Slash Retention Research Articles

Effects of harvesting intensity, vegetation control and fertilization on 5–20 year post-harvest N availability in boreal jack pine and black spruce forest soils in northern Ontario, Canada

The impact of harvesting intensity, vegetation control and fertilization on soil N availability 5–20 years post-harvest was determined at 18 jack pine and black spruce forests in northern Ontario, Canada. The study sites are affiliated with the North American long-term soil productivity (LTSP) study. Boreal forest growth is commonly N limited and young stands approaching crown closure acquire most of their N from the soil. The study sites represented a range of soil types and were harvested with three levels of organic matter removal (stem-only harvest, full-tree harvest and full-tree harvest with organic horizon removal by blading). Three study sites had multiple applications of glyphosate to control competing vegetation and one site included a trial with two applications of N, P, K fertilizer to the full-tree harvest treatment. We used short-term anaerobic laboratory incubations and in-situ closed top tube growing season incubations to determine mineralizable N and net N mineralization, respectively. Profile (organic plus mineral horizons) mineralizable N pools were higher in operational clearcut (stem-only and full-tree harvesting) treatments compared to the uncut reference forest 10–14 years post harvest. Full-tree harvesting with blading significantly reduced profile mineralizable N pools at this time compared to operational harvesting, while the same treatment at one upland site also reduced net N mineralization pools. Profile mineralizable N pools and microbial biomass C and N were lower in full-tree compared to stem-only harvesting treatments, suggesting that increased slash retention with stem-only harvesting was playing a role in enhanced soil N availability at a time when plantations were approaching a period of greater N demand. Fertilization and vegetation control treatments had few effects on N indices, although net N mineralization pools for the stem-only and full-tree harvest that also had the repeated herbicide treatment were consistently, but not significantly, lower than non-herbicided treatments. Soil N indices were useful predictors of dominant height growth for jack pine to a greater degree than for black spruce, and soil N concentrations were as effective at predicting height growth as more resource intensive measurements. Our results point to the importance of continued monitoring of soil and stand conditions at these LTSP sites as treatment effects on soil N cycling and site productivity will likely continue to evolve. At this stage of stand development, full-tree harvesting plus organic horizon removal, followed by operational harvesting has led to lower soil N availability than operational tree-length harvesting, and stands with understory vegetation control are now showing indications of reduced N availability.

Cumulative effects of salvage logging and slash removal on erosion, soil functioning indicators and vegetation in a severely burned area in NW Spain

Post-fire salvage logging, a common practice in productive forest stands, can lead to increased soil erosion and loss of soil quality. However, further field research is required to confirm and clarify these effects. This study was conducted in NW Spain, in productive mixed stands of P. sylvestris and P. pinaster affected in the summer of 2015 by a crown fire that caused a high level of soil burn severity. Salvage logging was carried out nine months after the fire, and the effects of logging slash retention and slash removal were compared after a further two years. The study objectives were: i) to assess soil loss in the pre-logging period and compare it with the soil losses accumulated during the two post-logging annual periods, ii) to determine whether logging slash can mitigate soil erosion after post-fire logging and iii) to compare the effects of slash cover and slash removal on soil physical, chemical and microbiological parameters and vegetation recovery and iv) to evaluate the short-term effect of salvage logging on soil functionality. Soil loss in the period between the wildfire and salvage logging (September-June) was 4.2 Mg ha−1. Soil loss was significantly lower (by 2.4 times) in the post-salvage logging period than in the pre-logging period. Slash cover significantly reduced soil loss (2.6 compared with 1.7 Mg ha−1), with no detrimental effect on soil properties and no effect on the recovery of natural vegetation cover.Around three years after a wildfire followed by logging, while soil physical parameters analyzed did not show values reflecting a severe impact, symptoms of delayed recovery of soil microbial and enzymatic attributes were detected. Our study also revealed a contrasting difference between the slow recovery of the soil physical and microbial properties and the rapid recovery of the vegetation. Further research will be needed to clarify that response.

