Long-term Site Productivity Research Articles

Paper Birch and Fireweed Stand Establishment Decision Aids

The Coastal Western Hemlock submaritime biogeoclimatic subzones (CWHds1 and ds2) occur in the inland valleys of coastal British Columbia, where the climate is commonly transitional between the province's coastal and interior regions. Vegetation management issues in these subzones include species such as fireweed (Epilobium angustifolium) and paper birch (Betula papyrifera). On wetter, rich sites of the CWH zone, management options for other vegetation (e.g., salmon-berry, devil's club, and elderberry) are well documented; however, little research has focussed on the competing species found primarily in the submaritime British Columbia subzones.A field guide for the region (Green and Klinka 1994) lists a number of vegetation concerns for the CWH's submaritime subzones. Practitioners, however, have identified a need for more information about paper birch and fireweed. The vegetation management Stand Establishment Decision Aids (SEDAS) included in this extension note are designed to fill this gap. Although initially developed to address the needs of coastal managers, much of the information presented here can be applied to other areas of the province experiencing similar issues related to successful conifer regeneration. Most of the content was adapted from previously published information on paper birch and fireweed management in the former Cariboo Forest Region (Swift and Turner 2002).The majority of the information provided for paper birch emanates from research conducted in the province's southern interior (Simard 1996; Simard and Henigman 1996), where birch is well distributed and can affect crop tree regeneration and survival. Earlier legislation required free-growing trees to be 150% of the height of competing vegetation; however, the relatively small canopy of paper birch compared to other hardwood species such as alder makes it a less serious deterrent to conifer development. Therefore, foresters working in the CWH's submaritime subzones must determine the effect that existing levels of birch will have on the short- and long-term growth of crop trees. Additionally, research has shown that birch within a regenerating stand of Douglas-fir are beneficial to ectomycorrhizal development (see Jones et al. 1997; Simard et al. 1997a and b). Licensees working in submaritime subzones, where a shift from timber-oriented objectives to ecosystem biodiversity and long-term site productivity objectives may occur, could greatly benefit from further research into the effects of paper birch on regeneration. This extension note provides a general guide for managing paper birch and fireweed in the Coast Forest Region. Peterson et al. (1997) provide a more complete discussion of paper birch throughout the majority of its range. Additional information sources for both paper birch and fireweed are listed in the Resource and Reference sections.

Wildfire and post-fire erosion impacts on forest ecosystem carbon and nitrogen: An analysis

Many ecosystem processes occurring in soils depend upon the presence of organic matter. Soil organic matter is particularly important for nutrient supply, cation exchange capacity, and water retention, hence its importance in long-term site productivity. However, wildfires consume large amounts of aboveground organic material, and soil heating can consume soil organic matter. In almost all ecosystems throughout the world, greater quantities of carbon (C) and associated nutrients such as nitrogen (N) are found belowground than aboveground In grasslands, savannas and tundra-covered areas, much greater quantities of organic C (>90%) are found in the underground plant parts and decomposed organic matter than aboveground. In forest ecosystems, C is more evenly distributed aboveground and belowground (e.g. temperate deciduous and boreal forests). In general, soils with larger proportions of organic matter in the aboveground biomass and on their forest floors are more prone to wildfire-caused disturbances in their nutrient and C regimes than those in which most of the C in the ecosystem is located belowground. Overall effects of wildfire on soil C and N are a function of fire temperatures, severity, and frequency. A metaanalysis By Johnson and Curtis (2001) on the results of 13 fire studies (mixture of wildfire, routine prescribed fire, and site preparation fire) completed between 1975 and 1997 showed that the C and N contents of both the A-horizon and the underlying mineral soil layers change only a small amount (<10%) in the long-term. These results agreed with the conclusions of a previous review (Johnson, 1992) that indicated the overall effect of fire was not significant although there was a significant effect of time since fire. Although small changes in soil C and N occurred in the soils during these studies, substantial amounts of both organic litter and duff (100% in high severity fires) were consumed during these fires, especially high severity areas of the wildfires. Erosion of mineral soil is a major concern after high-severity wildfires. After wildfires, restoration activities are aimed at reducing erosion that might adversely affect water resources and site productivity. Despite many concerns and expensive treatments to reduce soil loss, erosion of mineral soil, most common after high severity fires, actually adds little to total C and N losses from forest ecosystems since combustion alone can produce 90% removal of C and N in the upper 7.5 cm of mineral soil. Organic matter and N losses from the forest floor and surface mineral soil could have a lasting effect on the long-term productivity and sustainability of forest sites, particularly when they occur on nutrient-deficient sites. This paper examines a number of studies around the world, contrasting C and N losses due to combustion and erosion processes and estimating risk to ecosystem productivity.

