Variations in phenology and growth of European white birch (Betula pendula) clones

Phenology can have a profound effect on growth and climatic adaptability of northern tree species. Although the large interannual variations in dates of bud burst and growth termination have been widely discussed, little is known about the genotypic and spatial variations in phenology and how these sources of variation are related to temporal variation. We measured bud burst of eight white birch (Betula pendula Roth) clones in two field experiments daily over 6 years, and determined the termination of growth for the same clones over 2 years. We also measured yearly height growth. We found considerable genetic variation in phenological characteristics among the birch clones. There was large interannual variation in the date of bud burst and especially in the termination of growth, indicating that, in addition to genetic effects, environmental factors have a strong influence on both bud burst and growth termination. Height growth was correlated with timing of growth termination, length of growth period and bud burst, but the relationships were weak and varied among years. We accurately predicted the date of bud burst from the temperature accumulation after January 1, and base temperatures between +2 and -1 degrees C. There was large clonal variation in the duration of bud burst. Interannual variation in bud burst may have important consequences for insect herbivory of birches.     

Effects of root trenching of overstorey Norway spruce (Piceaabies) on growth and biomass of underplanted beech (Fagussylvatica) and Douglas fir (Pseudotsugamenziesii) saplings

In Central Europe, the conversion of pure Norway spruce stands (Picea abies [L.] Karst.) into mixed stands with beech (Fagus silvatica L.) and other species like e.g. Douglas fir (Pseudotsuga menziesii [Mirb.] Franco) is accomplished mainly by underplanting of seedlings beneath the canopy of overstorey spruce trees after partial cutting treatments what means exposure to shade and below-ground root competition by the overstorey to the seedlings. Particularly about the second factor, our knowledge is limited. Therefore, we carried out a below-ground competition exclusion experiment by root trenching and investigated the effects on soil resources, growth, and biomass partitioning of underplanted beech and Douglas fir saplings under target diameter and strip cutting treatments. The exclusion of overstorey root competition by trenching increased the soil water potential in the second year that had a fairly dry growing season and led to significantly higher foliar concentrations of most nutrients, particularly in Douglas fir, indicating an amended nutrient supply. Both improvements were accompanied by an increase in length and diameter increment of the underplanted saplings, appearing in both species only after having surpassed a species-specific threshold light value (Douglas fir 16% of above canopy radiation, beech 22%). We also found significant interactions between trenching and light for specific fine root length and further biomass and morphological parameters. Judged by the much steeper increase in height and diameter growth with increasing light after release from below-ground competition, Douglas fir saplings appeared to be more sensitive to root competition than beech saplings what conforms to older findings for beech. According to our results, a strip cutting seems to be more appropriate than a target diameter cutting treatment to replace a pure spruce stand by a mixed stand with beech and Douglas fir.     

Effects of sika deer (Cervus nippon yakushimae) population growth on saplings in an evergreen broad-leaved forest

An increase in size of sika deer populations has caused severe damage to forest vegetation in many regions of Japan. In a Natural World Heritage area on the western part of Yakushima Island, existing high densities of deer are thought to be increasing, but the impact of deer on forest vegetation has not been estimated. Roadside censuses of deer between 1988 and 2006 showed that encounter rates are continuing to increase. Long-term monitoring of individual tree saplings in a 0.13 ha area of the forest between 1992 and 2005 showed that many saplings still remain despite increases in sika deer density. However, differences in growth and mortality rates of preferred, neutral (neither preferred nor avoided) and avoided sika deer food species varied over time. As a group, preferred species continuously had a low growth rate, while their mortality rate increased through time. The growth rates of the neutral and avoided groups decreased especially in the second half of our study period at the same time that their mortality rates increased. These data suggest that in spite of food species preference, direct feeding pressures as well as indirect physical effects by increasing sika deer density are affecting growth and mortality of tree saplings. Clearly, it is difficult to predict how plant communities will respond to possible continued sika deer population growth, and it is therefore important to continue monitoring these forest communities in the interest of preserving the unique ecosystems on Yakushima Island.     

