Genomic and physiological approaches to advancing forest tree improvement

The recent completion of a draft sequence of the poplar (Populus trichocarpa Torr. & Gray ex Brayshaw) genome has advanced forest tree genetics to an unprecedented level. A "parts list" for a forest tree has been produced, opening up new opportunities for dissecting the interworkings of tree growth and development. In the relatively near future we can anticipate additional reference genome sequences, including the much larger Pinus genome. One goal is to use this information to define the genomic attributes that affect the phenotypic performances of trees growing in various environments. A first step is the definition of ideotypes that constitute optimal tree and stand-level performance. Following this, the genome can be systematically searched for genetic elements and their allelic variants that affect the specified traits. Knowledge of these alleles and their effects will facilitate the development of efficient tree improvement programs through genome-guided breeding and genetic engineering and further our mechanistic understanding of trait variation. Improved mechanistic understanding of tree growth and development is needed to develop process models that will allow us to anticipate and manage change in forest ecosystems. Here we consider the development of an ideotype for loblolly pine (Pinus taeda L.) and discuss genomic approaches for studying the component traits that will enable advances in process model development and the genetic improvement of this important conifer.     

Estimating decomposition rate constants for European tree species from literature sources

Coarse woody debris (CWD) is an important substrate in forests, provides habitat for a multitude of organisms and is also a sink and a source for nutrients and carbon. To assess the dynamics of this substrate equations to describe the course of decomposition have been developed. They can also be used to describe the release of carbon from dead wood into the atmosphere. Unfortunately few equations have been parameterised for European conditions, although there is a need to estimate the dynamics of CWD in this region. Therefore, a method (termed the LB-method) was invented to estimate suitable decomposition rate constants (DRC) from sources published in the CWD Global Database by systematically comparing features and conditions of the area and species of interest with the information given with the published equations. The DRC were weighed according to these comparisons and then sorted by the weight assigned to them. From the resulting distribution a plausible DRC for the region and species of interest was estimated. The LB-method was compared to results from a study on Norway spruce (Picea abies L.) decomposition in Thuringia, Germany, and to results from an expert elicitation on DRC for Scots pine (Pinus sylvestris L.) in the Federal State of Brandenburg, Germany. In both cases the method gave results within the confidence interval of the respective study used for comparison. Both methods are suitable for the estimation of preliminary DRC until more accurate constants can be derived by research. While the focus in this study was on density loss the method can be applied to other dead wood features (e.g. volume), and should be applicable to other regions of the world where information is also scarce.     

Prediction of stem profile of Picea abies using a process-based tree growth model

We built a simple tree growth model for Norway spruce (Picea abies (L.) Karst.) that describes the biomass and stem radial growth of one tree in a stand. Growth is controlled by an external height growth function that accounts for site quality. Crown recession is represented by an empirical function that accounts for the limitation to crown development caused by mechanical contacts with neighboring trees. The model describes biomass growth based on carbon budget (photosynthesis, respiration and senescence) and carbon partitioning between foliage, stem and root compartments. An internal regulation is introduced based on a functional balance between crown and root development. Stem annual growth is distributed along the stem by means of an empirical rule. Stem profile is the final output of the model and can be used to check the overall consistency of the model and as an aid in wood quality studies. The underlying assumptions of the model are described.     

