Ailanthus triphysa at different densities and fertiliser regimes in Kerala, India: growth, yield, nutrient use efficiency and nutrient export through harvest

A split plot trial involving Ailanthus triphysa (ailanthus) at four spacings (3 m×1 m, 2 m×2 m, 3 m×2 m and 3 m×3 m) and four fertiliser regimes (0:0:0, 50:25:25, 100:50:50 and 150:75:75 kg ha⁻¹ per year N, P₂O₅, K₂O) was initiated in June 1991. Objectives included evaluating the growth and yield potential of ailanthus grown under differing density and fertiliser regimes and to estimate the nutrient export through harvest. Ninety-six randomly selected average-sized trees were felled at 8.8 years of age for assessment. Results show that height, diameter, stand leaf area index, biomass production and volume yield were greater in the 2 m×2 m spacing. Repeated application of fertilisers at 1.2, 2.25 and 5.25 years after planting had little effect on biomass and volume yields, presumably because of weed competition (despite periodic weed control), higher pest incidence (in the heavily fertilised plots) and/or moderately adequate soil nutrient levels. Regarding partitioning of tree biomass, stem wood represented the principal component (>70%), while foliage contributed the least (<7%). Conversely, foliar N, P and K concentrations were the highest, followed by branch wood, coarse roots and stem wood. Denser stands showed greater accumulation of N, P and K with higher potential for nutrient export through harvest. However, as the bole fraction accounted for only about 56–64% of the total nutrients removed, leaving other biomass components (foliage and branches) at the site will reduce the associated nutrient export. Wider spacings (3 m×2 m and 3 m×3 m) were more efficient in N and K use, but P use efficiency was higher in 2 m×2 m. Likewise, trees in the no fertiliser plots exhibited greatest N, P and K use efficiencies. Available soil P, K and organic C levels declined with increasing tree density, while repeated fertilisation increased nutrient concentrations. Soil pH and available P levels declined in comparison to the pre-treatment values.     

Mineral nutrition, soil factors and growth rates of Gmelina arborea plantations in the humid lowlands of northern Costa Rica

The purpose of this study was to determine the factors which are responsible for clearly visible growth irregularities in Gmelina arborea stands by relating growth rates to soil and site properties. All Gmelina plantations under study, established on degraded pasture soils in the Atlantic lowlands of Costa Rica, showed the same growth pattern: very poor growth and chlorotic foliage of trees in mid-slope positions, and fast growth and healthy leaves of trees on hilltops and hillbases. The variation in site and soil properties and tree growth rates was analysed, investigating 24 plots in eight different plantations. Leaf samples were taken from 120 trees (five trees per site). Foliar analysis revealed that the tree growth is highly dependent on the supply of N, P, K and S, indicating that poorly growing trees suffer from a multiple nutrient disorder. To identify the most restricting soil factor, simple correlations between growth rates and soil chemical and physical properties were applied to the entire data set. The best correlation was obtained with exchangeable soil K (r=0.78, P<0.001). Subdivision of the data set into plots on brown soils (eight) and plots on red soils (16) and subsequent correlation analysis resulted in much stronger relationships. Growth depressions in mid-slope positions had other causes on brown soils than on red soils. On the acid brown soils the combination of the variables Al saturation and bulk density could explain 82% of the variation of tree growth. Al saturation of up to 80% in brown soils inhibits nutrient uptake, particularly of N and P. The red soils dominate on Ca---Mg-enriched, alluvial terraces and were among the first soils to be cultivated in the region. During the agricultural use and at the establishment of the tree plantations, they received dolomitic lime to reduce Al toxicity. Here, very low K/Mg ratios (less than 0.03) may induce K deficiency. Therefore, the best multiple regression model for tree growth rates on red soils is obtained with K/Mg ratio and thickness of the humic A-layer (r²=0.75, P<0.001). Inclusion of the variable bulk density resulted in a clear improvement of the model, explaining 91% of the growth variability.     