Effects of Thinning Practice, High Pruning and Slash Management on Crop Tree and Stand Growth in Young Even-Aged Stands of Planted Silver Birch (Betula pendula Roth)

The objective was to quantify the influence of thinning, high pruning and slash management on crop tree and stand growth in young even-aged stands of planted silver birch (Betula pendula Roth). This study was based on two field experiments, aged six and eleven years at initiation and re-measured after six and eight years, respectively. Treatments included the unthinned control, moderate thinning mainly from below (removing 28–33% of standing volume), point thinning to favor 300 trees per ha and with no thinning elsewhere in the plot (removing 16–25%), and heavy thinning leaving 600 evenly distributed potential future crop trees per ha (removing 64–75%). Slash management (extraction or retention) was applied to heavily thinned plots. High pruning removing 30–70% of the green crown was carried out in some plots with point or heavy thinning on 300 or 600 trees per ha, respectively. Stand volume growth increased with increasing pre-treatment mean annual volume increment and decreased with increasing thinning intensity as compared to the unthinned control. LS-means estimates indicated a reduction for moderate thinning by 14%, for point thinning by 12% and for heavy thinning (combined with pruning) by 62%. However, in the youngest experiment, heavy thinning (without pruning) reduced growth by 54%. Combining these results with results from a similar experiment in Canada, the reduction in stand volume growth (RedIv%) depending on thinning removal (RemV%), both expressed as a percentage of the unthinned control, was quantified as RedIv% = −23.67 + 1.16·RemV% (calibration range: 30–83%). For heavy thinning (large quantities of slash), slash extraction resulted in no reduction in stand volume growth as compared to slash retention. The instantaneous numeric reduction in the average stem diameter of the 300 thickest trees per ha (D300) due to thinning was 3.5, 15–21% and 955–11% with moderate, point and heavy thinning, respectively. The subsequent average annual increase in D300 during the observation period was 8.5%, 25 and 18%, respectively. In the youngest experiment, pruning in unthinned plots led to a reduction in the annual increase of D300 by 14%, and heavy thinning in unpruned plots led to an increase by 30%. The growth of pre-selected potential future crop trees increased with increasing thinning intensity. In heavily thinned plots, pruning reduced growth increasingly with increasing pruning severity; LS-means estimates indicated 21% larger growth on stem diameter for unpruned trees and 3% for pruned trees. As an adverse side effect, heavily thinned plots with only 600 trees per ha were at increased risk of windthrow for some years after the thinning intervention. In the oldest experiment, 95–21% of the trees in these plots were damaged by wind.

Compaction and organic matter retention in mixed-conifer forests of California: 20-year effects on soil physical and chemical health

Soil resilience following harvest disturbance was studied at nine Long-Term Soil Productivity sites across the Sierra Nevada mountain range of California. We tested whether soil compaction, surface organic matter (OM) retention, and plant diversity have lasting effects on soil C, N, bulk density, or porosity. Study treatments following clearcut harvesting included soil compaction (none, moderate, severe) in factorial combination with three levels of OM retention (harvest slash retained, [OM2]; no slash retained, [OM1]; bare soil, [OM0]) and two plant diversity treatments (trees only; trees and shrubs). The sites encompassed a wide range in soil OM (5 to 12%) and clay content (5 to 36%) and represented common soil types in Sierran forests. At year 10, soil bulk densities were 10–29% greater for severely compacted compared to non-compacted plots across sites, corresponding to an average loss of 9–11% total porosity. Partial recovery was found by year 20, as bulk densities were 2–17% higher for compacted compared to non-compacted plots. In contrast to these physical responses, compaction had no adverse effect on soil C and N content at either year 10 or 20. Slash retention (OM2 versus OM1) and plant diversity treatments also had few effects on soil properties with the exception of a small increase in bulk density in the absence of shrubs, presumably due to reduced root and surface litter input. A more pronounced response was found following complete removal of surface organics (OM0), which produced a study-wide decline (12–17%) in soil C, N, and bulk density by year 20. All treatment responses were fairly consistent across sites, regardless of site differences in clay or soil OM content. Our results suggest substantial tolerance of soil C and N to harvest disturbance in Sierra Nevada forests despite a sustained change in soil physical properties and highlight the importance of the O horizon to long-term soil health.