Tree-growth analyses to estimate tree species' drought tolerance

Climate change is challenging forestry management and practices. Among other things, tree species with the ability to cope with more extreme climate conditions have to be identified. However, while environmental factors may severely limit tree growth or even cause tree death, assessing a tree species' potential for surviving future aggravated environmental conditions is rather demanding. The aim of this study was to find a tree-ring-based method suitable for identifying very drought-tolerant species, particularly potential substitute species for Scots pine (Pinus sylvestris L.) in Valais. In this inner-Alpine valley, Scots pine used to be the dominating species for dry forests, but today it suffers from high drought-induced mortality. We investigate the growth response of two native tree species, Scots pine and European larch (Larix decidua Mill.), and two non-native species, black pine (Pinus nigra Arnold) and Douglas fir (Pseudotsuga menziesii Mirb. var. menziesii), to drought. This involved analysing how the radial increment of these species responded to increasing water shortage (abandonment of irrigation) and to increasingly frequent drought years. Black pine and Douglas fir are able to cope with drought better than Scots pine and larch, as they show relatively high radial growth even after irrigation has been stopped and a plastic growth response to drought years. European larch does not seem to be able to cope with these dry conditions as it lacks the ability to recover from drought years. The analysis of trees' short-term response to extreme climate events seems to be the most promising and suitable method for detecting how tolerant a tree species is towards drought. However, combining all the methods used in this study provides a complete picture of how water shortage could limit species.     

The importance of an indigenous tree to southern African communities with specific relevance to its domestication and commercialization: a case of the marula tree

Sclerocarya birrea (A. Rich.) Hochst. subsp.     

Dynamic response of tree growth to changing environmental pollution

Long-term investigations of Scots pine (Pinus sylvestris L.) growth were carried out in the vicinity of one of the biggest air pollution sources in Lithuania—mineral fertilizers plant “Achema.” It is detected that initial stages of the dynamic tree response to the external stress factors in the polluted environment with an increased quantity of nitrogen compounds have started with a stimulation stage, followed by a gradual transition to a depression of growth. The recovery of the damaged stands took place along with the reduced environmental pollution, and the overdraft of the “normal” annual increment was a characteristic feature of all the investigated stands. This phase is still continuing for the most damaged stands. A very high individual variability of the tree growth response to the environmental impact is a characteristic feature of the damaged Scots pine stands. The homeostatic mechanisms of the survived trees stipulated reaching approximately the same or even higher growth rate as prior to the depression period, and the tree growth rate before the depression period can serve as the most powerful predictor of tree growth recovery capacity under the reduced environmental pollution. Crown defoliation is the next most important predictor of individual tree recovery capacity. Lower stand density and lower competition cause higher recovery capacity of damaged trees. The conclusion is made that a reduction in the environmental pollution on the local and regional scale and especially a decrease in emissions and deposition of sulfur and nitrogen compounds caused the recovery of damaged forests. These trends should be taken into account while analyzing and modeling forest dynamics. Interaction of environmental pollution and climatic factors is very important for the response of tree growth to the environmental stressors.     

Use of modeled photosynthesis and decomposition to describe tree growth at the northern tree line

Growth of subarctic Scots pine (Pinus sylvestris L.) trees was investigated by a combination of process-based models and dendroecological approaches. Tree ring width indices were strongly autocorrelated and correlated with simulated photosynthetic production of the previous year and with organic matter N mineralization of the current year. An autoregressive model, with photosynthesis and N mineralization as external inputs, explained growth of the trees well. However, relationships for the period 1950-1992 differed significantly from relationships for the period 1876-1949; the slope of the regression of tree ring width index and photosynthesis was lower for the 1950-1992 period. Also, the autocorrelation structure of the data changed. First-order autocorrelation decreased and second-order autocorrelation increased from the earlier to the later period. This means that growth is becoming less sensitive to variations in photosynthetic production, whereas the relationships between growth and N mineralization are remaining fairly constant. We postulate that, although photosynthesis has increased in response to increasing CO2 concentrations, tree growth rate cannot parallel the increase in photosynthesis because potential growth rate is limited directly by temperature.     

