Variability in Mixed Conifer Spatial Structure Changes Understory Light Environments

Abstract

In fire-adapted conifer forests of the Western U.S., changing land use has led to increased forest densities and fuel conditions partly responsible for increasing the extent of high-severity wildfires in the region. In response, land managers often use mechanical thinning treatments to reduce fuels and increase overstory structural complexity, which can help improve stand resilience and restore complex spatial patterns that once characterized these stands. The outcomes of these treatments can vary greatly, resulting in a large gradient in aggregation of residual overstory trees. However, there is limited information on how a range of spatial outcomes from restoration treatments can influence structural complexity and tree regeneration dynamics in mixed conifer stands. In this study, we model understory light levels across a range of forest density in a stem-mapped dry mixed conifer forest and apply this model to simulated stem maps that are similar in residual basal area yet vary in degree of spatial complexity. We found that light availability was best modeled by residual stand density index and that consideration of forest structure at multiple spatial scales is important for predicting light availability. Second, we found that restoration treatments differing in spatial pattern may differ markedly in their achievement of objectives such as density reduction, maintenance of horizontal and tree size complexity, and creation of microsite conditions favorable to shade-intolerant species, with several notable tradeoffs. These conditions in turn have cascading effects on regeneration dynamics, treatment longevity, fire behavior, and resilience to disturbances. In our study, treatments with high aggregation of residual trees best balanced multiple objectives typically used in ponderosa pine and dry mixed conifer forests. Simulation studies that consider a wide range of possible spatial patterns can complement field studies and provide predictions of the impacts of mechanical treatments on a large range of potential ecological effects.

Keeping mixtures of Norway spruce and birch in production forests: insights from survey data

“…If the objective with growing the Norway spruce stand together with admixture of broadleaves is to increase nature conservation values, then this issue needs to be considered from the outset of the thinning regime. Simulations of thinning approach in mixtures demonstrate positive effects of maintaining clustering tree structures for maintained or increasing within-stand heterogeneity (Cannon et al 2019) as well as a general increase of species richness with increasing forest heterogeneity (Felton et al 2016;Latif et al 2020).…”

The aboveground and belowground growth characteristics of juvenile conifers in the southwestern United States

“…Due to increased need to access light in environments with a shorter growing season and greater canopy shading (Oliver and Ryker 1990, Cannon et al. 2019), we hypothesized that juveniles in high elevation mixed‐conifer forests would exhibit higher measures of aboveground growth or light capture compared with juveniles located at lower elevations. We found support for this hypothesis for firs which had the highest SLA of all tree groups.…”

“…2015, Cannon et al. 2019). We expected that these differences would be maintained across tree height classes, underscoring the sustained importance of resource limitation for juvenile conifers in these different environments, and supporting the conclusion that juvenile growth characteristics would largely be controlled by climatic factors and elevation across the SWUS.…”

“…(3) quantify the maximum rooting depth of juvenile conifers; (4) determine if height and root collar diameter field measurements can explain patterns of juvenile growth characteristics; and (5) investigate to what degree variation in growth characteristics was attributable to broadscale, regional climate, and landscape factors that vary across the SWUS, vs. factors that vary across finer local scales. We hypothesized that differences in climate would impart the strongest control on juvenile growth characteristics, such that junipers and piñon pines at lower elevations would exhibit higher belowground proportional growth due to their need to access soil moisture in drier environments (Burns 1983, Meeuwig et al 1990), whereas ponderosa pines and firs in higher elevation mixed-conifer forests with dense overstory canopy cover would exhibit higher aboveground proportional growth due to their need to access light (Oliver and Ryker 1990, Briggs et al 2015, Cannon et al 2019. We expected that these differences would be maintained across tree height classes, underscoring the sustained importance of resource limitation for juvenile conifers in these different environments, and supporting the conclusion that juvenile growth characteristics would largely be controlled by climatic factors and elevation across the SWUS.…”

Low‐ and moderate‐severity fire offers key insights for landscape restoration in ponderosa pine forests

“…2012, Cannon et al. 2019), and wildlife habitat (Kalies and Chambers 2010). Thus, restoration guidelines in many fire‐prone forests typically include objectives to increase or maintain variability in forest structure, such as increasing horizontal spatial complexity at both fine and coarse scales (Brockway et al.…”