Water and element fluxes during the regeneration of Norway spruce with European beech: Effects of shelterwood-cut and clear-cut

The regeneration of mature Norway spruce with European beech using the shelterwood silvicultural system is a good example of continuous cover forestry. In contrast, the regeneration may also start with clear-cut plots, which often occur after calamities like wind-throw or bark beetle attack. During regeneration the forest ecosystem becomes a highly dynamic system. Nutrient losses from the soil may occur as the element turnover is affected by the reduced nutrient uptake of forest trees as well as the enhanced mineralisation and nitrification due to higher soil temperature and soil moisture. Continuous cover forestry may help to reduce these nutrient losses. In order to test this, we investigated water and element fluxes of two chronosequences. The first investigated regeneration in the shelterwood system, while the second concerned itself with regeneration on clear-cut plots. In a shelterwood-cut about 30% of the mature spruce trees are removed and young beech trees are planted. Some 10 years later a secondary felling is carried out and at age 20 of the beech regeneration the final harvest of the mature trees occurs. Thus, the studied time steps were (a) the first 5 years after the initial cut and planting, (b) 10-year-old beech regeneration after the second shelterwood cut and (c) 20-year-old beech regeneration after the final harvest.Our results indicate that nutrient losses with seepage water – especially nitrogen, calcium and magnesium – occur during the first years after the clear cut and, to a lesser extent, after secondary felling on the selective-cut plot. This may temporarily affect seepage water quality due to elevated nitrate concentrations, which reached values of more than 100 mg l⁻¹. In the time span between planting and an age 20 of the beech regeneration, total losses of nitrogen from the main rooting zone reach 230 kg ha⁻¹ after clear cut. Preliminary estimates of the total nitrogen loss in the shelterwood system range between 150 and 230 kg ha⁻¹ indicating either significantly lower or equal losses of nutrients. In the second case, however, element output is distributed more equally over the 20-year-period than after clear felling where 85% of the nitrate leaching occurs during the first 3 years.     

Nitrogen budgets for Scots pine and Norway spruce ecosystems 12 and 7 years after the end of long-term fertilisation

The magnitude of nitrogen storage and its temporal change in forest ecosystems are important when analysing global change. For example, the accelerated growth of European forests has been linked to increased nitrogen deposition, but the changes in the N inputs that cause long-term changes in ecosystems have not yet been identified. We used two Swedish forest optimum nutrition experiments with Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) to study the long-term fate of N applied to these forest ecosystems. In the pine experiment, in addition to fertiliser (NPK) application, soil acidity was manipulated by application of lime and dilute sulphuric acid. From the spruce experiment, we selected treatments with similar fertiliser doses as in the pine experiment and with and without lime addition.We quantified various terms in the N budget 12 years (pine) and 7 years (spruce) after the last N addition. In the pine stand the NPK-treatment was the only treatment to produce a significant increase in N in the tree biomass (97% above control), whereas in the spruce stand the N additions increased tree N in all treatment combinations (207% above control). In the pine stand the relative distribution of nitrogen between trees and soil did not vary across treatments, with trees containing around 12% of ecosystem N and humus containing around 44% of soil N. The increases in N stocks in the pine stands were mainly in the soil. In contrast, in the spruce ecosystem trees accumulated most of the added N and the increase in the soil was restricted to the humus layer.In the pine ecosystem, large losses of added N (between 254 and 738 kg ha⁻¹ out of 1040 kg ha⁻¹ added as fertiliser) occurred, whereas in the spruce ecosystem we recovered more N than could be accounted for by inputs (between 250 and 591 kg ha⁻¹). There was no clear pattern in the interaction between acidification/liming and N additions.     

Wood volume yield and stand structure in Norway spruce understorey depending on birch shelterwood density