Modeling tree water deficit from microclimate: an approach to quantifying drought stress

Tree water deficit estimated by measuring water-related changes in stem radius (DeltaW) was compared with tree water deficit estimated from the output of a simple, physiologically reasonable model (DeltaWE), with soil water potential (Psisoil) and atmospheric vapor pressure deficit (VPD) as inputs. Values of DeltaW were determined by monitoring stem radius changes with dendrometers and detrending the results for growth. We followed changes in DeltaW and DeltaWE in Pinus sylvestris L. and Quercus pubescens Willd. over 2 years at a dry site (2001-2002; Salgesch, Wallis) and in Picea abies (L.) Karst. for 1 year at a wet site (1998; Davos, Graubuenden) in the Swiss Alps. The seasonal courses of DeltaW in deciduous species and in conifers at the same site were similar and could be largely explained by variation in DeltaWE. This finding strongly suggests that DeltaW, despite the known species-specific differences in stomatal response to microclimate, is mainly explained by a combination of atmospheric and soil conditions. Consequently, we concluded that trees are unable to maintain any particular DeltaW. Either Psisoil or VPD alone provided poorer estimates of DeltaW than a model incorporating both factors. As a first approximation of DeltaWE, Psisoil can be weighted so that the negative mean Psisoil reaches 65 to 75% of the positive mean daytime VPD over a season (Q. pubescens: approximately 65%, P. abies: approximately 70%, P. sylvestris: approximately 75%). The differences in DeltaW among species can be partially explained by a different weighting of Psisoil against VPD. The DeltaW of P. sylvestris was more dependent on Psisoil than that of Q. pubescens, but less than that of P. abies, and was less dependent on VPD than that of P. abies and Q. pubescens. The model worked well for P. abies at the wet site and for Q. pubescens and P. sylvestris at the dry site, and may be useful for estimating water deficit in other tree species.     

The arbuscular mycorrhizal fungi status of selected tree nurseries in the Ethiopian highlands

We investigated the arbuscular mycorrhizal fungi(AMF) status of ten nurseries suitable for restoration of dry evergreen Afromontane forests in Ethiopia. We quantified AMF root colonization(RC) and spore abundance(SA) in seedlings of nine native tree species namely Acacia abyssinica Hochst. ex Benth., Cordia africana Lam., Dovyalis abyssinica(A. Rich.) Warb., H agenia abyssinica J.F. Gmel., Juniperus procera Hochst. ex Endl., Millettia ferruginea(Hochst.) Baker, Olea europaea L. subsp. c uspidata(Wall. ex G. Don) Cif., Podocarpus falcatus(Thunb.) R. Br. ex Mirb. and Prunus africana(Hook. f.) Kalkman. We used the ink and vinegar method to stain AMF in roots. RC levels ranged from 8.00 to 99.67% and were generally higher than the RC levels reported from other similar nurseries in Ethiopia. SA levels ranged from 1 to 25 spores g~(-1) and werecomparable with some reports from the field in Ethiopia but they were lower than levels reported by another similar study. RC was more affected by host species than nursery location, while the reverse was true for SA. The results also showed that nursery management could improve AMF status among seedlings. When all nursery tree species were considered, RC and SA levels were unrelated. No strong correlation existed between the nursery management variables considered and RC or SA. However, considering C. africana, J. procera and P. falcatus separately, RC-age(r_s = 0.829, P = 0.042) correlation for O. europaea and RC-pot diameter( r_s = 0.820, P = 0.046), RC-pot volume( r_s = 0.928, P = 0.008) and SA-age( r_s = 0.943, P = 0.005) correlations for C. africana, were significant, strong and positive. Generally, most of the tree species and particularly, early-mid successional tree species had sufficient AMF inoculum. Hence, only the mid-late successional tree species; J. procera, P. falcatus, and P. africana may require AMF inoculation, preferably, during filed planting. Based on our results, age and pot volume were identified to be important variables potentially affecting RC and SA. To better understand the effects of these and other nursery management variables, additional study is required. We demonstrated for the first time that black Hero ink is suitable for staining root AMF and can be used in future AMF research.     

