Soil quality relationships with tree growth in exotic forests in New Zealand

Quantitative information on the relationships between site quality and plantation productivity (dominated by the exotic species Pinus radiata) is required to achieve goals for sustainable forest production. Soil quality is a key component of site quality. A nationwide study of soil quality measurements is reported for 35 representative forest sites, covering a wide range of climatic and edaphic conditions found throughout New Zealand's plantation forest estate, representing most of the soils used for plantation forestry in New Zealand. The objectives of the study were to find the most important soil properties that discriminated among eight New Zealand Soil Orders and determine relationships between Soil Orders and early tree growth rates for P. radiata and Cupressus lusitanica. Soil physical and chemical properties were measured to identify key soil indicators of soil quality related to tree productivity. Tree growth was measured after four years on small plots planted at very high stand density (40 000 stems ha⁻¹). A factorial design was used to examine the influence of three factors on tree productivity: two species, P. radiata D. Don (ectomycorrhizal) and C. lusitanica Miller (endomycorrhizal); with and without fertilizer; and low or high disturbance (soil compaction and/or topsoil scalping by machinery). Carbon content, Phosphorus (P) retention, and soil physical properties that index the degree of soil compactness were strongly correlated to Soil Order. These properties are similar to soil quality factors that correlated with tree growth. Discriminant analyses of soil quality parameters by Soil Order clustered soils based on P retention (phosphate absorption capacity), subsoil Carbon (C), and subsoil air capacity (volume % of voids at 10 kPa matric potential). Allophanic Soils and Podzols clustered (from plots of first versus second canonical variates) separately from the other Soil Orders, which were somewhat clustered on the second variate within a broad clustering on the first variate. Soil Orders were ranked for tree growth rates for both species: pumice Andisols > Inceptisols > tephric Andisols > Entisols > Ultisols > Spodosols (NZ classification: for P. radiata is Pumice > Brown > Pallic > Allophanic > Recent > Raw > Ultic > Podzol and for C. lusitanica Pumice > Pallic > Allophanic > Brown > Raw > Ultic > Recent > Podzol).     

Predicting the severity of Dothistroma on Pinus radiata under current climate in New Zealand

Despite being a damaging foliar disease of Pinus species little research has characterised spatial variation in disease severity of Dothistroma needle blight at a macroscale. Using an extensive dataset describing Dothistroma needle blight severity (S_sev) on plantation grown Pinus radiata stands distributed widely across New Zealand the objectives of this research were to (i) develop a regression model describing S_sev, (ii) use this model to identify key drivers of S_sev, their functional form and relative importance, and (iii) develop spatial predictions of S_sev for New Zealand P. radiata under current climate. Using an independent validation dataset, the final model accounted for 72% of the variance in S_sev using four significant (P < 0.001) explanatory variables and an isotrophic exponential model to account for the spatial covariance in the data. S_sev was most sensitive to mean air temperature from November to April (T_Nov-Apr), followed by mean relative humidity from October to April (RH_Oct-Apr), mean total November rainfall (P_Nov), and then stand age (A). There was a quadratic correlation between A and S_sev with S_sev increasing to a maxima at 12 years before declining. S_sev exponentially increased to a threshold with increases in both P_Nov and RH_Oct-Apr. The relationship between S_sev and T_Nov-Apr was quadratic with S_sev increasing to a maximum at T_Nov-Apr of 15.5 °C before declining at higher values of T_Nov-Apr. Spatial predictions of S_sev varied widely throughout New Zealand. Values of S_sev were highest in moderately warm wet environments in the North Island, and on the west coast of the South Island. In contrast, relatively low values of S_sev were predicted in drier eastern and southern regions of New Zealand.     