Growth responses by Pinus radiata to combinations of superphosphate, urea and thinning type

A 2³ factorial experimental testing the effects of urea (N), superphosphate (P) and two thinning types (diagonal-line thinning and thinning from below) on the basal-are increment of 18-year-old Pinus radiata (D. Don) is described. Although the trees had received localized applications of superphosphate shortly after establishment they were considered at the age of 18 years to be phosphate-deficient because concentrations of phosphorus in the foliage were low (0.065% P). Application of superphosphate (200 kg P ha⁻¹) increased concentrations in the foliage (0.132% P) and increased the 7-year basal-area increment by approximately 70%. No growth response was obtained when urea (476 kg N ha⁻¹) was applied alone. When both fertilizers were applied there was a further increase in basal-area increment of 81% on line-thinned plots and 26% on plots which had been thinned from below. This interaction reversed the normal trend in which plots thinned from below, by virtue of their greater initial basal area, produced 11–16% more increment than those which had been line-thinned.

A model of tree growth using initial basal area and a competition index as independent variables was adequate to describe differences in basal-area growth between thinning types except where urea and superphosphate had both been applied. In the case of N + P-treated plots, the reciprocal of the competition index was required as an additional variable. Analysis of the development of the growth responses over time indicated that application of superphosphate had produced sustained improvement in growth rates but that the response to urea occurred only in some growing-seasons. In line-thinned plots the response to urea in addition to phosphate occurred in the 2nd, 3rd and 4th growing-seasons, but the response occurred only in the 2nd and 4th growing-seasons in plots thinned from below. It is argued that this differential response between thinning types can be attributed to differences in stand density and water availability. In order to maximise the growth response to added nitrogen it is important to reduce competition by paying particular attention to both tree spacing and residual basal area.     

Structure, allometry, and biomass of plantation Metasequoia glyptostroboides in Japan

We quantified structural features and the aboveground biomass of the deciduous conifer, Metasequoia glyptostroboides (Hu and Cheng) in six plantations in central Japan. In order to derive biomass estimates we dissected 14 M. glyptostroboides trees into three structural components (stem wood, branch wood and foliage) to develop allometric equations relating the mass of these components and of the whole tree to diameter at breast height (DBH). We found robust relationships at the branch and whole tree level that allow accurate prediction of component and whole tree biomass. Dominant tree height was similar within five older (>40 years) plantations (27–33 m) and shorter in a 20-year-old plantation (18 m). Average stem diameter varied from 12.8 cm in the youngest stand to greater than 35 cm in the oldest stand.

Metasequoia have relatively compact crowns distributed over the top 30% of the tree although the youngest stand had the deepest crown relative to tree height (up to 38%). At the individual tree level in older stands, 87% of the aboveground biomass was allocated to the stem, 9% to branch wood and 4% to foliage. We found little difference in the relative distribution of above ground biomass among the stands with the exception of lower foliage biomass in larger diameter trees. Total aboveground biomass of the older stands varied twofold, ranging from a maximum of 450 Mg ha⁻¹ in a 42-year-old stand to a minimum of 196 Mg ha⁻¹ in a 48-year-old stand. Total above ground biomass of the 20-year-old stand was 176 Mg ha⁻¹.     

Competition control in juvenile hybrid poplar plantations across a range of site productivities in central Saskatchewan,Canada

The response of hybrid poplar plantations established on former agricultural land in Saskatchewan to competition from weeds on a range of site productivities was studied. The short-term impact of competition control on the growth of juvenile trees and how tree responses to competition control differed across the productivity gradient was of particular interest, as was the determination of which resource was most highly competed for and was most important in determining tree growth. Eight sets of paired plots in juvenile hybrid poplar plantations were established in central Saskatchewan across a range of site productivities. In each pair, one plot had complete weed control (weed-free) while in the other plot weeds were allowed to grow. The best soil predictor of tree growth was soil texture, represented by a combination of the percentage silt and clay, with finer textures showing better growth. Competition control significantly increased tree growth on all sites with the benefit being greatest on the higher productivity sites. Soil water appeared to be highly competed for between trees and weeds and was a dominant resource controlling growth. For soil nutrients, nitrogen and phosphorous were highly competed for between trees and weeds. However, leaf phosphorous concentration of the weed-free plots had a strong positive relation to tree growth while nitrogen did not, indicating that when trees are free of competition they can access sufficient nitrogen from these soils.     

