Comparison of Plantation Arrangements and Naturally Regenerating Mixed-Conifer Stands After a High-Severity Fire in the Sierra Nevada

A sharp escalation in wildfire frequency, severity, and scale in the western United States calls for the creation of forests that are resilient in the future. One reforestation method involves clustering trees into groups of two to four, instead of creating evenly spaced plantations, in an effort to increase structural heterogeneity and emulate natural regeneration patterns. There have been a limited number of studies on clustered plantations, and this study addresses this important research gap. In Eldorado National Forest in the Sierra Nevada, we compared growth and structure in several post-fire plantations, treated with and without pre-commercial thinning (PCT), and naturally regenerating stands. Using mixed-effects models, we tested for growth and structural differences between evenly spaced and clustered plantations, as well as comparing them to stands of naturally regenerating trees. Our results indicated that diameter and height growth were generally better maintained in the plantations compared to under natural stand conditions. When considering plantation arrangement, the annual basal area increment (BAI) thinning index ([BAI after thinning − BAI before thinning]/BAI before thinning) was generally higher in evenly spaced plantations (1.03) compared to clustered plantations (0.79). While high plant diversity would be important eventually from an ecological perspective, our study suggests that during the initial phases of plantation development, lower shrub diversity could assist with plantation establishment and growth. The frequency of yellow pines was an important, positively associated factor affecting BAI and height growth, but primarily in the high-elevation region, which demonstrates a facilitative legacy effect of prior stand composition. Our study highlighted the important legacy effect of prior stand density on the growth of yellow pines, but primarily in the low-elevation region, and only when the two plantation groups were examined. The negative association suggests that a lower initial density of plantations promotes better BAI growth and height growth after PCT. These findings thus have broad implications for effective post-fire restoration of young plantations to help ensure their future resilience to both post-fire restoration and climate change adaptation and biotic (i.e., plant competition) stress factors.