Wood volume yield and stand structure were investigated for Norway spruce understorey growing at 1500 trees ha⁻¹ under birch shelters of two different densities, 300 and 600 trees ha⁻¹, and Norway spruce growing without shelter, in a field trial in the boreal coniferous forest, 56 years after the establishment of the stand and 19 years after establishment of the trial.Wood volume yield in sheltered spruce (mean annual increments of 1.87 and 1.78 m³ ha⁻¹ year⁻¹ under the dense and sparse shelterwoods, respectively) was significantly lower than that of unsheltered spruce (mean annual increment 2.43 m³ ha⁻¹ year⁻¹). The loss in wood volume yield for sheltered spruce was more than compensated for by the additional wood volume yield in the shelterwoods (mean annual increments 3.26 and 1.88 m³ ha⁻¹ year⁻¹ for the dense and sparse shelterwood respectively).Shelterwood density did not produce any significant differences in inequality of the understorey stands, measured as skewness and the Gini coefficient for the wood volume distributions. This implies that two-sided competition for nutrients and water was more significant than competition for light.Immediately after trial establishment, trees in the no shelterwood treatment (i.e. where all overstory trees had been removed) showed a marked increase in diameter growth. Over time, the growth rate of unsheltered Norway spruce was reduced to a level comparable to that of sheltered spruce. The difference in average diameter has persisted during the trial period. There was no similar effect on height growth, resulting in an increased slenderness index (h/d) with increased shelterwood density for the understorey trees.     

Carbon sources and sinks in high-elevation spruce–fir forests of the Southeastern US

This paper examines carbon (C) pools, fluxes, and net ecosystem balance for a high-elevation red spruce–Fraser fir forest [Picea rubens Sarg./Abies fraseri (Pursh.) Poir.] in the Great Smoky Mountains National Park (GSMNP), based on measurements in fifty-four 20 m × 20 m permanent plots located between 1525 and 1970 m elevation. Forest floor and mineral soil C was determined from destructive sampling of the O horizon and incremental soil cores (to a depth of 50 cm) in each plot. Overstory C pools and net C sequestration in live trees was estimated from periodic inventories between 1993 and 2003. The CO₂ release from standing and downed wood was based on biomass and C concentration estimates and published decomposition constants by decay class and species. Soil respiration was measured in situ between 2002 and 2004 in a subset of eight plots along an elevation gradient. Litterfall was collected from a total of 16 plots over a 2–5-year period.The forest contained on average 403 Mg C ha⁻¹, almost half of which stored belowground. Live trees, predominantly spruce, represented a large but highly variable C pool (mean: 126 Mg C ha⁻¹, CV = 39%); while dead wood (61 Mg C ha⁻¹), mostly fir, accounted for as much as 15% of total ecosystem C. The 10-year mean C sequestration in living trees was 2700 kg C ha⁻¹ year⁻¹, but increased from 2180 kg C ha⁻¹ year⁻¹ in 1993–1998 to 3110 kg C ha⁻¹ year⁻¹ in 1998–2003, especially at higher elevations. Dead wood also increased during that period, releasing on average 1600 kg C ha⁻¹ year⁻¹. Estimated net soil C efflux ranged between 1000 and 1450 kg C ha⁻¹ year⁻¹, depending on the calculation of total belowground C allocation. Based on current flux estimates, this old-growth system was close to C neutral.     

Regeneration of Norway spruce in canopy gaps in Sphagnum-Myrtillus old-growth forests

Gap-associated spruce (Picea abies (L.) Karst.) regeneration in Sphagnum-Myrtillus stands of south taiga forests (Central Forest Biosphere reserve, Tver region, Russia) was studied to evaluate the role of different disturbances in spruce dynamics. Sampled gaps (n=70) ranged from 40 m² to 1.7 ha in size, and from 1 to 70 years since disturbance moment. Formation of gaps lead to increase in the number of stems per ha in all gap size classes (small: 40–200 m², medium: 200–3000 m², and large: >3000 m² gaps). Spruce was the most important species in gap refilling, although its role was not the same in different gap classes. The highest values of relative abundance (compared to other species) were recorded in small gaps, and much lower values – in middle and large gaps. However, as refilling of gaps proceeded, spruce showed rather active regeneration in middle and large gaps and partly regained its abundance in middle-age disturbances. In general, all types of gaps studied supported spruce regeneration into the forest canopy. Almost perfect correlation between predicted outcome of spruce dynamics in gaps and its current role in the canopy of Sphagnum-Myrtillus stands suggests a good adaptation of this species to the current disturbance regime and a steady state of the these forests.     