Interspecific variation in xylem vulnerability to cavitation among tropical tree and shrub species

In tropical moist forests, seasonal drought limits plant survival, productivity and diversity. Drought-tolerance mechanisms of tropical species should reflect the maximum seasonal water deficits experienced in a particular habitat. We investigated stem xylem vulnerability to cavitation in nine tropical species with different life histories and habitat associations. Stem xylem vulnerability was scored as the xylem water potential causing 50 and 75% loss of hydraulic conductivity (P50 and P75, respectively). Four shade-tolerant shrubs ranged from moderately resistant (P50=-1.9 MPa for Ouratea lucens Kunth. Engl.) to highly resistant to cavitation (P50=-4.1 MPa for Psychotria horizontalis Sw.), with shallow-rooted species being the most resistant. Among the tree species, those characteristic of waterlogged soils, Carapa guianensis Aubl., Prioria copaifera Griseb. and Ficus citrifolia Mill., were the most vulnerable to cavitation (P50=-0.8 to -1.6 MPa). The wet-season, deciduous tree, Cordia alliodora (Ruiz and Pav.) Oken., had resistant xylem (P50=-3.2 MPa), whereas the dry-season, deciduous tree, Bursera simaruba (L.) Sarg. was among the most vulnerable to cavitation (P50=-0.8 MPa) of the species studied. For eight out of the nine study species, previously reported minimum seasonal leaf water potentials measured in the field during periods of drought correlated with our P50 and P75 values. Rooting depth, deciduousness, soil type and growth habit might also contribute to desiccation tolerance. Our results support the functional dependence of drought tolerance on xylem resistance to cavitation.     

Green fluorescent protein as a tool for monitoring transgene expression in forest tree species

The gene coding for green fluorescent protein (GFP) from the jellyfish Aequorea victoria was successfully used as a vital marker for the transformation of three woody plant species, black spruce (Picea mariana (Mill.) BSP), white pine (Pinus strobus L.) and poplar (Populus spp.). The gfp gene and the gene conferring resistance to kanamycin (nptII) were introduced by microprojectile bombardment or Agrobacterium tumefaciens-mediated technology. Screening by fluorescence microscopy of the transformed plant material, under the selection of kanamycin, identified five to eight cell lines from each tree species that clearly expressed GFP. Expression of GFP was observed in somatic embryonal cells of the coniferous species and in stem sections of poplar. For all species, GFP transgene expression was stable over multiple subcultures. Stable integration of the gfp gene into plant genomes was confirmed by Southern hybridization or polymerase chain reaction (PCR) analysis. We conclude that GFP can be used as a vital marker and reporter protein in transformation experiments with gymnosperms and deciduous trees.     

增效剂对球孢白僵菌产孢量及对光肩星天牛致死速率的影响

在应用昆虫病原菌防治光肩星天牛（Ａｎｏｐｌｏｐｈｏｒａ ｇｌａｂｒｉｐｅｎｎｉｓＭｏｔｓｃｈ）的试验中，筛选出一种增效剂，可使病原菌的产孢量增加，光肩星天牛的致死速率加快。在生产病原菌的培养基内加入０．０２％的增效剂培养５天，产孢量比对照增加一倍多。在含孢量１０＾８／ｍｌ的球孢白僵菌的菌悬液中加入０．０２％的增效剂，以每头光肩星天牛幼虫接种１ｍｌ菌悬液，使光肩星天牛幼虫第６天开始发病，第１６天死亡     

A controlled test of the dual-isotope approach for the interpretation of stable carbon and oxygen isotope ratio variation in tree rings

