Influence of mechanical site preparation on regeneration success of planted conifers in clearcuts in Fennoscandia – a review

Abstract

In the Nordic countries Finland, Norway and Sweden, the most common regeneration method is planting after clearcutting and, often, mechanical site preparation (MSP). The main focus of this study is to review quantitative effects that have been reported for the five main MSP methods in terms of survival and growth of manually planted coniferous seedlings of Norway spruce ( (L.) Karst.), Scots pine ( L.) and lodgepole pine ( var. Engelm.) in clearcuts in these three countries. Meta analyses are used to compare the effects of MSP methods to control areas where there was no MSP and identify any relationships with temperature sum and number of years after planting. In addition, the area of disturbed soil surface and the emergence of naturally regenerated seedlings are evaluated. The MSP methods considered are patch scarification, disc trenching, mounding, soil inversion and ploughing. Studies performed at sites with predominately mineral soils (with an organic topsoil no thicker than 0.30 m), in boreal, nemo-boreal and nemoral vegetation zones in the three Fenno-Scandinavian countries are included in the review. Data from 26 experimental and five survey studies in total were compiled and evaluated. The results show that survival rates of planted conifers at sites where seedlings are not strongly affected by pine weevil ( L.) are generally 80–90% after MSP, and 15–20 percent units higher than after planting in non-prepared sites. The experimental data indicated that soil inversion and potentially ploughing (few studies) give marginally greater rates than the other methods in this respect. The effects of MSP on survival seem to be independent of the temperature sum. Below 800 degree days, however, the reported survival rates are more variable. MSP generally results in trees 10–25% taller 10–15 years after planting compared to no MSP. The strength of the growth effect appears to be inversely related to the temperature sum. The compiled data may assist in the design, evaluation and comparison of possible regeneration chains, i.e. analyses of the efficiency and cost-effectiveness of multiple combinations of reforestation measures.Picea abiesPinus sylvestrisPinus contortalatifoliaHylobius abietis

Effects of spot mounding and inverting on growth of conifers, exposed mineral soil and natural birch regeneration

“…In inverted areas, there were 21% fewer naturally regenerated birches compared to spot mounded ones. Site preparation has been reported to increase the number of naturally regenerated trees compared to controls where no site preparation was done (Karlsson et al 2002;Johansson et al 2013;Sikström et al 2020). Compared to previous studies, our observation of 29 086 ha -1 of naturally regenerated birch on spot mounded areas can be considered notable.…”

“…Site preparation results in a better survival rate and increased growth of seedlings in Nordic forests (Örlander et al 1996;Hallsby and Örlander 2004;Saksa et al 2005;Kankaanhuhta et al 2009;Hjelm et al 2019;Sikström et al 2020) but also in boreal forests of North America (Bedford and Sutton 2000;Simard et al 2003;Thiffault et al 2010). Site preparation creates favorable growing conditions for planted seedlings and improves germination conditions of seeds by, for example, increased soil temperature, enhanced root growth and nutrient uptake, reduced competition from vegetation, and decreased damage risk caused by the pine weevil (Hylobius abietis (L.)) (Örlander et al 1996;Nilsson and Örlander 1999; de Chantal et al 2003;Nordborg et al 2003;Heiskanen and Rikala 2006;Norlander et al 2011;Heiskanen et al 2013;Mjöfors et al 2015).…”

“…Site preparation also exposes seedlings to frost heave in fine-textured soil causing deformation and upward thrust of the ground surface (Heiskanen et al 2013). Mineral soil exposed in site preparation improves the germination conditions of seeds, enhancing abundant natural regeneration of unwanted deciduous trees (Karlsson and Örlander 2000;Lehtosalo et al 2010;Sikström et al 2020). In planted conifer stands, this response often leads to a need for young stand management operations to ensure growing conditions for economically more valuable conifer trees by removing competing deciduous trees (Uotila et al 2012).…”

“…Intermittently working but continuously advancing site preparation machines, such as continuously advancing mounders, expose considerably less mineral soil, normally 30 to 40% of the soil surface. With excavator-based site preparation equipment, the site preparation track can be precisely directed and the area of exposed mineral soil surface can be much lower (15-25% of the soil surface) (Sikström et al 2020).…”

Resource availability and physiological response of black spruce to scarification in two climatic regions of Québec (Canada)

“…Scarifiers consist of motorized disks with multiple blades that rip and lift surface organic layers, thus exposing the mineral soil along trenches in which tree seedlings are planted. Positive effects of scarification on tree seedling growth have been ascribed to decreased competition from vegetation (Hébert et al 2014), increased soil temperature (Thiffault and Jobidon 2006;Sikström et al 2020), improved light availability for seedlings (Willis et al 2015), improved soil drainage (Rappe George et al 2017) and increased soil nutrient mineralization (Johansson 1994). In the province of Québec (Canada), scarification is commonly used to improve the regeneration of planted black spruce (Picea mariana (Mill.)…”