Growth, physiological responses and wood production of an Acacia auriculiformis plantation in southern Vietnam following mid-rotation thinning, application of phosphorus fertiliser and organic matter retention

Acacia auriculiformis plantations are widely planted in Vietnam. Initially they were grown for wood chip production, but these plantations have potential to be managed for higher value sawing and/or peeling grade logs, through enhanced silvicultural management. This study sought to understand the impact of resource constraints on responses of A. auriculiformis plantations to thinning, phosphorus fertiliser application and slash retention at age four years on growth and physiology of A. auriculiformis trees to rotation end at age nine years. A trial in South Vietnam had a factorial combination of thinning (unthinned (planted at 1666 trees ha−1), or thinned to 833 trees ha−1), P fertilizer (Nil, or 50 kg P ha−1), and organic matter manipulation (slash and litter retained or removed).Thinning significantly increased photosynthetic rate (Amax) compared to control (unthinned) for at least 12 months after treatment application. Combined thinning and phosphorus fertiliser (P) application increased Amax, but Amax in the unthinned treatment did not significantly increase when supplied with additional P. Foliar nitrogen and phosphorus concentrations were greater in thinned than in unthinned treatments. Thinning commensurately reduced the leaf area, but subsequently, leaf area increased again in the thinned treatments compared to the unthinned treatments, such that twenty months after thinning there were no significant differences in leaf area index between thinned plus P fertiliser and unthinned treatments. There were significant differences in the mean diameter under different thinning regimes at one and five years after thinning. At nine years, average diameter of unthinned treatments (16.2 cm) was significantly less than the average diameter of the thinned treatments (19.0 cm). The application of P fertiliser or differences in management of slash and litter had no significant effect on mean tree diameter. The recovery of larger sawlogs was significantly higher in thinned (68%) than in unthinned treatments (25%). At nine years, the total stand volume of wood products from the thinned treatments (including the thinnings harvested in year four) was 340 m3 ha−1, which was 11 m3 ha−1 higher than for the unthinned treatments (329 m3 ha−1). The results suggest that a commercial mid-rotation thinning of A. auriculiformis in these environments can increase the value of these plantations to acacia growers.

The importance of harvest residue and fertiliser on productivity of Pinus patula across various sites in their first, second and third rotations, at Usutu Swaziland

BackgroundConcern is growing about the future of forestry productivity due to intensive nutrient removal, as a result of different harvesting operations. This study aimed to determine the effects on forest productivity when using different slash-retention scenarios with the recommended amounts of mineral fertiliser in Usutu forest. Usutu is a plantation forest that grows mostly softwood where the predominant species is Pinus patula Schiede ex Schltdl. & Cham.MethodsThe first trial series (F) comprised of one trial located in each of five forest blocks. It was established in 1971 and compared the effects of various site preparation scenarios (slash removal, slash retention and cultivation) on the early growth of Pinus patula for adjacent first (F1R) and second rotation (F2R) sites (i.e. grassveld and clearfelled first-rotation sites). The second (S) trial series was re-established in April 1991 on exactly the same position as the first trial series, and involved second (S2R) and third rotation (S3R) sites. Three main treatments, standard pitting through harvest residue (control); manual pitting after removal of harvest residue and forest floor (cleared); and manual pitting and broadcast application of dolomitic lime (2 t ha− 1) over the slash (lime), were undertaken in factorial combination with the application of phosphate and potassium fertiliser. The data reported here are for tree volume productivity across the five sites of the first trial series up to age 10 years and three of the five sites of the second trial series up to age 9 years.ResultsSlash removal decreased volume productivity by 9 and 13% in the F1R and F2R at 10 years of age and further by 21 and 33% in S2R and S3R, respectively at 9 years of age. However, fertiliser application increased volume productivity by 14 and 15% in the F1R and F2R at 10 years age and further by 18 and 10% in the S2R and S3R, respectively at 9 years of age.ConclusionsIn order to sustain or increase productivity, it is recommended that harvest residue slash should be conserved and fertiliser containing phosphate and potassium be applied at planting at Usutu.

