Reference conditions and historical fine-scale spatial dynamics in a dry mixed-conifer forest, Arizona, USA

To improve the knowledge of ecosystem dynamics within frequent-fire forests and to develop targets for forest restoration, we dendrochronologically reconstructed four 1-ha plots within dry mixed-conifer forests in northern Arizona, USA. Forest densities in the study area increased from 139.8 live trees ha^-1, 10.26 m² of basal area ha1, and 14.9% canopy cover in 1879 (the assumed year of fire exclusion) to 1,116.8 live trees ha^-1, 42.23 m² of basal area ha-1, and 55.3% canopy cover in 2014. Shade-tolerant species also became more prevalent. Initial increases in tree density occurred near the established overstory or randomly throughout each stand, rather than within canopy gaps. Tree spatial patterns were random or aggregated in 1879 and 88.3% of trees were isolated individuals or in groups of 2-4 trees. Sprouting hardwoods and shade-tolerant conifers were more likely than other tree species to have been members of groups, whereas shade-intolerant conifers were more likely to be isolated individuals. Relative shade tolerance and the reproductive strategies of component species contribute to fine-scale spatial patterns in mixed-species forests. This interaction between species silvics and fine-scale spatial patterns is an important consideration for management activities targeting heterogeneity and the natural ranges of variability in frequent-fire forests.