The effects of fertilization on the health status,nutrition and growth of Norway spruce forests with yellowing symptoms

We studied the effects of magnesium (Mg) NPK fertilizer applied in 2000 to maturing Norway spruce (Picea abies /L./ Karst.) stands with yellowing symptoms and situated on poor acidified soils in the ?umava mountains (the Czech Republic). Long-term defoliation, nutrient content in the foliage and diameter increment were evaluated in three specific variants: yellowing trees, trees without yellowing symptoms and fertilized trees. The results indicate differences in soil characteristics, nutrient content and radial growth with response to climate. Fertilization increased Mg content and base saturation in the soil and decreased the amount of exchangeable aluminum and hydrolytic acidity. Differences were significant especially in magnesium, calcium and phosphorus contents in the foliage of yellowing trees. There was also a significant difference in manganese content; yellowing trees showed deficiency in comparison with both fertilized and unfertilized healthy trees. Two years after the initial fertilization (to present), defoliation and yellowing symptoms in the unfertilized yellowing trees were more pronounced compared to other variants. These symptoms were more influenced by fertilization than foliation. The duration of significant positive effects of fertilization on diameter increment lasted for a shorter time than the effect on foliage. Simultaneously, higher sensitivity to climatic factors was observed in the fertilized trees.     

Influence of fertilizer timing on seasonal nutrient uptake and dry-matter production by young Pinus radiata in southern western Australia

Seasonal changes in above ground dry-matter, nitrogen (N), and phosphorus (P) accumulation were measured following application of N and P in autumn or spring to 1-year-old Pinus radiata (D. Don). Dry-matter production and nutrient accumulation were measured eight times over two years following fertilization.

All trees produced dry-matter throughout the year, but during the summer, fertilized trees produced more dry-matter than unfertilized trees. In contrast to dry-matter production, nutrient accumulation showed a distinctly seasonal pattern with maximum accumulation of N and P occurring in winter and spring, when rainfall and soil moisture were highest. Accumulation of N and P either slowed markedly or ceased during summer depending on fertilizer treatment. Continued dry-matter production during summer, when nutrient accumulation was low, resulted in the decline of N and P concentrations in needles, branches and stems of all trees. This indicated that nutrients required for new growth during summer were mobilized from existing foliage and wood. Fertilization increased the concentrations of N and P in foliage and wood, and these higher concentrations persisted through summer. Spring fertilization increased N accumulation to a greater extent than autumn fertilization, this effect lasting two years. The greater dry-matter production by fertilized trees during summer indicated that growth during summer was limited by nutrient supply.     

Stand density and fertilization effects on aboveground allocation patterns and stem form of Pinus sylvestris in young stands

The increasing demand for forest biomass for energy generation could be partially met by growing denser stands and use of fertilizer. Before this is done at large scale, more knowledge of the effects of stand density and fertilization on aboveground allocation patterns and stem form is needed. Therefore, effects of pre-commercial thinning (PCT) to 3000 stems ha^?1, an unthinned dense control (C), and PCT combined with two levels of fertilization (100 kg ha^?1 of nitrogen applied either during the establishment of the field experiment (F1) or annually (F2)) were examined in 23- to 26-year-old Scots pine (Pinus sylvestris L.) stands six years after the establishment of the field experiment. In total, 114 sample trees were harvested using destructive biomass sampling. The growth allocation and stem form of trees with diameter at breast height (DBH; 1.3 m height) >5.0 cm were not affected by either the PCT or fertilization. Small trees (DBH < 5 cm) in denser, unthinned control plots had more slender stems (lower DBH/height ratios) and allocated less growth to branches and foliage than trees in PCT plots. Fertilization had little effect on the stem form and growth allocation of the smallest trees. Therefore, effects of stem density and fertilization on stem form and growth allocation to foliage were only found for small suppressed trees, and the treatments had very little influence on dominant and codominant trees.     