Effects of experimental acidification,nitrogen addition and liming on ground vegetation in a mature stand of Norway spruce (Picea abies (L.) Karst.) in SE Sweden

During the period 1976–1991, a combined experiment of acidification, liming and nitrogen addition in a mature spruce stand was conducted at Farabol in south-east Sweden. The aim of this study was to investigate the effects of these treatments on the ground vegetation 0, 1, 5 and 15 years after experimental establishment. The treatment regimes were nitrogen (200 kg N ha⁻¹, repeated three times at 4–5-year intervals, totally 600 kg N ha⁻¹), sulphur powder (50 and 100 kg S ha⁻¹ a⁻¹, totally 600 and 1200 kg ha⁻¹), sulphur plus nitrogen (600+600 kg ha⁻¹) and limestone (500 kg ha⁻¹ a⁻¹, i.e. totally 6000 kg ha⁻¹). The results showed that nitrogen addition and liming promoted the abundance of the grass Deschampsia flexuosa, while acidification had a negative effect on D. flexuosa and herbs in the field layer. There was a negative reaction giving immediate damage to the bryophytes in connection with additions of nitrogen, sulphur powder and lime. The magnitude of damage and the capacity to recover varied among species as well as among treatments. The recovery from immediate damage after liming was much faster than after the treatments with sulphur powder and/or nitrogen. A negative interaction between sulphur powder and nitrogen was found for herbs and mosses where the combined effects were stronger than the effects of a single treatment alone. Acidification also had a negative effect on the total number of species. The results of this study showed that acidification and nitrogen deposition could negatively influence forest vegetation by changing the nutrient availability in the soils. Liming led to an improved growth of the forest ground vegetation and the flora changed towards a more nitrophilic species composition.     

Stand development and production dynamics of loblolly pine under a range of cultural treatments in north-central Florida USA

The objectives of this study were to examine the effects of stand development and soil nutrient supply on processes affecting the productivity of loblolly pine (Pinus taeda L.) over a period approximately equal to a pulpwood rotation (18 years). The experiment consisted of a 2×2 factorial combination of complete and sustained weed control and annual fertilization treatments (C: control treatment, F: fertilization, W: weed control, FW: combined fertilization and weed control), located on a Spodosol in north-central Florida, USA. The reduction of soil nutrient limitations through fertilization or control of competing vegetation resulted in dramatic increases in almost every measure of productivity investigated, including height (19.7 m in the FW treatment versus 12.5 m in the C treatment at age 18 years), basal area (FW=44.2 m² ha⁻¹, F=39.6 m² ha⁻¹, W=36.6 m² ha⁻¹, C=19.9 m² ha⁻¹ at age 16 years), stemwood biomass accumulation (114 Mg ha⁻¹ in FW versus 42.8 Mg ha⁻¹ in C at age 18 years), foliar nitrogen concentration (1.53% in plots receiving fertilization versus 1.06% in unfertilized plots at age 17 years) and leaf area index (age 16-year peak projected of approximately 3.3 at age 9–10 years in F and FW plots, 2.5 in the W treatment and 1.5 in the C plots). Cultural treatments also decreased the growth ring earlywood/latewood ratio, and accelerated the juvenile wood to mature wood transition. While soil nutrient supply was a major determinant of productivity, production changes that occurred within treatments over the course of stand development were equally dramatic. For example, between age 8 and 15 years, stemwood PAI in the FW treatment declined by 275%; similarly large reductions occurred in the F and W treatments over the same time period. The reductions in PAI in the treated plots were linearly related to stand BA, suggesting the decline in productivity was associated with the onset of inter-tree competition. Responses of stemwood PAI to re-fertilization treatments at age 15 years suggests that the declines in growth and growth efficiency with time were partially attributable to nutrient limitations.     

Long-term trends in loblolly pine productivity and stand characteristics in response to thinning and fertilization in the West Gulf region

Two levels each of thinning and fertilization were applied to a 7-year-old loblolly pine (Pinus taeda L.) plantation on a nitrogen- and phosphorus-deficient West Gulf Coastal Plain site in Louisiana. Levels of thinning were no thinning, or thinning applied 7 and 14 years after stand initiation. Levels of fertilization were no fertilization or broadcast fertilization with diammonium phosphate at age 7 years plus refertilization with urea, monocalcium phosphate, and potash at age 14 years. Long-term measurements of climate, stand development and productivity, projected leaf area index, and foliar nutrition were initiated at age 11 years. We found that by age 17 years, thinning increased mean live-crown length from 4.2 to 7.8 m, and mean tree diameter from 15.0 to 21.8 cm compared to the unthinned treatment. After rethinning at age 14 years, stand basal area increased 1.2 and 19.2% between ages 15 and 17 years on the unthinned and thinned plots, respectively. Refertilization at age 14 years reestablished foliar N, P and K sufficiency, which increased leaf area index from 4.2 to 6.0 m² m⁻² on the unthinned plots and from 3.2 to 3.8 m² m⁻² on the thinned plots, and subsequently, increased gross stand biomass from 114 to 141 Mg ha⁻¹ on the unthinned plots and from 78 to 95 Mg ha⁻¹ on the thinned plots by age 17 years. Leaf area was an important factor controlling loblolly pine productivity. At our study site, however, competition for light and water and nutrition-limited foliage growth influenced the variability and scope of this relationship. Our results suggest that a positive and linear relationship between leaf area and loblolly pine productivity does not universally occur on loblolly pine sites.     

