Increased Tree Growth Research Articles

Responses of Intrinsic Water-use Efficiency and Tree Growth to Climate Change in Semi-Arid Areas of North China

Tree-level intrinsic water-use efficiency (iWUE) is derived from the tree-ring 13C isotope composition (δ13C) and is an important indicator of the adaptability for trees to climate change. However, there is still uncertainty regarding the relationship between long-term forest ecosystem carbon sequestration capacity and iWUE. To determine whether elevated atmospheric CO2 concentration (Ca) increase iWUE and tree growth (basal area increment, BAI), dendrochronological methods and stable isotope analyses were used to examine annual changes in the tree-ring width and carbon isotope composition (δ13C) of Platycladus orientalis in northern China. The iWUE derived from δ13C has increased significantly (p < 0.01). Long-term iWUE trend was largely and positively driven by the elevated atmospheric CO2 concentration and temperature. We observed a general increase in averaged BAI, which had significant positive correlation with iWUE (R2 = 0.3186, p < 0.01). Increases in iWUE indeed translated into enhanced P. orientalis growth in semi-arid areas of northern China. Elevated atmospheric CO2 concentration significantly (p < 0.01) stimulated P. orientalis biomass accumulation when Ca was less than approximately 320 ppm in the early phase; however, this effect was not pronounced when Ca exceeded 320 ppm.

Interplay between N-form and N-dose influences ecosystem effects of N addition to boreal forest

AimsNitrogen (N) addition effects on boreal forest ecosystem are influenced by an interplay between N-form and N-dose. We hypothesize that trees take up organic N more efficiently than inorganic N and that unwanted side-effects of organic N are smaller. We predicted that 1) the tree growth response to arginine (ARG) addition is larger than to ammonium-nitrate (AN) and, 2) understory vegetation and ectomycorrhizal (EcM) changes following ARG addition are smaller than following AN addition.MethodsWe investigated the effects of AN and ARG addition (50 and 150 kg N ha−1) during five years on tree growth, understory vegetation and EcM fungi in a Pinus sylvestris L. forest (c 50 years old) in northern Sweden.ResultsN addition increased tree growth and changed understory vegetation composition with few significant differences between AN and ARG. Differences in responses mainly occurred for the bryophyte Pleurozium schreberi which decreased more from ARG, and for EcM sporocarps, which sharply declined from AN, but not from ARG.ConclusionsWe found very few differences in responses between AN and ARG addition with the exception of EcM and bryophytes. These species groups have several key functions in boreal forests and the differences in responses merits further investigations.

Thinning increases tree growth by delaying drought-induced growth cessation in a Mediterranean evergreen oak coppice

Abstract The Mediterranean evergreen oak coppices of Southern Europe are increasingly vulnerable to drought because of both the ongoing climate change that increases drought length and intensity, and the lack of forest management that induces a structural aging of the stands. Decreasing stand density through thinning has been widely regarded as a means to improve the resistance of evergreen oak forests to climate change by decreasing the competition for water amongst the remaining stems. Data from a 30-years thinning experiment, that includes a control and four thinning intensity treatments (from 25% to 80% of basal area removed), in a coppiced holm oak (Quercus ilex L.) forest of southern France, was used to quantify the effects of thinning on stem growth. Building on the ‘sink limitation’ paradigm, which proposes that tree growth is controlled by phenology and climatic constraints and decoupled from carbon assimilation, we investigated if the effect of thinning on stem growth was explained by a delayed drought-induced growth cessation. Using a water balance model, we simulated the date of drought-induced growth cessation, previously found to correspond to the day of the year when water potential drops below a threshold of −1.1 MPa, and used it to predict growth in the different treatments of the thinning experiment. Thinning increased long-term growth at the stem level but decreased the wood biomass at the stand level. Decreasing stem density, and hence the leaf area index, was simulated to delay the date of drought-induced growth cessation. A growth model based on the date of growth cessation explained 85% of the effect of thinning on stem growth over the 30-year period of the study, and 95% for the first five years after thinning. The canopy density for which the effect of thinning is the most beneficial was found to maximize the growth duration without lifting completely the water limitation in summer. Moderate thinning had a sustained beneficial effect on the growth of all stem size classes, whereas stronger thinning intensities increased the size asymmetry of competition and their overall effect dropped faster. Our simple predictive model based on the simulation of the water balance as a function of stand density opens the way to providing management guidelines for the optimization of tree density as a function of water limitation in Mediterranean evergreen woodlands.