Growth response and sapwood hydraulic properties of young lodgepole pine following repeated fertilization

We examined how tree growth and hydraulic properties of branches and boles are influenced by periodic (about 6 years) and annual fertilization in two juvenile lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) stands in the interior of British Columbia, Canada. Mean basal area (BA), diameter at breast height (DBH) and height increments and percent earlywood and sapwood hydraulic parameters of branches and boles were measured 7 or 8 years after the initial treatments at Sheridan Creek and Kenneth Creek. At Sheridan Creek, fertilization significantly increased BA and DBH increments, but had no effect on height increment. At Kenneth Creek, fertilization increased BA, but fertilized trees had significantly lower height increments than control trees. Sapwood permeability was greater in lower branches of repeatedly fertilized trees than in those of control trees. Sapwood permeabilities of the lower branches of trees in the control, periodic and annual treatments were 0.24 x 10(-12), 0.35 x 10(-12) and 0.45 x 10(-12) m2 at Kenneth Creek; and 0.41 x 10(-12), 0.54 x 10(-12) and 0.65 x 10(-12) m2 at Sheridan Creek, respectively. Annual fertilization tended to increase leaf specific conductivities and Huber values of the lower branches of trees at both study sites. We conclude that, in trees fertilized annually, the higher flow capacity of lower branches may reduce the availability of water to support annual growth of the leader and upper branches.     

Impacts of long-term elevation of atmospheric carbon dioxide concentration and temperature on the establishment,growth and mortality of boreal Scots pine branches

Abstract

The aim of this study was to investigate the impacts of elevated carbon dioxide (CO₂) and temperature on the establishment, growth and mortality of the branches of Scots pine (Pinus sylvestris L). In 1997, 16 young trees were individually enclosed in chambers, in eastern Finland, for a period of 5 years (1997–2001), in an environment that simulated the future climate for the region. There were four replicates of each treatment, including combinations of ambient and elevated CO₂ and temperature. Measurements were carried out on the establishment of new branches, branch diameter growth and branch mortality. Elevated temperature and elevated CO₂ had no positive effect on the number of branches that established each year or branch diameter growth. They were, instead, related to tree height growth and stem diameter growth, respectively. However, elevated CO₂ and temperature caused an increase in branch mortality.     

Branch mortality and potential litterfall from Douglas-fir trees in stands of varying density

Forest floor characteristics influence nutrient cycling and energy flow properties of forest ecosystems, and determine quality of habitat for many forest plants and animals. Differential crown recession and crown development among stands of differing density suggest that an opportunity may exist to control the input of fine woody litter into the system by manipulating stand density. The objective was to measure the rate of branch mortality among stands of differing density and to estimate the range in total per hectare necromass inputs. Although litter traps are reliable for estimating per hectare rates of litterfall, branch mortality dating on sectioned stems uniquely allows assessment of several other litterfall components: (1) individual tree contributions to total litterfall; (2) the amount of branch material released by mortality, regardless of whether the branches are shed to the forest floor; (3) the distribution of basal diameters characterizing the litterfall from a given tree and stand. Twenty-four trees were felled and sectioned on permanent plots that were part of a silvicultural study of stand density regimes in Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco.). Whorl branches were dissected out of bole sections to determine the dates of mortality, and a branch biomass equation was applied to estimate potential rate of litterfall. Periodic annual rates were expressed in four ways: (1) number of branches per tree; (2) mass of branches per tree; (3) mass of branches per unit of crown projection area; (4) mass of branches per hectare. For the growth periods investigated, larger trees and trees growing on denser plots tended to release a greater necromass through branch mortality. Average branch basal diameter generally decreased with increasing stand density. Annual branch mortality ranged from 33 to 430 g m⁻² crown projection area for individual trees, and from 236 to 1035 kg ha⁻¹ for individual plots. These rates approached the low end of the range of previously published fine litterfall rates for Douglas-fir. Rates on these plots were relatively low owing to the temporary delay in crown recession imposed by artificial thinning. A conceptual model of branch litter dynamics is presented to depict consistencies with crown development among stands managed under different density regimes.     

