Reproductive maturity and cone abundance vary with tree size and stand basal area for two widely distributed conifers

Understanding potential limitations to tree regeneration is essential as rates of tree mortality increase in response to direct (extreme drought) and indirect (bark beetle outbreaks, wildfire) effects of a warming climate. Seed availability is increasingly recognized as an important limitation for tree regeneration. High variability in seed cone production is a trait common among many northern temperate conifers, but few studies examine the determinants of individual tree cone production and how they vary with stand structure. In subalpine forests in the southern Rocky Mountains, USA, we monitored >1600 Picea engelmannii (Engelmann spruce) and Abies lasiocarpa (subalpine fir) trees for cone presence (an indicator of reproductive maturity) and a subset of those trees for cone abundance (an indicator of seed production) from 2016 to 2018. We constructed mixed models to test how individual tree cone presence and cone abundance were affected by tree size and age as well as forest attributes at the neighborhood- and stand-scales. The probability of cone presence and cone abundance increased with tree size and age for A. lasiocarpa and P. engelmannii. The youngest ages of trees with cones present were more than 100 yr later for individuals in high basal area (BA) stands (>65 m(2)/ha) relative to low BA stands (<25 m(2)/ha). P. engelmannii produced many more cones than A. lasiocarpa at similar sizes, especially in young, low BA stands. Our findings reveal how differences in tree sizes and stand structures typically associated with time since last disturbance can affect seed production patterns for decades to well over a century. The consistent regional pattern of earlier and more abundant postfire establishment of P. engelmannnii vs. the delayed postfire establishment by A. lasiocarpa may be partially explained by species' differences in cone abundance by stand structure. The increasing loss of large, dominant cone-producing trees will significantly reduce seed production to support future tree regeneration and maintain forest cover. However, seed availability and resilience following disturbances may be less limiting than expected for species like P. engelmannii that have the capacity to produce more cones in open-canopy forests, such as recently disturbed areas.