The impact of hemlock looper (Lambdina fiscellaria fiscellaria (Guen.)) on balsam fir and spruce in New Brunswick, Canada

In 1989, the first recorded outbreak of hemlock looper (Lambdina fiscellaria fiscellaria (Guen.)) occurred in New Brunswick, Canada. Data were collected from ten plots established in an area infested from 1992–1994, to assess impacts of hemlock looper. Ocular and branch sample assessments of current defoliation and ocular assessments of total defoliation (all age classes of foliage) were conducted for balsam fir (Abies balsamea [L.] Mill.), white spruce (Picea glauca [Moench] Voss), and black spruce (Picea mariana (Mill.) B.S.P.). Stand response was assessed and related to cumulative defoliation. Ocular assessments were found to accurately estimate defoliation, which was significantly related to tree mortality. Ninety-two percent of balsam fir trees that had cumulative defoliation >90% died. Mortality of balsam fir was significantly (p < 0.05) related to tree size, in both lightly and severely defoliated plots; trees with DBH <11 cm sustained 22–48% higher mortality than larger trees. Mortality of balsam fir, in terms of both percent stems/ha and m³/ha merchantable volume, increased exponentially in relation to three estimates of cumulative (summed) plot mean defoliation. The strongest relationships (r² = 0.75–0.79) were between mortality and the ocular defoliation assessment for 1990–1993 foliage. Tree mortality caused by the looper outbreak ranged from 4–14% stems/ha in lightly defoliated and from 32–100% in severely defoliated plots; merchantable volume killed was 3–14 m³/ha and 51–119 m³/ha, respectively. Relationships between mortality and defoliation were similar when defoliation was assessed for 1987–1993 and 1990–1993 foliage age classes.     

基于分层的典型中亚热带天然阔叶林林木竞争关系研究

[目的]通过分析不同林层林木之间的竞争情况,以期揭示林木间相互竞争的类型、林木受到的竞争压力与林木垂直分布之间的关系,为森林资源的保护与利用提供科学依据。[方法]在福建万木林自然保护区内设5块典型中亚热带天然阔叶林样地,采用Hegyi的竞争指数,按分层和不分层两种情形研究林木间的竞争强度。[结果]在1—5号样地,当取样半径超过4 m后,林木间的竞争强度随取样半径的增大而减小的趋势变缓,4 m是分析林木竞争的适宜半径;以4 m为半径分析林木间的竞争,发现有一半以上的组成树种只发生种间竞争,剩余同时发生种间和种内竞争的树种,其竞争木构成中数量占优势的普遍是与其相异的树种,且种间竞争强度普遍强于种内竞争;各样地的乔木层都可以划分出由高到低的3个林层,在平均水平,越高层的林木距离竞争木越远、在竞争木的组成中较矮小的第3林层林木数量也越多;越高层的林木,平均每株对象木与其全部竞争木之间的竞争压力越小。[结论]在调查的典型群落中,普遍发生的是种间竞争,而林木受到的竞争压力及其竞争木的组成都与它们的垂直分布关系密切,在森林资源保护和经营中应对垂直分布予以更多的关注;乔木层中的优势树种未发生激烈种间或种内竞争;可以发生激烈种间竞争的乔木树种有米槠、庆元冬青、少叶黄杞和杜英,不存在特定的竞争木与它们竞争,它们与竞争木相近、主要分布在第2或第3林层;种内竞争过于激烈的有山杜英、华杜英、山黄皮、野含笑和羊舌树,也主要分布在第2或第3林层。     

Impact of mistletoes (Amyema miquelii) on host (Eucalyptus blakelyi and Eucalyptus melliodora) survival and growth in temperate Australia

The damaging effects of mistletoes Amyema spp. on host eucalypts in rural Australia concern many landholders, but few data are available to evaluate mistletoe impacts or formulate management strategies. We used an experimental disinfection approach to determine the effects of a putative pest mistletoe, Amyema miquelii, on its two principal hosts, Blakely's red gum (Eucalyptus blakelyi) and yellow box (Eucalyptus melliodora), on pastoral properties in northern New South Wales. For each host species, pairs of nearby trees were matched for heigth, diameter at breast height, level of mistletoe infestation and distance to neighbouring trees. One tree per pair was chosen at random and all the mistletoes pruned. After 33 months all treated trees were still alive, but among control trees seven of 29 red gums and one of 20 yellow box trees had died. The difference in survival between treated and control trees was significant for red gum. Among surviving pairs of trees, treated trees of both species had significantly greater diameter increments and significantly more foliage than untreated trees. After 33 months, the average increase in relative host foliage biomass attributable to removal of mistletoes was 22% in red gum and 24% in yellow box, compared with untreated controls. The average increase in radial growth attributable to mistletoe removal was 55% in red gum and 49% in yellow box. Diameter increment was negatively and linearly related to mistletoe infestation level in control trees of both species. The differences between infected and disinfected trees in this experiment may underestimate the impact of Amyema miquelii on host growth because small amounts of host biomass were unavoidably lost during disinfection. In a separate experiment, loss of host biomass had a significant negative impact on diameter increment. Although the results indicate that control of serious Amyema miquelii infestation of individual trees will be worthwhile in terms of immediate host survival and higher growth rate, prudent long-term management may well allow for the loss of small numbers of farm trees to mistletoes when planning tree recruitment and utilisation.     