Fertilization at planting influences seedling growth and vegetative competition on a post-mining boreal reclamation site

Reclamation of post-mining sites is challenged by limiting factors including adverse soil chemical and physical properties, along with weed competition. Fertilization may alleviate nutrient deficiencies, but broadcast fertilization with immediately available fertilizers (IAF) results in generally low rates of nutrient recovery for planted trees. Directed application of controlled-release fertilizer (CRF) to the rhizosphere offers an alternative to extend nutrient longevity while reducing nutrient leaching or uptake by competing vegetation. We evaluated white spruce (Picea glauca (Moench) Voss) and aspen (Populus tremuloides Michx.) development on a mine reclamation site in the Oil Sands Region of northern Alberta in response to IAF and polymer-coated CRF (3–4 and 8–9 month release). IAF and CRF (each NPK plus other macro- and micro-nutrients) were applied at 20 and 40 g N seedling^?1 and 2 and 4 g N seedling^?1, respectively. No effect on seedling survival occurred. White spruce year-1 height and diameter growth responded positively to both IAF and CRF relative to non-fertilized controls, but in year-2 seedlings treated with CRF (i.e., 8–9 month) outperformed those fertilized with IAF, which were similar to controls. Aspen height growth did not respond strongly to fertilization in either year, but diameter growth showed positive responses to all fertilizer treatments in year-1 and to all CRF treatments and the high IAF rate in year-2. Responses were attributed to a pronounced increase in vegetative competition observed for IAF relative to CRF or controls following the first growing season, and generally higher levels of foliar N in year-1 for CRF compared to IAF or unfertilized trees. Thus, directed root zone application of CRF promoted seedling growth responses similar to or better than those induced by broadcast IAF applications, but at 90–95 % lower N application rates.     

Decreased needle longevity of fertilized Douglas-fir and grand fir in the northern Rockies

Changes in nutrient availability significantly affect canopy dynamics in conifers. To elucidate these effects, we experimentally fertilized mixed conifer stands at several sites across the northern Rocky Mountains. We measured needle longevity, total branch length and foliated length along the main branch axis, and determined mean retained cohort length on mid-canopy branches of shade-intolerant Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) and shade-tolerant grand fir (Abies grandis Lindl.). Needle longevity ranged from 6 to 8 years in Douglas-fir and from 7 to 8 years in grand fir on unfertilized plots. Fertilization significantly decreased needle longevity by 26 and 27% in Douglas-fir and grand fir, respectively. However, the foliated branch length remained unchanged following fertilization and was similar for both species, indicating a 33% increase in mean branch length per needle cohort in Douglas-fir and a 27% increase in grand fir. These data are consistent with the theory that foliated branch length and needle longevity are a result of the ecological light compensation point (ELCP), which links the inherent physiology of the leaf with the availability of resources in the leaf environment. Mid-canopy ELCP was approximately 74 and 71 cm from the branch terminus in Douglas-fir and grand fir, respectively, regardless of fertilization. We hypothesize that fertilization-enhanced needle production and annual shoot growth resulted in a higher rate of shading of older needles. The shaded needles were unable to maintain a positive carbon balance and abscised. The results demonstrate that foliated branch length of Douglas-fir and grand fir in the northern Rocky Mountains can be treated as a homeostatic response to fertilization, whereas foliar turnover is plastic.     

