Introduction
There are only few effective ways to increase carbon (C) sequestration in forests. One is reclamation of previously nonforested areas for timber production by afforestation (
Vilén et al. 2016;
Menard et al. 2023). Another is to improve the growth of low-productive forests in a permanent manner by, e.g., using genetically improved material in forest regeneration (
Serrano-Leon et al. 2021) or using fertilizers to increase the nutrient availability for trees.
Concerning the latter option, nitrogen (N) fertilization has resulted in increased tree stand C sequestration in experimental studies (
Shryock et al. 2014;
Jörgensen et al. 2021). However, because the response to N fertilization lasts less than 10 years, repeated treatments are needed to enhance long-term tree growth significantly. More permanent improvement in timber productivity can be achieved with fertilization treatment resulting in Type II growth response as defined by
Snowdon (2002). Type II or long-term (i.e., 20–35 years) growth responses have been reported in different studies after phosphorus (P) fertilization in P-deficient soils (
Pritchett and Comerford 1982;
Snowdon 2002;
Fox et al. 2006;
Trichet et al. 2009). Comparable decades long (30–60 years) responses have also been found in drained N-rich peatland forests after P and potassium (K) or wood ash fertilization (
Moilanen et al. 2002;
Hökkä et al. 2012). Several studies have shown that stores of mineral nutrients, such as P, K, and boron (B) are low in peat, especially in relation to the N stores (e.g.,
Kaunisto and Paavilainen 1988;
Laiho and Laine 1995;
Kaunisto and Moilanen 1998). Peat N content can vary among different sites, mainly depending on site fertility and the peat degree of humification. Because nutrients are released from peat through decomposition, besides peat nutrient content, decomposition rate affects nutrient availability for tree growth (
Charman 2002).
Wood ash is used as a forest fertilizer and it contains all the nutrients that trees need to grow, e.g., P, K, B, and calcium (Ca), except N. In N-rich drained peatland sites, such as type II
Vaccinium-vitis idaea- and
Vaccinium myrtillus-sites (classification according to
Laine et al. 2012), growth responses to commercial PK fertilizer or wood ash have been shown significant and long-lasting (
Moilanen 1993;
Hökkä et al. 2012;
Moilanen et al. 2015). In these sites the surface peat is commonly well humified and the quantity of N in a 20 cm top peat layer can be 50–100-fold compared to that of K and 30-fold compared to that of P on hectare basis (
Kaunisto and Paavilainen 1988;
Westman and Laiho 2003). The drawback to the use of fertilization to improve tree growth and consequent C storage in N-rich peatland sites is the increased decomposition rate in peat soil and consequent increased CO
2 production (
Moilanen et al. 2012;
Ojanen et. al 2019).
In N-poor peatland sites, the surface peat usually consists of poorly humified
Sphagnum peat, in which the amount of plant available N is low. Many studies have reported that in N-poor drained peatland sites (Dwarf shrub- and
Cladonia-sites;
Laine et al. 2012) PK or NPK fertilization result in only a modest tree growth increase during the first 10–20 years (e.g.,
Moilanen and Issakainen 1990;
Silfverberg and Moilanen 2008). The response of tree growth to N alone is also known to be minor and short-lived (e.g.,
Hökkä et al. 2012). It has been assumed that in N-poor sites the availability of K in relation to N is more balanced, and thus PK fertilization treatment may not significantly impact stand nutrition and growth (
Silfverberg and Moilanen 2008). Based on these results application of PK fertilizers in N-poor drained peatlands has not been recommended.
However, some fertilization studies have proved that in poor drained peatland sites PK fertilization and ash fertilization may also result in significant growth responses. For instance, tentative findings of
Veijalainen (2000) showed a growth increase of 1.5–2.0 m
3·ha
−1·a
−1 over a 20-year period in southern Finland poor pine peatland sites due to PK fertilization.
Sikström et al. (2010) reported that adding wood ash (2.5 t·ha
−1) or PK fertilizer increased pine growth 1.6–1.9 m
3·ha
−1·a
−1 over 26 years in a poor oligotrophic peatland in southern Sweden.
Ernfors et al. (2010) reported a significant basal area growth response of a pine stand to 6.6 t·ha
−1 dose of wood ash in 5 years on a southern Swedish pine bog. Also,
Hytönen and Hökkä (2020) found significant increase in volume growth of a pine sapling stand over a 15-year period following ash fertilization (5 t·ha
−1) in a poor bog in Central Finland.