A nutrient balance model (NuBalM) to predict biomass and nitrogen pools in Pinus radiata forests

Forest managers are increasingly required to enhance the productivity and profitability of plantation management while simultaneously reducing the negative ecological effects associated with forest operations. NuBalM (from Nutrient Balance Model) is presented here as a decision support tool that has the potential to assist forest managers in meeting these requirements in Pinus radiata D. Don (radiata pine) plantations. NuBalM incorporates nutrient dynamics and allocation into projections of growth, allowing management techniques to be optimised for productivity and nutrient pool retention over single or multiple rotations.NuBalM was developed using data from biomass, nutrient allocation and soil nutrient dynamics studies conducted in New Zealand radiata pine plantations. The capability of NuBalM to predict stem wood mass based on nitrogen supply and demand was tested against data from multiple trial sites established to examine the effects of variations in stocking, thinning and fertilization regimes. NuBalM satisfactorily predicted stem wood masses across a range of stand ages, with the exception of a trial examining ultra-high applications of nitrogen fertilizer. With the exclusion of the data from this trial, the predicted stem wood masses underestimated the observed figures by a mean value of 1.1 ± 1.0 t ha⁻¹ (95% CI, n = 92).The utility of NuBalM as a tool to predict biomass allocation in radiata pine and nitrogen pools in the forest floor and soil was assessed using comprehensive biomass, nutritional and site data collected from two radiata pine trial sites subjected to differences in organic matter removal at site establishment. NuBalM performed acceptably, generating accurate estimates of stem mass (mean overestimate of 5.5 ± 7.4 t ha⁻¹, 95% CI, n = 6) and total above ground biomass (mean overestimate of 3.1 ± 9.6 t ha⁻¹, 95% CI, n = 6). The effects of organic matter removal and fertilization on total nitrogen pools were also predicted with a reasonable degree of accuracy (mean overestimate of 52 ± 53 kg N ha⁻¹, 95% CI, n = 9).From these results we conclude that NuBalM can be utilised to provide projections of productivity and nitrogen pools in radiata pine plantations, and enables the effects of various management practices to be predicted with a reasonable degree of confidence.     

Decomposition of woody debris in managed Pinus radiata plantations in New Zealand

Decay rates of stems, branches and roots were assessed in Pinus radiata (D. Don) plantation forests located throughout New Zealand. Stem and branch decay rates were obtained using (1) post-harvest material from two central North Island locations (Kaingaroa and Tarawera Forests) based on a 10-year chronosequence (in ground contact or suspended) and (2) post-thinning stems and attached branch material from five sites covering a range of climatic conditions across New Zealand (Woodhill, Puruki, Hokonui, Nemona and Selwyn) with up to 5 years of decay. Stem, stump and root decay rates were determined from two central North Island locations (Kinleith and Puruki) from thinnings with 0, 5, and 10 years of decay (Kinleith) and mature trees at 0 and 11 years of decay (Puruki). Stem and branch post-harvest material decayed faster when in contact with the ground than when elevated above the ground. The proportion of material elevated or in ground contact was not estimated in this study. P. radiata discs from young trees and post-harvest residue showed no significant diameter effect on decay rate and could be used confidently to predict whole stem decay rate. Discs from older trees covering a larger diameter range at Puruki showed a significant effect of diameter on decay rate. Decay rates of coarse roots at the two central North Island sites were faster than above-ground whole stem decay rates. Exponential models incorporating mean annual temperature for P. radiata stems in ground contact arising from thinning and harvest currently provide the best estimate of residue decay in New Zealand. There was no increase in carbon concentration with decay, suggesting that live stem values may be applied to all dead wood for determining the mass of carbon change with decomposition. Nitrogen concentrations increased with decay.     

Potential for Cleopus japonicus to control the weed Buddleja davidii in plantation forests in New Zealand

Buddleja davidii (buddleia) is an invasive weed in commercial Pinus radiata D. Don plantation forests in New Zealand. For the first time, forestry managers are assessing the potential of a newly established biological control agent to manage B. davidii. Initial releases of the leaf feeding weevil Cleopus japonicus Wingelmüller (Coleoptera: Curculionidae) were made in 2006 at three P. radiata plantations located in cool temperate areas within the North Island in New Zealand. At each site, 1000 adults were released at a central point and their movement monitored along four 150 m transects, radiating outwards from the point of release, for three years. Measurements included the establishment and rates of spread of C. japonicus and damage to B. davidii by the weevils.C. japonicus established at all sites. Feeding damage to B. davidii was strongly seasonal, with damage peaking each autumn, and achieving up to 95% defoliation within three years. A model of adult dispersal probability from the central release point predicted the population front was moving on average 65 m, 42 m and 27 m per annum at each of the three sites. There was an initial time lag of approximately one year between the arrival of the weevil dispersal front, and the agent causing damage of greater than 30% defoliation to B. davidii.Effective B. davidii control needs to occur in the first few years after planting. Our results suggest that C. japonicus could be quite effective at controlling B. davidii during this time if integrated with a complementary weed management technique, such as the spot application of herbicides around plantation seedlings, rather than broadcast application. This would maximize C. japonicus establishment throughout stands.     