Stand and landscape level effects of a major outbreak of spruce beetles on forest vegetation in the Copper River Basin,Alaska

From 1989 to 2003, a widespread outbreak of spruce beetles (Dendroctonus rufipennis) in the Copper River Basin, Alaska, infested over 275,000 ha of forests in the region. During 1997 and 1998, we measured forest vegetation structure and composition on one hundred and thirty-six 20-m × 20-m plots to assess both the immediate stand and landscape level effects of the spruce beetle infestation. A photo-interpreted vegetation and infestation map was produced using color-infrared aerial photography at a scale of 1:40,000. We used linear regression to quantify the effects of the outbreak on forest structure and composition. White spruce (Picea glauca) canopy cover and basal area of medium-to-large trees [≥15 cm diameter-at-breast height (1.3 m, dbh)] were reduced linearly as the number of trees attacked by spruce beetles increased. Black spruce (Picea mariana) and small diameter white spruce (<15 cm dbh) were infrequently attacked and killed by spruce beetles. This selective attack of mature white spruce reduced structural complexity of stands to earlier stages of succession and caused mixed tree species stands to lose their white spruce and become more homogeneous in overstory composition. Using the resulting regressions, we developed a transition matrix to describe changes in vegetation types under varying levels of spruce beetle infestations, and applied the model to the vegetation map. Prior to the outbreak, our study area was composed primarily of stands of mixed white and black spruce (29% of area) and pure white spruce (25%). However, the selective attack on white spruce caused many of these stands to transition to black spruce dominated stands (73% increase in area) or shrublands (26% increase in area). The post-infestation landscape was thereby composed of more even distributions of shrubland and white, black, and mixed spruce communities (17–22% of study area). Changes in the cover and composition of understory vegetation were less evident in this study. However, stands with the highest mortality due to spruce beetles had the lowest densities of white spruce seedlings suggesting a longer forest regeneration time without an increase in seedling germination, growth, or survival.     

Vegetation change and forest regeneration on the Kenai Peninsula,Alaska following a spruce beetle outbreak, 1987–2000

Forests of the Kenai Peninsula, Alaska experienced widespread spruce (Picea spp.) mortality during a massive spruce beetle (Dendroctonus rufipennis) infestation over a 15-year period. In 1987, and again in 2000, the U.S. Forest Service, Pacific Northwest Research Station, Forest Inventory and Analysis Program conducted initial and remeasurement inventories of forest vegetation to assess the broad-scale impacts of this infestation. Analysis of vegetation composition was conducted with indirect gradient analysis using nonmetric multidimensional scaling to determine the overall pattern of vegetation change resulting from the infestation and to evaluate the effect of vegetation change on forest regeneration. For the latter we specifically assessed the impact of the grass bluejoint (Calamagrostis canadensis) on white spruce (Picea glauca) and paper birch (Betula papyrifera) regeneration. Changes in vegetation composition varied both in magnitude and direction among geographic regions of the Kenai Peninsula. Forests of the southern Kenai Lowland showed the most marked change in composition indicated by relatively large distances between 1987 and 2000 measurements in ordination space. Specific changes included high white spruce mortality (87% reduction in basal area of white spruce >12.7 cm diameter-at-breast height (dbh)) and increased cover of early successional species such as bluejoint and fireweed (Chamerion angustifolium). Forests of the Kenai Mountains showed a different directional change in composition characterized by moderate white spruce mortality (46% reduction) and increased cover of late-successional mountain hemlock (Tsuga mertensiana). Forests of the Gulf Coast and northern Kenai Lowland had lower levels of spruce mortality (22% reduction of Sitka spruce (Picea sitchensis) and 28% reduction of white spruce, respectively) and did not show consistent directional changes in vegetation composition. Bluejoint increased by ≥10% in cover on 12 of 33 vegetation plots on the southern Kenai Lowland but did not increase by these amounts on the 82 plots sampled elsewhere on the Kenai Peninsula. Across the Kenai Lowland, however, regeneration of white spruce and paper birch did not change in response to the outbreak or related increases in bluejoint cover from 1987 to 2000. Although some infested areas will be slow to reforest owing to few trees and no seedlings, we found no evidence of widespread reductions in regeneration following the massive spruce beetle infestation.     

