Comparative Drought Resistance of Five Conifers and Foliage Moisture Content as a Viability Index

Abstract

Whole plant lethal points of foliage moisture content (FMC) for ponderosa pine, sugar pine, Douglas—fir, grand fir, and incense—cedar were established from foliage tissue of various ages. Exact lethal level depended upon the age of the tissue, but there was a general plateau between the ages 6 and 13 months for sugar pine and Douglas—fir and 3 and 13 months for incense—cedar. Whole plant relative turgidity (RT) lethal points were set for the pines and firs using 13—month—old needles. Dry—weight loss upon soaking the needles was found to increase with the severity of the drought, especially for the firs. Soil moisture content (SMC) lethal points were established for all five species on builders' sand (death began at an SMC probably equivalent to tension above 24 atm) and on pumice soil (death points equivalent to tensions from 15 to 19 atm). Foliage moisture content declined with decreasing soil moisture stress in a straight—line relationship; FMC may be used as an index of soil moisture stress within a rather narrow range of SMC and may also be calibrated with leaf tissue diffusion pressure deficits developing under soil moisture stress to give an index of the water balance of the plant. Based on the SMC at the death point and also upon some species' ability to outlive other under competitive conditions, ponderosa pine, incense—cedar, and Douglas—fir were the most drought resistant, sugar pine the least, and grand fir intermediate. Ways are suggested in which the mixed—conifer forests could become established and retain their mixed composition through maturity. The findings suggest that if planting is used to regenerate clearcuts, ponderosa pine and incense—cedar would be first choice, but that sugar pine would also be a preferred species if seeding is used.