Seedlings of a conifer (Pinus radiata D. Don) and a broad leaf angiosperm (Eucalyptus globulus Labill.) were grown for 100 days in two growth cabinets at 45 or 65% relative humidity. The seedlings were exposed to treatments designed to modify carbon assimilation rates and capacities, stomatal conductance and transpiration to test conceptual models that attempt to clarify the interpretation of carbon isotope discrimination (Δ(13)C) by using oxygen isotope enrichment (Δ(18)O). Differences in relative humidity and within-cabinet treatments (including lower irradiance, lower nitrogen inputs, higher leaf temperature and lower moisture status than control seedlings) produced significant differences in assimilation rates, photosynthetic capacities, stomatal conductance, leaf transpiration rates and leaf evaporative enrichment. The dual-isotope approach accurately interpreted the cause of variation in wood cellulose Δ(13)C for some of the treatments, but not for others. We also tested whether we could use Δ(13)C variation to constrain the interpretation of δ(18)O variation. Carbon isotope discrimination appears to be influenced by transpiration (providing information on leaf evaporative enrichment), but the results did not provide a clear way to interpret such variation. The dual-isotope approach appears to be valid conceptually, but more work is needed to make it operational under different scenarios.     

Long-term respiration in relation to growth and maintenance processes of the aboveground parts of a hinoki forest tree

Over a three-year period, CO(2) exchange rates were measured continuously on the aboveground parts of a 13-year-old hinoki (Chamaecyparis obtusa (Sieb. et Zucc.) Endl.) tree in the field, using an open gas-exchange system tracking ambient temperature. The relationship between daily aerial respiratory consumption and daily gross production, which was analyzed monthly, closely resembled McCree's equation. The value of the coefficient of growth respiration ranged between 0.0 in winter and 0.16 in summer and increased with increasing monthly mean temperature. A clockwise loop was observed for monthly change in the relationship between the coefficient of growth respiration and temperature. Maintenance respiration could be formulated as a power function of aboveground dry weight. The exponent of the equation ranged from 0.3 to 1.1. A value of 1.1 in May and June, when trees were growing most actively, indicated that maintenance respiration was directly proportional to aboveground weight. In March, April, July, and August, maintenance respiration was not proportional to aboveground weight, but it was closely proportional to surface area. The exponent value exhibited seasonal change with a clockwise loop in response to monthly average temperature. During the dormant season, respiration was used only for maintenance purposes, whereas during the growing season both growth and maintenance respiration occurred. Annual growth and maintenance respiration increased with increasing tree age. The average annual contribution to total respiratory consumption was 21% for growth respiration and 79% for maintenance respiration.     

Modeling individual tree height growth of Norway spruce and Scots pine from national forest inventory data in Norway

We developed individual tree height growth models for Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) in Norway based on national forest inventory data. Potential height growth is based on existing dominant height growth models and reduced due to competition by functions developed in this study. Three spatially explicit and two spatially non-explicit competition indices were tested. Distance effects and diameter ratio effects were estimated from the data simultaneously with parameters of the potential modifier functions. Large height measurement errors in the national forest inventory data caused large residual variation of the models. However, the effects of competition on height growth were significant and plausible. The potential modifier functions show that height growth of dominant trees is largely unaffected by competition. Only at higher levels of competition, height growth is reduced as a consequence of competition. However, Scots pine also reduced height growth at very low levels of competition. Distance effects in the spatially explicit competition indices indicated that the closest neighbors are most important for height growth. However, for Scots pine also competitors at larger distance affected height growth. The five competition indices tested in this study explained similar proportions of the variation in relative height growth. Given that unbiased predictions can only be expected for the same plot size, we recommend a spatially explicit index, which describes the distance function with a negative exponential, for use in growth simulators.     