Efficacy of Anaerobic Soil Disinfestation for Control of Prunus Replant Disease.

Prunus replant disease (PRD) is an important soilborne complex that suppresses growth and productivity of replanted stone fruit and nut orchards. It is effectively managed with preplant soil fumigation but, due to regulatory challenges, nonfumigant-based control strategies for PRD and other soilborne disease problems may become increasingly important, especially in California. We examined the potential of preplant anaerobic soil disinfestation (ASD) for control of PRD in four repeated orchard replant trials on sandy loam soil near Parlier, CA. After removal of the old orchard trees, alternative ASD treatments, all using rice bran as the main carbon source, were implemented, starting in late September. The alternative treatments incorporated rice bran at (i) 20 t ha-1, alone, in 3.0-m-wide row strips; (ii) 20 t ha-1, preceded by incorporation of a sudangrass cover crop and followed by drip application of molasses (10 t ha-1), in 3.0-m-wide row strips; (iii) 20 t ha-1, alone, in 1.8-m-wide strips; or (iv) 12 t ha-1, alone, in 1.8-m-wide strips. All ASD-treated areas were covered with clear tarp and drip irrigated with 25 cm of water. Tarps remained for 6 weeks, during which the soil moisture level was kept at or above field capacity by drip irrigation. All trials included nontreated control and fumigated standard treatments. ASD raised temperature and reduced redox potential in soil at 15- and 46-cm depths for 6 weeks. Fumigation and ASD treatments both nearly eradicated bioassay inoculum of Pythium ultimum in the soil before almond trees were replanted and significantly affected almond tree root communities of fungi and oomycetes after planting. Fumigation treatments and ASD treatments with rice bran at 20 t ha-1 in 3.0-m strips increased tree growth significantly and by similar magnitudes. Among repeated experiments, mean increases in trunk cross-sectional area growth due to fumigation ranged from 137 to 264%, while the increases due to ASD at 20 t ha-1 in 3.0-m strips ranged from 148 to 214%, compared with controls. ASD offers effective control of PRD and is worthy of further optimization and testing for management of PRD and additional orchard replant problems.

Climate-Induced Tree Growth Variations under the RCP 2.6 Scenario: A Case Study on the Southeastern Tibetan Plateau

The relationship between climate and radial tree growth is traditionally used to reconstruct past climate conditions based on interannual tree-ring variations. However, few studies have used these climate-growth relationships to model the radial tree growth based on future climate projections. To detect the future forest dynamics, the climate-induced tree growth from 2006 to 2100 was projected using temperature changes under representative concentration pathway (RCP) 2.6 for the southeastern Tibetan Plateau. Radial tree growth was mainly controlled by annual mean temperature in this region. Based on the relationship between regional annual mean temperature and radial tree growth, a regression model was built that explained 62.5% of variance in the observed temperature record over the period 1911–2005. A period of unprecedented radial tree growth was found after 1998 when compared with the tree growth in the past 700 years. We found that radial tree growth would increase in the period 2006–2045 and decline after that period due to the projected temperature decrease. As forest productivity and biomass are expected to increase with the increased tree growth, these results suggest that temperature-limited systems could see future increases in productivity as growth limitations are lessened. The results of this research could be used to predict regional forest dynamics in the future.

Responses of key understory plants in the boreal forests of western North America to natural versus anthropogenic nitrogen levels