Growth and physiological response of eastern white pine seedlings to partial cutting and site preparation

We examined how white pine (Pinus strobus L.) seedlings planted under a mature cover of white and red (Pinus resinosa Ait.) pine in eastern Ontario (Canada) responded to treatments aimed at improving light and soil conditions for seedling growth. The treatments were: (a) three levels of partial cutting (no cut or CS0, cut to one-crown spacing between residual trees or CS1, cut to two-crown spacing or CS2); (b) two levels of vegetation control (without herbicide or H0, with herbicide or H1); and (c) two levels of soil scarification (S0 and S1). On the third growing season after planting, total growth of seedlings was lowest in CS0 treatment and similar in CS1 and CS2 treatments. The CS2 created better growing light conditions than the CS1, with and average of 50% of full light at seedling height, which corresponded to the maximum height and diameter growth rates of seedlings. However, CS2 also stimulated the growth of competing woody vegetation (both understory trees and shrubs), and resulted in greater microsite heterogeneity of light availability. Scarification warmed the soil (approximately 1–3 °C in the middle of the growing season), decreased the density of competing trees, but increased the shrub density, with no impact on white pine seedling growth. The treatments had no effect on light-saturated photosynthetic rate (A) of current-year foliage of seedlings, nor on their midday shoot water potential. Leaf N was higher in partial cuts and with vegetation control, but the relationship between N and A was weak to non-existent for the different foliage classes. Measures of the proportion of aboveground biomass allocated to foliage (leaf-mass ratio) suggest an acclimation response of young white pine that improves growth under moderate light availability and compensates for the lack of leaf-level photosynthetic plasticity. We suggest a combination of soil scarification under a one-crown spacing partial cut (corresponding to 14 m² ha⁻¹ of residual basal area, or an average of 32% of available light at seedling height) as an establishment cut. This should provide optimum growth conditions for planted understory white pine, while also favoring natural regeneration and providing some protection against damage from insects and disease.     

甘肃连城国家级自然保护区青海云杉种内种间竞争的研究

采用Hegyi提出的单木竞争指数模型对甘肃连城国家级自然保护区的青海云杉种内、种间的竞争指数进行定量分析。结果表明:青海云杉种内竞争指数随着林木径级的增大而逐渐减小,种内竞争远远大于种间竞争,种内种间竞争指数的大小顺序为青海云杉>青杄>桦叶四蕊槭>山杨>红桦>柳树;竞争指数与对象木的胸径之间服从幂函数关系CI=AD~B,当青海云杉胸径达到45cm以上时,竞争指数变化很小;青海云杉的竞争指数与海拔和郁闭度呈显著正相关性。     

Needle and stem wood production in Scots pine (Pinus sylvestris) trees of different age,size and competitive status

We studied effects of tree age, size and competitive status on foliage and stem production of 43 Scots pine (Pinus sylvestris L.) trees in southern Finland. The tree attributes related to competition included foliage density, crown ratio and height/diameter ratio. Needle mass was considered to be the primary cause of growth through photosynthesis. Both stem growth and foliage growth were strongly correlated with foliage mass. Consequently, differences in growth allocation between needles and stem wood in trees of different age, size, or position were small. However, increasing relative height increased the sum of stem growth and foliage growth per unit foliage mass, indicating an effect of available light. Suppressed trees seemed to allocate more growth to stem wood than dominant trees, and their stem growth per unit foliage mass was larger. Similarly, trees in dense stands allocated more growth to stem wood than trees in sparse stands. The results conformed to the pipe model theory but seemed to contradict the priority principle of allocation.     

