Douglas-Fir Biomass Allocation and Net Nutrient Pools 15–20 Years after Organic Matter Removal and Vegetation Control

K M Littke,E C Turnblom,S M Holub,W R Littke,R B Harrison,T B Harrington

doi:10.3390/f11091022

Abstract

Douglas-fir (Pseudotsuga menziesiivar. menziesii (Mirbel) Franco) plantation forests of the coastal Pacific Northwest have been intensively managed to improve the yield of forest products. However, the long-term effects of these management techniques have received limited research attention in this region. Three affiliate Long-Term Soil Productivity study sites were installed in Douglas-fir forests to understand the impacts of organic matter removals and vegetation control on soil productivity over time. Matlock and Fall River are located in Washington, USA and Molalla is located in Oregon. Organic matter removal treatments included traditional bole-only harvest (BO), whole tree removals (WT), and a whole tree plus coarse woody debris removal (WT+) (Fall River only). Five years of annual vegetation control (AVC) was compared with a conventional initial vegetation control (IVC) treatment at all sites. Douglas-fir biomass allocation to foliage, branch, and stem components was modeled using 15- to 20-year-old trees from this study along with 5- to 47-year-old trees from previous studies on these sites. Across all sites, model predictions indicated that the WT treatment had 7.1 to 9.7 Mg ha−1 less Douglas-fir biomass than the BO treatment. There was 1.5 to 20.5 Mg ha−1 greater Douglas-fir biomass in the AVC treatment than in the IVC treatment at all sites. Douglas-fir carbon and nitrogen biomass were consistently lower in the WT treatment, but there were no significant changes in overall site nutrient pools. The AVC treatment resulted in greater Douglas-fir nutrient pools yet there was a net loss in site calcium, magnesium, and potassium due to lower forest floor and soil base cation pools. While WT removals did not significantly affect site nutrition, the decrease in Douglas-fir biomass at all sites and increase in invasive Scotch broom (Cytisus scoparius (L.) Link) biomass at Matlock suggests that the standard practice of retaining harvest residuals is beneficial. The use of intensive vegetation control to improve Douglas-fir biomass and nutrition must be balanced with retaining soil base cations.

Highlights

The Long-Term Soil Productivity (LTSP) study has aimed to understand the effects of intensive forest management on soil productivity over time [1,2]
The experimental and intensive whole tree removal treatments applied at these three study sites consistently resulted in modestly lower Douglas-fir biomass and aboveground C and N pools relative to the more industry-standard bole only removal treatments
Despite the observed aboveground growth differences, soil and total site nutrient pools were minimally affected by the level of experimental biomass removal, even though the whole tree removals were far in excess of any normal operational procedures

Summary

Introduction

The Long-Term Soil Productivity (LTSP) study has aimed to understand the effects of intensive forest management on soil productivity over time [1,2]. Results from LTSP studies have shown that management effects on productivity are variable by region, treatment intensity, and time since harvest [3,4,5,6]. Forests 2020, 11, 1022 feed growing bioproduct markets yet the long-term effects on forest plantations are highly variable [4]. The short-term losses of nutrients through whole tree removals have resulted in long-term reductions in soil productivity especially during canopy closure when nutrient demands are greatest [4]. Whole tree harvesting has been linked to greater long-term cover of invasive species, such as Scotch broom (Cytisus scoparius (L.)

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