Fertilization—Effects on deciduous tree growth and browsing by moose

Knowledge of plant–herbivore interactions is fundamental for understanding foraging patterns of herbivores and their effects on the ecosystem as well as the human use of natural resources. The nutrient resources available for plants may determine not only growth rate, but also the amounts of secondary compounds in the plant tissues. As a consequence, browsing pressure on plant communities by large herbivores can be affected by fertilization and nitrogen deposition. We conducted a field fertilization experiment to test the hypothesis that fertilization leads to increased sapling growth and increased browsing by a large herbivore, the moose (Alces alces). The study was conducted on fertilized and unfertilized plots (50 m × 50 m) in northern Sweden. The plots were located in Scots pine (Pinus sylvestris) stands of young succession stages with an intermixture of deciduous species. Fertilization with 600 kg calcium–ammonium–nitrate per ha (=200 kg N ha⁻¹), was conducted in spring on one of the plots in each of 25 paired plots. The other plot in each pair was kept as control. The effect of fertilization on plants and browsing pressure was evaluated on five deciduous tree species in June and August the year after fertilization. Both browsing from preceding winter (bites) and current summer (leaf strips and bites) was recorded. Shoot size and other shoot variables increased with fertilization. Fertilization increased winter browsing for most species, but few differences were statistically significant. The late summer recording (August) showed pronounced differences in browsing pressure (leaf strips and bites) between control and treatment. In June, only a trend of increased browsing (leaf strips) was found for all species on fertilized plots. Generally, the pressure of leaf stripping was low in June, probably due to the short time the annual shoots had been exposed to moose browsing. Number of summer bites (recorded in August) was significantly higher on fertilized plots for the Betula spp., and the trend was similar for the other tree species. Within species, there was no difference in strip length between the treatments. The result indicates that fertilization can be used as a habitat improvement for ungulates within wildlife management. But, fertilization is also likely to lead to an increased risk of browsing damage, which is undesirable if forests are being fertilized for increased forest production.     

Effects of feathermoss removal, thinning and fertilization on lodgepole pine growth, soil microclimate and stand nitrogen dynamics

The influence of a continuous feathermoss layer (dominated by Pleurozium schreberi) on soil temperature, soil water potential and tree growth was assessed in a 5-year study. The feathermoss layer was manually removed from 900 m² plots in lodgepole pine (Pinus contorta) stands of northwestern Alberta. The interception and retention of nutrients by moss was estimated in N fertilization treatments with and without moss removal. As well, the potential for intraspecific competition to affect nutrient dynamics was assessed through a thinning treatment. Removal of the moss layer increased soil temperatures in summer and increased the period when soil was frost free, but the plots without moss had soil temperature as low as −13 °C in one winter period with little snow cover. Moss removal did not significantly affect N concentrations of the tree foliage but did reduce needle weight. Thinning had little effect on the rate of diameter growth after the first 3 years, but produced a significant increase in growth by year 4. Fertilization had a consistently positive effect on radial stem increments and N content of foliage, regardless of the presence of moss on the plot. Overall, the intraspecific competition between trees was apparently greater than interspecific competition between moss and trees.     

Fertilization of lodgepole pine trees increased diameter growth but reduced root carbohydrate concentrations

Tree growth variables and more complex derived variables such as vigor index have all been linked to the ability of lodgepole pine (Pinus contorta var latifolia) trees to defend against insect herbivores, particularly mountain pine beetle (Dendroctonus ponderosae Hopkins, Coleoptera: Curculionidae). These variables are considered indirect measures of carbohydrate reserves. Trees with high vigor index values have high growth rates and are assumed to have high carbohydrate reserves. However, this critical assumption was untested for lodgepole pine. In this paper we evaluated the relationship between carbohydrate concentration and tree characteristics. We measured concentrations of root starch in 10 pure lodgepole pine stands of different ages in the Canadian Rockies which had been fertilized, thinned, fertilized and thinned or left untreated in a 2 × 2 factorial design. Both fertilization and thinning increased basal area increment while only fertilization increased lateral branch growth in the crown. Contrary to expectations, fertilization decreased root starch reserves although it increased basal area increment; thinning had no effect on root carbohydrates. Root starch reserves were positively related to basal area increment and vigor index in both fertilized and unfertilized plots, however, the best predictors of starch reserves were height-to-live-crown and cambial surface area below the live crown; starch reserves declined as both of these variables increased. These data suggest that large carbon sinks associated with long boles below the crown, and rapid growth of the crown and stem as a result of fertilization are detrimental to building starch reserves in the roots.     

