Raija Laiho

Humus in northern forests: friend or foe?

Abstract Organic matter is of primary importance to the sustainability of long-term site productivity in forest ecosystems. In boreal forests, organic matter accumulates at the surface as mor humus. This may represent a substantial portion of the total nutrient capital of a site, and its decomposition is essential for the short-term availability of nutrients for tree growth and long-term site fertility. However, organic matter accumulation at the soil surface can also effect the forest ecosystem by immobilizing nutrients making them unavailable for plant uptake, and by creating physical and environmental conditions that can impede seedling establishment and survival. Therefore, it is necessary to understand the processes of humus formation and decomposition in order to manage these soils in a manner that will maintain or improve site productivity. This paper provides an overview of (i) the composition of humus, (ii) the conditions in the boreal forest that result in the surface accumulation of humus, (iii) decomposition processes and (iv) the effects of humus on nutrient (especially nitrogen) availability. Questions relating to the detrimental role of surface organic matter accumulation, the effects of natural disturbances (e.g., fire) and harvesting disturbances on humus loss and accumulation and management practices that can maintain long-term site productivity will also be discussed.

Changes in root biomass after water‐level drawdown on pine mires in southern Finland

Changes in living and dead root biomass were monitored in the succession created by drainage for forestry on six sites from undrained to 55‐yr‐old drained tall‐sedge pine mires in southern Finland. This was done to evaluate the role of root biomass in carbon cycling in drained peatland ecosystems. The total living Scots pine (Pinus sylvestris L.) root and stump biomass varied from 190 g m2 on an undrained site to 3060 g m2 on the oldest drained site. The pine fine root biomass reached the maximum level within 22 yrs of drainage with a pine stand volume of 81 m3 ha‐1. The pine coarse root and stump biomass increased with the age of the stand, comprising 90% of the total living root biomass on the 55‐yr‐old drained site. The drainage also increased the root biomass of field layer species; however, the roots of sedges were replaced by those of shrubs and trees. The dead fine root biomass was smallest on the two oldest drained sites, which could have resulted from a higher decomposition rate after improved ae...

Tree stand biomass and carbon content in an age sequence of drained pine mires in southern Finland

Abstract Biomass and carbon accumulation into tree stand and distribution between tree stand components was studied in two undrained and four drained Scots pine ( Pinus sylvestris L.) dominated peatland stands in southern Finland. On the drained sites, the amount and distribution of biomass above-ground was rather similar to pine-dominated stands on upland sites when drainage age of the site was thought to represent the stand age. The proportion of estimated below-ground biomass of the total pine biomass was ca. 30% on all sites studied which is more than on upland sites with supposedly similar growth potential. Due to the bigger amount of below-ground biomass, there is, on average, more biomass and thus also carbon in relation to stem volume in peatland stands than upland stands, when southern boreal Scots pine stands are examined. Equations for estimating the amount of carbon accumulating in the tree stand along with increasing stem volume are presented.

The effect of forestry drainage on vertical distributions of major plant nutrients in peat soils

Vertical distributions of total N, P, K, Ca and Mg in a 0–60 cm surface peat layer were studied at 80 pine mire sites in southern Finland. The sites fell into two categories according to the soil nutrient regime: Meso-oligotrophic and oligo-ombrotrophic, and formed a chronosequence from undrained sites to sites drained 55 years ago. A statistically significant drainage age effect on the gravimetric (mg g-1) concentration profile forms was detected for all nutrients except K. In oligo-ombrotrophic sites the concentration of N increased following drainage in the topmost layer (0–10 cm) and that of P in all layers. In meso-oligotrophic sites the changes in N and P profiles were obscure. The concentration profiles of K remained clearly surface-enriched in both site type groups, but there was a general drop in the concentration values immediately after drainage. Ca and Mg decreased, especially in the 10–20 and 25–35 cm layers in both site type groups. The volumetric (kg m-3) nutrient concentrations clearly reflected the increase in the bulk density of the surface peat occurring after drainage. The compaction of peat had compensated for the effect of the processes removing nutrients from the soil (increased tree stand uptake, leaching); for Ca and Mg to a lesser degree than for the other nutrients. It was concluded that the N, P and K profiles did not show changes that would be likely to affect site productivity, whereas the net loss of Ca and Mg may cause problems in the longer term. As the total K capital of the sites was in general rather small, a disturbance in the biological cycle, such as cutting of the tree stand, may be critical.