The understory plants of the boreal forests have evolved divergent strategies to succeed under low nitrogen conditions, with their ability to respond to nitrogen addition varying greatly. We examined the response of 5 key understory North American boreal forest species in the Yukon (4 shrubs and a herb) to low levels of nitrogen that could occur from natural pulses and to high levels that could occur because of anthropogenic deposition from either atmospheric deposition or as a management strategy to increase tree growth. We fertilized 240 one m2 plots for the three dwarf shrubs and one herb and 60 three m2 plots for one tall shrub at 0, 0.5, 1.0, 2.0, 4.0, and 17.5g nitrogen/m2 in June 2004 and May 2005. We measured changes in cover, growth, leaf metrics, and berry production in three dwarf shrubs (Arctostaphylos rubra [deciduous] and A. uva-ursi and Empetrum nigrum [both evergreen]), one herb (Geocaulon lividum), and one tall shrub (Shepherdia canadensis). We predicted the first three would be strongly affected by variation in nitrogen levels but that neither Geocaulon (a hemi-parasite) nor Shepherdia (a nitrogen fixer) would be. To examine for long-term effects of nitrogen application, we also fertilized four 2.8ha plots in 2004 and 2005 with 1.0 and 2.0g/m2 and then measured berry production in 2006 and 2007 on the two Arctostaphylos species. Nitrogen addition at all levels had no effect on either Geocaulon or Shepherdia. There were general positive effects at low nitrogen levels and negative ones at high levels on the other 3 dwarf shrub species in a species-specific manner. There were marked year effects in some plant growth metrics and in berry production, emphasizing the role of climate, but also clear evidence that individual species tradeoff somatic growth for reproduction. There was a pronounced long-term negative effect of low nitrogen addition on the large plots, causing marked reductions in berry production in both Arctostaphylos species. We conclude that these dwarf shrubs are constrained to function optimally within a narrow range of nitrogen levels normally encountered, but are unable to cope with higher levels. Both Geocaulon and Shepherdia function independently of nitrogen limitation. Thus the life history adaptations of these understory plants to variation in nitrogen levels are species-specific and understanding their individual responses is key in predicting their fate and the biodiversity and organization of boreal forest ecosystem when challenged with higher anthropogenic levels of nitrogen.

Drought stress increased survival and development of emerald ash borer larvae on coevolved Manchurian ash and implicates phloem‐based traits in resistance

AbstractEmerald ash borer (Agrilus planipennis Fairmaire, EAB) is causing widespread ash (Fraxinus spp.) mortality as it invades North America and Eastern Europe. Resistance of its coevolved hosts, including Manchurian ash (Fraxinus mandshurica Rupr.), is considered to limit EAB outbreaks and ash mortality in its native Asia, although an understanding of resistance mechanisms is still developing. Such knowledge may facilitate breeding for resistance and management of EAB in its invaded ranges. In the present study, controlled egg inoculations were used to investigate resistance mechanisms impacting larval performance, as well as to characterize the effects of water and nutrient availability on inter‐ and intra‐specific variation in resistance phenotypes based on larval outcomes. Larval survival and growth rates were lower on coevolved Manchurian ash than on evolutionarily naïve white ash (Fraxinus americana L.). Water stress decreased tree growth and resistance of Manchurian ash to EAB, although it had little effect on resistance of the already highly susceptible white ash. High nutrient availability increased tree growth but had no effect on larval performance. These results show that the higher resistance of Manchurian ash to EAB is conferred by phloem traits that decrease larval performance, in addition to lower oviposition preference.

Evidence that higher [CO2] increases tree growth sensitivity to temperature: a comparison of modern and paleo oaks.

To test tree growth sensitivity to temperature under different ambient CO2 concentrations, we determined stem radial growth rates as they relate to variation in temperature during the last deglacial period, and compare these to modern tree growth rates as they relate to spatial variation in temperature across the modern species distributional range. Paleo oaks were sampled from Northern Missouri, USA and compared to a pollen-based, high-resolution paleo temperature reconstruction from Northern Illinois, USA. Growth data were from 53 paleo bur oak log cross sections collected in Missouri. These oaks were preserved in river and stream sediments and were radiocarbon-dated to a period of rapid climate change during the last deglaciation (10.5 and 13.3cal kyr BP). Growth data from modern bur oaks were obtained from increment core collections paired with USDA Forest Service Forest Inventory and Analysis data collected across the Great Plains, Midwest, and Upper Great Lakes regions. For modern oaks growing at an average [CO2] of 330ppm, growth sensitivity to temperature (i.e., the slope of growth rate versus temperature) was about twice that of paleo oaks growing at an average [CO2] of 230ppm. These data help to confirm that leaf-level predictions that photosynthesis and thus growth will be more sensitive to temperature at higher [CO2] in mature trees-suggesting that tree growth forest productivity will be increasingly sensitive to temperature under projected global warming and high-[CO2] conditions.