Sugar maple (Acer saccharum Marsh.) decline during 1979–1989 in northern Pennsylvania

Sugar maple decline has been observed in northern Pennsylvania since the early 1980s. We investigated the interactions between soil moisture stresses in sugar maple and other factors, such as soil chemistry, insect defoliation, geology, aspect, slope, topography, and atmospheric deposition. In the summer of 1998, we sampled 28 sugar maple (Acer saccharum Marsh.) plots drawn from the USFS Forest Inventory and Analysis (FIA) plots, containing declining and non-declining sugar maple trees across northern Pennsylvania for a variety of soil physical and chemical parameters, site characteristics, and tree health. Foliage from declining plots was found to have significantly lower base cations and higher Mn as compared to that from non-declining plots. Soils in declining plots had lower base cations and pH, a Ca:Al⁻≤1, lower percent clay and higher percent sand and rock fragments than soils on non-declining plots, suggesting that trees on declining plots are at risk of nutritional and drought stress. Regression relationships between foliar and soil chemistry indicated that foliar nutrition was highly correlated with soil chemistry in the upper 50 cm of the soil. Declining sugar maple plots in this study occurred at higher elevations on sandstone dominated geologies. Soils were found to be base poor-sandy soils that contained high percentages of rock fragments. Soils below 50 cm on declining plots had lower soil pH and foliar chemistry indicated lower foliar base cations. A trend, while not significant was found with declining plots experiencing a greater number of and more severe insect defoliations.     

Competition for nitrogen between adult European beech and its offspring is reduced by avoidance strategy

Plant growth, reproduction, and biomass allocation may be affected differently by nitrogen availability depending on tree size and age. In this context, competition for limited N may be avoided by different strategies of N acquisition between different vegetation components (i.e., seedlings, mature trees, other woody and herbaceous understorey). This study investigated in a field experiment whether the competition for N between different vegetation components in beech forests was prevented via seasonal timing of N uptake and affected by microbial N use. For this purpose, a removal approach was used to study the seasonal effects on N uptake and N metabolites in adult beech trees and beech natural regeneration, as well as soil microbial processes of inorganic N production and utilisation. We found that the competition for N between beech natural regeneration and mature beech trees was reduced by seasonal avoidance strategies (“good parenting”) of N uptake regardless of the N sources used. In spring, organic and inorganic N uptake capacity was significantly higher in beech seedlings compared to adult beech trees, whereas in autumn mature beech trees showed the highest N uptake rates. Removal of vegetation components did not result in changes in soil microbial N processes in the course of the growing season. Thus, N resources released by the removal of vegetation components were marginal. This consistency in soil microbial N processes indicates that competition between plants and soil microorganisms for N was not avoided by timing of acquisition during the vegetation period, but existed during the entire growing season. In conclusion, N nutrition in the studied forest ecosystem seems to be optimally attuned to European beech.     

Biomass and carbon sequestration of ponderosa pine plantations and native cypress forests in northwest Patagonia

Fast growth tree plantations and secondary forests are considered highly efficient carbon sinks. In northwest Patagonia, more than 2 million ha of rangelands are suitable for forestry, and tree plantation or native forest restoration could largely contribute to climate change mitigation. The commonest baseline is the heavily grazed gramineous steppe of Festuca pallescens (St. Yves) Parodi. To assess the carbon sequestration potential of ponderosa pine (Pinus ponderosa (Dougl.) Laws) plantations and native cypress (Austrocedrus chilensis (Don) Flor. et Boutl.), individual above and below ground biomass models were developed, and scaled to stand level in forests between 600 and 1500 annual rainfall. To calculate the carbon sequestration baseline, the pasture biomass was simulated. Also, soil carbon at two depths was assessed in paired pine-cypress-pasture sample plots, the same as the litter carbon content of both forest types. Individual stem, foliage, branch and root log linear equations adjusted for pine and cypress trees presented similar slopes (P>0.05), although some differed in the elevations. Biomass carbon was 52.3 Mg ha⁻¹ (S.D.=30.6) for pine stands and 73.2 Mg ha⁻¹ (S.D.=95.4) for cypress forests, given stand volumes of 148.1 and 168.4 m³ ha⁻¹, respectively. Soil carbon (litter included) was 86.3 Mg ha⁻¹ (S.D.=46.5) for pine stands and 116.5 Mg ha⁻¹ (S.D.=38.5) for cypress. Root/shoot ratio was 19.5 and 11.4%, respectively. The low r/s value for cypress may account for differences in nutrient cycling and water uptake potential. At stand level, differences in foliage, taproot and soil carbon compartments were highly significative (P<0.01) between both forest types. In pine stands, both biomass and soil carbon were highly explained by the rainfall gradient (r²=0.94). Nevertheless, such a relationship was not found for cypress, possibly due to stand and soil disturbances in sample plots. The carbon baseline estimated in pasture biomass, including litter, was 2.6 Mg ha⁻¹ (S.D.=0.8). Since no differences in soil carbon were found between pasture and both forest types, additionality should be accounted only by biomass. However, the replacement of pasture by pine plantations may decrease the soil carbon storage, at least during the first years. On the other hand, the soil may be a more relevant compartment of sequestered carbon in cypress forests, and if pine plantation replaces cypress forests, soil carbon losses could cause a negative balance.     