Response of the soil microbial community and soil nutrient bioavailability to biomass harvesting and reserve tree retention in northern Minnesota aspen-dominated forests

Intensive forest biomass harvesting, or the removal of harvesting slash (woody debris from tree branches and tops) for use as biofuel, has the potential to negatively affect the soil microbial community (SMC) due to loss of carbon and nutrient inputs from the slash, alteration of the soil microclimate, and increased nutrient leaching. These effects could result in lowered forest productivity and threaten the long-term sustainability of forest management. Retaining organic material post-harvest, including greater amounts of harvesting slash and live trees, within harvested areas may ameliorate some negative effects of biomass harvesting on soil processes. We evaluated the effects of biomass harvests with reserve tree and slash retention on the SMC and soil nutrient bioavailability (assessed using plant-root simulator probes) in trembling aspen (Populus tremuloides Michx.) forests in northern Minnesota during the spring and summer, 1–3 years after harvest. Variable biomass removal levels tested include complete removal (whole tree harvest of boles and branches), complete slash retention (bole only harvest), and 20% slash retention (amount suggested by regional biomass harvesting guidelines). Compared to the unharvested control, biomass harvests had no effect on the multivariate SMC composition or microbial biomass, but did result in a 1–4% increase in arbuscular mycorrhizal fungal abundance and reduced bacterial stress two and three years after harvest. Additionally, biomass harvesting increased NH4 bioavailability during year one, and reduced NO3 bioavailability during year two when compared to unharvested controls. Among the three biomass harvests with differing levels of slash removal there were few differences in overall SMC composition, microbial biomass, and soil nutrients; however, the abundance of arbuscular mycorrhizal fungi, gram positive and actinomycete bacteria were significantly higher in harvested treatments with more slash retained. These results are specific to single rotation biomass harvesting in aspen stands due to the unique relationships between plants and their associated SMCs, and may not be directly applicable to forest biomass harvesting of other commercial forest tree species, or multiple rotations.

Fifteen-Year Patterns of Soil Carbon and Nitrogen Following Biomass Harvesting

The substitution of forest-derived woody biofuels for fossil fuel energy has garnered increasing attention in recent years, but information regarding the mid- and long-term effects on soil productivity is limited. We investigated 15-yr temporal trends in forest floor and mineral soil (0-30 cm) C and N pools in response to organic matter removal treatments (OMR; stem-only harvest, SOH; whole-tree harvest, WTH; and whole-tree plus forest floor removal, FFR) at three edaphically distinct aspen (Populus tremuloides Michx. and P. grandidentata Michx.) forests in the Great Lakes region. The OMR and temporal effects were generally site specific, and both were most evident in the forest floor and combined profile (mineral soil and forest floor) compared with the mineral soil alone. Forest floor and combined profile C and N pools were generally similar in the SOH and WTH treatments, suggesting that slash retention has little impact on soil C and N in this time frame. Temporal changes in C and N at one of the three sites were consistent with patterns documented following exotic earthworm invasion, but mineral soil pools at the other two sites were stable over time. Power analyses demonstrated that significant effects were more likely to be detected for temporal differences than the effects of OMR and in the combined profile than in the mineral soil. Our findings are consistent with previous work demonstrating that OMR effects on soil C and N pools are site specific and more apparent in the forest floor than the mineral soil.

Long-Term Growth Responses of a Jeffrey Pine Stand to Mechanized Thinning and Prescribed Fire

Forest thinnings implemented with cut-to-length and whole-tree harvesting systems followed by underburning were evaluated for their effects on individual tree and stand level growth responses in pure, uneven-aged Jeffrey pine (Pinus jeffreyi Grev. & Balf.) accompanied by isolated California white fir (Abies concolor var. lowiana [Gord.] Lemm.). Based on both dimension and volume measures, trees of the unburned whole-tree treatment combination exhibited the greatest individual growth responses. At the stand level, a diminished volume growth response in the whole-tree treatment was especially pronounced in the burned portion, mostly attributable to exaggerated stocking losses, while a superior response in the unburned cut-to-length combination likely reflected not only the absence of detrimental fire impacts but also benefits of on-site slash retention. For stand level biomass, diminished growth in the whole-tree treatment was again evident, with that in the burned portion again most pronounced, while biomass accrual in the unburned cut-to-length treatment combination was generally comparable to that in the unthinned control. Increasingly utilized in forest restoration efforts in the western USA, the responses presented herein to these thinning and burning practices provide natural resource managers insight into potential compromised outcomes when implemented in Jeffrey pine and similar dry site forest types.