Productivity of three successive rotations of P. radiata plantations in South Australia over a century

In the southeast of South Australia and the adjacent south west of Victoria (called the Green Triangle Region), planting of Pinus radiata D. Don (radiata pine) plantations began about 130 years ago and these now occupy 175,000 ha. They are growing on sites with generally podsolised sandy soils, low in soil fertility and where evaporation typically exceeds precipitation for five to six months of the year. Regional softwood industries producing a range of value added products are completely dependant on the versatile radiata pine wood from these plantations.     

Comparison of spatial prediction techniques for developing Pinus radiata productivity surfaces across New Zealand

Spatial interpolation is frequently used to predict values across a landscape enabling the spatial variation and patterns of a property to be quantified. Inverse distance weighting (IDW), ordinary kriging (OK), regression kriging (RK), and partial least squares (PLS) regression are interpolation techniques typically used where the region of interest's spatial extent is relatively small and observations are numerous and regularly spaced. In the current era of data ‘mining’ and utilisation of sparse data, the above criteria are not always fully met, increasing model uncertainties. Furthermore, regression modelling and kriging techniques require good judgement, experience, and expertise by the practitioner compared with IDW with its more rudimentary approach. In this study we compared spatial predictions derived from IDW, PLS, RK, and OK for Pinus radiata volume mean annual increment (referred to as 300 Index) and mean top height at age twenty (referred to as Site Index) across New Zealand using cross-validation techniques. Validation statistics (RMSE, ME, and R²) show that RK, OK, and IDW provided predictions that were less biased and of greater accuracy than PLS predictions. Standard deviation of rank (SDR) and mean rank (MR) validation statistics showed similar results with OK the most consistent (SDR) predictor, whereas RK had the lowest mean rank (MR), closely followed by IDW. However, the mean performance rankings for validation observations classified according to their distance to the nearest model data point indicate that although PLS provided the poorest predictions at relatively close separation distances (<2 km), in the medium range (∼4–8 km) performance was of similar ranking to that of the other techniques, and at greater separation distances PLS outperformed the other techniques. Maps illustrating the spatial variation of P. radiata forest productivity are provided.     

Harvest residue management and fertilisation effects on soil carbon and nitrogen in a 15-year-old Pinus radiata plantation forest

Growing interest in the use of planted forests for bioenergy production could lead to an increase in the quantities of harvest residues extracted. We analysed the change in C and N stocks in the forest floor (LFH horizon) and C and N concentrations in the mineral soil (to a depth of 0.3 m) between pre-harvest and mid-rotation (stand age 15 years) measurements at a trial site situated in a Pinus radiata plantation forest in the central North Island, New Zealand. The impacts of three harvest residue management treatments: residue plus forest floor removal (FF), residue removal (whole-tree harvesting; WT), and residue retention (stem-only harvesting; SO) were investigated with and without the mean annual application of 190 kg N ha⁻¹ year⁻¹ of urea-N fertiliser (plus minor additions of P, B and Mg). Stocks of C and N in the forest floor were significantly decreased under FF and WT treatments whereas C stocks and mass of the forest floor were significantly increased under the SO treatment over the 15-year period. Averaged across all harvesting treatments, fertilisation prevented the significant declines in mass and C and N stocks of the forest floor which occurred in unfertilised plots. The C:N ratio of the top 0.1 m of mineral soil was significantly increased under the FF treatment corresponding to a significant reduction in N concentration over the period. However, averaged across all harvesting treatments, fertilisation prevented the significant increase in C:N ratio of the top 0.1 m of mineral soil and significantly decreased the C:N ratio of the 0-0.3 m depth range. Results indicate that residue extraction for bioenergy production is likely to reduce C and N stocks in the forest floor through to mid-rotation and possibly beyond unless fertiliser is applied. Forest floors should be retained to avoid adverse impacts on topsoil fertility (i.e., increased C:N ratio). Based on the rate of recovery of the forest floor under the FF treatment, stocks of C and N in the forest floor were projected to reach pre-harvest levels at stand age 18-20. While adverse effects of residue extraction may be mitigated by the application of urea-N fertiliser, it should be noted that, in this experiment, fertiliser was applied at a high rate. Assessment of the sustainability of harvest residue extraction over multiple rotations will require long-term monitoring.     