Aboveground and belowground competition between intercropped cabbage and young Eucalyptus torelliana

An expanding market for planted timber in the Philippines is providing a strong incentive for upland farmers to incorporate trees into their farming systems. Farmers often intercrop young timber species with well-fertilized annuals in expectation that inter- species competition for nutrients and light will be minimal while the trees are small, and that the trees will benefit from intensive nutrient and weed management of the intercrop. The relative level of aboveground and belowground competition in a vegetable/timber intercropping system was investigated in the uplands of Mindanao, the Philippines. Eight 5-m² microplots were established containing one nine-month-old Eucalyptus torelliana and four rows of cabbage (two on each of the north and south sides of the tree, 0.5 and 1.0 m from the stem base). The tree canopy shaded north rows. Monocrop cabbage microplots (2 m²) were also installed. Four tree/cabbage microplots and all cabbage-only plots were fertilized with ¹⁵ 5N-labeled ammonium sulfate (100 kg N ha⁻¹); remaining microplots received unlabeled fertilizer. Cabbage yields were reduced by 16% in the north rows when compared to the south rows, and by 15% in rows closer to the tree when compared to rows further from the trees. Belowground competition in the first cabbage row, possibly for moisture, is supported by the high proportion of tree roots found in the top 30 cm of soil. Competition did not appear to be for N or other nutrients. Foliar analyses revealed no row differences in mineral concentrations in cabbage, uptake of applied N, or percent of N derived from fertilizer. The modest amount of 15N found in aboveground tree parts (4.5% of N applied to four cabbage rows) improved overall N-use efficiency in the intercropped plots. An improved understanding of the tradeoffs between improved nutrient efficiency and depressed intercrop growth, as well as management options to reduce competition, will help farmers design systems to improve efficiency without increasing competition. This revised version was published online in June 2006 with corrections to the Cover Date.     

Light,soil, and seedling characteristics associated with varying levels of competition in a red pine plantation

Red pine (Pinus resinosa Ait.) seedlings growing under differing levels of competition were evaluated during the fifth growing season following planting, and placed into low, moderate, or high tree classes, as a function of levels of competing vegetation. Tree growth, moisture status, and nutrition were monitored over the growing season. Additionally, site characteristics such as soil temperature and moisture, inorganic nitrogen concentration, mineralized soil nitrogen, and light were measured. Red pine seedlings growing under low competition had an absolute volume growth increase of 795% over the seedlings growing under the heaviest competition. The associated relative volume growth increase during the fifth growing season was 44%. Discriminant analysis was used to describe three classes of trees representing low, moderate, and high levels of competition. Trees growing under low competition had longer and heavier needles, but generally lower nutrient concentrations. Pre-dawn plant moisture status did not vary among competition levels. Soil variables indicated that, in general, the trees growing under low competition occupied warmer, drier, and less fertile microsites, as inorganic soil N and mean monthly mineralized NO₃-N and NH₄>-N tended to be lower on these microsites. The suite of independent variables was effective in classifying the model data into three tree competition classes, with percent correct classifications of 92, 75, and 100 for low (tree class one), moderate (tree class two), and heavy (tree class three) competition, respectively.     