Effects of elevated CO(2) on chloroplast components, gas exchange and growth of oak and cherry

Specific chloroplast proteins, gas exchange and dry matter production in oak (Quercus robur L.) seedlings and clonal cherry (Prunus avium L. x pseudocerasus Lind.) plants were measured during 19 months of growth in climate-controlled greenhouses at ambient (350 vpm) or elevated (700 vpm) CO(2). In both species, the elevated CO(2) treatment increased the PPFD saturated-rate of photosynthesis and dry matter production. After two months at elevated CO(2), Prunus plants showed significant increases in leaf (55%) and stem (61%) dry mass but not in root dry mass. However, this initial stimulation was not sustained: treatment differences in net assimilation rate (A) and plant dry mass were less after 10 months of growth than after 2 months of growth, suggesting acclimation of A to elevated CO(2) in Prunus. In contrast, after 10 months of growth at elevated CO(2), leaf dry mass of Quercus increased (130%) along with shoot (356%) and root (219%) dry mass, and A was also twice that of plants grown and measured at ambient CO(2). The amounts of Rubisco and the thylakoid-bound protein cytochrome f were higher in Quercus plants grown for 19 months in elevated CO(2) than in control plants, whereas in Prunus there was less Rubisco in plants grown for 19 months in elevated CO(2) than in control plants. Exposure to elevated CO(2) for 10 months resulted in increased mean leaf area in both species and increased abaxial stomatal density in Quercus. There was no change in leaf epidermal cell size in either species in response to the elevated CO(2) treatment. The lack of acclimation of photosynthesis in oak grown at elevated CO(2) is discussed in relation to the production and allocation of dry matter. We propose that differences in carbohydrate utilization underlie the differing long-term CO(2) responses of the two species.     

Effects of litter quality and climate change along an elevational gradient on litter decomposition of subalpine forests,Eastern Tibetan Plateau,China

Temperature and freeze-thaw events are two key factors controlling litter decomposition in cold biomes.Predicted global warming and changes in freeze-thaw cycles therefore may directly or indirectly impact litter decomposition in those ecosystems. Here, we conducted a2-year-long litter decomposition experiment along an elevational gradient from 3000 to 3600 m to determine the potential effects of litter quality, climate warming and freeze-thaw on the mass losses of three litter types [dragon spruce(Picea asperata Mast.), red birch(Betula albosinensis Burk.), and minjiang fir(Abies faxoniana Rehd. et Wild)]. Marked differences in mass loss were observed among the litter types and sampling dates. Decay constant(k) values of red birch were significantly higher than those of the needle litters. However, mass losses between elevations did not differ significantly for any litter type.During the winter, lost mass contributed 18.3–28.8 % of the net loss rates of the first year. Statistical analysis showed that the relationships between mass loss and litter chemistry or their ratios varied with decomposition periods. Our results indicated that short-term field incubations could overestimate the k value of litter decomposition.Considerable mass was lost from subalpine forest litters during the wintertime. Potential future warming may not affect the litter decomposition in the subalpine forest ecosystems of eastern Tibetan Plateau.     

西南桦对菌根的依赖性及其接种效应研究

为了摸清西南桦引种驯化过程中苗木菌根存在的可能性及其对菌根的依赖程度,分别对西南桦幼苗实施ＶＡ菌根和ＥＣＭ菌根的接种试验。试验结果表明：西南桦幼苗对两个类型的菌根均可受感染,对菌根均有较强的依赖性,尤以对外生菌根的依赖性较强,而对ＶＡ菌根的依赖性属中等。西南桦幼苗接种外生菌根菌后１８０ｄ平均苗高比对照增另了９２．９８￣１０６．８５％,地上干质量增另２０６．４３￣５５４．６９％,地下中２０２．８３￣     

