Adult Forest Trees Research Articles

Using unmanned aerial vehicle‐based multispectral, RGB and thermal imagery for phenotyping of forest genetic trials: A case study inPinus halepensis

The assessment of genetic differentiation in functional traits is fundamental towards understanding the adaptive characteristics of forest species. While traditional phenotyping techniques are costly and time‐consuming, remote sensing data derived from cameras mounted on unmanned aerial vehicles (UAVs) provide potentially valid high‐throughput information for assessing morphophysiological differences among tree populations. In this work, we test for genetic variation in vegetation indices (VIs) and canopy temperature among populations ofPinus halepensisas proxies for canopy architecture, leaf area, photosynthetic pigments, photosynthetic efficiency and water use. The interpopulation associations between vegetation properties and above‐ground growth (stem volume) were also assessed. Three flights (July 2016, November 2016 and May 2017) were performed in a genetic trial consisting of 56 populations covering a large part of the species range. Multispectral (visible and near infrared wavelengths), RGB (red, green, blue) and thermal images were used to estimate canopy temperature and vegetation cover (VC) and derive several VIs. Differences among populations emerged consistently across flights for VC and VIs related to leaf area, indicating genetic divergence in crown architecture. Population differences in indices related to photosynthetic pigments emerged only in May 2017 and were probably related to a contrasting phenology of needle development. Conversely, the low population differentiation for the same indices in July 2016 and November 2016 suggested weak interpopulation variation in the photosynthetic machinery of mature needles ofP. halepensis. Population differences in canopy temperature found in July 2016 were indicative of variation in stomatal regulation under drought stress. Stem volume correlated with indices related to leaf area (positively) and with canopy temperature (negatively), indicating a strong influence of canopy properties and stomatal conductance on above‐ground growth at the population level. Specifically, a combination of VIs and canopy temperature accounted for about 60% of population variability in stem volume of adult trees. This is the first study to propose UAV remote sensing as an effective tool for screening genetic variation in morphophysiological traits of adult forest trees.

Can mimicking sexual reproduction solve problems with recalcitrance in in vitro propagation of tree species?

Clonal propagation by in vitro means of adult forest trees is often difficult or not possible with current technology. This problem, generally described as recalcitrance, has been approached from several angles by researchers and reviewers. However, it has not yet been reviewed from the point of view that it may eventually be possible to overcome recalcitrance by mimicking the rejuvenation process that occurs during sexual reproduction. It is suggested that somatic cell nuclear transfer, in a manner similar to the one used for animal cloning, could be helpful. Furthermore, application of controlled stress and autophagy, or inducing apomixes by halting natural or artificially induced meiosis before chromosome segregation, could perhaps assist in overcoming recalcitrance. The discussion below indicates that mimicking the sexual or apomictic process in vitro is worthy of further exploration and should be evaluated as a means of genetically improving tree species, especially those of high economic value. Studies along these lines may also be helpful in improving our knowledge of the epigenetic, cytogenetic, and genomic mechanisms involved.

Branch cuvettes as means of ozone risk assessment in adult forest tree crowns: combining experimental and modelling capacities

The branch autonomy principle has been referred to extensively for using branch cuvettes as a technique of studying ozone (O3) effects within the canopy of adult forest trees. However, this principle may not hold in general regarding biochemical interactions between O3-impacted branches exposed inside cuvettes and neighbouring crown parts under the unchanged ambient O3 regime. After reviewing relevant cuvette studies conducted to date, we will provide evidence that cuvette-exposed branches may serve, given awareness of outlined pre-requisites and restrictions, as surrogates for examining the crown-level response of trees to elevated O3 regimes. Such a conclusion is based on the defence metabolism of branches, which seems to be autonomous to some extent from neighbouring crown sections. Cuvette studies may, therefore, be used to derive dose response functions as measures of O3 sensitivity. On such grounds, also validation and improvement of stomatal O3 uptake modelling becomes feasible. The branch-level approach, however, does not substitute whole-tree free-air O3 fumigation and related flux assessments, as branches in view of representativeness and boundary layer characteristics represent one stage in scaling O3 flux between leaf and tree level. Branch level-based flux scaling should be backed, therefore, by independent trunk sap-flow assessment techniques that offer derivation of FO3 at the whole-tree level.