Productivity of radiata pine (Pinus radiata D. Don.) clones in monoclonal and clonal mixture plots at age 12 years

Productivities of monoclonal plots and clonal mixtures of 10 radiata pine (Pinus radiata D. Don.) clones were compared in a trial established in 1993 at Dalethorpe, Canterbury, New Zealand. Ten monoclonal and one mixture of the 10 clones were planted in a complete randomised block design with three replications using 40-tree plots (un-thinned, pruned to 2.5 m, stocking of 1250 stems per hectare). The study was conducted to determine if mode of deployment (monoclonal versus clonal mixture) affected overall productivity and how or if each clone was affected by mode of deployment.     

Intensive,site-specific silviculture: Manipulating resource availability at establishment for improved stand productivity. A review of South African research

An important window of opportunity to increase and sustain productivity in short-rotation plantations is the period from felling through re-establishment to canopy closure. This paper explores the effects, interactions and response mechanisms of intensive silvicultural practices on plantation productivity and sustainability, using five South African case studies (a–e). (a) Land preparation trials showed that complete surface cultivation by ploughing had a significant beneficial effect when afforestation is done for the first time in grasslands, improving basal area growth by 11–52% over pitting only. However, similar treatments have not resulted in significant growth responses under re-establishment conditions. (b) Stand growth suppression resulting chiefly from soil compaction during mechanised harvesting operations is strongly related to soil type, soil textural class and residue management options. Volume growth reduction in short-rotation eucalypt crops ranged from 25% on compaction sensitive loamy soils to less than 2% in resistant sandy soils. (c) The response mechanism whereby vegetation management improves stand productivity is a reduction in both inter-specific and intra-genotypic competition for resources, as well as a decrease in stand variability. Operationally, the most important criteria in a vegetation management programme relate to the timing of control operations across diverse site conditions. In local trials, the primary factors controlling the time taken for competition-induced tree growth suppression to occur were related to altitude, slash burning and the interaction between these factors, which facilitated the development of regional vegetation management strategies. (d) Empirical fertilizer trials in short-rotation hardwood stands have shown significant improvements in final productivity (commonly 20–90 m³ ha⁻¹ in eucalypts and 30–50 m³ ha⁻¹ in Acacia), as well as wood density (15–30 kg m⁻³ for eucalypts) following improvements in early nutrition. Improved nutrition was achieved through fertilization at planting or indirectly through residue management. The response mechanism is primarily due to early canopy development and associated increases in light capture, coupled with a more modest increase in canopy quantum efficiency and above-ground carbon allocation on a dry site. On sites with abundant water supply, increased quantum efficiency is likely to be the dominant response mechanism. (e) A series of operational gains trials tested the interactive effect of genetic tree improvement, site–genotype interaction, stand density and vegetation management + fertilization on eucalypt stand growth across five sites. There were no significant interactions between factors, but importantly, the results were additive, emphasizing the need to optimise each practice in the value chain to achieve maximum productivity.     