Root-zone temperatures affect phenology of bud break, flower cluster development, shoot extension growth and gas exchange of 'Braeburn' (Malus domestica) apple trees

We investigated the effects of root-zone temperature on bud break, flowering, shoot growth and gas exchange of potted mature apple (Malus domestica (Borkh.)) trees with undisturbed roots. Soil respiration was also determined. Potted 'Braeburn' apple trees on M.9 rootstock were grown for 70 days in a constant day/night temperature regime (25/18 degrees C) and one of three constant root-zone temperatures (7, 15 and 25 degrees C). Both the proportion and timing of bud break were significantly enhanced as root-zone temperature increased. Rate of floral cluster opening was also markedly increased with increasing root-zone temperature. Shoot length increased but shoot girth growth declined as root-zone temperatures increased. Soil respiration and leaf photosynthesis generally increased as root-zone temperatures increased. Results indicate that apple trees growing in regions where root zone temperatures are < or = 15 degrees C have delayed bud break and up to 20% fewer clusters than apple trees exposed to root zone temperatures of > or = 15 degrees C. The effect of root-zone temperature on shoot performance may be mediated through the mobilization of root reserves, although the role of phytohormones cannot be discounted. Variation in leaf photosynthesis across the temperature treatments was inadequately explained by stomatal conductance. Given that root growth increases with increasing temperature, changes in sink activity induced by the root-zone temperature treatments provide a possible explanation for the non-stomatal effect on photosynthesis. Irrespective of underlying mechanisms, root-zone temperatures influence bud break and flowering in apple trees.     

滇西地区西南桦无性系早期测定与生长节律研究

[目的]开展西南桦无性系早期生长变异及生长节律研究,揭示其生长规律及适应性,为西南桦无性系选育及速生丰产林营建提供参考。[方法]以滇西地区20个无性系参试的3年生西南桦无性系测定林为研究对象,每月定期测定其树高、胸径,探讨各无性系的生长变异,应用聚类分析划分生长类型,运用有序聚类分析进一步揭示各类型无性系的生长节律特征,通过相关性分析探明影响生长节律的主要气象因子。[结果]20个西南桦无性系间树高和胸径差异极显著(P0.01),根据生长表现,可将其划分为速生、中等和慢生3种类型,其中,B3、A10、Q3、Q4、Q2无性系属于速生型,其树高、胸径年均生长量大多超过2 m和2 cm;与中等和慢生型无性系相比,速生型无性系的生长优势并非完全表现在速生期,其在缓生期和滞生期的生长优势更大;各类型西南桦无性系的生长节律基本一致,其树高生长高峰在7—9月,低谷为4月和12月;其胸径生长高峰为5、10月,8月和12月为低谷,尤以8月生长最慢,这与该月的日照时数低有关。[结论]在滇西地区,B3、A10、Q3、Q4、Q2等5个无性系生长最迅速,颇具发展潜力。从经营措施看,宜在树高、胸径生长高峰之前,即5月和9月加强幼林抚育施肥,促进林木生长。     

内蒙古大兴安岭南段山杨纯林与白桦纯林的枯落物层水文效应

目的 在暖干化气候变化导致干旱胁迫带来大量树木死亡的赛罕乌拉国家级自然保护区,选择典型林分,研究其枯落物层蓄积量和水文功能。 方法 选取山杨（Populus davidiana Dode）纯林与白桦（Betula platyphylla Suk.）纯林2种林分,各设立9块典型样地,测定枯落物现存量,并采用室内浸泡法测定其枯落物层持水能力,探究2种林分的枯落物层水文效应。 结果 研究表明：（1）在相同郁闭度条件下,山杨纯林的枯落物总储量为12.41 t·hm−2,稍高于白桦纯林的10.97 t·hm−2。（2）山杨纯林、白桦纯林的枯落物层的最大持水量为26.85、26.77 t·hm−2,有效拦蓄量为11.96、11.07 t·hm−2,二者相近。（3）枯落物的动态持水量与浸水时间呈对数函数关系,吸水速率与浸水时间呈幂函数关系。 结论 内蒙古大兴安岭南段山杨纯林的枯落物储量稍大于白桦纯林,但二者的枯落物层持水能力相近。     

