Metsätalouden vesistövaikutukset

Koeta Kurki miettiä asiaa yhden vesistön valuma-alueen kannalta. Muuttuuko näkymä yhtään?

Kurki ei uskalla lukea Härkönen ym. 2023 juttua, kun joutuisi mahdollisesti muuttamaan käsityksiään. Haaste on heitetty.

Härkönen ym. 2023 on erittäin hyvä yhteenveto hiilikysymyksistä, suosittelen luettavaksi.

Kts.

Fig. 2. Processes behind organic carbon and iron leaching in traditional, even-aged peatland forest management
Table 1. Impacts of peatland forestry practices on organic carbon (OC) and iron (Fe) loading to surface waters.

Maailma muuttuu, ja tummentavan kuorman määrät ovat kasvussa. Ilmiössä on paikallisen vaihtelun ohella ajallinen vaihtelu. Vähälumisen talven jälkeen kuorma on pienempi. Lauhana talvena kuormaa voi valua metsistä ja pelloilta läpi vuoden. Jos ymmärrän oikean suuntaisesti, orgaanisen hiilen kuorman kannalta sekä paksun turvekerroksen kuivilla olo että veden pinnan vaihtelu suolla on haitaksi.

Vesistökuormitukseen kai pätee se periaate, että asetetaan tavoitteet, mutta jätetään auki millä keinoilla tavoitteeseen pitää päästä. Paikasta riippuen käypiä keinoja voivat olla: sekametsät, ennallistaminen, leveät poimintahakattavat vesistöjen suojakaistat, metsänhoidon suositusten perinteiset vesiensuojelun rakenteet, jatkuva kasvatus ja veden kulun hidastaminen suolla.

Härkönen et al. 2023. (Viitteet poistettu tekstistä. OC = organic carbon = orgaaninen hiili = eliöissä kuten kasveissa tai kasvinjäänteissä tai humuksessa sitoutuneena oleva hiili; CCF = continuous cover forestry = jatkuvapeitteinen kasvatus; WTD = water table depth = vesipinnan syvyys suolla; DNM = ditch network maintenance = kunnostusojitus.)

Sivu 5:

Together with the increased hydrological connectivity, the increased tree biomass associated with drainage appears as important. Drainage has significantly increased tree growth in boreal coniferous forests with both peat and waterlogged mineral soils by improving aeration in the rooting zone if other factors, such as nutrients, are not limiting. An increased forest biomass may increase OC export by increasing litter input into the soil. In forest litter, the share of labile DOC can exceed 60%, which by comparison is less than 5% in peat, and this may partly explain the increased OC leaching from drained peatlands converted into forests. OC export may also be enhanced due to increasing tree stand evapotranspiration and the subsequent thickness of the oxidized peat layer in old, drained forests. While leaching is amplified in afforested, drained peatlands, also the presence and increased abundance of conifer forests seems to increase the OC and Fe yield. High OC and Fe concentrations are most frequent in freshwaters where coniferous tree cover dominates the land area.

Also, the way of treating peatland forest biomass matters. In Finland, drained peatlands account for nearly 25 % of the total forest growth. The importance of peatland forests in timber production will be pronounced during the next decades, as the fellings will be increasingly targeted to their maturing stands. Forest harvesting increases nutrient and OC export by decreasing the WTD and increasing oxygen deficiency, by enhancing microbial activity in the upper soil layers, and by enhancing soil temperatures due to increased site exposure to solar radiation. Conifers have a significant interception capacity for snowfall, and intensive tree removal may promote the accumulation of snow in the clear-cut area, consequently increasing runoff and associated OC export.

The effects of clear-cutting and subsequent soil harrowing on OC loading may be negligible in boreal, organo-mineral coniferous forests, since the podzolic soil horizon effectively retains OC despite its increased leaching. In peatland forests, final felling clearly and significantly increases OC export to surface waters from peatland forests for several years. The negative impacts are pronounced on fertile minerotrophic peatland forest sites with high microbial activity. Exports of both Fe and OC from minerotrophic sites may especially increase after harvesting due to the decreasing WTD and subsequent anoxia. In addition to increased OC export, clear-cutting can further increase Hg export to streams. Although buffer areas along the water courses are usually required during harvest operations, the buffer widths are often left too narrow (less than 15 m), and there can be marks left by machine tracks, stream crossings, drainage ditches and soil preparation consequently hindering their efficiency in water protection.

Sivu 9:

In terms of preventing excess OC leaching, management practices that prevent future peat decomposition (hot spots) and diminish hydrological connectivity, especially during high runoff (hot moments), are essential. Although the long-term impacts and processes related to the untraditional forestry practices such as CCF and favouring mixed forests still require additional research, it appears that both the naturalization of practices and reducing artificial hydrological connectivity between the landscape and water bodies would be beneficial in counteracting future freshwater ecosystem browning.