was strongly
influenced by tree- and stand-level factors such as diameter, age, and
local neighborhood density, all of which were associated with the mean,
interannual variability, and between-tree synchrony of cone production at
the individual-level. Larger and older trees produced more cones, more
frequently, and with less synchrony than smaller and younger trees.
Open-grown trees experiencing lower levels of neighborhood competition
also produced more cones with less interannual variability, but with
higher between-tree synchrony. Because tree- and stand-level traits appear
to regulate seed production more strongly than climate or weather in this
species, management interventions targeting these factors could be
powerful tools to manage future tree recruitment. Thus, current efforts to
reduce stand density and conserve large trees in some ponderosa pine
forests may enhance tree-level seed production and reduce variability in
seed crops among years.