Phosphorus in accumulated harvest residues on skid trails☆
Introduction
Phosphorus is susceptible to mismanagement as it is scarce, an essential macronutrient, and is replenished slowly through weathering and limited atmospheric deposition (Walker and Syers, 1976, Newman, 1995, Föllmi, 1996). Given these properties, cycling of P between soil and biomass through litterfall and roots is necessary to maintain forest nutrient stocks (Attiwill and Adams, 1993, Fox et al., 2011). Harvesting interferes with this continuous exchange of P by removing biomass (Tiessen et al., 2011). Yet how much P is exported by a harvesting system depends not only on the quantity of biomass extracted, but also on the P concentrations of that biomass. Fractions such as branches, leaves, and roots are relatively rich in P, and thus harvesting systems that extract larger quantities of these fractions risk depleting a stand’s nutrient stock (Kimmins, 1977, Achat et al., 2015). Illustrative examples of this include depleted P stocks in agriculture (Flueck, 2009), intensive plantation forestry (Tiessen et al., 2011), and ‘whole-tree harvesting’ (WTH) spurred by ever increasing demand for woody biomass (Richter et al., 2009, Thiffault et al., 2011, Helmisaari et al., 2014).
However, economic efficiency and protection of soil and water also govern the selection of harvesting systems, not only yield and nutrient stocks. Mechanized felling and forwarding offer substantial gains in cost effectiveness yet risk critical damage to soils (Cambi et al., 2015). Such traffic is now increasingly restricted to skid trail networks used over multiple rotations (von Wilpert and Schäffer, 2006). In those systems, which are common in Central Europe, harvested stems are processed on the skid trail, thus creating semi-protective brash mats (Hutchings et al., 2002, Han et al., 2009) and maintaining an orderly work area. This accumulation of harvest residues concentrates nutrients — including P — on compacted skid trails.
To what extent this redistribution of P affects stand nutrient stocks is unknown. Wall (2008) reported that removing residues did not matter to soil P pools. Yanai (1998) also deemed the portion of stand P stocks in biomass fractions that constitute harvest residues insignificant in relation to total P in the mineral soil, yet found that P removal during harvesting was huge in relation to P stored in the forest floor. Likewise Laiho and Prescott (2004) showed that coarse woody debris could retain P for decades. And, curiously, shifts in soil P stocks following harvesting have been observed predominately in larger, ‘slowly-cycling’ pools (Richter et al., 2006). Even then, there are four possibilities as to what happens to the accumulated P: it is (1) not mineralized and retained in the residues, (2) mineralized and lost through runoff or leaching, (3) mineralized and immobilized in mineral soil, or (4) mineralized and taken up by vegetation.
Processing on skid trails could simulate systems where residues and P are either left on-site in the stand or moved off-site — retention and export of P, respectively. Understanding how concentrating harvest residues on skid trails affects P stocks lies in answering the following questions:
- 1.
How much P is redistributed to skid trails?
- 2.
What happens to that P?
In this study we address Questions 1 and 2 by (i) quantifying the P input from harvest residues and (ii) quantifying P stocks in a stand and skid trail’s forest floor and mineral soil. Changes within these pools of P have implications for harvest residue management.
Section snippets
Study site
Litter and soil samples originated from a one hectare, thirty year-old, planted silver fir (Abies alba Mill.) stand on the Schönberg, a 650 m foothill west of the Back Forest, Germany (7°47′ 44″ E, 47°56′ 30″ N). Though few in number, additional species in decreasing count include larch (Larix decidua Mill.), beech (Fagus sylvatica L.), oak (Quercus sp.), and spruce (Picea abies [L.] Karst.). Average yearly temperature is 9°C with 15–16 °C during the vegetation period, and yearly precipitation is
Biomass and phosphorus redistribution
From modeling, 27.6 kg ha−1 of P in 95,660 kg ha−1 of biomass was harvested in the 2009 thinning (Table 2). The largest compartment of harvested biomass was wood, but a majority of harvested P was in branches and needles.
An estimated 60.4% of harvested P was deposited on the skid trail; in comparison only 34.4% was exported and 5.3% was left in the stand (Table 2). The resulting post-harvest forest floor P stocks were 8.9 and 2.4 g m−2 on the skid trail and stand, respectively (includes pre-harvest P
Fate of phosphorus from skid trail harvest residues
As outlined in the introduction, the following scenarios are what can happen to P deposited on skid trails:
- 1.
P is not mineralized and retained in harvest residues.
- 2.
P is mineralized and lost through runoff or leaching.
- 3.
P is mineralized and strongly fixed in mineral soil.
- 4.
P is mineralized and taken up by vegetation.
Despite previous observations of P immobilization within the forest floor and woody debris for decades or longer (Qualls et al., 1991, Laiho and Prescott, 2004), Scenario 1 was not evident
Acknowledgments
We sincerely thank Kai Stüwe, Clara Wild, and Hannes Warlo for their field and laboratory work; Petra Grossman and Petra Wiedemer for their technical expertise; and the anonymous reviewers for their helpful comments. The University of Freiburg’s Gesellschaft zur Förderung der forst- und holzwirtschaftlichen Forschung funded this study; they had no other involvement.
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This article is part of a special feature entitled “The characteristics, impacts and management of forest fire in China”