Impacts of intensive forestry on early rotation trends in site carbon pools in the southeastern US

Abstract The effects of different silvicultural practices on site, especially soil, carbon (C) pools are still poorly known. We studied changes in site C pools during the first 5 years following harvesting and conversion of two extensively managed pine-hardwood stands to intensively managed loblolly pine plantations. One study site was located on the lower Atlantic Coastal Plain in North Carolina (NC) and another on the Gulf Coastal Plain in Louisiana (La). Four different harvesting-disturbance regimes were applied: stem only harvest (SO), whole tree harvest (WT), whole tree harvest with forest floor removal (WTFF), and full amelioration, i.e. whole tree harvest, disking, bedding and fertilization (FA; only in NC). Each harvesting-disturbance regime plot was split and one-half received annual herbicide treatments while the other half received no herbicide treatments. In NC, soil C decreased slightly with WT, and increased with FA, otherwise no significant changes were detected. In La, there was a consistent decrease in soil C content from the pre-harvest value in all cases where herbicides were applied. All treatments caused a reduction in the forest floor C pool in NC. In La, the most intensive treatments also resulted in a decrease in the forest floor C, but to a smaller extent. In contrast, there was no net change in forest floor C with the SO and WT treatments, even though significant amounts of logging slash were added to the forest floor at harvest in the SO plots and not in the WT. Herbicide treatment clearly decreased the C pool of hardwoods and understory, and more than doubled that of planted pines. Carbon accumulation in the planted pines was similar for trees growing in the SO, WT, and WTFF treatments on both the LA and NC sites. The full amelioration treatment (only applied at the NC site) led to a significant increase in C sequestration by the planted pine component. Due to a large amount of voluntary pines, total 5-year pine C pool was highest on the non-herbicided intensive management plots on the NC site, however. The differing response patterns of soil and forest floor C pools between the two sites may be due to their differing drainage-summer rainfall regimes. Our results suggest that while poor drainage-wet summer conditions may be impeding carbon loss from the soil component it may be accelerating the rate of decomposition of the forest floor and slash on the soil surface.

Litter quality and its response to water level drawdown in boreal peatlands at plant species and community level.

Changes in the structure of plant communities may have much more impact on ecosystem carbon (C) cycling than any phenotypic responses to environmental changes. We studied these impacts via the response of plant litter quality, at the level of species and community, to persistent water-level (WL) drawdown in peatlands. We studied three sites with different nutrient regimes, and water-level manipulations at two time scales. The parameters used to characterize litter quality included extractable substances, cellulose, holocellulose, composition of hemicellulose (neutral sugars, uronic acids), Klason lignin, CuO oxidation phenolic products, and concentrations of C and several nutrients. The litters formed four chemically distinct groups: non-graminoid foliar litters, graminoids, mosses and woody litters. Direct effects of WL drawdown on litter quality at the species level were overruled by indirect effects via changes in litter type composition. The pristine conditions were characterized by Sphagnum moss and graminoid litters. Short-term (years) responses of the litter inputs to WL drawdown were small. In long-term (decades), total litter inputs increased, due to increased tree litter inputs. Simultaneously, the litter type composition and its chemical quality at the community level greatly changed. The changes that we documented will strongly affect soil properties and C cycle of peatlands.

