Herbicide, fertilization, and planting density effects on intensively managed loblolly pine early stand development

Production forestry in the southeast US has been partially transitioned to
intensively managed short rotations (~10 years), in which
multiple silvicultural interventions are performed during forest
development. Understanding the responses to silvicultural practices and
continued refinement of site-specific recommendations is critical to
sustainably maximize forest production. We evaluated the effects of
silvicultural practices (herbicide, fertilization, and planting density)
on growth, stand homogeneity, and above- and belowground biomass
accumulation and partitioning of loblolly pine (Pinus taeda) throughout
early stand development (age 5 years) in the southeastern US.
Five treatments with eight replications each were tested: no herbicide and
no fertilization (C); herbicide only (H); herbicide and half-reduced
fertilization rate (R); herbicide and full fertilization (F); and
increased stand density (60+ %; 1346 vs. 2152 trees per
hectare) with herbicide and full fertilization rate (D). Allometric
equations generated from destructive harvests were applied to annual
diameter measurements to estimate plot-level biomass and allocation.
Herbicide was crucial to promote stand uniformity and increase yield
(~600+ % stem biomass compared with C at age
5). Aboveground biomass was similar
in R and F treatments, which was, on
average, ~25% higher than in H at age 5. Increasing
planting density along with multiple herbicide and fertilizer applications
yielded higher biomass without compromising individual tree size (diameter
and height). There was little effect of silviculture practices on
allocation patterns. Our results parallel what was found for fertilization
with herbicide from a number of loblolly stands under similar conditions
and indicate a ~28% volume gain with fertilization during early
stand development. Similarly, our results were consistent with other
studies implementing similar differences in planting density and suggest
a ~26% volume gain through early stand development with an
initial 60% increase in planting density. Our study helps to understand
complex relationships between production and silvicultural practices
during early stand development and demonstrates that silvicultural
prescriptions can be optimized to increase sustainability of production
forestry.