Distinct roles of savanna and forest tree species in regeneration under fire suppression in a Brazilian savanna

Questions: Has fire suppression relaxed barriers to the exchange of species between savanna and forest? Do all species or a subset of species participate in this exchange? Would current vegetation structure persist if fire suppression were to cease? Location: A gallery forest edge in the Cerrado region of central Brazil that burned only once in the past 35 years. Methods: Density of tree seedlings, saplings and adults, leaf area index (LAI), tree basal area and diameter were surveyed in 12, 10m � 70m transects centred on and perpendicular to the forest‐savanna boundary. Community composition was assessed using non-metric multi-dimensional scaling (NMDS). Results: Basal area and LAI declined substantially from forest to savanna, with an associated shift in species composition. Savanna tree species were nearly absent in the forest, but accounted for the majority of stems in the savanna. In contrast, forest species comprised 14% of adults and more than one-third of juveniles in the savanna. Despite the high diversity of trees (85 species) in the forest, five species play a particularly large role in this initial phase of forest expansion. Reintroduction of fire, however, would result in widespread topkill of juveniles and the majority of adult forest trees, thereby interrupting the succession towards forest. Conclusions: After 35 years during which the site burned only once, the savanna still remains dominated by savanna species. Nevertheless, the dominance of forest juveniles in border and savanna tree communities suggests that with a continued policy of fire suppression, the forest will continue to expand.

A free-air system for long-term stable carbon isotope labeling of adult forest trees

Stable carbon (C) isotopes, in particular employed in labeling experiments, are an ideal tool to broaden our understanding of C dynamics in trees and forest ecosystems. Here, we present a free-air exposure system, named isoFACE, designed for long-term stable C isotope labeling in the canopy of 25 m tall forest trees. Labeling of canopy air was achieved by continuous release of CO2 with a δ13C of −46.9‰. To this end, micro-porous tubes were suspended at c. 1 m distance vertically through the canopy, minimizing CO2 gradients from the exterior to the interior and allowing for C labeling exposure during periods of low wind speed. Target for CO2 concentration ([CO2]) increase was ambient +100 μmol mol−1. Canopy [CO2] stayed within 10% of the target during more than 57% of the time and resulted in a drop of δ13C in canopy air by 7.8‰. After 19 labeling days about 50% of C in phloem sugars and stem CO2 efflux were turned over and 20–30% in coarse root CO2 efflux and soil CO2. The isoFACE system successfully altered δ13C of canopy air for studying turn-over of C pools in forest trees and soils, highlighting their slow turn-over rates.

Tuning growth versus defence–belowground interactions and plant resource allocation

As substrate and energy resources in plants are limited, there is a need for plants for a well balanced trade-off, which allows growth but does not exclude defense mechanisms. This is addressed by the “Growth-Differentiation-Balance Theory” (GDB; Loomis 1953; Lorio 1988) as accentuated by Herms and Mattson (1992) and adapted by Matyssek et al. (2002, 2005). However, this theory suffers from substantial deficits in the understanding of mechanisms that underlie the balance between resource sequestration (as mediated through the plant’s competitiveness related to growth) and retention (as a result of defence). Therefore since 1998 an integrative research center, entitled “Growth and Parasite Defense–Competition for Resources in Economic Plants from Agronomy and Forestry” (Sonderforschungsbereich 607, SFB 607, funded by the ‘Deutsche Forschungsgemeinschaft’, DFG) has been established in the Munich area (Germany). The mechanistic and ecological perspective of GDB makes understandable that “third-party trade-offs” between further internal (e.g. reserve storage, reproduction) or external resources (e.g. driven through symbionts or other associative soil micro-organisms), may intensely impact on and modulate the principal resource trade-off addressed by GDB between growth and defence (Stitt and Schulze 1994; Heil and Baldwin 2002; Stamp 2003; Matyssek et al. 2005). This integrated view on GDB requires spatiotemporal process scaling that bridges gene regulation, metabolism, whole-plant functionality and resource flux at the stand level, being aware of the intense resource interchange that occurs between plants and their environment at the specific field sites. The scarcity of knowledge about the mechanistic and ecological foundations of GDB (Heil and Baldwin 2002, Stamp 2003) is evident, in particular, in forest trees, as their longevity and dimension pose a particular spatiotemporal challenge on research. Process scaling is to be extended, therefore, across plant-ontogenetic stages and a range of growth conditions, the latter being most distinctively represented by controlled, short-term chamber studies (where most of the currently available evidence is derived from, while being restricted to juvenile trees; e.g., Kolb and Matyssek 2001) versus long-term experimentation understand conditions with adult forest trees (e.g., Matyssek et al. 2007). In view of such challenges, scaling concepts that integrate Plant Soil (2009) 323:1–5 DOI 10.1007/s11104-009-0070-6