Does increasing intrinsic water use efficiency (iWUE) stimulate tree growth at natural alpine timberline on the southeastern Tibetan Plateau?

Little is known about whether increasing iWUE (intrinsic water use efficiency) can stimulate tree growth in the temperature-limited natural timberlines. Here, we presented the basal area increment (BAI) and iWUE chronologies of Smith fir (Abies georgei var. smithii) from 1900 to 2006 at a high-elevation (ca. 4400ma.s.l.) timberline in the humid Sygera Mountains, southeastern Tibetan Plateau (TP). The commonality analysis model was applied to investigate the relationships among BAI, temperatures, atmospheric CO2 concentration (Ca) and iWUE during 1961–2006, taking into account of both pure and joint effects. As illustrated by the commonality analysis model, the pure effect of Ca (39.15%) had more stronger influence on iWUE than that of the Tmean (annul mean temperature, 0.12%), but the joint effect between Ca and Tmean (49.79%) on iWUE was stronger than any pure effect for the raw data with an increasing trend. For the first-difference data with year-to-year variations, the pure effect of Ca (7.72%) on iWUE was stronger than that of Tmean (0.59%) and the joint effect between them (0.59%). All above imply the Ca is the dominant factor for iWUE both for the 46-year trend and interannual variations. In addition, as showed by the commonality analysis model, the pure effect of iWUE (17.57%) played a much more important role on BAI than that of temperatures (smt, mean temperature during June, July, August of current year, 5.92%; amt, mean temperature during September, October, November of previous year, 3.04%), while joint effects of iWUE and temperatures contributed more (27.96%; 13.90%; 16.47%) to the BAI than their pure effects for the raw data with an increasing linear trend. For the first-difference data with interannual variations, the pure effect of smt (12.45%) had much more effect on BAI than that of iWUE (5.49%), at the same time the joint iWUE and temperatures contributed less (3.56%; 1.9%; 1.31%) to the BAI than their pure effects. These results suggest that an increasing iWUE could enhance 46-year increasing tree growth trend at humid and high-elevation timberlines, supporting the CO2 fertilization hypothesis, while temperatures dominate the interannual variations of tree growth.

Fortifying the forest: thinning and burning increase resistance to a bark beetle outbreak and promote forest resilience.

Fire frequency in low-elevation coniferous forests in western North America has greatly declined since the late 1800s. In many areas, this has increased tree density and the proportion of shade-tolerant species, reduced resource availability, and increased forest susceptibility to forest insect pests and high-severity wildfire. In response, treatments are often implemented with the goal of increasing ecosystem resilience by increasing resistance to disturbance. We capitalized on an existing replicated study of fire and stand density treatments in a ponderosa pine (Pinus ponderosa)-Douglas-fir (Pseudotsuga menziesii) forest in western Montana, USA, that experienced a naturally occurring mountain pine beetle (MPB; Dendroctonus ponderosae) outbreak 5yr after implementation of fuels treatments. We explored whether treatment effects on tree-level defense and stand structure affected resistance to MPB. Mortality from MPB was highest in the denser, untreated control and burn-only treatments, with approximately 50% and 39%, respectively, of ponderosa pine killed during the outbreak, compared to almost no mortality in the thin-only and thin-burn treatments. Thinning treatments, with or without fire, dramatically increased tree growth and resin ducts relative to control and burn-only treatments. Prescribed burning did not increase resin ducts but did cause changes in resin chemistry that may have affected MPB communication and lowered attack success. While ponderosa pine remained dominant in the thin and thin-burn treatments after the outbreak, the high pine mortality in the control and burn-only treatment caused a shift in species dominance to Douglas-fir. The high Douglas-fir component in the control and burn-only treatments due to 20th century fire exclusion, coupled with high pine mortality from MPB, has likely reduced resilience of this forest beyond the ability to return to a ponderosa pine-dominated system in the absence of further fire or mechanical treatment. Our results show treatments designed to increase resistance to high-severity fire in ponderosa pine-dominated forests in the Northern Rockies can also increase resistance to MPB, even during an outbreak. This study suggests that fuel and restoration treatments in fire-dependent ponderosa pine forests that reduce tree density increase ecosystem resilience in the short term, while the reintroduction of fire is important for long-term resilience.