Impacts of post-harvest slash and live-tree retention on biomass and nutrient stocks in Populus tremuloides Michx.-dominated forests, northern Minnesota, USA

Globally, there is widespread interest in using forest-derived biomass as a source of bioenergy. While conventional timber harvesting generally removes only merchantable tree boles, harvesting biomass feedstock can remove all forms of woody biomass (i.e., live and dead standing woody vegetation, downed woody debris, and stumps) resulting in a greater loss of biomass and nutrients as well as more severe habitat alteration. To investigate the potential impacts of this practice, this study examined the initial impacts (pre- and post-harvest) of various levels of slash and live-tree retention on biomass and nutrient stocks, including carbon (C), nitrogen (N), calcium (Ca), potassium (K), and phosphorus (P), in Populus tremuloides Michx.-dominated forests of northern Minnesota, USA. Treatments examined included three levels of slash retention, whole-tree harvest (WTH), 20% slash retention (20SR), and stem-only harvest (SOH), factored with three levels of green-tree retention, no trees retained (NONE), dispersed retention (DISP), and aggregate retention (AGR).Slash retention was the primary factor affecting post-harvest biomass and nutrient stocks, including woody debris pools. Compared to the unharvested control, stocks of biomass, carbon, and nutrients, including N, Ca, K, and P, in woody debris were higher in all treatments. Stem-only harvests typically contained greater biomass and nutrient stocks than WTH, although biomass and nutrients within 20SR, a level recommended by biomass harvesting guidelines in the US and worldwide, generally did not differ from WTH or SOH. Biomass in smaller-diameter slash material (typically 2.5–22.5cm in diameter) dominated the woody debris pool following harvest regardless of slash retention level. Trends among treatments in this diameter range were generally similar to those in the total woody debris pool. Specifically, SOH contained significantly greater amounts of biomass than WTH while 20SR was not different from either WTH or SOH.Within P. tremuloides systems, we observed high stocks of smaller diameter slash material for all prescribed slash retention treatments. Most notably, WTH retains much more material than anticipated, up to 50% of available slash. These results reflect the high levels of breakage during winter harvest operations in these stands and, consequently, warrant consideration when anticipating the impacts of biomass harvesting on woody debris pools. Further investigation is necessary to understand how deliberate slash retention levels and season-of-harvest impact woody debris in other forest systems.

Influence of variable organic matter retention on nutrient availability in a 10-year-old loblolly pine plantation

The effects of varying forest floor and slash retention at time of regeneration were evaluated 10 years after the establishment of a loblolly pine plantation near Millport, Alabama. Treatments included removing, leaving unaltered, or doubling the forest floor and slash material. Forest floor and litterfall mass and nutrient concentrations, available soil N, foliar nutrient concentrations and stand yield were all impacted by the treatments. Forest floor mass and nutrient contents in the doubled treatment were significantly greater than the other two treatments. The doubled treatment accumulated 25, 45 and 350% more forest floor mass and 56, 56, and 310% more N than the control treatment in the Oi, Oe, and Oa layers, respectively. The other nutrients followed similar patterns. Potentially mineralized NO 3 −-N in the mineral soil was also significantly higher in the doubled treatment. The positive effect of doubling the forest floor on soil N availability was reflected in greater foliage production, 30% more litterfall and 25% more stand yield for this treatment. This study shows that increasing the forest floor retention has resulted in increased nutrient availability and improved tree growth.