Effect of wide spacing on increment and branch properties of young Norway spruce

The effects of stand density on increment and branch properties were studied in three spacing experiments of Norway spruce [Picea abies (L.) Karst.]. The stand densities ranged from 350 stems ha⁻¹, regarded as open-grown trees, up to 1,600 stems ha⁻¹, corresponding to the density recommended for forestry practice. Properties of all the branches were measured from the stem apex downwards. The study material included a total of 5,661 branches from 45 trees. Increasing stand density resulted in a decrease in radial increment as well as shorter and narrower crowns, but it had no effect on height increment. The average number of spike knots per tree was 0.87, 0.27, and 0.33 in densities of 350, 700 and 1,600 ha⁻¹, respectively. Additionally, in the widely spaced stands of 350 stems ha⁻¹, the fraction of trees having spike knots was high (over 50%). At a density of 1,600 ha⁻¹, the sample trees had somewhat less branches in a whorl compared with the more widely spaced plots. The most pronounced effect of stand density was the increase in branch diameter with decreasing stand density. At a density of 350 ha⁻¹, the maximum branch diameter of all the sample trees exceeded the diameter limit of quality class B in the European quality requirements for round wood. The results give some indication that trees subjected to severe competition would produce smaller branches per unit of crown projection area. However, the possibilities for reducing branch dimensions relative to stem and crown size through competition appear quite restricted.     

The impact of climate,drainage and fertilization on the survival and growth of pinus sylvestris L. in afforestation of open,low‐production peatlands

The objective of this study was to analyse the conditions for forest production on open, low‐production peatlands in Sweden with respect to climate, and water and nutrient regimes. The study focused on survival and growth of Scots pine (Pinus sylvestris L.) seedlings, planted 18 yrs ago in five experimental areas evenly distributed between south and north Sweden. Different ditch spacing and NPK fertilizer treatments were combined systematically in all experiments. Survival was positively correlated with temperature sum, fertilization and drainage intensity. Tree growth was not influenced by variations in temperature sum after merely draining, but in combination with fertilization, growth was strongly correlated with climate. In the southern experiments, fertilization increased stand growth eight to nine times, whereas stands on the northern sites did not respond to fertilization. The most important fertilizer element was P. The application of N had no effect on growth. More intensive drainage increased stand growth by 60%. In the southern areas, height development in the most intensive drainage and fertilizer treatments indicates a mean annual increment of 6–7 m³ ha^?1, and no sign of decline in growth was seen. Turf‐planting had positive effects on both survival and growth, especially in less intensively drained plots. A large proportion of damaged trees was observed in the experiment. The frequency of damaged trees was positively correlated with treatment intensity but negatively correlated with temperature sum. The results show clearly that merely draining is not sufficient for successful afforestation of low‐production peatland sites. Fertilization by P and K is a necessary prerequisite, but the effect of fertilization varies with climatic conditions, probably owing to the amount of plant‐available N.     

Crown characteristics of juvenile loblolly pine 6 years after application of thinning and fertilization

Total foliage dry mass and leaf area at the canopy hierarchical level of needle, shoot, branch and crown were measured in 48 trees harvested from a 14-year-old loblolly pine (Pinus taeda L.) plantation, six growing seasons after thinning and fertilization treatments.

