Growth responses of ponderosa pine to long-term exposure to ozone, wet and dry acidic deposition, and drought

Abstract

A 3-year field study of the cumulative effects of ozone (O3), wet and dry acidic deposition, and soil water availability was conducted on ponderosa pine (Pinusponderosa Laws.) in the Sierra Nevada of California from 1988 to 1990. Thirty-six 2-year-old potted seedlings were placed in each of 30 chambers and exposed from May through October to three levels of O3 (charcoal-filtered (CF), nonfiltered (NF), and NF plus 1.5 times ambient O3 (NF150)); three levels of acidity in simulated rain (pH 3.5, 4.4, 5.3); two levels of dry deposition (60 or 90% filtration), and two levels of soil water availability (well watered (WW) or drought stressed (DS)). An additional six plots served as ambient air (AA) controls. One-third (432) of the trees were harvested at the end of each exposure season. Low soil water availability was the only stress factor to significantly affect growth following the first exposure season. After the second season, O3 significantly reduced foliar biomass in WW–NF150 trees, but DS seedlings did not respond to O3. After 3 years of exposure, WW–NF150 trees averaged 70% loss of 1988 needles and 48% loss of 1989 foliage. Ozone-injured seedlings compensated for these losses by increased growth of current-year needles and stems and also increased growth of fine feeder roots. Radial stem growth and coarse-root growth were significantly reduced in O3-injured trees. DS trees in NF150 chambers averaged half the needle loss of WW trees and showed no reduction in radial growth in response to O3. Rain pH and dry deposition had no direct effects on growth of ponderosa pine. These cumulative responses to interacting stresses indicate the importance of multifactorial, long-term studies to evaluate forest tree responses to atmospheric deposition.