Effects of Slash, Machine Passes, and Soil Moisture on Penetration Resistance in a Cut-to-length Harvesting

Multiple entries into forest stands are often needed for fire hazard reduction and ecosystem restoration treatments in the Inland-Northwest U.S.A. region. However, soil compaction occurring from mechanized harvesting operations often remains for many years and may contribute to a decline in long-term site productivity. A controlled experiment on a silt loam soil was conducted to determine (a) the effectiveness of logging slash to buffer compaction, (b) the influence of the number of machine passes, and (c) the contribution of soil moisture to changes in penetration resistance during a cut-to-length harvest in northern Idaho. Penetration resistance was measured at three soil depths (10, 20, and 30 cm) for three different moisture contents (low, medium, and high) and slash amounts (none, light, and heavy) after each of 12 machine passes (one pass each with a harvester and an empty forwarder, and 10 passes with a fully-loaded forwarder). At all three soil depths the main effect of moisture content and machine passes on penetration resistance was significant, but slash amounts alone did not significantly affect penetration resistance. After 12 passes, we measured the greatest penetration resistance in the medium soil moisture treatment at 5 to15 cm of soil depth. When evaluated at similar moisture contents after harvesting, the soil that was driest during machine traffic (low moisture treatment) had the lowest penetration resistance. Slash was important for protecting the soil against compaction in the medium and high soil moisture treatments. Penetration resistance did not significantly increase after the second pass of a fully-loaded forwarder (31,752 kg) at any moisture content or slash level. Managing felling operations to take advantage of dry soil conditions or using slash when soils are moist may help reduce ruts and avoid long-term compaction impacts on this soil type.

Long-term effects of helicopter and ground-based skidding on site properties and stand growth in a tupelo–cypress wetland

Ground-based timber harvesting operations in forested wetlands have the potential to cause soil disturbances. Soil disturbances on upland sites have been linked to reduced site productivity, but the effects of such disturbances on the long-term site productivity of bottomland hardwoods is not well documented. In 1986, a long-term research project was established to compare the effects of helicopter and skidder timber harvesting on the regeneration, growth, and development of naturally regenerated water tupelo ( Nyssa aquatica)–baldcypress ( Taxodium distichum) stands. At stand age 16-years, both treatments have well-stocked, vigorously growing stands composed of coppice-regenerated water tupelo, Carolina ash ( Fraxinus caroliniana), baldcypress, and seed-origin black willow ( Salix nigra). Although both treatments are well stocked, the skidder treatment favored the establishment and growth of water tupelo, while the helicopter treatment had greater densities and growth rates for Carolina ash. Apparently, the initial effects of the skidder traffic puddled the soils and resulted in more reduced soil conditions, which favored the very flood tolerant water tupelo rather than the less flood tolerant species. Stand growth parameters suggest that both treatments will produce stands that will be similar to the previous stand in terms of species and volumes. Recovery in this area was speeded by annual inputs of nutrient rich sediment, the shrink-swell nature of the soil, and rapid growth of the coppice regeneration.