Developing resilient ponderosa pine forests with mechanical thinning and prescribed fire in central Oregon's pumice region

Thinning and prescribed burning are common management practices for reducing fuel buildup in ponderosa pine forests. However, it is not well understood if their combined use is required to lower wildfire risk and to help restore natural ecological function. We compared 16 treatment combinations of thinning, prescribed fire, and slash retention for two decades across a site quality gradient of second-growth pine stands, measuring changes in forest vegetation growth, structure, and composition. Thinning alone doubled the diameter growth increment of ponderosa pine, moderately stimulated shrub production, and resulted in lower tree mortality compared with unthinned plots. In contrast, repeated fire alone did not substantially alter stand structure or increase tree vigor, herbaceous production, or plant diversity. The combined use of thinning and repeated burning reduced shrub cover, yet produced no changes in herbaceous production, plant diversity, stand structure, or tree vigor compared with thin-only treatments. Additional findings identified (1) inconsequential effects of thinning residues on site productivity, (2) the need for multiple entries of prescribed fire if the abatement of shrubs is required, (3) the ineffectiveness of repeated burning to stimulate plant growth, and (4) that the thinning treatment served as an effective surrogate to fire for managing central Oregon forest vegetation.

Effects of slash retention and wood ash addition on fine root biomass and production and fungal mycelium in a Norway spruce stand in SW Sweden

In the study reported here we examined the short-term effects (1–3 years) of slash retention (SR) and the long-term effects (13–15 years) of wood-ash application (A) on fine roots and mycorrhizae in a 40-year-old Norway spruce forest in southwest Sweden. Soil cores were used to obtain estimates of the biomass (g m −2) of roots in three diameter classes (<0.5, 0.5–1 and 1–2 mm), root length density (RLD), specific root length (SRL) and mycorrhizal root tip density (RTD). Fine root (<1 mm) length production and mortality, and mycelium production, were estimated using minirhizotron and mesh bag techniques, respectively. Compared with the control plots (C), the biomass of fine roots in diameter classes <0.5 mm and 0.5–1 mm was significantly higher in A plots, but lower in SR plots. In addition, RLD was significantly lower in the humus layer of SR plots than in the humus layers of C and A plots, but not in the other layers. None of the treatments affected the SRL. In all soil layers, the SR treatment resulted in significant reductions in the number of ectomycorrhizal root tips, and the mycelia production of fungi in mesh bags, relative to the C treatment, but the C and A treatments induced no significant changes in these variables. Fine root length production in the C, A and SR plots amounted to 94, 87 and 70 mm tube −1 during the 2003 growing season, respectively. Fine root mortality in treated plots did not change over the course of the study. We suggest that leaving logging residues on fertile sites may result in nitrogen mineralisation, which may in turn induce reductions in root biomass, and both root and mycelium production, and consequently affect nutrient uptake and the accumulation of organic carbon in soil derived from roots and mycorrhizae.

Effects of fire frequency and mowing on a temperate, derived grassland soil in south-eastern Australia

Frequent disturbances such as fire are widely considered important drivers of plant composition and diversity in productive grassy ecosystems. Effects of fire frequency on grassland soils, however, are less well understood. We established replicated disturbance regimes in a high-quality, representative Themeda australis–Poa sieberiana-derived grassland in south-eastern Australia that had historically been burnt every 4–8 years. Effects on soil chemical, physical and biological properties were measured after 10 years of application of 2-, 4-, and 8-yearly burning, 2-yearly mowing and an undisturbed treatment. Contrary to other grassy ecosystems, there were no detectable effects of disturbance regime on total soil nitrogen and carbon, or a range of other soil chemical properties in the top 10 cm. However, a cumulative effect of burning on the grassland soil was evident from a suite of changes to soil surface properties, available nutrients and biological activity. In particular, on biennially burnt plots, reduced litter and plant protective cover were associated with increased soil surface compaction, decreased infiltration and decreased soil biological activity, which in turn were related to poor sward recovery after fire and drought. These relationships indicate potential for positive feedbacks whereby repeated removal of soil protective cover and changes to soil surface chemistry through very frequent burning ultimately lead to further reduction in soil protective cover through reduced productivity. However, this is only likely in extreme cases: data from unburnt plots indicated that soils that had historically been burnt every 4–8 years had not passed a threshold beyond which such soil changes were irreversible or damaging. Contrary to other predictions, cessation of burning for 13 years did not lead to detectable soil nutrient release through senescence of dominant grasses. Biennial mowing with slash retention was an effective alternative disturbance for maintaining sward vigour while avoiding soil surface damage.