Regeneration dynamics of Sitka spruce in artificially created forest gaps

This study examined the variation in the development of naturally regenerated and planted seedlings of Sitka spruce (Picea sitchensis (Bong.) Carr.) within gaps cut in a 32-year-old stand of the same species. The circular gaps were 20 m in diameter and designed to allow sunlight into only half of the gap floor at midsummer given the latitude of 56°45′N. Eight plots (8 m × 3 m) were laid out along a north–south transect through each gap (four within the gap and two each under the closed canopy north and south of the gap). Each plot was sub-divided and seedlings were planted into one part and the other part was left to naturally regenerate. In subsequent seasons, plots were further subdivided into ‘weed free’ and ‘vegetation left untouched’. Results showed that while the two central plots within the gaps had the highest value of canopy openness, the highest accumulated temperature and lowest soil moisture were recorded in plots that received direct sunlight. However, level of germination was significantly higher in the shaded area of the gap than in the part that received direct sunshine suggesting that higher moisture levels in shaded areas are important to successful germination. Minimal germination was recorded in the plots beneath the canopy. Seedling survival was significantly influenced by the influx of competing vegetation, but only in the part of the gaps that received direct sunlight. The success of Sitka spruce regeneration within gaps appears to depend on sufficient moisture and light to support regeneration and early growth, but not too much light to encourage the development of competing vegetation. The permanently shaded areas of the gaps appeared to offer ground conditions with sufficient moisture and light to ensure successful germination and early growth of seedlings, but without excessive competition from other vegetation.     

Stand development after different thinnings in two uneven-aged Picea abies forests in Sweden

Two field experiments, located in Central and Northern Sweden, were used to study the influence of standing volume on volume increment and ingrowth in uneven-aged Norway spruce (Picea abies (L.) Karst.) stands subjected to different thinnings. Each experiment had a 3 × 2 factorial block design with two replications. Treatments were thinning grade, removing about 45, 65, and 85% of pre-thinning basal area, and thinning type, removing the larger or the smaller trees, respectively. Each site also had two untreated control plots. Plot size was 0.25 ha. Volume increment was 0.5–6.8 m³ ha⁻¹ year⁻¹ for the plots, and significantly positively (p < 0.01) correlated with standing volume. Within treatment pairs, plots thinned from Above had consistently higher volume increment than plots thinned from Below. Ingrowth ranged from 3 to 33 stems ha⁻¹ year⁻¹, with an average of 14 and 21 stems ha⁻¹ year⁻¹ at the northern and southern site, respectively. At the southern site ingrowth was significantly negatively (p < 0.01) correlated with standing volume, but not at the northern site. Mean annual mortality after thinning was 2 and 7 stems ha⁻¹ year⁻¹at the northern and southern site, respectively.     

Changes in soil,seepage water and needle chemistry between 1984 and 2004 after liming an N-saturated Norway spruce stand at the Höglwald,Germany

In 1984, a liming experiment with a surface application of 4 t ha⁻¹ of dolomitic limestone was started at the acidic N-saturated Norway spruce forest “Höglwald” in southern Germany and monitored until 2004. The decay of surface humus due to the accelerated mineralisation accounted for 18.5 ± 2.7 t ha⁻¹ C or 50% of the initial pool and 721.6 ± 115.0 kg ha⁻¹ N or 46% for N. Due to some translocation of organic material to the mineral soil the values to 40 cm depth are slightly lower (13.5 ± 4.4 t ha⁻¹ C or 15% of the initial pool and 631.6 ± 192.8 kg ha⁻¹ N or 13% for N). In the control plot NO₃⁻ concentrations at 40 cm depth were above the European level of drinking water (0.8 mmolc l⁻¹ or 50 mg NO₃⁻ l⁻¹) for nearly the whole investigation period. Liming increased NO₃⁻ concentrations in seepage water for approximately 15 years, and accelerated leaching losses by 396.2 NO₃⁻–N kg ha⁻¹ from 1984 to 2003. The increase in pH of the soil matrix was more or less restricted to the humus layer and the upper 5 cm of the mineral soil during the whole time span, while the base cations Ca and Mg reached deeper horizons with seepage water. From 1984 to 2003, an amount that nearly equalled the applied Mg, was leached out of the main rooting zone, while most of the applied Ca was retained. The time series of the elemental concentrations in needles showed minor changes. Ca concentrations in needles increased with liming, while Mg remained nearly unchanged, and P decreased in older needles.     