Response of five temperate deciduous tree species to water stress

Gas exchange, tissue water relations, and leaf/root dry weight ratios were compared among young, container-grown plants of five temperate-zone, deciduous tree species (Acer negundo L., Betula papyrifera Marsh, Malus baccata Borkh, Robinia pseudoacacia L., and Ulmus parvifolia Jacq.) under well-watered and water-stressed conditions. There was a small decrease (mean reduction of 0.22 MPa across species) in the water potential at which turgor was lost (Psi(tlp)) in response to water stress. The Psi(tlp) for water-stressed plants was -1.18, -1.34, -1.61, -1.70, and -2.12 MPa for B. papyrifera, A. negundo, U. parvifolia, R. pseudoacacia, and M. baccata, respectively. Variation in Psi(tlp) resulted primarily from differences in tissue osmotic potential and not tissue elasticity. Rates of net photosynthesis declined in response to water stress. However, despite differences in Psi(tlp), there were no differences in net photosynthesis among water-stressed plants under the conditions of water stress imposed. In A. negundo and M. baccata, water use efficiency (net photosynthesis/transpiration) increased significantly in response to water stress. Comparisons among water-stressed plants showed that water use efficiency for M. baccata was greater than for B. papyrifera or U. parvifolia. There were no significant differences in water use efficiency among B. papyrifera, U. parvifolia, A. negundo, and R. pseudoacacia. Under water-stressed conditions, leaf/root dry weight ratios (an index of transpiration to absorptive capacity) ranged from 0.77 in R. pseudoacacia to 1.05 in B. papyrifera.     

Root recovery of five tropical tree and shrub species by sieves of different mesh sizes

Accurate quantitative assessment of roots is key to understanding the belowground plant productivity as well as providing an insight of the plant-soil interactions. In this study, root recoveries by sieves of different mesh sizes (2.0, 1.0, 0.5 and 0.25 mm) were measured for five tropical tree and shrub species grown in monoculture stands: crotalaria (Crotalaria grahamiana Wight and Arn.), pigeonpea [Cajanus cajan (L.) Millsp.], sesbania [Sesbania sesban (L.) Merr.], tephrosia (Tephrosia vogelii Hook F.), siratro [Macroptilium atropurpureum (DC.) Urb.] and tithonia [Tithonia diversifolia (Hemsl.) Gray]. Root samples were take from 0-15 cm soil depth. Recovery of coarser roots (>1.0 mm) ranged from 70 to 93% and 90 to 98% of the cumulative root length and biomass respectively. The proportion of root length of the finer roots (<1.0 mm) was greater for pigeonpea (30%), tithonia (22%) and siratro (18%) compared with other species, but contributed negligibly to the cumulative total root biomass for all species. The use of 0.5 mm sieve improved the recovery of root length for most species but had little effect on root biomass. The 0.25 mm sieve was most effective in capturing finer roots (<0.5 mm) of pigeonpea which represented 16% of cumulative root length and 4% of root biomass recorded for this species. Recovery of roots of different diameter classes depended on species, suggesting that for an improved estimation of root parameters especially when sieves of large mesh sizes (>0.25 mm) are used, a correction factor could be useful for root length measurements but not root biomass measurements for a particular species in each site and for a specific study. This revised version was published online in June 2006 with corrections to the Cover Date.     

Growth and photosynthetic responses of four Virginia Piedmont tree species to shade

To determine the effects of shade on biomass, carbon allocation patterns and photosynthetic response, seedlings of loblolly pine (Pinus taeda L.), white pine (Pinus strobus L.), red maple (Acer rubrum L.), and yellow-poplar (Liriodendron tulipifera L.) were grown without shade or in shade treatments providing a 79 or 89% reduction of full sunlight for two growing seasons. The shade treatments resulted in less total biomass for all species, with loblolly pine showing the greatest shade-induced growth reduction. Yellow-poplar was the only species to show increased stem height growth in the 89% shade treatment. The shade treatments increased specific leaf area of all species. Quantum efficiency, dark respiration and light compensation point were generally not affected by the shade treatments. Quantum efficiency, dark respiration, maximum photosynthesis and light compensation point did not change consistently between the first and second growing seasons. We conclude that differences in shade tolerance among these species are not the result of changes in the photosynthetic mechanism in response to shade.     

园林绿化和工业用材的优良树种--美国梧桐

美国梧桐广泛分布于北美，适应性广，尤其耐水湿。其树形优美，是良好的庭园绿化树种；生长迅速，也是重要的短周期工业用材树种。