Site quality and intensive early stand management practices affect growth dominance, structural complexity, and tree growth in ponderosa pine plantations

Abstract

During stand initiation, the influences of silvicultural practices such as competing vegetation control and fertilization have been primarily examined in terms of stand-level growth rather than structural complexity. Understanding how intensive silvicultural treatments alter the long-term trajectory of stand development may be critical to meeting management objectives, such as habitat restoration, minimizing ladder fuels, or carbon sequestration. Previous studies have found mixed evidence for an effect of vegetation control and fertilization on structural complexity. Investigating how early pulse treatments drive tree size differentiation in young stands, as indicated by growth dominance (GD), could potentially allow managers to anticipate or design treatments to alter the development of structural complexity later in stand development. We reanalyzed data from the Garden of Eden experiment, a full-factorial study investigating how competing vegetation and nutrients impact ponderosa pine growth and yield over a gradient of site quality in northern California, USA. Our goals were to determine whether these silvicultural practices alter GD and structural complexity, and whether GD and structural complexity created potential feedbacks to alter stand productivity. We examined data for the first 20 years of stand development across 6 study sites. For each site and observation period, we calculated the growth dominance coefficient of total cubic volume to represent GD and the Gini coefficient of inequality in basal area distribution (GC) to represent structural complexity. We created a series of competing generalized additive mixed models (GAMMs) to disentangle the roles of site, stand age, and treatment on GD and GC. We created a set of competing individual-tree GAMMs at age 20 to isolate the influences of GC and GD on volume increment, controlling for the effects of competition, tree size, site, and treatment influences. In agreement with previous research, we found that GD deviated little from size-symmetry (tree growth being roughly proportional to tree size) and did not vary consistently with treatment. Competing vegetation control and higher site quality promoted lower GC, indicative of lower structural complexity. Fertilization increased structural complexity relative to other treatments on two sites in particular, which may reflect unintentional increases in drought stress from fertilization on sites with metasedimentary soils. While GD had limited influence on individual-tree growth, GC interacted with tree size such that small-medium sized trees grew more rapidly on structurally simple plots. Our results have implications for balancing objectives such as fire risk, carbon sequestration, and habitat during reforestation projects.