Resource-Dependent Growth Models for Sierran Mixed-Conifer Saplings

Abstract

Thinning to reduce wildfire hazard is a common management practice in frequent-fire forests of the American west, but it is uncertain whether projects will help regenerate fire-resistant, shade-intolerant pines. We studied naturally established saplings of six conifer species in mixed-conifer forest in northern California, USA to determine how three environmental resources - light, soil moisture, and soil mineralizable nitrogen - affect stem radial growth. Likelihood principles were used to select among models of growth as a function of resources, age and size. There was support for models of growth as an exponential function of light for three species. Pinus jeffreyi growth was slow even at 40% full sun but increased sharply at higher light. Light-dependent growth of Pinus ponderosa and Calocedrus decurrens was contingent upon age: some young (<20 y) saplings grew rapidly at 16% full sun, but older saplings required higher light for rapid growth. No relationship of light or other resources to growth was detected for three other species; instead, null models of size-dependent (Abies concolor, Pseudotsuga menziesii) or age-dependent (Pinus lambertiana) growth provided the most parsimonious explanations of growth patterns. These results suggest the latter species will not grow rapidly under more open canopies, but the scope of this inference is limited because few individuals of these more tolerant species were found in high-light environments. In contrast, large openings or patchy canopy are required for rapid growth of P. jeffreyi; smaller openings will increase growth of P. ponderosa, but increases may be modest unless saplings are young.