Effects of site management treatments on growth of six-year-old, second-rotation Chinese fir (Cunninghamia lanceolata) plantations

The effects of five different site management treatments on the productivity of a six-year-old and secondrotation Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) plantations planted after harvesting a 29-year-old and first-rotation Chinese fir plantation in Xiayang State Forest Farm, Nanping, Fujian Province, were studied. Results showed that the Chinese fir grew best on plots treated with the double slash treatment (BL3), followed by the whole tree harvest (BL1) and the slash burning treatment (SB), and poorest on treatment BL2 (normal slash retention) and BL0 (removal of all organic matter aboveground). The site index of the second rotation Chinese fir plantations in BL3 and BL0 treatments increased by 0.56 and 0.27, respectively, compared with the first rotation, and decreased in the rest of the three treatments. Compared with the first rotation, the site index of the second rotation treated with BL1, SB and BL2 treatments decreased by 0.39, 0.45 and 0.63, respectively. Differences among the treatments were not statistically significant.

Effect of site management on leaf area, early biomass development, and stand growth efficiency of a Eucalyptus grandis plantation in South Africa

The effects of intensive site management operations on leaf area index (LAI), aboveground woody biomass (WB) development, and stand growth efficiency were studied in a South African Eucalyptus grandis (Hill ex Maiden) stand from establishment to 3.5 years of age. Site management treatments included slash removal, slash retention, fertilization, slash burning, and slash disturbance through mechanized harvesting operations. Stand LAI development responded strongly and significantly to the site management treatments imposed at establishment. Responses were driven by nutrient supply but were apparently limited by a severe drought after establishment. At 1.3 years, LAI in the slash removal treatment reached 2.1, compared to 4.5 following fertilization or slash burning. Treatments also significantly affected the development of aboveground WB, which ranged from 53.6 (slash removal) to 64.9 (fertilization) and 65.8 Mg·ha–1 (slash burning) at 3.5 years. Biomass allocation to roots and stand growth efficiency (i.e., WB increment per unit of LAI) did not differ significantly between treatments. We conclude that the fast-growing treatments produced more WB because of more rapid development of maximum LAI. The possible implications of this response mechanism for management operations that aim to improve growth through improved nutrient supply are discussed, with reference to unthinned, short-rotation crops.

Silvicultural effects on the productivity and wood quality of eucalypt plantations

The productivity of most forest plantations is less than their physiological potential as defined by the prevailing climate, because the supply or capture of light, water and nutrients is less than optimal. However, maximum growth does not equate to maximum wood value. The silvicultural challenge is to design and use management regimes that achieve target growth rates and wood quality by manipulating resource supply, capture or use. It has been and remains possible to identify and ameliorate factors limiting growth, sometimes on a large scale, e.g. by soil cultivation, residue management, fertiliser, weed control, irrigation, coppice management, thinning and pruning. This review examines some of these silvicultural practices for eucalypt plantations. During the past two decades, an increase in research on silvicultural practices in relation to soil and plant processes has improved our understanding of their effects on tree growth and wood quality, and on non-wood values including the potential for adverse on- or off-site effects. Eucalypt silviculture usually targets high growth rates and short rotations. High growth rates, and to some extent low nutrient-use efficiencies of eucalypts, lead to high nutrient demand and a high potential for nutrient depletion if management does not facilitate nutrient conservation. A detailed examination of nutrient management is presented. We indicate how silvicultural information is incorporated into current empirical- and process-based systems of productivity prediction, and identify potential improvements to these models. Silvicultural options are included adequately in some empirical models, and some process-based productivity models adequately cater for pruning and thinning, but process-based predictions of other silvicultural options, e.g. cultivation, slash retention, fertiliser, weed control and pest management, remain in their infancy.