Deriving ozone dose–response of photosynthesis in adult forest trees from branch-level cuvette gas exchange assessment

Branch-level gas exchange provided the basis for assessing ozone flux in order to derive the dose–response relationship between cumulative O 3 uptake (COU) and carbon gain in the upper sun crown of adult Fagus sylvatica. Fluxes of ozone, CO 2 and water vapour were monitored simultaneously by climatized branch cuvettes. The cuvettes allowed branch exposure to an ambient or twice-ambient O 3 regime, while tree crowns were exposed to the same O 3 regimes (twice-ambient generated by a free-air canopy O 3 exposure system). COU levels higher than 20 mmol m −2 led to a pronounced decline in carbon gain under elevated O 3. The limiting COU range is consistent with findings on neighbouring branches exposed to twice-ambient O 3 through free-air fumigation. The cuvette approach allows to estimate O 3 flux at peripheral crown positions, where boundary layers are low, yielding a meso-scale within-crown resolution of photosynthetic foliage sensitivity under whole-tree free-air O 3 fumigation.

Synopsis of the CASIROZ Case Study: Carbon Sink Strength of Fagus sylvatica L. in a Changing Environment ‐ Experimental Risk Assessment of Mitigation by Chronic Ozone Impact

Databases are needed for the ozone (O(3)) risk assessment on adult forest trees under stand conditions, as mostly juvenile trees have been studied in chamber experiments. A synopsis is presented here from an integrated case study which was conducted on adult FAGUS SYLVATICA trees at a Central-European forest site. Employed was a novel free-air canopy O(3) fumigation methodology which ensured a whole-plant assessment of O(3) sensitivity of the about 30 m tall and 60 years old trees, comparing responses to an experimental 2 x ambient O(3) regime (2 x O(3), max. 150 nl O(3) l (-1)) with those to the unchanged 1 x ambient O(3) regime (1 x O(3)=control) prevailing at the site. Additional experimentation on individual branches and juvenile beech trees exposed within the forest canopy allowed for evaluating the representativeness of young-tree and branch-bag approaches relative to the O(3) sensitivity of the adult trees. The 2 x O(3) regime did not substantially weaken the carbon sink strength of the adult beech trees, given the absence of a statistically significant decline in annual stem growth; a 3 % reduction across five years was demonstrated, however, through modelling upon parameterization with the elaborated database. 2 x O(3) did induce a number of statistically significant tree responses at the cell and leaf level, although the O(3) responsiveness varied between years. Shade leaves displayed an O(3) sensitivity similar to that of sun leaves, while indirect belowground O(3) effects, apparently mediated through hormonal relationships, were reflected by stimulated fine-root and ectomycorrhizal development. Juvenile trees were not reliable surrogates of adult ones in view of O(3) risk assessment. Branch sections enclosed in (climatized) cuvettes, however, turned out to represent the O(3) sensitivity of entire tree crowns. Drought-induced stomatal closure decoupled O(3) intake from O(3) exposure, as in addition, also the "physiologically effective O(3) dose" was subject to change. No evidence emerged for a need to lower the "Critical Level for Ozone" in risk assessment of forest trees, although sensitive tree parameters did not necessarily reflect a linear relationship to O(3) stress. Exposure-based concepts tended to overestimate O(3) risk under drought, which is in support of current efforts to establish flux-related concepts of O(3) intake in risk assessment.

Exemplifying whole-plant ozone uptake in adult forest trees of contrasting species and site conditions

Whole-tree O 3 uptake was exemplified for Picea abies, Fagus sylvatica and Larix decidua in stands at high and low altitude and contrasting water availability through sap flow measurement in tree trunks, intrinsically accounting for drought and boundary layer effects on O 3 flux. O 3 uptake of evergreen spruce per unit foliage area was enhanced by 100% at high relative to low elevation, whereas deciduous beech and larch showed similar uptake regardless of altitude. The responsiveness of the canopy conductance to water vapor and, as a consequence, O 3 uptake to soil moisture and air humidity did not differ between species. Unifying findings at the whole-tree level will promote cause–effect based O 3 risk assessment and modeling.

Comparison of ozone uptake and sensitivity between a phytotron study with young beech and a field experiment with adult beech (Fagus sylvatica)

Chamber experiments on juvenile trees have resulted in severe injury and accelerated loss of leaves along with reduced biomass production under chronically enhanced O3 levels. In contrast, the few studies conducted on adult forest trees in the field have reported low O3 sensitivity. In the present study, young beech in phytotrons was more sensitive to O3 than adult beech in the field, although employed O3 regimes were similar. The hypotheses tested were that: (1) differences in O3 uptake were caused by the ontogenetically higher stomatal conductance of young compared to adult trees, (2) the experimental settings in the phytotrons enhanced O3 uptake compared to field conditions, and (3) a low detoxification capacity contributes to the higher O3 sensitivity of the young trees. The higher O3 sensitivity of juvenile beech in the phytotrons is demonstrated to relate to both the experimental conditions and the physiological responsiveness inherent to tree age.