Factors influencing production efficiency of intensively managed loblolly pine plantations in a 1- to 4-year-old chronosequence

Changes in biomass, nutrient accumulation and production efficiency (PE—total biomass increment per unit leaf area) were investigated in intensively managed plantations of loblolly pine ( Pinus taeda L.) using an age sequence of replicated 1-, 2-, 3-, and 4-year-old stands ( n = 13). All stands, located on sandy Spodosols in the Coastal Plain of southern Georgia, were managed using a similar prescription that included a common genetic source, fertilization (years 1 and 3) and understory competition control. Total biomass accumulation (above- and below-ground) ranged from about 13 Mg ha −1 at age 2 years to 49.7 Mg ha −1 at age 4 years. Dry matter distributions averaged 30%, 34%, 18%, 6%, 15%, 12% and 3% for foliage, stemwood, branches, bark, taproots, coarse roots and fine roots, respectively, at age 4 years. Intensive management and rapid growth rates markedly increased soil nutrient demands, being about 15-fold higher than that documented in extensively managed stands of comparable age. Nutrient accumulations in tree biomass at age 4 years averaged 195, 22, 86, 72 and 28 kg ha −1 for N, P, K, Ca and Mg, respectively. Large declines (∼50%) in PE (2.6 Mg ha −1 year −1/unit versus 1.3 Mg ha −1 year −1/unit LAI) were apparent among all sites between the second and third years, with a noted recovery occurring at age 4 years. Changes in branch and taproot PE were most apparent, whereas stemwood PE did not change appreciably between ages 2 and 3 years. Correlation analyses suggested that larger declines in PE were associated with decreasing foliar nutrient levels (dilution), although changes in growth dynamics (carbon allocation) associated with advancing stand development may have also contributed. Collectively, these results suggest that intensive management may induce multiple nutrient limitations on sandy Spodosols, and that a better understanding of nutrient requirements, including macro- and micronutrient supply, will be necessary to maintain and enhance soil quality and long-term site productivity on these soils.

Defining sustainability of plantation forests through identification of site quality indicators influencing productivity—A national view for New Zealand

New Zealand is committed to developing sustainable forest management practices as evidenced through Government involvement in international forestry agreements such as the Montreal Process, and the forestry sector's adoption of forest certification mechanisms. In support of this commitment, it has been identified that there is little quantitative evidence of the interactions of plantation forestry on site quality and long-term site productivity. To address this issue, a nationwide study of site quality was initiated at 35 key sites covering the range of edaphic and environmental conditions representing the productivity envelope for New Zealand plantation forests. At each location, within the productivity envelope, eight short-term site quality plots were planted at a very high stand density (40,000 stems ha −1) to rapidly identify key soil indicators of growth which may be useful for determining site sustainability. In addition, a permanent sample plot was established by planting seedlings at conventional stem densities (500–1100 stems ha −1). At each site, a factorial design was applied with the following three factors: species ( Pinus radiata and Cupressus lusitanica), fertiliser (no fertiliser and nutrients supplied in excess of crop demands) and disturbance (low and high disturbances). After two years of increment, initial analyses are presented which partition treatment and site effects on increment and identify key soil properties that influence increment of the two species. Volume increment over the two-year period was most strongly influenced by site, ranging 50-fold and 15-fold across sites for C. lusitanica and P. radiata, respectively. For the treatments, species accounted for most of the variance in increment, with mean volume increment across all sites of P. radiata significantly exceeding that of C. lusitanica by 56%. Fertilisation also significantly influenced volume increment inducing mean gains of 30%. Disturbance had a significant, but comparatively weak effect, reducing mean volume increment by 9%. After correction had been made for environment (temperature and rainfall), soil properties that were most strongly related to volume increment for both species included CN ratio, total soil nitrogen, total soil phosphorus, organic phosphorus and depth of the A horizon. When soil properties were included in combination, the best predictive models of volume increment formulated for both species included rainfall, temperature, the product of total soil nitrogen and total soil phosphorus and depth of the A horizon.