Nitrogen and phosphorus stores in Peatlands drained for forestry in Finland

Nitrogen and phosphorus concentrations were determined for surface peat samples for 78 sites on both undrained and drained pine mires. The oldest areas had been drained 55 years earlier. Generally, the N and P stores in a 0–50 cm peat layer increased with increasing drainage age in oligo‐ombrotrophic sites, and remained unchanged in meso‐oligotrophic sites, even if large quantities were bound up in the increasing tree stand biomasses. This was mainly caused by post‐drainage subsidence of the mire surface and the consequent compaction, which increased the peat bulk densities and brought new stores from deeper peat layers into the layer under observation. Measured stand volumes and published values for N and P concentrations of tree stand compartments were used to estimate the amounts of N and P bound up in the tree stands. The estimates of N and P in the tree stands of the oldest drainage areas were considerable (ca. 400 kg N and 40 kg P ha‐1 in the meso‐oligotrophic and ca. 300 kg N and 30 kg P ha‐1 in th...

Actual state of European wetlands and their possible future in the context of global climate change

The present area of European wetlands is only a fraction of their area before the start of large-scale human colonization of Europe. Many European wetlands have been exploited and managed for various purposes. Large wetland areas have been drained and reclaimed mainly for agriculture and establishment of human settlements. These threats to European wetlands persist. The main responses of European wetlands to ongoing climate change will vary according to wetland type and geographical location. Sea level rise will probably be the decisive factor affecting coastal wetlands, especially along the Atlantic coast. In the boreal part of Europe, increased temperatures will probably lead to increased annual evapotranspiration and lower organic matter accumulation in soil. The role of vast boreal wetlands as carbon sinks may thus be suppressed. In central and western Europe, the risk of floods may support the political will for ecosystem-unfriendly flood defence measures, which may threaten the hydrology of existing wetlands. Southern Europe will probably suffer most from water shortage, which may strengthen the competition for water resources between agriculture, industry and settlements on the one hand and nature conservancy, including wetland conservation, on the other.

Changes in mesofauna abundance in peat soils drained for forestry

Abstract Soil mesofauna community structure was studied in a drainage-succession continuum of peatland sites supporting Scots pine (Pinus sylvestris L.) dominated stands in southern Finland. The numbers of Enchytraeidae, Collembola, Oribatida, Mesostigmata and Prostigmata in an 8-cm-deep surface peat layer were compared among a pristine undrained pine mire site, comparable sites drained for forestry 12, 26 and 60 years earlier, and a 42-year-old drained site re-wetted two years earlier. Drainage and the consequent water-level draw-down clearly increased the numbers of all soil animals studied, the numbers showing highly significant correlation with the water-table level of the sites. On the older drained sites, the populations were generally about ten times higher than on the undrained site; for Collembola, almost 100 times higher. After re-wetting, the numbers dropped abruptly, falling between those of the undrained and the youngest drained site. Oribatida were the most frequently found animals on the sites studied: on the undrained site almost 60% of the total number were these mites, the relative proportion somewhat decreasing along the drainage succession. The proportion of Enchytraeidae also decreased slightly after drainage, whereas that of Collembola clearly increased. Most of the soil fauna was found close to the soil surface. In general, >80% of Collembola, Oribatida, Mesostigmata and Prostigmata, and >60% of Enchytraeidae were found in the topmost 4-cm layer. In the course of the drainage succession, the soil fauna community structure became more similar to that of upland sites with similar tree-stand growth potential.

Effects of water level and nutrients on spatial distribution of soil mesofauna in peatlands drained for forestry in Finland

We investigated the within-site distribution of Enchytraeidae, Collembola, Oribatida, Mesostigmata and Prostigmata relative to varying water level and substrate quality on pine mire sites forming a drainage succession continuum. Collembolans were most intolerant of wetness, favoring drier locations at all stages of the drainage succession. In general, the effect of water level variation on the within-site distribution of the soil fauna was strongest when the site was at an early stage of either progressive or regressive water level change. When the average water level was below 20 cm, it no longer had a significant effect on the distribution of mites, but still affected that of Enchytraeidae and Collembola. Boron was positively correlated with faunal density in several cases, and thus may be the growth limiting nutrient affecting substrate quality for decomposers in these sites. The overall changes in the soil mesofauna in drained peatlands depicted here show that these peatland forests are converging ecologically on upland forests where decomposition in general is much faster than in pristine peatlands. The change caused by restoration shows how labile these ecosystems are also with respect to the mesofaunal community.