Long-term effects of repeated urea fertilization in Douglas-fir stands on forest floor nitrogen pools and nitrogen mineralization

In six Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] stands in the Puget Sound Region in Western Washington/USA, forest floor C and N pools were quantified on control plots and on plots that had been fertilized repeatedly with urea 8–30 years ago (total amount of applied N 0.9–1.1 Mg ha⁻¹). Additionally, net N mineralization and nitrification rates were assessed in field and laboratory incubation experiments. Forest floor C/N ratios were decreased on the fertilized plots of all sites compared to the respective control plots. The decreases were particularly strong at sites with initial C/N ratios larger than 30. On sites with low productivity (site index at age 50: <33 m), N fertilization resulted in considerable increases in forest floor N pools. Net N mineralization and nitrification during 12-week field incubation was negligible for the unfertilized and fertilized plots of all except one site (Pack Forest), where the stand had been clear-cut 2 years ago. The increases in N mineralization rates during 12-week laboratory incubation induced by repeated N fertilization showed an inverse relationship to the time elapsed since the last fertilizer application, and were generally larger at sites with initial forest floor C/N ratios >30. For the investigated sites, fertilization effects on net N mineralization sustained for at least 11 years after the last fertilizer application. Nitrification correlated strongly with the forest floor pH; significant formation of NO₃⁻ was observed only for O layers with a pH (H₂O) higher than 4.5.     

Effects of chronic nitrogen amendment on dissolved organic matter and inorganic nitrogen in soil solution

Increased atmospheric deposition of N to forests is an issue of global concern, with largely undocumented long-term effects on soil solution chemistry. In contrast to bulk soil properties, which are typically slow to respond to a chronic stress, soil solution chemistry may provide an early indication of the long-term changes in soils associated with a chronic stress. At the Harvard Forest, soil solution was collected beneath the forest floor in zero tension lysimeters for 10 years (1993–2002) as part of an N saturation experiment. The experiment was begun in 1988 with 5 or 15 g N m⁻² per year added to hardwood and pine forest plots, and our samples thus characterize the long-term response to N fertilization. Samples were routinely analyzed for inorganic nitrogen, dissolved organic nitrogen (DON), and dissolved organic carbon (DOC); selected samples were also analyzed to determine qualitative changes in the composition of dissolved organic matter. Fluxes of DOC, DON, and inorganic N were calculated based on modeled water loss from the forest floor and observed concentrations in lysimeter samples. The concentration and flux of inorganic N lost from the forest floor in percolating soil solution are strongly affected by N fertilization and have not shown any consistent trends over time. On average, inorganic N fluxes have reached or exceeded the level of fertilizer application in most plots. Concentrations of DOC were unchanged by N fertilization in both the hardwood and pine stands, with long-term seasonal averages ranging from 31–57 mg l⁻¹ (hardwood) and 36–93 mg l⁻¹ (pine). Annual fluxes of DOC ranged from 30–50 g m⁻² per year. DON concentrations more than doubled, resulting in a shift toward N-rich organic matter in soil solution percolating from the plots, and DON fluxes of 1–3 g m⁻² per year. The DOC:DON ratio of soil solution under high N application (10–20) was about half that of controls. The organic chemistry of soil solution undergoes large qualitative changes in response to N addition. With N saturation, there is proportionally more hydrophilic material in the total DON pool, and a lower C:N ratio in the hydrophobic fraction of the total DOM pool. Overall, our data show that fundamental changes in the chemistry of forest floor solution have occurred in response to N fertilization prior to initiation of our sampling. During the decade of this study (years 5–14 of N application) both inorganic N and dissolved organic matter concentrations have changed little despite the significant biotic changes that have accompanied N saturation.     