FOREST SITE PREPARATION EFFECTS ON SOIL AND NUTRIENT LOSSES IN EAST TEXAS

Site preparation practices are frequently utilized in Southern pine ecosystems to facilitate planting and seedlingestablishment. Soil and nutrient losses were monitored in 12 bordered erosion plots following four site preparation treatmentsin a clearcut harvested loblolly pine (Pinus taeda L.) forest in east Texas. Three replications of four site preparation treatmentswere used: (1) chemical herbicide followed by prescribed fire and mechanical tillage, (2) chemical herbicide followed byprescribed fire, (3) chemical herbicide only, and (4) unprepared control. Annual soil loss from the mechanical tillage andprescribed fire treatments (1273 kg ha-1 and 885 kg ha-1, respectively) was significantly greater than annual soil loss fromthe chemical herbicide and control treatments (240 kg ha-1 and 219 kg ha-1, respectively). During the first post-treatmentyear, sediment concentration and overland flow increased significantly in the mechanical tillage and prescribed firetreatments with respect to control. Nutrient (N, P, K, Ca, Mg, and S) concentrations and losses in sediment and overland flowtemporarily increased after the mechanical tillage and prescribed fire treatments, but not after the chemical herbicide andcontrol treatments. Nutrient concentrations and losses for all site preparation treatments were relatively small and shouldhave little or no effects on water quality and long-term site productivity.

The effect of harvesting operations, slash management and fertilisation on the growth of aEucalyptusclonal hybrid on a sandy soil in Zululand, South Africa

Harvesting operations during clearfelling and extraction of timber from commercial plantations result in many processes that may affect long-term site productivity such as soil compaction and residue manipulation. The objective of this work was to evaluate the effects of various harvesting operations and fertiliser applications on soil physical properties and the subsequent growth of a Eucalyptus clonal hybrid ( Eucalyptus grandis x camaldulensis ) on the sandy soils of the Zululand coastal plain in northern KwaZulu- Natal. Increases in bulk density and penetrometer soil strength and decreases in aeration porosity were recorded on plots where a wheeled harvester had been used and on extraction roads, but these were not of a sufficient magnitude to limit tree growth. Available water capacity increased on the compacted plots. At six years of age there were no significant differences in stand volume between the plots harvested by different methods or between fertiliser treatments, nor was there any interaction between the treatments. However, a significant response to fertilisation was evident after one year. In contrast to the other treatments, trees in plots subjected to the wheeled harvesting operation did not respond to fertilisation, suggesting that an early release of nutrients occurred, caused by the breakdown and incorporation of slash in the topsoil. Key Words: Harvesting impacts; soil compaction; Zululand; soil physical properties; Eucalyptus grandis x camaldulensis; long-term site productivity Southern African Forestry Journal No.203 2005: 15-26

Harvest residue and competing vegetation affect soil moisture, soil temperature, N availability, and Douglas-fir seedling growth

Decisions made during stand regeneration that affect subsequent levels of competing vegetation and residual biomass can have important short-term consequences for early stand growth, and may affect long-term site productivity. Competing vegetation clearly affects the availability of site resources such as soil moisture and nutrients. Harvest residues can also impact the availability of site resources. We examined second- and third-year seedling performance of a Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) plantation with different vegetation control and biomass retention treatments on a highly productive site in the coast range of Washington. Treatments included a bole-only harvest without vegetation control (BO − VC), a bole-only harvest with complete vegetation control (BO + VC), and a total tree harvest with complete vegetation control that also included removal of all coarse woody debris and harvest residues (TTP + VC). The objectives of the study were to determine if vegetation control and residue retention treatments affected soil moisture, soil temperature, and apparent nitrogen (N) availability, and whether these differences in site resources were correlated with seedling size and growth. In both the second and third growing seasons, volumetric soil moisture at 0–20 cm depth was lowest on plots that did not receive vegetation control (BO − VC). Seedlings on these plots also had the lowest diameter and volume growth. In year 2, which was fairly moist, volume growth on TTP + VC plots was slightly higher than on BO + VC plots. TTP + VC plots did have lower soil moisture, but soil temperatures were slightly warmer. In year 3, a drier year, growth was greatest on BO + VC plots, which had consistently higher soil moisture levels. Apparent N availability in year 3 also varied with vegetation control. Douglas-fir foliar N concentrations averaged 2.3% on the plots where competing vegetation was eliminated, compared to 1.8% on plots where competing vegetation was not controlled. Douglas-fir foliar N concentrations did not differ between residue retention treatments, although N concentrations of competing vegetation were higher where residual biomass was retained. Higher apparent N availability was correlated with greater seedling growth. Based on results from years 2 and 3, it appears that soil moisture, particularly late in the growing season, had the greatest effect on seedling growth in both years. Available N may also have played a role, although the effects of N cannot be completely separated from those of soil moisture. When soil moisture is adequate, it appears that available N and soil temperature exert greater influence on growth. Vegetation control and residue retention can influence all three of these factors. The relative importance of each factor may depend on the year-to-year variation in environmental conditions.