Potential knowledge gain in large-scale simulations of forest carbon fluxes from remotely sensed biomass and height

Global vegetation models (GVMs) simulate CO₂, water and energy fluxes at large scales, typically no smaller than 10 × 10 km. GVM simulations are thus expected to simulate the average functioning, but not the local variability. The two main limiting factors in refining this scale are (1) the scale at which the pedo-climatic inputs - temperature, precipitation, soil water reserve, etc. - are available to drive models and (2) the lack of geospatial information on the vegetation type and the age of forest stands. This study assesses how remotely sensed biomass or stand height could help the new generation of GVMs, which explicitly represent forest age structure and management, to better simulate this local variability. For the ORCHIDEE-FM model, we find that a simple assimilation of biomass or height brings down the root mean square error (RMSE) of some simulated carbon fluxes by 30-50%. Current error levels of remote sensing estimates do not impact this improvement for large gross fluxes (e.g. terrestrial ecosystem respiration), but they reduce the improvement of simulated net ecosystem productivity, adding 13.5-21% of RMSE to assimilations using the in situ estimates. The data assimilation under study is more effective to improve the simulation of respiration than the simulation of photosynthesis. The assimilation of height or biomass in ORCHIDEE-FM enables the correct retrieval of variables that are more difficult to measure over large areas, such as stand age. A combined assimilation of biomass and net ecosystem productivity could possibly enable the new generation of GVMs to retrieve other variables that are seldom measured, such as soil carbon content.     

Identification of key soil indicators influencing plantation productivity and sustainability across a national trial series in 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. Despite this, there is little quantitative evidence of how plantation forestry affects site quality and long-term site productivity. To address this issue, a nationwide study of site quality was established at 31 key sites covering the range of edaphic and environmental conditions for New Zealand plantation forests. At each location, 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. The plots at each site were arranged in a factorial design with the following three factors: species (Pinus radiata D. Don and Cupressus lusitanica Mill.), fertiliser (no fertiliser and nutrients supplied in excess of crop demands) and disturbance (low and high disturbance). These plots were harvested after 4 years and data was used to (i) examine treatment and site effects on volume mean annual increment (MAI) and (ii) identify key soil properties that influence volume MAI for the two species.Volume MAI significantly ranged 7-fold across sites. For the treatments, species accounted for most of the variance in volume MAI, with values for P. radiata significantly exceeding those of C. lusitanica by 95%. Volume MAI was significantly affected by fertilisation which induced gains of 33%. Disturbance did not significantly affect volume MAI. After correction had been made for climatic variables, soil properties that were most strongly related to volume MAI for both species included C:N ratio, total phosphorus (P), and organic P. When soil properties were included in combination, the best predictive models of volume MAI formulated for P. radiata included total P and C:N ratio while the best predictive model for C. lusitanica included C:N ratio and Olsen P. Variation in species sensitivity to total P and C:N ratio is likely to be attributable to their different mycorrhizal associations. For both species, the most important soil physical property influenced by management operations was total porosity. This research suggests that total P, C:N ratio, Olsen P and total porosity should be used as indicator properties for determining sustainability of plantation grown P. radiata and C. lusitanica.     

Why is the productivity of Douglas-fir higher in New Zealand than in its native range in the Pacific Northwest,USA?

Douglas-fir (Pseudotusga menziesii (Mirb.) Franco), a native to the Pacific Coast Range in North America, is recognized as a tree that is long-lived and can grow rapidly to standing volumes that approach the highest recorded for temperate conifers. Managed plantations in western Oregon register maximum periodic annual increments (PAI) at ages between 20 and 40 years of ∼30 m³ ha⁻¹ year⁻¹. The same seed source, when planted in New Zealand and elsewhere in the Southern Hemisphere, may attain a PAI of ∼50 m³ ha⁻¹ year⁻¹. Is this higher productivity mainly related to climate or to isolation from native pests? To evaluate the role of climate, we obtained meteorological data from plantation sites in New Zealand and Oregon, from which we established relationships between mean monthly temperature extremes and solar irradiance, air humidity deficits, and frost frequency. Using these empirical relationships, long-term weather records were converted to the meteorological variables required to drive a process-based forest growth model, 3-PG, for sites approaching the most productive in New Zealand and in Oregon. Annual precipitation is similar in both areas, but sites in Oregon receive only 10% during the growing season, resulting in humidity deficits 30% larger than those recorded in New Zealand. According to sensitivity analyses and direct measurements, the more productive Douglas-fir forests in Oregon avoid the limiting effects of summer drought by obtaining water from the subsoil and fractured bedrock. If such forests were under a reduced evaporative demand similar to that in New Zealand, we show that they would exhibit comparable productivity.     