Organic amendments decrease bulk density and improve tree establishment and growth in roadside plantings

Soil compaction significantly impacts urban tree survival and herbaceous vegetation recruitment along roadsides. Soil samples were collected at two highway tree planting sites in Ontario, Canada. The average soil bulk density in the 0–10 and 20–30cm range was 1.45gcm−3 and 1.55gcm−3, respectively, at Site 1. At site 2, the average soil bulk density in the 0–10 and 20–30cm range was 1.49gcm−3 and 1.67gcm−3, respectively. Six treatments were established on each site to determine the amount of organic amendment required to decrease bulk density and increase tree growth as a measurement of successful tree establishment. Compost was incorporated into the soil at 0%, 10%, 25% and 50% on a volume-to-volume basis at both sites. In 2014 Acer x freemanii Autumn Blaze® ‘Jeffersred’ whips were planted into each treatment bed (Site 1, n=36 and Site 2 n=42). Bulk density, porosity, soil pH, EC, tree survival, height, shoot extension and chlorophyll content were measured for the 2014 and 2015 growing seasons. Incorporation of 25% v/v compost consistently decreased bulk density to below-root restricting thresholds, resulting in improved tree growth. Significant differences were observed in chlorophyll content between organic-amended remediated treatment beds compared to the control; chlorophyll content was significantly higher (p<0.05) for the 25% and 50% treatments throughout the 2015 growing season at both sites. Incorporation of compost at 10% and 25% v/v effectively reduced soil bulk density, leading to increased growth and reduced tree stress at both sites. This study demonstrates the importance of understanding the baseline of soil physical structure in prescribing remediation strategies, including organic amendments, in urban tree planting

Microbial communities after wood ash fertilization in a boreal drained peatland forest

Wood ash contains all the essential plant nutrients except nitrogen, and thus supports tree growth especially on peatlands where mineral nutrients often are in short supply. Here we report the effects of wood ash fertilization on the soil microbial community in a boreal drained peatland, characterized by phospholipid fatty acid (PLFA) composition, and more specifically the fungal community, characterized with PCR-DGGE fingerprinting and sequencing methods. We investigated the microbial communities in unfertilized and ash-fertilized sample plots (loose wood ash 5000 or 15 000 kg ha−1) 6 years after fertilization. Microbial function measured as heterotrophic basal respiration increased over 30% in the surface peat layer after wood ash fertilization, yet the result was statistically insignificant. The higher amount of wood ash induced a shift in the microbial community in the surface peat layer (0–10 cm) along with an increase in pH. A higher relative proportion of fungal-specific PLFA 18:2ω6 and an increased fungal:bacterial ratio indicated that fungi benefited from the higher amount of wood ash in the topmost layer. Also, the lower amount of wood ash seemed to have stimulated actinobacteria. Our results indicated that higher pH and extra nutrients due to wood ash fertilization promoted the appearance of mycorrhizal and wood-decomposing fungi directly or via increased tree growth. We suggest that wood ash as a fertilizer would be a natural and economical choice to improve the nutrient status of drained peatlands used for forestry since it maintains the positive feedback between fungi and trees.

Improved fallow: growth and nitrogen accumulation of five native tree species in Brazil

Small-holding farmers of the Brazilian Amazon often use a rotation of secondary forest, slash-and-burn land-clearing and fallow phase regeneration for agriculture. In recent decades reduction of the fallow phase from ~20 to ~5 years has limited nutrient accumulation by fallow vegetation to sustain future crop growth. Slash-and-mulch and improved fallow schemes, including use of native nitrogen-fixing species, have been investigated to address the issue. In the current study in eastern Amazonia of Brazil, a 7-year old forest site was slash-and-mulched and four treatments applied; no fertilizer and no N-fixer as the control; no fertilizer with N-fixer; P + K fertilizer with no N-fixer; P + K fertilizer with N-fixer. Manioc was planted in all plots at establishment. After manioc harvest and 4 years of fallow, a total of 6 years after planting, use of P + K fertilizer increased tree growth of four of five planted species. In the presence of the N-fixer I. edulis, trends of increased growth and survival among these four tree species were observed. After 6 years, fertilization with P + K significantly increased tree volume and biomass. The N-fixer lowered survival of other species yet increased estimated N uptake of planted trees. Use of P + K fertilizer without N-fixer might allow for commercial harvest of S. amazonicum at the end of one 7-year crop-fallow cycle without jeopardizing agroecosystem N stocks. Use of P + K fertilizer in the presence of I. edulis can increase planted-tree N-content but increases in competition may limit commercial harvest yet may better sustain future food crop growth.