Effects of afforestation of a páramo grassland on soil nutrient status

Plantations of radiata pine ( Pinus radiata) cover more than 4,000,000 ha worldwide [P.B. Lavery, D.J. Mead, Pinus radiata: a narrow endemic from North America takes on the world, in: D.M. Richardson (Ed.), Ecology and Biogeography of Pinus, Cambridge University Press, Cambridge, 1998, pp. 432–449]. In many areas, including the Ecuadorian Andes, these plantations have been established on former grasslands. Although this land use has grown over the past four decades in Ecuador, little is known about the effect of the change in vegetation cover on nutrient cycling in the high-altitude grassland systems where the plantations are frequently established. We examined changes in soil nitrogen, phosphorus, and acidity with stand age, using a chronosequence of stands ranging from 0 to 25 years of age. The effects of planting the grasslands with pine were most notable near the soil surface (0–10 cm), although in some cases changes deeper in the A horizon were also large. Total nitrogen concentrations became increasingly depleted along the chronosequence at intermediate depth ( P=0.04), while N was gained in the litter ( P=0.02) and upper-A horizon ( P=0.001) until the plantations reached 20 years, at which point it again declined. In the top 10 cm, concentrations of NO 3 − increased dramatically under pine ( P<0.001), while NH 4 + was lower than in grasslands ( P=0.04). Unlike nitrogen, neither total nor available phosphorus was significantly altered by the change in vegetation. Soil pH was higher in the grassland soils (5.5) than under pine stands of any age ( P<0.01), all of which had a mean pH of 5.2. Acidification under pine occurred only in the top 10 cm, with no differences in pH at other depths, indicating that it is being driven by soil processes that predominate in the near-surface environment. These results demonstrate that the change of vegetation can affect soil properties on a decadal time scale, with implications for long-term site productivity.

Effects of organic matter removal, soil compaction, and vegetation control on Collembolan populations

Collembola can be among the most numerous meso-invertebrates in the forest floor and, through their interaction with primary decomposers in the decomposition food web, may affect litter decomposition and consequently site productivity. This study was conducted to determine whether Collembolan abundance could be impacted by organic matter removal, compaction, and vegetation control on a loblolly pine ( Pinus taeda L.) plantation. Monthly soil and litter samples were taken over 2 years and the fauna extracted from them using modified Tulgren funnels. Organic matter removal and vegetation control generally caused a significant decrease in Collembolan populations, while compaction did not significantly affect Collembolan populations. These results indicate that habitat was the primary influence on population abundance in this experiment, possibly via its influence on desiccation. Sensitivity of collembolan populations to habitat changes caused by organic matter removal indicates a potential effect on long-term site productivity.

Disturbances and the sustainability of long-term site productivity in lodgepole pine forests in the central interior of British Columbia—an ecosystem modeling approach