Effects of ozone on growth and gas exchange of Eucalyptus globulus seedlings

To study the effects of a low concentration of ozone on growth and gas exchange in Eucalyptus globulus Labill. seedlings, ozone was applied for 37 days at a concentration of 50 ppb for 7 h daily under conditions of low light (250 micro mol m(-2) s(-1) (PAR)) and controlled temperature (20 degrees C). The seedlings exhibited extreme sensitivity to ozone. The ozone treatment reduced total plant biomass but had no effect on the partitioning of assimilate. Photosynthesis, stomatal conductance and internal CO(2) concentration were all reduced by ozone. The decline in photosynthesis was partly the result of direct effects of ozone on the stomata.     

Influence of light availability on growth, leaf morphology and plant architecture of beech ( Fagus sylvatica L.), maple ( Acer pseudoplatanus L.) and ash ( Fraxinus excelsior L.) saplings

In a field study, we measured saplings of beech, ash and maple growing in a fairly even-aged mixed-species thicket established by natural regeneration beneath a patchy shelterwood canopy with 3–60% of above canopy radiation reaching the saplings. Under low light conditions, maple and ash showed a slight lead in recent annual length increment compared with beech. With increasing light, ash and maple constantly gained superiority in length increment, whereas beech approached an asymptotic value above 35% light. A suite of architectural and leaf morphological attributes indicated a more pronounced ability of beech to adapt to shade than ash and maple. Beech displayed its leaves along the entire tree height (with a concentration in the middle crown), yielding a higher live crown ratio than ash and maple. It allocated biomass preferentially to radial growth which resulted in low height to diameter ratios, and expressed marked plagiotropic growth in shade indicating a horizontal light-foraging strategy. In addition, beech exhibited the highest specific leaf area, a greater total leaf area per unit tree height, a slightly greater leaf area index, and a greater plasticity to light in total leaf area. Ash and maple presented a “gap species” growth strategy, characterized by a marked and constant response in growth rates to increasing light and an inability to strongly reduce their growth rates in deep shade. In shade, they showed some plasticity in displaying most of their leaf area at the top of the crown to minimize self-shading and to enhance light interception. Through this, particularly, maple developed an “umbrella” like crown. These species-specific responses may be used for controlling the development of mixed-species regeneration in shelterwood systems.     

Ecophysiology of Cecropia schreberiana saplings in two wind regimes in an elfin cloud forest: growth,gas exchange,architecture and stem biomechanics

The hypothesis that the short, stunted development of elfin cloud forest trees is influenced by strong winds was tested by studying growth, gas exchange and biomechanics of potted saplings of Cecropia schreberiana Miq. ex. C. peltata L. (Cecropiaceae) exposed to two natural wind regimes (exposed and protected) at high elevation sites in the Luquillo Experimental Forest, Puerto Rico. The wind-exposure treatment produced several thigmomorphogenetic responses, including reductions in plant stature and crown area, changes in allocation patterns, and increased root to shoot ratio, leaf abrasion and leaf epinasty. Wind-exposure decreased maximum photosynthetic rate and respiration on an area basis, but not on a leaf-mass basis. Wind-exposed plants had lower apparent quantum yields, and higher light compensation points than wind-protected plants. Photosynthetic nitrogen-use efficiency was lower in wind-exposed plants, but such plants had higher leaf nitrogen concentration than wind-protected plants. There were no effects of treatments on stomatal conductance, transpiration rate and water-use efficiency. Stems of wind-exposed plants had lower second moment of area, apparent modulus of elasticity, flexural stiffness and stem density, but higher water content than wind-protected plants. Tissue-density-specific stiffness and the calculated critical height were not affected by the treatments. Wind-exposed plants were biomechanically less predisposed to bending and failing under their own weight than wind-protected plants because their safety factors were smaller, indicating that maintenance of an ontogenetically less developed structure enables plants to cope with wind loading. Windward trees showed a lower scaling component of the allometric relationship between diameter and height than leeward trees.     

