- Forest Vegetation Management (VM) is an important tool used in the Pacific Northwest (PNW) for reforestation. It has been well documented that VM increases seedling survival and crop tree volume growth. What is less understood, is how altering the plant community and successional trajectory affects the way the ecosystem uses and distributes nutrients in the long term. In this study, we investigate long-term effects of vegetation management on nutrient concentration and content of various tissues and ecosystem components of Douglas-fir, western hemlock, western redcedar, and grand fir growing in Oregon’s central Coast Range (CR) and Douglas-fir and western redcedar growing in Oregon’s Cascade mountain foothills (CF) under two contrasting VM treatments. This is the first study of its kind to investigate how VM affects distribution of several nutrients throughout both plant derived tissue and soil.
The two VM treatments represent operational extremes of VM regimes and consist of: Control, which received no herbicide application post planting, and VM, which received five years of spring release herbicide application. Both treatments include a fall site preparation herbicide application. The ecosystem was broken down into crop trees (separated into foliage, live branches, stembark, and stemwood), midstory species (separated into foliage and stem), understory, forest floor, fine roots, and mineral soil (with depth increments 0.0-0.2 m, 0.2-0.4 m, 0.4-0.6 m, and 0.6-1.0 m). Samples of crop tree, understory, forest floor, and soil from 0.0-0.2 m samples were taken during the 16th and 17th growing season (the CF site was planted one year later). Midstory and remaining mineral soil samples were collected during the 18th and 19th growing season.
All samples were analyzed for concentration of carbon (C), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), boron (B), copper (Cu), iron (Fe), manganese (Mn), sodium (Na), and zinc (Zn). Concentrations of CNS for all samples was determined by dry combustion. The remaining nutrients were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) from dry ash extractions (for plant tissue) and nitric acid microwave digestion (for determination of total soil nutrients). These concentrations were scaled to tissue content using empirically derived biomass equations from Gonzalez-Benecke et al. (2018) and updated biomass measurements taken at year 16 by Flamenco et al. (2019).
VM effects on tissue concentration varied by nutrient, overstory species (species), tissue, and site. Forest floor and crop tree stembark, followed by fine roots, were the tissue types that showed the greatest number of treatment effects (greatest number of nutrients affected) across all species. Soil concentrations were generally unaffected by treatment, except for surface soil concentrations of Mg, and Ca (which were only significant for certain species) and deep soil carbon and N (which was only detected for Douglas-fir at the CR site). All detectable concentration differences between treatments showed higher concentrations in VM plots. The exception to this trend was soil N for WRC at the CR site which had significantly lower concentrations in VM plots for the 0.2-0.4 and 0.4-0.6 depth increments. Soil concentrations showed much greater variation between sites than between species or treatments.
VM effects on total plant derived nutrients masses were more prominent than differences in concentrations. Ca was the only nutrient for which all species showed higher plant derived masses in the VM condition. Plant derived tissue content of, C, Cu, P, and B, all tended to be higher in VM plots, with the exception of western redcedar plots at the CR site. This case was an outlier due to the fact that Control plots developed significantly more biomass due to high midstory biomass, whereas the VM plots developed relatively little midstory and crop tree biomass. There were few differences in soil content between species and treatments, and those that were significant were unable to be explained by differences in uptake by plant species. Notably, total soil N of WRC at the CR site, however was significantly lower for VM plots. This may indicate the potential for VM applied to a slow growing species, such as WRC, to reduce ecosystem retention of N. With the exception of C and N, total soil nutrient reserves were orders of magnitude greater than total plant derived masses. This indicated that there is low probability of an adverse effect of VM on soil nutrient stores.
Treatment effects on foliar nutrition varied by site and species, though crop trees at the CF site tended to have diluted concentrations of P and K and increases in Ca, Mg, and Fe in the VM treatment. Nutrient use efficiency (NUE) for the different stands was computed from ratios between the mass of all plant derived carbon and all other plant derived nutrients. VM significantly increased the NUE of N, P, Mg, S, and Cu across all species. When the NUE was calculated with only the carbon stored in crop tree stemwood, VM increased the NUE of all nutrients.
The results of this analysis indicate that sustained VM during the first five years of stand establishment affected nutrient content of various pools more than concentration, though both tissue concentration and content vary more strongly by site and species. While total plant derived masses of Ca, C, Cu, P, and B all tended to be higher in VM plots, trends varied greatly by nutrient, site, and species. Overstory species exert control over the nutrient requirements of the ecosystem, but VM does tend to increase the NUE, especially with respect to N, P, Mg, S, and Cu. Stands growing under sustained competing vegetation control did tend to produce more harvestable and plant-derived carbon per unit nutrient fixed in plant tissues, improving the efficiency of nutrient use for stands that are being managed for carbon sequestration as well as for timber harvest. While total soil reserves were generally unaffected by VM treatment and are unlikely to be adversely affected by VM in the long term, it is possible that VM can reduce soil N retention for slow growing species like WRC. Managers can use this information to make better decisions about site preparation treatments.