Microbial activity,nutrient dynamics and litter decomposition in a Canadian Rocky Mountain pine forest as affected by N and P fertilizers

To study the specific effects of N and P fertilizers on soil microbial processes under field conditions, a pine forest in southern Alberta was fertilized with ammonium nitrate and urea (0 and 188 kg N ha⁻¹, respectively) singly and in combination with triple super phosphate (0 and 94 kg P ha⁻¹, respectively). Microbial respiration (BR), substrate induced respiration (SIR), metabolic quotient (qCO₂) and rates of microbially mediated key processes were monitored in the forest floor FH material during the growing periods of spring 1990 to fall 1992. A transient increase in soil NH₄⁺ availability was detected following N addition but significant increases in PO₄³⁻ availability were detected throughout the study as a result of P enhancement. Microbial variables were unaffected by N addition, whereas soil organic matter and SIR was increased with P fertilization. Microbial BR and qCO₂ were reduced with P fertilization suggesting more efficient utilization of C. Increased net mineralization of P in the P-fertilized plots was found during the second and third summers after fertilization, following immobilization of P during the first year. In contrast, NH₄⁺-N mineralization in the N-fertilized plots was significantly increased only during the first year. Rates of nitrification were unaffected by either N or P addition. Decomposition of pine litter was enhanced with N addition only during the first year and P had no effect on decomposition. Addition of N and P at these rates does not appear to impose a major ecological stress to the soil ecosystem.     

Effects of enhanced nitrogen deposition on foliar chemistry and physiological processes of forest trees at the Bear Brook Watershed in Maine

Effects of enhanced nitrogen deposition on nutrient foliar concentrations and net photosynthesis of sugar maple (Acer saccharum Marsh.), American beech (Fagus grandifolia Ehrh) and red spruce (Picea rubens Sarg.) were evaluated at the Bear Brook Watershed in Maine (BBWM). The BBWM is a paired-watershed forest ecosystem study with one watershed treated since 1989 with bimonthly dry ammonium sulfate ((NH₄)₂SO₄) additions at a rate of 25.2 kg N ha⁻¹ year⁻¹, while the other watershed serves as a reference. The (NH₄)₂SO₄ treatment resulted in significant increases in foliar N concentrations for all three species and significant reductions in foliar Ca, Mg and Zn concentrations for American beech and red spruce. Treatment effects on foliar concentrations of other nutrients were not significant in any species. Despite higher N concentrations in all species, only treated sugar maple showed significantly higher photosynthetic rates. The non-response in net photosynthesis to higher foliar N in American beech and red spruce might be attributed to their low foliar Ca and/or Mg concentrations. Higher net photosynthetic rates in sugar maple might be explained by the higher foliar N and by the ability of this species to maintain an adequate Ca and Mg supply. Results suggested that nutrient imbalances due to inadequate supply of Ca and Mg might have counteracted a potential increase in net photosynthesis induced by higher N concentrations in American beech and red spruce.     

Long term growth responses of loblolly pine to optimal nutrient and water resource availability

A factorial combination of four treatments (control (CW), optimal growing season water availability (IW), optimum nutrient availability (FW), and combined optimum water and nutrient availability (FIW)) in four replications were initiated in an 8-year-old Pinus taeda stand growing on a droughty, nutrient-poor, sandy site in Scotland County, NC and maintained for 9 years. Results for the first 4 years after treatment initiation at this study were first reported by Albaugh et al. [For. Sci. 44 (1998) 317]. The site is primarily nutrient limited and all measured stand parameters (height, basal area, leaf area index, live crown length, stem mass accumulation, current annual stem mass increment) were increased with fertilization throughout the study period. Irrigation effects were also positive for these parameters but the increases were much smaller than those found with fertilization. For example, 9 years after treatment initiation, standing stem mass was increased 100 and 25% by fertilization and irrigation, respectively, while current annual increment of stem biomass production was increased 119 and 23% by fertilization and irrigation, respectively. Interestingly, stem density (stems ha⁻¹) was not significantly affected by treatment in any year of the study. Growth efficiency (stem mass increment per unit leaf area index) was 1.9 Mg ha⁻¹ per year per LAI for CW and influenced by treatment with IW, FW, and FIW achieving growth efficiencies of 2.4, 2.7 and 2.9 Mg ha⁻¹ per year per LAI, respectively. Growth efficiency appeared to be relatively stable in the last 4 years of the study. Ring specific gravity was measured in the third, fourth, and fifth years after treatment initiation. An average reduction in ring specific gravity of 7.5% was observed with fertilization while irrigation had little effect on specific gravity in any year measured. The continuation of high growth rates with no observable growth decline in the treated stands throughout the 9-year study may be a function of the age of the stands when treatments were initiated (8 years), the very poor initial nutrient and moisture availability, and/or the application of an ongoing optimum nutrient regime at the site. The fertilized plots are now at or near an age and a size when a commercial harvest would be feasible. For the stand conditions at this site, then, the optimum nutrient availability plots have achieved high productivity throughout the economic life of the stand without measurable declines in stand productivity.     