Effects of shade on growth and mortality of maple (Acer pseudoplatanus), ash (Fraxinus excelsior) and beech (Fagus sylvatica) saplings

We compared shade tolerance of maple, ash and beech in the saplingstage from two sites with rich soils differing in water supply,growing in dense thickets underneath a beech shelterwood ofvarying canopy densities. Shade tolerance was described by twocomponents: mortality in shade and height growth in high light.At low light, beech showed the least mortality, maple the highestand ash in between on both sites. The decline with increasinglight was steepest in beech and more gradual with ash and maple.At 15 per cent above canopy light, all three species approachedzero mortality. Beech as the most shade-tolerant species hadthe highest survival rate under low light and the least lengthgrowth rate under high light (>17 per cent). Ash had a lowersurvival rate at low light than beech and a highest growth rateat high light. Maple showed a bit weaker trade-off with thelowest survival rate but a growth rate inferior to ash. On thebetter water-supplied site, height growth was significantlysuperior in all three species only under high light. On thebasis of these results, silvicultural conclusions are drawnwith respect to appropriate light levels and cutting types.     

NaCl胁迫对青檀种子发芽率、叶气体交换和苗木生长的影响

在水培条件下,本文探讨了NaCl胁迫对青檀种子发芽率、叶气体交换和苗木生长的影响。研究结果表明,NaCl胁迫延长了青檀种子的发芽时间,特别是当NaCl浓度超过17mM(1.0g/L)时,发芽率明显降低;NaCl胁迫降低了青檀叶的呼吸速率、气孔导度和净光合速率,并随着NaCl胁迫浓度的提高,下降的幅度更大,但NaCl胁迫苗木胞间隙CO2浓度明显高于对照;与对照相比,在NaCl胁迫50天后,青檀苗木的成活率、苗高和地径生长以及根系、茎和叶生物量显著降低,但茎和叶生物量下降更为明显。本文的研究结果认为,青檀是一种对盐胁迫较敏感的植物,一年生幼苗的耐盐阈值在34mM(2.0g/L)左右。图2表3参27。     

Effects of NaCl stress on seed germination, leaf gas exchange and seedling growth of Pteroceltis tatarinowii

Effects of NaCl on seed germination, leaf gas exchange and seedling growth were investigated. The germination of P. tatarinowii seeds was delayed in the presence of NaCl, and was significantly inhibited when NaCl concentration exceeded 17 mM (1.0 g·L⁻¹). NaCl treatments induced reductions of transpiration rate, stomatal conductance and net photosynthetic rate and their reductions were significant with the increasing of NaCl concentration. However, intercellular CO₂ concentration in seedling leaves under salt stress was significantly higher than that of controls. There was a significant reduction in survival, basal diameter increment and height increment with increasing NaCl concentration 50 days after NaCl treatment, and a significant reduction in the dry weight increment of roots, stems, and leaves was also observed in the NaCl treatments. The deleterious effect on the biomass increment was more evident in the case of stems and leaves. Results from this study suggest that P. tatarinowii is sensitive to salinity stress and the salinity threshold for seedling growth might be less 34 mM (2.0 g·L⁻¹). Biography: FANG Sheng-zuo (1963–), male, Professor, College of Forest Resources and Environment, Nanjing Forestry University, Nanjing 210037, P. R. China.     