The ecosystem-management model FORECAST was used to compare some ecological impacts of natural disturbance (wildfire) and timber harvesting. The scientific objective of the study was to assess whether or not two types of timber harvesting at various rotation lengths would have biogeochemical and biomass implications that are within the natural range of variation caused by wildfire. The practical objective was to identify management strategies that would sustain or improve long-term site productivity in lodgepole pine forests in the central interior of British Columbia. We defined three fire severity categories (low, medium and high), three fire return intervals (40, 80 and 120 years), two utilization levels (including stem-only harvesting [SOH] and whole-tree harvesting [WTH]), and three timber production rotation lengths (40, 80 and 120 years). Differences in simulated productivity, decomposing litter mass, total available soil nitrogen and nitrogen removals were compared for all 15 combinations of the five levels of disturbance at the three frequencies. The simulated nutritional impacts of timber harvesting were within the simulated range of impacts caused by the wildfire defined in this study. They were similar to the simulated long-interval, low-severity wildfire regimes. Simulations suggest that ecological rotation lengths for long-term site productivity for lodgepole pine forests in the study area would be 80–120 years. These rotation lengths are close to the average wildfire return intervals (100–125 years) in the study area, supporting the idea that the present harvesting strategies should sustain tree growth at this frequency of harvest and severity of harvesting impacts. Both WTH and SOH are acceptable harvesting methods for the maintenance of long-term site productivity in these lodgepole pine forests if harvest intervals are 80 years or longer. However, SOH is a more nutrient conservative harvest method, and should be used instead of WTH for rotations less than 80 years. The importance of initial site quality in assessing sustainable long-term site productivity by modeling is demonstrated.

Survival, growth, and growth allocation of planted Scots pine trees after different levels of biomass removal in clear-felling

A great concern in forestry today is whether whole-tree harvesting influences site productivity and whether it is consistent with the principle of sustainable use of forest resources. To evaluate this a randomised field experiment established 24 years ago in Scots pine ( Pinus sylvestris L.) in southern Sweden was used. Treatments were conventional stem harvest (CH), whole-tree harvest (WTH), and branch and stem harvest (BSH). Seedling survival was unaffected by treatments. The total basal area over bark at breast height (1.3 m, m 2 ha −1) was significantly reduced following WTH from the 15th year after planting. Sample trees on CH plots produced 20% more wood biomass than WTH and BSH plots, while biomass produced within the crown was unaffected by treatment. Height growth was greater for sample trees on CH plots during the last year of measurement, while basal area and volume under bark were larger from the 12th year onwards when compared with the WTH treatment. BSH showed a decreased basal area growth under bark during the two 4-year-periods, 13–16 and 17–20 years after planting, and a decreased volume growth under bark from year 9 onwards in comparison with CH. Radial growth was increased for CH up to 3 m of the stems during the 9–12-year period and at 3 m during the 13–16-year period in comparison with the other two treatments. The implications of the results are discussed in terms of long-term site productivity and overall silvicultural outcome.

The effect of land preparation at re-establishment on the productivity of fast growing hardwoods in South Africa

Appropriate land preparation at re-establishment is of major importance to forest managers to ensure maximum survival and vigorous early growth of South African plantation forests. This work revisits a series of trials carried out over the last two decades in an attempt to evaluate the long-term effects of land preparation at re-establishment on the productivity of fast—growing hardwoods (Acacia mearnsii and Eucalyptus spp.). A wide range of land preparation techniques was tested: ploughing, ripping, subsoiling, ridging and complete site preparation (de-stumping, ripping and discing) inter alia. Of the eleven trials originally planted, five reached rotation age (between 9 and 11 years) and, apart from one which was destroyed by a storm at 5 years, the rest, at time of writing, were close to clearfelling. With the exception of one trial where significant growth responses were recorded, and apart from a few significant lower-order interactions, final survival, basal area and volume were not affected by land preparation when compared to a standard pitting operation. Overall, growth responses in terms of basal area and volume to various forms of surface cultivation, ripping, subsoiling and terracing ranged between +21% and—23% for all species tested. Intensive site preparation (e.g. de-stumping, ripping/subsoiling and discing) has generally not resulted in improved growth of E. grandis or A. mearnsii across a range of sites and in some cases even resulted in a reduction in site productivity. Erratic growth responses of Eucalyptus spp. and A. mearnsii to ripping were recorded. Forest management should focus efforts on less intensive and more cost-effective methods of re-establishment such as pitting, shallow ripping to mark a planting line, or surface ploughing unless a known growth limitation exists (such as a root-limiting compacted B horizon overlying a horizon with available water). It is conjectured that the lack of major growth responses to intensive site preparation is due to a combination of the good physical condition of forest soils in the summer-rainfall growing region of South Africa, and beneficial perforation effects of tree roots from previous rotations. The results imply that topsoil disturbance and mixing of soil layers caused by intensive site preparation may pose a threat to long-term site productivity.