Seasonal nutrient retranslocation in reforested <Emphasis Type="Italic">Pinus halepensis</Emphasis> Mill. stands in Southeast Spain

Retranslocation, resorption and relocation of nutrients are important adaptive mechanisms developed by plants to acquire the amount of the nutrients required for growth. They are usual mechanisms in deciduous and conifer trees that occur in Mediterranean regions where drought periods are usual. Soil factors, environmental characteristics and species factors are key drivers of nutrient retranslocation in conifers but is not well understood how soil fertility or intraspecific competition influences the process. We studied retranslocation in Pinus halepensis Mill. stands showing different site quality (differences in climate and intraspecific competition) occurring in Southeast Spain. We monitored reforested mature Aleppo pine forests in stands with differences in site quality, climate and intraspecific competition. Stands were characterised, the content of nutrients of soil and green samples (twigs and pine needles) were recorded, and seasonal nutrient retranslocation was obtained. Site characteristics were related to growth rate and nutrient content of foliage and soil. We evaluated whether the retranslocation of nutrients from older to younger foliage was related to the current-year growth rate and to the nutritional status of the plant as influenced by intraspecific competition. Foliar macronutrient concentrations and the amount of retranslocated macronutrients were seasonal, with differences related to site quality and tree density. As a general trend, nutrient concentrations increased after drought (autumn) and decreased during the growth period (spring). However, some micronutrients (mainly Na and Fe) decreased during both periods. The retranslocation pattern in Aleppo pine reinforced the hypothesis that pine adaptations to drought- and fire-prone habitats are linked to the resilience of these forest types. We developed scientific knowledge to assist decision making in adaptive forest management; e.g. fertilizer recommendations or reforestation programmes.     

Density-dependent effects of northern hardwood competition on selected environmental resources and young white spruce (Picea glauca) plantation growth, mineral nutrition, and stand structural development – a 5-year study

The effects of competition from red raspberry (Rubus idaeus L.) and northern hardwood tree species on white spruce (Picea glauca (Moench) Voss) seedlings were examined on a clearcut site of the boreal mixedwood forest of the Bas-Saint-Laurent region of Quebec, Canada. A controlled experiment involving a gradient of five vegetation densities on the basis of the leaf area index (LAI) was established in a completely randomized plot design with six replications. Each of the five levels of vegetation cover (including vegetation-free plots) were examined to evaluate how they affected environmental factors (quantity and quality of light reaching the spruce seedlings, and soil temperature), spruce growth (height, basal diameter, volume index, and above-ground biomass), spruce mortality, browsing damage, spruce foliar mineral nutrition, as well as the stand structural development, during the first 5 years after seedling planting.

Each spruce growth variable analyzed in this study, according to a RMANOVA procedure, followed a negative hyperbolic form of density dependence of competitive effects. Loss of growth in young white spruce plantations in competition with northern hardwoods is likely to occur with the first few competitors. In cases where higher levels of competing vegetation were maintained over time, loss of spruce growth was extremely severe, to an extent where the exponential growth character of the young trees has been lost. At the end of the fifth year, spruce growing with no interference were larger in mean total above-ground biomass by a factor of 9.7 than those growing with the highest level of vegetation cover. Spruce did not develop a strategy of shade avoidance by increasing tree height, on the contrary. Spruce mortality differed among treatments only in the fifth year, indicating that early evaluation of spruce survival is not a strong indicator of competitive effects, when compared to diameter growth. Spruce foliar N and Ca contents were significantly reduced by the first level of competing vegetation cover, while K increased with the density of the vegetation cover, and P and Mg were not affected. Nitrogen nutrition of young white spruce planted on recently disturbed sites is discussed in relation to the potential root discrimination of this species against soil nitrate, a reaction observed by Kronzucker et al. [Kronzucker, H.J., Siddiqi, M.Y., Glass, A.D.M., 1997. Conifer root discrimination against soil nitrate and the ecology of forest succession. Nature London 385, 59–61]. The effects of hardwood competition indicate a prevalence of competition for light over a competition for nutrients, as revealed by the substantial increase in the h/d ratio of white spruce. Two indicators, h/d ratio and the quantity of light received at the tree seedling level, are suggested as a basis for the management of hardwood competition in a white spruce plantation.