Recent reduction in the frequency of frost accounts for most of the increased growth of a high elevation spruce forest in northwestern China

Key message Using process-based models in combination with dendrochronological measurements provides a way to explain recent increased tree growth in northwestern China.

Habitat associations drive species vulnerability to climate change in boreal forests

Species climate change vulnerability, their predisposition to be adversely affected, has been assessed for a limited portion of biodiversity. Our knowledge of climate change impacts is often based only on exposure, the magnitude of climatic variation in the area occupied by the species, even if species sensitivity, the species ability to tolerate climatic variations determined by traits, plays a key role in determining vulnerability. We analyse the role of species’ habitat associations, a proxy for sensitivity, in explaining vulnerability for two poorly-known but species-rich taxa in boreal forest, saproxylic beetles and fungi, using three IPCC emissions scenarios. Towards the end of the 21st century we projected an improvement in habitat quality associated with an increase of deadwood, an important resource for species, as a consequence of increased tree growth under high emissions scenarios. However, climate change will potentially reduce habitat suitability for ~9–43 % of the threatened deadwood-associated species. This loss is likely caused by future increase in timber extraction and decomposition rates causing higher deadwood turnover, which have a strong negative effect on boreal forest biodiversity. Our results are species- and scenario-specific. Diversified forest management and restoration ensuring deadwood resources in the landscape would allow the persistence of species whose capacity of delivering important supporting ecosystem services can be undermined by climate change.

Linnebjer—A South Swedish Oak Forest and Meadow Area—Revisited after Half a Century

An oak forest and three wet meadows/fens were reinvestigated after 50 years concerning tree vitality, biomass and productivity, and soil chemistry. Sulphur and nitrogen deposition has changed dramatically during these years, and the aim was to analyse the differences in both the oak forest and the open field ecosystems. Trees were re-measured and soil profiles were resampled. Important visible changes in the oak forest were stated concerning the vitality of oaks. Aboveground there was a decrease in tree biomass, production and litter fall, but a huge increase in standing dead logs. During the years, the deposition of sulphur had decreased drastically, but nitrogen deposition was still high. Soil acidification in the forest had decreased, reflected in an increased base saturation in the forest, in spite of slightly lowered pH-values. Strongly increased amounts of exchangeable Ca and Mg now appeared in the forest soil, and a substantial transport of calcium and magnesium had obviously taken place from the forest soil to the meadow and fens during the years. However, the most important soil change was the accumulation of organic matter. The increased accumulation of organic matter in turn meant increased amounts of colloid particles and microsites for ion exchange in the soil. This favoured 2-valence base cations, and especially Ca and Mg that increased very much in all the studied ecosystems. Carbon as well as nitrogen had strongly increased in the forest, meadow and fen soils. This was interpreted as a natural result of increased vegetation growth due to high nitrogen deposition, increased global annual temperature and increased carbon dioxide concentration in air. It was concluded that the decreased deposition of sulphur had had a positive effect on soil chemistry, and that the deposition of nitrogen probably had stimulated vegetation growth in general, and contributed to increased amount of organic matter in the soils. However, in this studied oak forest, the decreased vitality and many killed trees were also suspected to be a result of high nitrogen deposition. Obviously increased tree growth was counteracted by decreased stress resistance, and increased appearance of pathogens in the oak trees.

Influence of weed composition, abundance, and spatial proximity on growth in young hybrid poplar plantations