These findings suggest that even in N-poor drained sites the peat N availability can be high enough to enable a significant growth response to added PK or wood ash at least in the long-term. Although the growth response appears to be smaller than in N-rich sites, the possibility that poor pine peatlands may also benefit from ash fertilization offers a significant possibility to increase forest C sequestration in these forests, where the soil CO
2 emissions are found to be negligible or negative (i.e., soil is a C sink) even when drained (
Minkkinen et al. 2018). Results of
Ojanen et al. (2019) also show that application of fertilizer on poor drained peatland site still maintains the soil as a CO
2 sink, which contrasts with the observed change of the N-rich sites to a CO
2 source. In Finland, ash fertilization of drained peatlands has been considered as one of the most important and cost-effective ways to increase the C sinks of land use, land use change, and forestry-sector (
Lehtonen et al. 2021). So far, the national fertilization recommendations and forestry policies (e.g., government subsidies for ash fertilization) have directed fertilization efforts to N-rich sites (fertility level
Vaccinium-vitis idaea or higher). Extending ash fertilization to N-poor drained sites would increase the potential fertilization area by ca. 718 000 ha (
Korhonen et al. 2017). To our knowledge, no studies have been conducted on the financial performance of fertilization of drained poor peatlands. The C sequestration achieved via ash fertilization of these low-productive forests can be considered truly additional, which could allow their use as C offset.
The aim of this study was to investigate the long-term response of drained poor Scots pine peatland sites to ash fertilization in terms of timber production, financial performance, and stand nutrition. The data were based on field experiments followed for several decades after fertilization treatments.
Discussion
The results showed that in N-poor Scots pine peatlands fertilization with wood ash increases average MAI and volume yield to more than two-fold when compared to the nonfertilized control plots in the long-term (approximately 40 years). This is a new result, contradicting the previous understanding that insufficient availability of N in nutrient-poor peatland sites is limiting the response of trees to ash fertilization. Our findings indicate a distinct growth response to ash in all sites of our data.
Some sites (Sievi, Valtimo, and Leivonmäki) represent so poor peatlands that drainage merely cannot increase their growth sufficiently to lift them to forest land category, i.e., according to Finnish definition, their average MAI in 100 years rotation remains below 1 m3·ha−1·a−1. In Vilppula, Pelso, and Lestijärvi, however, also the nonfertilized stands were forest land (MAI varied between 1.2 and 4.6 m3·ha−1·a−1). Nevertheless, ash fertilization appeared to increase timber productivity of such poor sites significantly.
Testing the differences in MAI and volume yield between control and fertilized plots in each site separately indicated significant differences in three of the six sites. In the combined data the effect of ash treatment became clearer and the mixed ANOVA models showed that both MAI and total yield were significantly higher in ash-fertilized plots that in the control plots. Also, the amount of K in the ash had a significant impact on both.
In N-rich pine peatlands, the average long-term growth response to ash application has varied between 1.0 and 3.0 m
3·ha
−1·a
−1 for 35–40 years (
Hökkä et al. 2012;
Moilanen et al. 2015). Much higher responses (up to 6.5 m
3·ha
−1·a
−1;
Moilanen et al. 2002), depending on the ash dose and peat properties, have been reported also. However, a clear difference can be observed between N-rich and N-poor sites in the rate of the response. Applying ash or PK on N-rich peat soils, the maximum growth response was achieved in 10–15 years (
Moilanen 1993;
Hökkä et al. 2012;
Moilanen et al. 2012). The nonlinear mixed effect model (
Table 6) showed that in these data the average annual volume yield developed at a much slower rate, i.e., at 20 years since fertilization the additional average volume yield was 1.5 m
3·ha
−1·a
−1, assuming equal initial volumes. However, the average increase in MAI at 40-year time point was 2.5 m
3·ha
−1·a
−1. This is comparable to the mean growth response previously found in N-rich sites but achieved within a much longer time. Longer response time of poor oligotrophic sites was also found by
Hökkä et al. (2012).
The more specific model of
Table 7 showed that the amount of K in the ash explained the Asym and xmid, and as a result, both the Asym and the form of the curve were different if different doses of K were applied, thus making it possible to capture a large part of the variability of the responses among plots and sites (
Fig. 3). Still, there was significant residual variation not explained by the model. We know that ditching intensity has only a marginal impact on stand growth when compared to, e.g., the effect of fertilization (
Ahtikoski and Hökkä 2019). Also, the sites were considered rather homogeneous in terms of timber productivity, all belonging into scrub land or low-productive forest land category even when drained. No trend was found in model residuals with respect to these site quality classes. As all the sites were located within a narrow climatic range, the geographical location (temperature sum) did not explain the response.