In the unthinned treatment, upper crown needles were heavier and had more leaf area than lower crown needles. Branch- and crown-level leaf area of the thinned trees increased 91 and 109%, respectively, and whole-crown foliage biomass doubled. The increased crown leaf area was a result of more live branches and foliated shoots and larger branch sizes in the thinned treatment. Branch leaf area increased with increasing crown depth from the top to the mid-crown and decreased towards the base of the crown. Thinning stimulated foliage growth chiefly in the lower crown. At the same crown depth in the lower crown, branch leaf area was greater in the thinned treatment than in the unthinned treatment. Maximum leaf area per branch was located nearly 3–4 m below the top of the crown in the unthinned treatment and 4–5 m in the thinned treatment. Leaf area of the thinned-treatment trees increased 70% in the upper crown and 130% in the lower crown. Fertilization enhanced needle size and leaf area in the upper crown, but had no effect on leaf area and other variables at the shoot, branch and crown level. We conclude that the thinning-induced increase in light penetration within the canopy leads to increased branch size and crown leaf area. However, the branch and crown attributes have little response to fertilization and its interaction with thinning.     

Influence of nitrogen fertilization on minerals, carbohydrates, amino acids and phenolic compounds in beech (Fagus sylvatica L.) leaves

Beginning in 1985, plots in a 120-year-old beech (Fagus sylvatica L.) stand in the province of Scania, southern Sweden were fertilized annually with ammonium nitrate. In August 1989, leaves from both fertilized and unfertilized trees were analyzed for mineral nutrients, soluble carbohydrates, amino acids and phenolic compounds. Leaf concentrations of total nitrogen were increased by N fertilization. Ammonium was not detected in the leaves, but NO(3) (-) was increased by up to 65% as a result of N fertilization. Nitrogen fertilization decreased foliar concentrations of P and Cu, leading to a nutrient imbalance relative to N. Of the free amino acids, glutamic acid and aspartic acid predominated and, together with glutamine and asparagine, were significantly increased by N fertilization. The concentrations of amides were 2-4 times higher in fertilized trees than in unfertilized trees. Soluble carbohydrates did not change in response to N fertilization, but total phenolic compounds decreased markedly. The reduced concentration of phenolics may explain the greater susceptibility to parasite attacks of trees exposed to N deposition and excess N.     

Carbon assimilation and nitrogen in needles of fertilized and unfertilized field-grown Scots pine at natural and elevated concentrations of CO2

Effects of elevated CO2 concentration ([CO2]) on carbon assimilation and needle biochemistry of fertilized and unfertilized 25-30-year-old Scots pine (Pinus sylvestris L.) trees were studied in a branch bag experiment set up in a naturally regenerated stand. In each tree, one branch was enclosed in a bag supplied with ambient [CO2] (360 micromol mol(-1)), a second branch was enclosed in a bag supplied with elevated [CO2] (680 micromol(-1)) and a control branch was left unbagged. The CO2 treatments were applied from April 15 to September 15, starting in 1993 for unfertilized trees and in 1994 for fertilized trees, which were treated with N in June 1994. Net photosynthesis, amount and activity of Rubisco, N, starch, C:N ratio and SLA of needles were measured during the growing season of 1995. Light-saturated net photosynthetic rates of 1-year-old and current-year shoots measured at ambient [CO2] were not affected by growth [CO2] or N fertilization. Elevated [CO2] reduced the amount and activity of Rubisco, and the relative proportion of Rubisco to soluble proteins and N in needles of unfertilized trees. Elevated [CO2] also reduced the chlorophyll concentration (fresh weight basis) of needles of unfertilized trees. Soluble protein concentration of needles was not affected by growth [CO2]. Elevated [CO2] decreased the Rubisco:chlorophyll ratio in unfertilized and fertilized trees. Starch concentration was significantly increased at elevated [CO2] only in 1-year-old needles of fertilized trees. Elevated [CO2] reduced needle N concentration on a dry weight or structural basis (dry weight minus starch) in unfertilized trees, resulting in an increase in needle C:N ratio. Fertilization had no effect on soluble protein, chlorophyll, Rubisco or N concentration of needles. The decrease in the relative proportions of Rubisco and N concentration in needles of unfertilized trees at elevated [CO2] indicates reallocation of N resources away from Rubisco to nonphotosynthetic processes in other plant parts. Acclimation occurred in a single branch exposed to high [CO2], despite the large sink of the tree. The responses of 1-year-old and current-year needles to elevation of growth [CO2] were similar.     