Response patterns in adult forest trees to chronic ozone stress: identification of variations and consistencies

The responsiveness of adult beech and spruce trees to chronic O 3 stress was studied at a free-air O 3 exposure experiment in Freising/Germany. Over three growing seasons, gas exchange characteristics, biochemical parameters, macroscopic O 3 injury and the phenology of leaf organs were investigated, along with assessments of branch and stem growth as indications of tree performance. To assess response pattern to chronic O 3 stress in adult forest trees, we introduce a new evaluation approach, which provides a comprehensive, readily accomplishable overview across several tree-internal scaling levels, different canopy regions and growing seasons. This new approach, based on a three-grade colour coding, combines statistical analysis and the proficient ability of the “human eye” in pattern recognition.

Dendroremediation of trinitrotoluene (TNT). Part 2: fate of radio-labelled TNT in trees.

Problems of long-term existence of the environmental contaminant 2,4,6-trinitrotoluene (TNT) and necessities for the use of trees ('dendroremediation') in sustainable phytoremediation strategies for TNT are described in the first part of this paper. Aims of the second part are estimation of [14C]-TNT uptake, localisation of TNT-derived radioactivity in mature tree tissues, and the determination of the degree of TNT-degradation during dendroremediation processes. Four-year-old trees of hybrid willow (Salix spec., clone EW-20) and of Norway spruce (Picea abies) were cultivated in sand or ammunition plant soil (AP-soil) in wick supplied growth vessels. Trees were exposed to a single pulse application with water solved [U-14C]-TNT reaching a calculated initial concentration of 5.2 mg TNT per kg dry soil. Two months after application overall radioactivity and extractability of 14C were determined in sand/soil, roots, stem-wood, stem-bark, branches, leaves, needles, and Picea May sprouts. Root extracts were analysed by radio TLC. 60 days after [14C]-TNT application, recovered 14C is accumulated in roots (70% for sand variants, 34% for AP-soil variant). 15-28% of 14C remained in sand and 61% in AP-soil. 3.3 to 14.4% of 14C were located in aboveground tree portions. Above-ground distribution of 14C differed considerably between the angiosperm Salix and the gymnosperm Picea. In Salix, nearly half of above-ground-14C was detected in bark-free wood, whereas in Picea older needles contained most of the above-ground-14C (54-69%). TNT was readily transformed in tree tissue. Approximately 80% of 14C was non-extractably bound in roots, stems, wood, and leaves or needles. Only quantitatively less important stem-bark of Salix and Picea and May shoots of Picea showed higher extraction yields (up to 56%). Pulse application of [14C]-TNT provided evidence for the first time that after TNT-exposure, in tree root extracts, no TNT and none of the known metabolites, mono-amino-dinitrotoluenes (ADNT), diaminonitrotoluenes (DANT), trinitrobenzene (TNB) and no dinitrotoluenes (DNTs) were present. Extractable portions of 14C were small and contained at least three unknown metabolites (or groups) for Salix. In Picea, four extractable metabolites (or groups) were detected, where only one metabolite (or group) seemed to be identical for Salix and Picea. All unknown extractables were of a very polar nature. Results of complete TNT-transformation in trees explain some of our previous findings with 'cold analytics', where no TNT and no ADNT-metabolites could be found in tissues of TNT-exposed Salix and Populus clones. It is concluded that 'cold' tissue analysis of tree organs is not suited for quantitative success control of phytoremediation in situ. Both short rotation Salicaceae trees and conifer forests possess a dendroremediation potential for TNT polluted soils. The degradation capacity and the large biomass of adult forest trees with their woody compartments of roots and stems may be utilized for detoxification of soil xenobiotics.

Stomatal oscillations in adult forest trees in natural environment

Conductance of abaxial epidermis was measured in leaves ofAcer campeatre, Carpinus betulus, Quercus cerris andQ. petraea sample trees growing in an oak-hornbeam forest. Measurements were performed on several summer days during morning and midday hours. Stomata tended to oscillations only inQ. cerris. The occurrence of stomatal oscillations was controlled by environmental conditions,e.g. weather.