Inhibition of conifers growing under a deciduous canopy : degrees, seasonality and causes of suppression

Abstract

The physiological response of conifers to a deciduous overstory is unstudied in the cool temperate zone despite the widespread occurrence of the association. The object of this study is therefore to determine the degree that understory conifers are inhibited by a deciduous overstory, and to identify the factors responsible. Thus, three conifer species (Juniperus scopulorum, Pseudotsuga menziesii, Abies lasiocarpa) growing with and without a deciduous Populus canopy were contrasted in regards to environment, seasonal performance, and physiological response to environmental factors. Understory conifer stands were darker, and especially so when the overstory had leaves. Understory environments of P. menziesii and A. lasiocarpa were cooler and had lower vapor pressure deficits than open sites, but were similar in relative humidity and soil water. In contrast, understory sites of J. scopulorum had similar temperature, vapor pressure deficit, and soil water. Relative to conifers in the open, photosynthesis of understory conifers was reduced by roughly the same amount (~10%), regardless of photosynthetic rate. While at intermittent times photosynthesis was reduced by much greater amounts, the expected increase of summertime suppression (>10%) was not seen. Understory conifer suppression was not linked to any seasonally limited resource, such as light, water, or nutrients. The slight suppression over the year was due to an unidentified factor, perhaps the lower temperature of the understory environment (-2C), or seemingly unlikely, an allelopathic effect from Populus. In instantaneous measures, photosynthesis was better correlated with day of year and RH than light, VPD, or temperature. The correlated phenomena relate to season and precipitation rather than canopy condition under study. Slight photosynthetic inhibition of understory conifers is supported by similar observations of suppression in the diameter growth and one-year's twig growth.