Hybrid poplar plantations have the potential to produce a large amount of biomass for the forest industry; however, competition for resources by neighbouring vegetation can severely reduce that potential. Intensive vegetation control of all neighbouring vegetation is potentially costly and an inefficient use of limited resources, thus identifying the characteristics of competing vegetation most detrimental to tree growth is essential for optimizing plantation inputs and yields. Our objectives were to model tree growth losses as a function of neighbouring vegetation composition, abundance, and spatial proximity. We then tested different vegetation control treatments for their effectiveness with different neighbouring vegetation. Field work took place on four operational plantation sites in northeastern Alberta over the course of three years, and commenced at the beginning of the second growing season after plantation establishment. Regression tree and Nonmetric Multidimensional Scaling (NMDS) analyses indicated that perennial grasses in general, and Elytrigia repens in particular, were detrimental to hybrid poplar growth – especially for younger trees. Planting site also played a large role in determining tree productivity, although it is difficult to quantify its influence relative to competition because site and vegetation community differed among sites. Increasing intensity of vegetation control treatments did not increase tree survival rates in any of the sites, but did result in commensurate increases in tree growth on most sites. Intensive vegetation control with herbicides dramatically increased tree growth on sites containing populations of perennial grasses. Using only cultivation as a form of vegetation control was the least effective option, and was ineffective for control of perennial grasses compared to other treatments. These results suggest that control of perennial grasses by means of herbicides over the other forms of competition control tested will have the greatest impact on reducing losses in tree growth due to competition.

Getting to the root of urban tree health

Getting to the root of urban tree health

Phosphorus fertilisation and large legume species affect jarrah forest restoration after bauxite mining

Re-establishing nutrient-cycling is often a key goal of mine-site restoration. This goal can be achieved by applying fertilisers (particularly P) in combination with seeding N-fixing legumes. However, the effect of this strategy on other key restoration goals such as the establishment and growth of non-leguminous species has received little attention. We investigated the effects of P-application rates either singly, or in combination with seeding seven large understorey legume species, on jarrah forest restoration after bauxite mining. Five years after P application and seeding, legume species richness, density and cover were higher in the legume-seeded treatment. However, the increased establishment of legumes did not lead to increased soil N. Increasing P-application rates from 0 to 80kgPha−1 did not affect legume species richness, but significantly reduced legume density and increased legume cover: cover was maximal (∼50%) where 80kgPha−1 had been applied with large legume seeds. Increasing P-application had no effect on species richness of non-legume species, but increased the density of weeds and native ephemerals. Cover of non-legume species decreased with increasing P-application rates and was lower in plots where large legumes had been seeded compared with non-seeded plots. There was a significant legume×P interaction on weed and ephemeral density: at 80kgPha−1 the decline in density of these groups was greatest where legumes were seeded. In addition, the decline in cover for non-legume species with increasing P was greatest when legumes were seeded. Applying 20kgPha−1 significantly increased tree growth compared with tree growth in unfertilised plots, but growth was not increased further at 80kgha−1 and tree growth was not affected by seeding large legumes. Taken together, these data indicate that 80kgha−1 P-fertiliser in combination with (seeding) large legumes maximised vegetation cover at five years but could be suboptimal for re-establishing a jarrah forest community that, like unmined forest, contains a diverse community of slow-growing re-sprouter species. The species richness and cover of non-legume understorey species, especially the resprouters, was highest in plots that received either 0 or 20kgha−1 P and where large legumes had not been seeded. Therefore, our findings suggest that moderation of P-fertiliser and legumes could be the best strategy to fulfil the multiple restoration goals of establishing vegetation cover, while at the same time maximising tree growth and species richness of restored forest.

Effect of three partial cutting practices on stand structure and growth of residual black spruce trees in north-eastern Quebec

Partial cutting practices are increasingly used in boreal forests for two major reasons: (1) maintaining age structure and tree diameter distribution according to the principles of ecosystem-based management and (2) increasing tree growth by decreasing competition. This study evaluated the effects of three different partial cutting treatments applied to even- and uneven-aged black spruce (Picea mariana (Mill.) B.S.P.) stands in north-eastern Quebec, Canada. The effect of partial cutting was assessed by comparing treated and control plots in terms of age structure, diameter and spatial distribution, amount of deadwood and tree radial growth. Age structure and diameter distribution were not different from control plots after partial cuttings applied in both uneven-aged and even-aged stands, but lower deadwood basal area was observed. Tree radial growth generally increased in uneven-aged stands but can be limited by tree age and inter-tree competition. In even-aged stands, tree removal was more uniformly distributed and the overall reduction in inter-tree competition resulted in an increased tree radial growth. Overall, these results suggest that the studied partial cuttings were adequate for maintaining structural attributes and increasing tree growth, but adjustments should be made to treatments to increase the amount of deadwood to a level observed in natural forests and to lower inter-tree competition.