Sustained productivity in intensively managed forest plantations

Because of the rapid increase in the world’s population, demand for forest products is increasing while large amounts of forest land are being lost or degraded. In addition, timber harvest is being restricted in many of the world’s natural forests. The use of plantations managed for timber production must increase to meet the world’s increasing demand for wood and fiber from this reduced land base. Concentrating timber production on the best-adapted sites will allow the world’s demands to be met on fewer acres. Intensive management of plantation forests is perhaps the only way to meet the increasing demand for forest products and still reserve large areas of native forests for conservation and preservation purposes. Maintaining long-term soil productivity in these intensively managed plantations is critical. The impacts of intensive management on soil quality and subsequent tree growth can be positive, neutral or negative; the direction and magnitude of the impact depends on the specific management practice soil physical, chemical and biological properties. In order to understand and predict the impacts of intensive management, the factors limiting productivity on each specific site must be understood. Forest harvesting by itself tends to have minor impacts on soil quality and long-term site productivity. Compaction during timber harvesting can degrade the soil quality. However, tillage during site preparation can in most cases restore soil physical properties to pre-disturbance levels. Site preparation practices that remove large quantities of organic matter and surface soil can detrimentally impact soil quality, most notably on sandy soils. Intensive management practices such as fertilization can improve soil quality and increase site productivity by ameliorating factors limiting growth. The increased growth rates in intensively managed stands can indirectly improve soil quality by increasing organic matter. The increased production of coarse roots in intensively managed plantations is particularly important in this regard. These changes can lead to long-term improvement in soil quality and site productivity, especially on the degraded soils on which many forest plantations are established. Based on the available data, intensive management can be practiced sustainably on many soils. Land classification systems are needed to identify soils that are suitable for intensive management. Site-specific management regimes must then be developed to insure that intensive management is practiced sustainably on these soils.

Effects of selection harvest and prescribed fire on the soil nitrogen status of ponderosa pine forests

One hundred years of timber harvest and reduced fire frequency have resulted in the conversion of once open stands of ponderosa pine ( Pinus ponderosa) forests to dense forests dominated by Douglas-fir ( Pseudotsuga menziesii). Selection harvest and harvest with prescribed fire have been identified as possible tools to restore ponderosa pine stands to pre-settlement stand structures. Case studies were performed at three separate sites in western Montana to assess the influence of selection harvest and prescribed burning on soil N dynamics. These sites had been exposed to either selection harvest, selection harvest with prescribed burning, or a no-treatment control 0 (Lubrecht Experimental Forest), 2 (E/L Ranch), or 11 (Lick Creek Demonstration Site) years prior to initial soil analyses. Replicate soil samples were collected over at least two growing seasons at each site and analyzed for total C and N, potentially mineralizable N (PMN), short-term soil respiration rates, soil microbial biomass N, extractable NH 4 + and NO 3 −, and soluble sugars (measured as 0.5 M K 2SO 4 extractable anthrone reactive carbon (KARC)). Selection harvest without prescribed burning had little or no influence on levels of available N or microbial activity relative to the control at all three sites. Selection harvest with prescribed fire, however, significantly increased extractable NH 4 +, NO 3 − and KARC immediately following treatment. Such differences were not observed 2 or 11 years following treatment. Potentially mineralizable N was significantly increased immediately following fire, but decreased to levels lower than the control 1 year following treatment. Levels of PMN were also found to be less than the control 2 and 11 years after treatment. Similarly, microbial biomass N was elevated immediately following prescribed burning, but was significantly lower than the control for up to 11 years following prescribed burning. Levels of mineralizable N were lowered within a year of treatment as a result of (1) N loss during soil heating, (2) N loss to plant uptake, and (3) potential leaching losses. The effect of reduced mineralizable N on long-term site productivity is not clear, however, these losses of N from the ecosystem should be considered along with stand mortality and yield when assessing the potential sustainability of forest management strategies.

