Influence of light and soil moisture on Sierran mixed-conifer understory communities

Abstract

Sierra Nevada forests have high understory species richness yet we do not know which site factors influence herb and shrub distribution or abundance. We examined the understory of an old-growth mixed-conifer Sierran forest and its distribution in relation to microsite conditions. The forest has high species richness (98 species sampled), most of which are herbs with sparse cover and relatively equal abundance. Shrub cover is highly concentrated in discrete patches. Using overstory tree cover and microsite environmental conditions, four habitats were identified; tree cluster, partial canopy, gap, and rock/shallow soil. Herb and shrub species were strongly linked with habitats. Soil moisture, litter depth and diffuse light were the most significant environmental gradients influencing understory plant distribution. Herb cover was most strongly influenced by soil moisture. Shrub cover is associated with more diffuse light, less direct light, and sites with lower soil moisture. Herb richness is most affected by conditions which influence soil moisture. Richness is positively correlated with litter depth, and negatively correlated with direct light and shrub cover. Disturbance or management practices which change forest floor conditions, shallow soil moisture and direct light are likely to have the strongest effect on Sierran understory abundance and richness.

Development of Vegetation and Surface Fuels Following Fire Hazard Reduction Treatment

“…However, potential increases in shrub growth as a result of reduced canopy cover and increased microsite availability may be limited. In the Teakettle Experimental Forest, a southern Sierra Nevada site, North et al [65] determined that mixed conifer shrubs were associated with diffuse light and low soil moisture levels. Cover was reduced in both closed canopy and canopy gaps with shrubs preferentially occupying an ecotone between the two cover types.…”

Long‐term vegetation responses to reintroduction and repeated use of fire in mixed‐conifer forests of the Sierra Nevada

“…Direct effects include physical consumption of above-ground structures (Whelan 1995, Agee 2003, mortality of root systems through soil heating (Brown and Smith 2000), and stimulation of soil seed banks (Leck et al 1989). Indirect effects include changes in microclimate (e.g., light and temperature) and soil resources (moisture and nutrient) via reductions in tree density or consumption of surface fuels (Wan et al 2001, North et al 2005, FitesKaufman et al 2006, Ma et al 2010. The relative importance of these effects can be mediated by the severity, frequency, or spatial heterogeneity of burning.…”

Mapping global seasonal forest background reflectivity with Multi‐angle Imaging Spectroradiometer data

“…Thus, factors affecting the growth of understory plants such as canopy openness [Ricard et al, 2003;Strengbom et al, 2004] and stand conditions such as litter depth, mass, and soil moisture [North et al, 2005;Yu and Sun, 2013] will change forest background reflectivity to some extent. These variations could be observed in the distribution of forest background reflectivity under different canopy LAI in June for Northern Hemisphere and December for Southern Hemisphere (Figure 2).…”