Soil Compaction Reduced the Growth of Lodgepole Pine and Douglas-fir Seedlings in Raised Beds after Two Growing Seasons

Growth of lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) seedlings after two growing seasons was reduced by medium and high levels of compaction in loamy sand and silt loam soils that received one of three compaction treatments (low = 0.70 relative bulk density [RBD], medium 0.79–0.82 RBD, and high 0.84 RBD). Survival was reduced on the loamy sand, but not on the silt loam. Soil water content was adjusted with irrigation to levels associated with plant water stress (near wilting point), reduced aeration (near 10% air-filled porosity), and intermediate conditions. Lodgepole pine survival on loamy sand was increased at high water content, but was unaffected by water regime on silt loam. For both soil types, the best lodgepole pine growth was observed for the intermediate watering level. The detrimental effects of compaction were consistent across all water regimes. We also evaluated the response of Douglas-fir [Pseudotsuga menziesii var. glauca (Beissn.) Franco] on silt loam and it was similar to lodgepole pine, except that survival was lower on the compacted silt loam, and under dry conditions. For both species, limitations to growth and survival at medium and high compaction levels were consistent with expectations based on the least limiting water range. Our results, however, are also consistent with a continuously declining growth response due to increasing compaction. The RBD was a good predictor of limiting soil conditions for both soil types and species, and substantially reduced survival and growth was observed at RBD levels higher than 0.80.