西藏高原典型生态系统退化及植被恢复技术综述

全球变暖和人类活动导致西藏高原脆弱生态系统遭到不同程度的干扰和破坏, 对正在进行的国家生态安全屏障保护与建设构成严重的潜在威胁。文中在全面梳理西藏高原典型退化生态系统的主要表现(草地退化、湿地退化、工程扰动区植被破坏、风沙化土地不断扩张)、识别其驱动因素(气候变化、过度放牧和工程扰动)的基础上, 总结了西藏高原典型退化生态系统植被恢复的技术措施(围栏封育、人工种草、工程扰动分类恢复、适生植物种选择、沙障设置等), 以期为进一步加强西藏高原退化生态系统保护与恢复治理、促进国家生态安全屏障建设提供决策依据和技术支撑。     

Effects of the interaction between drought and shade on water relations, gas exchange and morphological traits in cork oak (Quercus suber L.) seedlings

The combined effect of drought and light on different physiological and biochemical traits was assessed in cork oak (Quercus suber L.) seedlings grown under two levels of light availability and submitted to a long-standing drought. Watering was withdrawn after germination and seedlings were allowed to dry to a water content of ca. 50% of field capacity. At this point, water-stressed seedlings were grown under moderate drought and two light regimes: high light (HL—50%) and low light (LL—2%). Soil water in control plants was kept close to field capacity (90–100%) for both light environments. Water-relations parameters derived from P–V curves, gas exchange and water status at predawn (Ψ_pd) were evaluated at twice during the experiment. Nitrogen and chlorophyll contents were determined in the same leaves used for the gas exchange measurements. In addition, maximum rate of carboxylation (V_cmax) and electronic transport (J_max) were derived from A–C_i curves in well-watered seedlings.

The variation on moisture availability during the experiment was the same under both light environments. In control plants, Ψ_pd was over −0.3 MPa at the two harvests, while stressed seedlings decreased to −0.9 MPa, with no differences between light treatments. Water stress decreased osmotic potentials at full (Ψπ₁₀₀) and zero turgor (Ψπ₀). The regressions between both potentials and Ψ_pd showed a higher intercept in shade grown seedlings. This fact will point out the higher osmoregulation capacity in sun seedlings whatever water availability.

Nitrogen investment on a per leaf mass (N_mass), chlorophyll content (Chl_mass) and SLA tended to show a typical pattern of sun-shade acclimation. Thus, the three parameters increased with shade. Only for N_mass there was a significant effect of watering, since water stress increased N_mass.

LL plants showed a lower photosynthetic capacity in terms of maximum net photosynthesis at saturating light (A_max), which was related to a decrease in V_cmax and J_max. Both parameters varied with specific leaf area (SLA) in a similar way. The low-light environment brought about a higher nitrogen investment in chlorophyll, while under high-light environment the investment was higher in carboxylation (V_cmax) and electronic transport (F_max).

Stomatal conductance to water vapour (g_wv) and A_max were lower in low-light seedlings independently of watering. In addition, there was a trend to keep higher intrinsic water use efficiency (IWUE) under high light environment. The increase of IWUE under water stress was higher in HL seedlings. This was as consequence of the steeper decline in g_wv as Ψ_pd decreased. The decrease of A_max with Ψ_pd occurred in a similar way in LL and HL seedlings. Thus, the HL seedlings tended to sustain a higher ability to increase IWUE than LL seedlings when they were submitted to the same water stress.     