Agricultural site productivity: principles derived from long-term experiments and their implications for intensively managed forests

Long-term agricultural field experiments provide relevant information for questions being asked about the sustainability of managed forests, particularly as management regimes become more intensive. In this paper, concepts and criteria related to sustainable site productivity are reviewed and findings from a range of long-term agricultural field experiments are evaluated. Based on this evaluation, the following site productivity principles are identified: (1) soil organic matter is the link between most management systems and sustainable site productivity; (2) nutrient deficiencies can be corrected; (3) soil texture is a key variable affecting soil organic matter and site productivity; (4) return of crop residues enhances soil organic matter and site productivity; and (5) productive cropping systems have environmental benefits. While technological advances have proven to be highly successful for enhancing long-term crop yields, they also have the potential to mask underlying declines in site productivity. This hypothesis needs to be rigorously tested. Agricultural field experiments show that crop productivity can be sustained for long time periods when appropriate management approaches are applied. Management requirements to achieve sustainability differ, however, depending on site-specific edaphic and climatic characteristics and the needs of the crop. Site productivity principles derived from agricultural field studies are also highly relevant to sustaining site productivity of managed forests. Several characteristics of managed forests should be considered when extrapolating conclusions from agricultural experiments. These include harvest removals of biomass and nutrients, residue type and distribution, management and system characteristics, and soil types used. Many characteristics associated with even intensively managed forests are sought as goals of conservation cropping systems and should contribute toward sustaining long-term site productivity.

Simulations on impacts of different management strategies on long-term site productivity in lodgepole pine forests of the central interior of British Columbia

Impacts of different management strategies on long-term site productivity over a period of 240 years were simulated using the ecosystem Model FORECAST for lodgepole pine forests of the central interior of British Columbia (BC). Results suggested that the sustainable management strategies for maintaining long-term site productivity in lodgepole pine forests would be 80–120-year rotation lengths, with either stem-only or whole-tree harvesting. However, because of more nutrient conservation stem-only harvesting can achieve higher productivity (3.5–8.5%) than whole-tree harvesting does. The thinning strategy (from 4000 to 1000 stems/ha) would not increase total productivity over a period of 240-year simulation. Thinning, however, can increase the habitat value for Caribou. The best thinning strategies await further simulations on effects of various stocking levels in a broader context. The simulations also showed that organic matter and below-ground woody debris are critical for maintaining long-term site productivity in lodgepole pine forests in the study area, and a significant removal of such material may cause yield decline.

Humus in northern forests: friend or foe?

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.

Cations in solution from forest soils subjected to forest floor removal and compaction treatments

Degree of compaction and loss of organic matter from the forest floor have direct influence on the weathering rates of minerals, nutrient mineralization and consequently of plant growth. However, due to the complicated nature of nutrient depletion and uptake, continued study is essential in order to quantify the components involved in the long-term effects of forest operations (e.g., harvesting and site preparation) on nutrient supply. The objective of this investigation is to compare the amounts of Ca 2+, Mg 2+, K +, and Al 3+ in leachates from forest soils subjected to forest floor removal and soil compaction treatments in the long-term site productivity sites of the BC Ministry of Forests. A leaching experiment was conducted to simulate the long-term impacts of forest operations on soil fertility on 15 cm diameter undisturbed soil core collected from surface soil down to the lower boundary of Ae horizons. Approximately 25 l of water were leached through each column from October 1997 to April 1998. Leachates collected during the experiment were analyzed for Ca 2+, K +, Mg 2+ and Al 3+ and other cations using inductively coupled plasma-atomic emission spectrometry. Results showed that the amounts of Ca 2+, K +, Mg 2+ and Al 3+ in soil solution were highest in the treatment where forest floor was retained and the soil was not compacted and least in soils stripped of forest floor and subjected to compaction treatments. The availability indices for K + and Ca 2+ also decreased with the removal of forest floor. The slopes of linear fits between the amount of cations leached against cumulative volume of leachates could be used as indicators for the long-term effects of forest operations on soil fertility. For instance, and based on these slopes in Topley site, the long-term reduction in the concentration of K + in soil solution due to forest floor removal and compaction treatments could be as high as 88%.