Influences of forest tending works on carbon distribution and cycling in a Pinus densiflora S. et Z. stand in Korea

This study was conducted to determine carbon (C) dynamics following forest tending works (FTW) which are one of the most important forest management activities conducted by Korean forest police and managers. We measured organic C storage (above- and below-ground biomass C, forest floor C, and soil C at 50 cm depth), soil environmental factors (soil CO₂ efflux, soil temperature, soil water content, soil pH, and soil organic C concentration), and organic C input and output (litterfall and litter decomposition rates) for one year in FTW and non-FTW (control) stands of approximately 40-year-old red pine (Pinus densiflora S. et Z.) forests in the Hwangmaesan Soopkakkugi model forest in Sancheonggun, Gyeongsangnam-do, Korea. This forest was thinned in 2005 as a representative FTW practice. The total C stored in tree biomass was significantly lower (P < 0.05) in the FTW stand (40.17 Mg C ha⁻¹) than in the control stand (64.52 Mg C ha⁻¹). However, C storage of forest floor and soil layers measured at four different depths was not changed by FTW, except for that at the surface soil depth (0–10 cm). The organic C input due to litterfall and output due to needle litter decomposition were both significantly lower in the FTW stand than in the control stand (2.02 Mg C ha⁻¹ year⁻¹ vs. 2.80 Mg C ha⁻¹ year⁻¹ and 308 g C kg⁻¹ year⁻¹ vs. 364 g C kg⁻¹ year⁻¹, respectively, both P < 0.05). Soil environmental factors were significantly affected (P < 0.05) by FTW, except for soil CO₂ efflux rates and organic C concentration at soil depth of 0–20 cm. The mean annual soil CO₂ efflux rates were the same in the FTW (0.24 g CO₂ m⁻² h⁻¹) and control (0.24 g CO₂ m⁻² h⁻¹) stands despite monthly variations of soil CO₂ efflux over the one-year study period. The mean soil organic C concentration at a soil depth of 0–20 cm was lower in the FTW stand (81.3 g kg⁻¹) than in the control stand (86.4 g kg⁻¹) but the difference was not significant (P > 0.05). In contrast, the mean soil temperature was significantly higher, the mean soil water content was significantly lower, and the soil pH was significantly higher in the FTW stand than in the control stand (10.34 °C vs. 8.98 °C, 48.2% vs. 56.4%, and pH 4.83 vs. pH 4.60, respectively, all P < 0.05). These results indicated that FTW can influence tree biomass C dynamics, organic C input and output, and soil environmental factors such as soil temperature, soil water content and soil pH, while soil C dynamics such as soil CO₂ efflux rates and soil organic C concentration were little affected by FTW in a red pine stand.     

Species substitution for carbon storage: Sessile oak versus Corsican pine in France as a case study

Species choice is potentially an important management decision for increasing carbon stocks in forest ecosystems. The substitution of a slow-growing hardwood species (Quercus petraea) by a fast-growing conifer plantation (Pinus nigra subsp. laricio) was studied in central France. Simulations of carbon stocks in tree biomass were conducted using stand growth models Fagacées for sessile oak and PNL for Corsican pine. The changes in soil carbon were assessed using the Century model and data from two European soil monitoring networks: 16 km × 16 km grid and RENECOFOR. Carbon in wood products was assessed with life cycle analysis and lifespan of final products. However, only carbon stocks and their variation were accounted for: effects of energy-consuming materials or fossil fuel substitution are excluded from the analysis. To compare the growth of these two types of forest stands, an important part of the study was to assess the productivity of both species at the same site, using National Forest Inventory data.     