The used ashes varied in terms of their nutrient contents from very poor to very rich.
Moilanen et al. (2005) expressed the growth response of Scots pine stands to ash fertilization on N-rich drained peatlands in relation to the given amount of P, but as shown in their results, P and K were almost equally correlated with growth. In our data the volume yield correlated slightly higher with K (0.34) than for P (0.33). According to the previous studies, K is the most growth limiting nutrient in drained peatlands (
Laurén et al. 2021). After PK fertilization, a relatively low amount of P (40–45 kg·ha
−1) is sufficient to sustain sufficient foliage P concentrations for decades, while K concentrations tend to decrease after 20 years (
Silfverberg and Moilanen 2008). That was also shown in our data, where the foliar P concentration was still satisfactory with the lowest given dose (16–50 kg·ha
−1) (
Fig. 6). As a water-soluble nutrient, K may leach from the ash easier than P (e.g.,
Callesen et al. 2017) and become limiting again. Thus, we assumed that beyond this minimum amount of P given in fertilizer, the quantity of K in the ash becomes important and affects the magnitude of the response.
Figure 3 showed that with poor ash, i.e., ash with low K concentration, the volume development was not much better than that of the nonfertilized control plots. Accordingly, high ash dose did not compensate for poor quality (low nutrient content;
Fig. 4).
Granulated ash was used in Sievi and in Pelso, while in the other experiments loose wood ash was used. Currently, granulated ash is almost entirely available for ash fertilization. Based on results of
Hytönen and Hökkä (2020), the form of ash will influence the growth response time: with loose ash the response is several years quicker. Except the Lestijärvi site, the nutrient amounts applied in these data were also rather high. Because of these reasons, responses slower (and lower financial outcome) than those found in this study can be expected in practice if granulated ash is applied. It is, however, possible that the time ash promotes growth will be longer because nutrients will be released at slower pace from granulated ash (
Callesen et al. 2017;
Hytönen and Hökkä 2020).
In general, the foliar nutrient concentrations of the fertilized plots were at a satisfactory level, considering the long time elapsed since fertilization and on average, the concentrations of K and P were mostly higher in the fertilized stands than in the nonfertilized controls. Despite the fact that significant differences still appeared in the concentrations of P, the effect of fertilization on the trees’ nutrient status has mostly vanished. Based on the very low K and P concentrations in needles, there was a need of re-fertilization in Lestijärvi and Leivonmäki sites (
Fig. 4b). In Lestijärvi the low foliar concentrations can be explained by the use of low-quality mixed ash in which the given quantities of K and P remained clearly below the current recommendations for ash fertilization (
Table 2). Although these were nutrient-poor sites, the availability of N on control plots was good in Vilppula and very good in Leivonmäki (c.f.
Moilanen et al. 2010).
The financial performance related to ash fertilization demonstrated to be promising: the average of the highest break-even fertilization cost among the study locations was 902 € ha
−1 (interest rate 5%), which is more than a double of the current average fertilization cost, 388 € ha
−1. In other words, ash fertilization tends to be financially lucrative. This result coheres with earlier studies (
Rantala and Moilanen 1993;
Moilanen et al. 2015) on the profitability of fertilization on peatland forests. A recent study (
Ahtikoski and Hökkä 2019) showed that fertilization was more important single factor for overall profitability than ditch network maintenance (DNM) on peatland forests. Further, the relative importance of fertilization (compared to DNM) increased toward North (
Ahtikoski and Hökkä 2019).
The data are exceptional because the study period was distinctively long, up to 60–85 years of monitoring after ash application. The clearly shorter follow-up periods (5–15 years) in
Moilanen and Issakainen (1990) and
Moilanen (1993) studies may be the main reason for the previous conclusion that the growth response to fertilization was modest in N-poor sites. The data of this study did not show the stand development over the whole rotation either. There were only few observations from stands approaching maturity, which in fact may have affected the estimate of Asym. Only in Vilppula and Valtimo, the fertilized plots were mature in terms of basal area weighted mean DBH (23.0–28.8 cm) based on forest management recommendations (
Äijälä et al. 2019). The other stands were thinning stands (mean diameter 13.0–18.2 cm) and could be grown for 10–30 years more before maturity. Of the nonfertilized controls, only Vilppula control plot was mature (mean diameter 26.0 cm). In other control plots it is unlikely that the mean diameter limit for regeneration maturity will be reached. It is thus expected that the difference in mean MAI and volume and saw log yields will increase further until all the fertilized plots have reached maturity.