Fine-root distribution and morphology in an acidic Norway spruce (Picea abies (L.) Karst.) stand in SW Sweden in relation to granulated wood ash application

Wood ash is recommended as a compensatory fertiliser to counteract the effects of acidic deposition on forest ecosystems. Spatial distribution of biomass, necromass and morphology parameters of the fine roots (diameter classes <1, 1–2, <2 mm) of Norway spruce (Picea abies (L.) Karst.) were analysed in response to fertilisation with granulated wood ash (GWA) in a long-term field experiment in SW Sweden. GWA was applied as a single dose of 3200 kg ha⁻¹ and the fine roots were sampled 9 years later by soil coring. Soil cores were divided into 1-cm strata within the top 0–2.5 cm humus limits, the lower humus below 2.5 cm (with varying thickness) and the mineral soil to 50 cm depth (from ground surface). Total fine-root biomass in the control (C) and GWA treatment, 256 ± 20 and 258 ± 25 g m⁻², respectively, and length 2072 ± 182 and 1800 ± 198 m m⁻², respectively, did not differ statistically from each other. Total fine-root necromass in the 1–2 mm fraction was significantly higher in C than in the GWA treatment, 130 ± 12 and 80 ± 10 g m⁻², respectively. Fine-root biomass in the <1 mm fraction was significantly greater in the lower humus in the GWA treatment, but this did not affect the total biomass in the <1 mm fraction in the whole soil profile. The biomass-to-necromass ratio (1–2 mm) was significantly higher in the GWA treatment in the 0–30 cm soil layer than in the corresponding layer of the control. Specific root length (SRL) was lower in the GWA treatment than in the control in the 0–5 cm soil layer. The lower necromass and SRL were more clearly related to the GWA treatment, whereas the difference in the vertical distribution of biomass may have been related to the thicker humus layer in the GWA plots.     

Long-term growth and ecophysiological responses of a southeastern Oklahoma loblolly pine plantation to early rotation thinning

A thinning levels study was initiated in a 9-year-old loblolly pine (Pinus taeda L.) plantation containing 26.6 m² ha⁻¹ basal area during the spring of 1984 in southeastern Oklahoma. Thinning treatments consisted of (1) three control plots (BA100), (2) three plots thinned to approximately 50% of the original basal area (BA50) and (3) three plots that were thinned to 25% of the original basal area (BA25). In 1987 the BA50 and BA25 plots were both rethinned to a basal area of 12 m² ha⁻¹. No other thinnings were done through age 24.The control plots have attained a basal area of 45.3 m² ha⁻¹ and basal area is now starting to decline. The BA25 and BA50 plots have basal areas between 34 and 35 m² ha⁻¹. Mortality has averaged about 90 trees ha⁻¹ per year from age 10 to age 24 on the control plot, declining from 2078 trees ha⁻¹ at age 10 to 827 trees ha⁻¹ at age 24. Mortality losses in the BA25 and BA50 plots have been only 3.2–7.7 trees ha⁻¹ per year over the entire study period. Cumulative stem biomass lost to mortality was 10.5, 16.0 and 61 Mg ha⁻¹, respectively, for the BA25, BA50 and BA100 treatments. Cumulative standing live biomass at age 24 in the BA100 treatment is 132 Mg ha⁻¹. Cumulative standing live biomass in the BA25 and BA50 treatments at age 24 is 86 and 79%, respectively, of that observed in the BA100 treatment. These results suggest wide ranges of residual stand densities left after an early thinning will produce a high percentage of the potential total maximum standing stem biomass. Diameter distributions at age 24 show only 33% of the trees in the BA100 treatments have the dimensions to be sawtimber (≥30 cm) but 92 and 95% of the trees in the BA25 and BA50, respectively, are sawtimber dimension or larger. Mean annual stem biomass production (MAI) of the BA100 treatment is 7.5 Mg ha⁻¹ per year at age 24. MAI of the thinned treatments is about 5.1 Mg ha⁻¹ per year and is converging to that of the BA100 treatment. The basis for this convergence is not that the live trees in the BA100 treatment are producing live biomass less rapidly than the thinned plots, but that mortality losses in the BA100 plot are much higher. Current annual stemwood production in all treatments is often limited by the severe summer droughts that occur in this region. The wide variations in weather experienced at this site also result in variations in earlywood:latewood ratio and ring specific gravity.