Long-term effects of nitrogen fertilization on the productivity of subsequent stands of Douglas-fir in the Pacific Northwest

The carryover effects of N fertilization on five coastal Pacific Northwest Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) plantations were studied. “Carryover” is defined as the long-term impact of N fertilizer added to a previous stand on the growth of a subsequent stand. Average height and diameter at 1.3 m above-ground (DBH) of 7–9-year-old Douglas-fir trees and biomass and N-content of understory vegetation were assessed on paired control (untreated) and urea-N-fertilized plots that had received cumulative additions of 810–1120 kg N ha⁻¹ to a previous stand. Overall productivity was significantly greater in the fertilized stands compared to the controls. In 2006, the last growth measurement year, mean seedling height was 15% greater (p = 0.06) and mean DBH was 29% greater (p = 0.04) on previously fertilized plots compared to control plots. Understory vegetation biomass of fertilized plots was 73% greater (p = 0.005), and N-content was 97% greater (p = 0.004) compared to control plots. These results show that past N fertilization markedly increased seedling growth in these plantations as well as biomass and N-content of understory vegetation in a subsequent rotation. These findings suggest that N fertilization could potentially increase site productivity of young Douglas-fir stands found on low quality sites in the Pacific Northwest 15–22 years after application by a carryover effect. These plantations have not yet reached the age where marketable materials can be harvested from them, and the growth of trees should be monitored over a longer time period before potential impacts on older stands, if any, can be determined.     

Loblolly pine growth and soil nutrient stocks eight years after forest slash incorporation

Incorporation of forest slash during stand establishment is proposed as a means of increasing soil carbon and nutrient stocks. If effective, the increased soil carbon and nutrient status may result in increased aboveground tree growth. Eight years after study installation, the impact of forest slash incorporation into the soil on soil carbon and nutrient stocks, foliar nutrients and loblolly pine growth are examined on mineral and organic sites on the North Carolina Lower Coastal Plain. Treatments include leaving forest slash on the surface and flat planting (control); V-shear and bedding (conventional), mulch forest slash followed by bedding (strip mulch) and mulch forest slash and till into the soil followed by bedding (strip mulch till). After eight years, mulching and/or tillage did not have a significant impact (p > 0.05) on soil bulk density or soil chemical properties (pH, cation exchange capacity, soil nutrients). Additionally, neither tree foliar nutrients nor stand volume were significantly impacted. However, significant effects were observed for soil phosphorus contents and stand volume between the control plots and the other treatment plots. For example, the mean stand volumes on the mineral site were 24.49 ± 1.28, 38.16 ± 2.90, 44.59 ± 3.07 and 46.96 ± 2.74 m³ ha⁻¹ for the control, conventional, strip mulch and strip mulch till plots. These observations are more likely due to the effect of bedding rather than mulching or tillage of the forest slash. These results are consistent for the mineral and the organic sites. Considering the greater expense to install the mulch and tillage treatments, the lack of a treatment effect on soil carbon and nutrient stocks and tree growth does not justify these treatments on these sites.     

Impact of stand density on water status and leaf gas exchange in Quercus ilex

Tree thinning reduces tree-to-tree competition and likely contributes to the improvement of tree water status and productivity in water-limited systems. In this study, we examined the importance of competition for water among Quercus ilex trees in open woodlands by comparing the water consumption and physiological status of trees located along stand density gradients which ranged from 10% (low density; LD) to 100% (high density; HD) of canopy cover. The study was carried out at two sites which differed in mean annual rainfall (506 and 816 L m⁻²; D_site and W_site, respectively). Predawn and midday leaf water potential (ψ_d and ψ_m, respectively) and CO₂ assimilation rate (A) were measured every two weeks from mid May to mid September, in eight trees located along a stand density gradient at each site. Sap flow and soil moisture were measured only at D_site. Sap flow was continuously recorded by sap flowmeters (constant heating method) installed in 12 trees along two stand density gradients. Soil moisture (?) was measured every 20 cm for the first meter and then every 50 cm up to 250 cm. Measurements were conducted in 18 soil profiles, 6 located in HD and 12 in LD (six beneath and six out the canopy). At W_site, differences among stand densities for ψ and A were very small and emerged only at the end of the dry season. At D_site, ψ (both predawn and midday), A, ?, and sap flow density were significantly higher in LD trees than in HD ones. At D_site, some water remained unused in the soil at the end of the dry season beyond the canopy in the LD areas, and trees did not experienced such an acute water deficit (ψ_d > −1 MPa) as the HD trees did (ψ_d < −3 MPa). Summer tree transpiration at the stand level (E_stand) tended to saturate with the increase of canopy cover. E_stand increases by 32% when canopy cover goes from 50% to 100%. Results confirmed that the increase of tree-to-tree competition with stand density was much more significant at dry sites. In these sites, tree thinning is recommended as a way to maintain tree functioning.     