This study was not able to explain all the factors behind the rather good, long-lasting growth responses to ash drained of N-poor pine peatlands. The quality of ash, i.e., K amount appeared to be one important factor. It is likely that improved N availability, induced by the long-term increase in peat decomposition rate also plays a significant role. Firstly, drainage itself increases peat decomposition, as the water level drawdown decreases anoxia in the soil. All the studied sites have been drained for forestry for a long time, ranging from 46 years (Lestijärvi) to 113 years (Vilppula). Additionally, the subsidence and compaction of the peat layer bring the nutrient stores of deeper peat layers accessible to the trees, both making poor drained sites generally more suitable for tree growth.
Moilanen et al. (2010) pointed out that N deficiency, when defined from foliar nutrient samples, is becoming less common also in N-poor drained peatlands.
Laiho and Laine (1995) observed an increase in peat N and P stores of N-poor sites with increasing time since drainage. Since peat decomposition rate is highly depended on temperature, the warmer and longer growing seasons may accelerate it even further (
Updegraff et al. 2001).
Moilanen et al. (2010) found that in peatland Scots pine stands higher foliar N concentrations were related to higher temperature sum. Decomposition processes, driven by microbial activity in the soil, are also enhanced by the added Ca, which increases soil pH (
Moilanen et al. 2002;
Peltoniemi et al. 2016), which in turn release more N for vegetation. Concurrently, the foliar N was higher in ash-fertilized plots in three study sites (Pelso, Lestijärvi, and Valtimo), indicating increased N availability. On the other hand, some studies have shown that also commercial PK fertilizers have clearly enhanced tree growth in poor peatlands (
Veijalainen 2000;
Sikström et al. 2010). This result was supported by the Lestijärvi site of this study, where PK fertilization has significantly improved stand growth (data not shown) suggesting that addition of Ca may not be a crucial requirement for enhanced growth.
The data were not enough for building representative yield equations, but sufficient to visualize and quantify the average change in long-term timber productivity resulting from ash fertilization through the nonlinear mixed effect modelling. Our results showed that ash fertilization clearly increases timber production in N-poor drained peatlands, that with a higher amount of K in the ash, a quicker and greater response can be achieved, and that the increase in timber production is comparable to that observed in N-rich peatland sites but takes place at a much slower rate. Nevertheless, the growth response was sufficient to make these fertilization investments financially feasible in most of the studied sites.
The observed growth response may offer a valuable tool for additional C sequestration from the atmosphere into the growing tree stock and therefore aiding climate mitigation efforts. Since fertilization increases tree growth, and thus C storage in living trees, it also creates an opportunity to gain revenues by sequestering C in forests. If there was a price mechanism for private forest owners to receive payment for sequestered C, this would increase forestry revenues as well (see
Assmuth et al. 2018;
Fig. 5) Currently, such a mechanism for private forest owners does not exist in Europe (cf. The European Union Emissions Trading System, EU ETS “gap and trade” system that excludes private forest owners).
However, in considering whether ash fertilization has potential for climate change mitigation, the net effect of ash fertilization on ecosystem C balance needs to be accounted for. Further, the C balance must be compared against a reference system, i.e., unfertilized, drained peatland forests. In drained peatland forests the net CO
2 emissions from the soil due to loss of peat are often significant. Previous research has shown that even after drainage, most peatland forests continue to act as C sinks because of the enhanced tree growth (
Ojanen et al. 2013). In N-poor sites, tree growth is slow, but the forests remain a C sink due to low or negligible CO
2-emissions from the soil (
Minkkinen et al. 2018). Conversely, in N-rich sites, the C sink is driven by high level of tree growth, outweighing substantial soil CO
2 emissions. The concern is whether ash fertilization changes the ecosystem function in such a way that the N-poor sites, similarly to N-rich sites, become a net source of CO
2 from the soil. Consequently, although ash fertilization shows significant promising prospects for climate change mitigation, a degree of uncertainty remains regarding its potential long-term effects on the soil processes.