Some Shrub Shading Effects on the Mid-Summer Microenvironment of Ponderosa Pine Seedlings in Central Oregon

Abstract

Recent studies of natural regeneration dynamics in ponderosa pine (Pinus ponderosa var. ponderosa Dougl. ex Laws.) forests of central Oregon's Cascade Range reveal positive associations between shrubs and the survival of new tree seedlings. To shed light on the potential mechanisms of this possible facilitative relationship, we conducted a simple experiment in which ambient air temperature, relative humidity, soil temperature, and shrub-canopy light penetration were recorded beneath shrub-shaded and shrub-free locations during a typical mid-summer day in four stands at two sites. Air temperature and relative humidity were unaffected by shrubs, but soil temperatures were 6.2 °C to 6.6 °C lower beneath shrubs between 12 pm and 4 pm. Percent shading by shrubs averaged 2.3 to 3.0 times that of adjacent shrub-free locations. Amelioration of midday soil temperature appears to be one of the mechanisms involved in theorized shrub-seedling facilitations in this region.