The role of fire and nutrient loss in the genesis of the forest soils of Tasmania and southern New Zealand

The dominant soil patterns in forested or previously forested landscapes in southern New Zealand and Tasmania are described. Soil properties on adjacent sunny and shady aspects in hill country of the South Island of New Zealand are compared to soil properties under adjacent ‘dry’ and ‘wet’ eucalypt forest in Tasmania.

A soil contrast index or SCI is defined for comparing soil contrasts on parent materials of different absolute nutrient contents. Three soil groups are defined using the SCI. Group 1 soil pairs are stable New Zealand soils in which exchangeable Ca + Mg + K values are higher on drier sunny aspects than on moister shady aspects. Group 2 soil pairs are New Zealand soils in which soils on sunny aspects display evidence of topsoil erosion by wind; consequently some soil pairs on dry (sunny) aspects have lower levels of exchangeable Ca + Mg + K than soils on moister (shady) aspects. Group 3 soil pairs are Tasmanian. Soils on drier sites (under dry eucalypt forest) invariably have lower exchangeable Ca + Mg + K values than soils on moister sites (under wet eucalypt forest), which is the reverse of the pattern in SCI Group 1 soils in New Zealand.

Except on clay-rich parent materials, Tasmanian soils under dry forest generally have texture-contrast profiles and a mean C/N ratio in topsoils (A1 horizons) of 29. Soils under wet forest generally have uniform or gradational texture profiles and a mean topsoil C/N ratio of 15. The texture-contrast soils show strong clay eluviation with sand or sandy loam textures in upper horizons and clayey textures in lower horizons. However, in New Zealand texture-contrast soils are all but absent, and do not occur in the previously forested areas described in this paper. Topsoils (Ah horizons and soils sampled to 7.5 cm depth) in New Zealand areas sampled in this study have a mean C/N ratio of 15, regardless of whether they occur on sunny or shady aspects.

We propose that the frequency and spatial occurrence of fire are the dominant processes causing: (1) the marked difference in levels of nutrients and different topsoil C/N ratios in soils of Tasmania; (2) the development of texture-contrast soils under dry forests in Tasmania; and (3) the difference between soil patterns in New Zealand and Tasmania. Fire depletes nutrients in forests by causing losses to the atmosphere, losses by runoff, and losses by leaching. Nutrient loss by fire encourages fire-tolerant vegetation adapted to lower soil nutrient status, so frequent fire is a feedback mechanism that causes progressive soil nutrient depletion. By destroying organic matter and diminishing organic matter supply to the soil surface fire inhibits clay–organic matter linkages and soil faunal mixing and promotes clay eluviation. Fire frequency is likely to have increased markedly with the arrival of humans at ca. 34 000 years B.P. in Tasmania and ca. 800 years B.P. in New Zealand. We argue that texture-contrast soils have not formed in New Zealand because of the short history of frequent fires in that country. A corollary of this conclusion is that texture-contrast soils in Tasmania are, at least in part, anthropogenic in origin.     

Changes in soil acidity and organic matter following the establishment of conifers on former grassland in New Zealand

Effects of a land use change from grassland to coniferous plantation forestry (Pseudotsuga menzieii [Douglas fir]; Pinus radiata [radiata pine]) on soil acidity and organic matter were assessed at two sites in New Zealand. The sites differed with respect to soils, climate, vegetation cover and type, relative maturity and management of the forest stands. Results obtained at the different sites were, therefore, not directly comparable, although they represented a comparison of a similar change in land use and some overall trends were evident. The change from grassland to conifers decreased levels of organic carbon, total nitrogen and exchangeable cations and increased exchangeable acidity in the upper 20–30 cm of soil. Exchangeable aluminium and exchangeable acidity were more sensitive measures of the effects of afforestation on soil acidity than pH.