Effects of light availability on leaf gas exchange and expansion in lychee (Litchi chinensis)

Effects of photosynthetic photon flux density (PPFD) on leaf gas exchange of lychee (Litchi chinensis Sonn.) were studied in field-grown "Kwai May Pink" and "Salathiel" orchard trees and young potted "Kwai May Pink" plants during summer in subtropical Queensland (27 degrees S). Variations in PPFD were achieved by shading the trees or plants 1 h before measurement at 0800 h. In a second experiment, potted seedlings of "Kwai May Pink" were grown in a heated greenhouse in 20% of full sun (equivalent to maximum noon PPFD of 200 micromol m(-2)xs(-1)) and their growth over three flush cycles was compared with seedlings grown in full sun (1080 micromol m(-2)xs(-1)). Young potted plants of "Kwai May Pink" were also grown outdoors in artificial shade that provided 20, 40, 70 or 100% of full sun (equivalent to maximum PPFDs of 500, 900, 1400 and 2000 micromol m(-2)xs(-1)) and measured for shoot extension and leaf area development over one flush cycle. Net CO2 assimilation increased asymptotically in response to increasing PPFD in both orchard trees and young potted plants. Maximum rates of CO2 assimilation (11.9 +/- 0.5 versus 6.3 +/- 0.2 micromol CO2 m(-2) s(-1)), dark respiration (1.7 +/- 0.3 versus 0.6 +/- 0.2 micromol CO2 m(-2) s(-1)), quantum yield (0.042 +/- 0.005 versus 0.027 +/- 0.003 mol CO2 mol(-1)) and light saturation point (1155 versus 959 micromol m(-2) s(-1)) were higher in orchard trees than in young potted plants. In potted seedlings grown in a heated greenhouse, shoots and leaves exposed to full sun expanded in a sigmoidal pattern to 69 +/- 12 mm and 497 +/- 105 cm(2) for each flush, compared with 27 +/- 7 mm and 189 +/- 88 cm(2) in shaded seedlings. Shaded seedlings were smaller and had higher shoot:root ratios (3.7 versus 3.1) than seedlings grown in full sun. In the potted plants grown outdoors in 20, 40, 70 or 100% of full sun, final leaf area per shoot was 44 +/- 1, 143 +/- 3, 251 +/- 7 and 362 +/- 8 cm(2), respectively. Shoots were also shorter in plants grown in shade than in plants grown in full sun (66 +/- 5 mm versus 101 +/- 2 mm). Photosynthesis in individual leaves of lychee appeared to be saturated at about half full sun, whereas maximum leaf expansion occurred at higher PPFDs. We conclude that lychee plants can persist as seedlings on the forest floor, but require high PPFDs for optimum growth.     

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.     

The contribution of structural indices to the modelling of Sitka spruce (Picea sitchensis) and birch (Betula spp.) crowns

Crown dimensions are important for the quantification of tree interactions in some growth models. This study investigates the potential for structural indices and other spatial measures to improve the prediction of crown radius and crown length for birch (Betula spp.) and Sitka spruce (Picea sitchensis (Bong.) Carr.) in forests in Wales. Crown dimensions were measured for 125 birch and 154 spruce in six fully stem-mapped research plots. These data were used to test the performance of a crown radius model and a crown length model which estimated crown dimensions on the basis of allometric relationships with stem dimensions. Spatial data from the six plots were used to calculate the structural indices mean directional index, diameter correlation index, species mingling, dbh and height dominance, and dbh differentiation, as well as the Hegyi competition index, and basal area of neighbours and larger neighbours, for each crown measurement sample tree, using various numbers of nearest neighbours. Two non-spatial indices, BAL and BALMOD, were also calculated for all sample trees for comparison. These spatial and non-spatial variables were then incorporated into modified crown dimension models. Model performances, in terms of efficiency and relative bias, were compared to determine whether the inclusion of spatial or non-spatial variables resulted in any improvements over models using tree dimensions alone. Crown length and radius were found to be correlated with most of the spatial measures studied. Models incorporating spatial variables gave improvements in performance over allometric models for every data set, and performed more consistently than models containing non-spatial variables. The greatest improvements were achieved for suppressed birch in unthinned forests which had irregularly shaped and strongly displaced crowns. The spatial variable contributing to the most efficient model for each data set varied widely. This points to the complexity of tree spatial interactions and indicates that there is a great deal of scope for investigating other structural indices and crown dimension model forms.