Analysis of the stand structural development indicates that spruce height distribution was affected only by moderate or dense cover of vegetation, while diameter distribution, when compared to competing vegetation-free plots, was affected by the lowest level of vegetation cover. This study shows that competition influenced the stand structural development in the same way as genetic and micro-site factors by aggravating the amplitude of size inequality. The impact of hardwood competition is discussed in view of reaching an equilibrium between optimal spruce plantation growth and benefits from further silvicultural treatments, and maintaining hardwood species known to improve long term site quality, within a white spruce plantation.     

History of fire and Douglas-fir establishment in a savanna and sagebrush–grassland mosaic, southwestern Montana, USA

Over the past century, trees have encroached into grass- and shrublands across western North America. These include Douglas-fir trees (Pseudotsuga menziesii (Mirb.) Franco var. glauca (Beissn.) Franco) encroaching into mountain big sagebrush (Artemisia tridentata Nutt. ssp. vaseyana (Rydb.) Beetle) from stable islands of savanna in southwestern Montana. Our objectives were to quantify the relative area occupied by mountain big sagebrush and grasslands versus Douglas-fir savanna in the Fleecer Mountains of southwestern Montana today and in the past, and to identify the historical role of fire and other factors in maintaining this distribution. To do this, we reconstructed a multicentury history of tree establishment and surface fires from 1120 trees sampled on a grid of 50 plots covering 1030 ha of a modern mosaic of sagebrush–grasslands and Douglas-fir trees. We compared these histories to time series of climate and land use, and to spatial variation in topography and soil moisture availability. Beginning in the mid-1800s, Douglas-fir trees established in areas in which trees did not persist historically. In 1855, less than half the plots (42%) had trees whereas today, most plots do (94%). This encroachment was synchronous with the cessation of frequent surface fires, likely caused by the advent of domestic livestock grazing, and perhaps by the start of several decades of relatively dry summers. Douglas-fir savannas historically occurred on fire-safe sites more often than did sagebrush–grass. Our inference that frequent fire excluded Douglas-fir in the past was supported by the results of a simulation model of fire and associated vegetation dynamics. In the continued absence of fire, mountain big sagebrush and grasslands in southwestern Montana are likely to become more homogeneous as Douglas-fir trees continue to encroach.     

Soil properties and tree growth on rehabilitated forest landings in the interior cedar hemlock biogeoclimatic zone: British Columbia

We studied operational landing rehabilitation programs in three forest districts of interior British Columbia (BC). Winged subsoiling and grass/legume seeding, followed by planting of lodgepole pine (Pinus contorta var. Latifolia) generally resulted in successful re-establishment of forest cover on landings. In the Boundary district, fifth year tree heights on landings were not significantly different from those in adjacent plantations, and fifth year growth increments were also similar despite evidence of delayed seedling establishment caused by cattle grazing damage. Trees on landings in the Kalum district were shorter than those in plantations, but fifth year increments were similar. Landings in the Kalum with >20% clay had lower stocking densities, tree heights, and probe depths compared to landings with <20% clay content. In the Kispiox district, average fifth year tree heights and fifth year increments were lower on landings than those on plantations. In the Kispiox district, landings with >20% clay content had shorter trees growing on them when compared to tree heights on landings with <20% clay. Landings in the Kispiox had the least probe depth of the three districts, and the greatest difference in height and increments between landings and plantations, supporting field reports of poor decompaction effectiveness there. In all districts, there was no forest floor present on landings 7 years after subsoiling, and cover of non-coniferous vegetation was lower than for plantations. Higher soil temperatures and lower soil moisture contents were recorded on landings relative to plantations in all districts during the growing season of 1998. Landing soils also had lower total C, N and mineralizable-N (min-N) than plantation soils in the Boundary and Kispiox, but such differences were not statistically significant in the Kalum. Reduced levels of total C, N and min-N on landings did not appear to have affected current foliar nutrient status of trees growing on these sites, as most nutrients appeared in concentrations considered adequate or only slightly deficient, except for foliar S on all sites. Average concentrations of N, P, K and S in foliage were either similar to or significantly higher for trees on landings relative to plantations, but these differences were attributed to dilution in plantation foliage rather than deficiency. Our results support the conclusion that operationally feasible techniques for soil rehabilitation can create conditions suitable for establishment of a new forest on sites that otherwise would be considered non-productive. Stocking levels and tree growth rates observed in our study are consistent with the conclusion that a commercial forest may be produced on some of the rehabilitated areas.