Effect of thinning and soil treatments on Pinus ponderosa plantations: 15-year results

• Thinning did not significantly compact the soils.
• Neither sub-soiling nor rototilling had a significantly positive effect on tree growth.
• Short-term plantation growth was not improved with chip returns or chips with fertilization.
• Plantation thinning and soil treatments did not reduce soil carbon sequestration.

Thinning with removal of whole trees in a plantation or natural forest stand raises two main concerns – soil compaction from the ground-based machinery and nutrient depletion particularly with whole tree harvest as is often practiced for attendant fuels reduction. To address these concerns, two sets of experimental treatments were imposed in young ponderosa pine plantations. In the first set, we applied four treatments to test the effects of thinning with biomass removal using progressively more soil manipulations: (I) control, (II) thinning only with all biomass removed, (III) same as (II) but followed by sub-soiling in traffic lanes, and (IV) same as (III) but with nitrogen and phosphorus fertilization within traffic lanes prior to sub-soiling. In the second experiment set we applied four combination treatments to test the further effects of soil manipulations with wood chips and fertilizer on traffic lanes. In thinned stands: (i) the harvested trees were chipped, and spread onto traffic lanes followed by sub-soiling and rototilling, (ii) same as (i) but traffic lanes also received N and P with the chips prior to the sub-soiling, (iii) traffic lanes were sub-soiled, then thinning chips were returned to just the surface of traffic lanes, and (iv) same as (iii) but traffic lanes also received N and P fertilizer with the chips. Tree height and diameter were measured three times, starting immediately following treatments and again at 5 and 15 years post-treatment. In addition, soil bulk density was measured at 6 years and soil chemistry (C, N, and P) was measured at 6 and 16 years. Our results indicate: (1) thinning by itself with no subsoiling did not compact the soils, but increased growth rate of residual trees, although the periodic annual increment of basal area and volume was still higher in the control than other main-plot treatments; (2) neither subsoiling nor rototilling, both of which might mitigate soil compaction, enhanced tree growth; (3) short-term plantation growth was not improved with chip returns or chips with fertilization; (4) since thinning and soil treatments showed more insect damage and higher mortality, any management operations that involve cutting or damaging trees or roots should be avoided during active periods of bark beetle flight; (5) both thinning and soil treatments did not reduce carbon sequestration in the mineral soils. A lack of growth benefits from returning thinning chips, rototilling, and direct fertilization for a longer period appeals to further study.