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.     

Effect of pruning on branch production and water relations in widely spaced ponderosa pines

This study dealt with the effects of pruning on branch and leaf area (F _a) production of ponderosa pines growing in silvopastoral systems in Patagonia. We hypothesized that pruning positively influences the number of branches per whorl and their basal area growth rate, changing F _a production. In addition, we studied some water relations in order to explain potential differences in branch growth rates between treatments. Two mathematical models were developed to estimate branch and total F _a. The averaged diameter at the third year of pruning was, for high-pruned trees 3.1 and 3.6 cm at the bottom and middle of the crown, against 4 and 4.4 cm for low-pruned trees. Pruning did not produce changes in the number of branches per whorl (approximately 7.6 branches per whorl). Water stress may be responsible of this lower branch growth in pruned trees. Water potential, stomatal conductance and transpiration were lower in high- than in low-pruned trees.     

Five native tree species and manioc under slash-and-mulch agroforestry in the eastern Amazon of Brazil: plant growth and soil responses

Throughout the Amazon of Brazil, manioc (Manihot esculenta) is a staple crop produced through slash-and-burn agriculture. Nutrient losses during slash-and-burn can be large and nutrient demand by food crops so great that fields are often abandoned after two years. In recent decades, farmers have reduced the fallow phase from 20 to ~5 years, limiting plant nutrient accumulation to sustain crop yields. Improved fallows through simultaneous planting of trees with food crops may accelerate nutrient re-accumulation. In addition, slash-and-mulch technology may prevent loss of nutrients due to burning and mulch decomposition may serve as a slow-release source of nutrients. This study in Pará, Brazil, in a 7-year-old secondary forest following slashing and mulching of the vegetation, involved two main plot treatments (with and without P and K fertilizers) and two sub-plot treatments (with or without a N₂-fixer Inga edulis). A mixed-culture of trees and manioc was planted in all plots. P and K fertilizer increased tree mortality due to weed competition but growth of surviving trees in four of the five tree species tested also increased as did biomass production of manioc. In the N₂-fixer treatment trends of greater growth and survival of four of five tree species and manioc biomass were also observed. Fertilization increased the biomass of competing vegetation, but there was a fertilizer by N₂-fixer interaction as I. edulis caused a reduction in competing biomass in the fertilized treatment. After one year, fertilization increased decomposition of the mulch such that Ca, Mg, and N contents within the mulch all decreased. In contrast, P and K contents of mulch increased in all treatments. No influence of the N₂-fixer on 0–10 cm soil N contents was observed. Two years after establishment, this agroforestry system succeeded in growing a manioc crop and leaving a well-maintained tree fallow after the crop harvest.     

Effects of Irrigation and Fertilization on Soil Microbial Biomass and Functional Diversity

Abstract

Optimal water and nutrient treatment effects on soil mi-crobial characteristics, including microbial functional diversity and mi-crobial biomass carbon (C) and nitrogen (N) were assessed at a loblolly pine plantation on a Sandhills site after 6 years of continuous fertilization and irrigation. Fertilization significantly increased soil C and N and microbial C and N. Irrigation significantly increased soil C and N, and microbial C. Fertilization, irrigation, and their interaction changed soil microbial selection of carbon compounds, but did not influence the numbers of carbon compounds utilized by soil microbes, as measured by the BIOLOG method suggesting that soil microbial species may have changed. The increases in soil microbial biomass and soil C and N indicate that the fertilization and irrigation treatments have had a positive effect on soil productivity on this very sandy site.