Genome-wide identification of the PFK gene family and their expression analysis in Quercus rubra

The glycolytic pathway involves phosphofructokinase (PFK), a rate-limiting enzyme that catalyzes the phosphorylation of fructose-6-phosphate. In plants, the two PFK members are ATP-dependent phosphofructokinase (PFK) and pyrophosphate-fructose-6-phosphate phosphotransferase (PFP). However, the functions of the PFK family members in Quercus rubra are not well understood. The purpose of this study was to investigate the genome-wide distribution of the PFK family members and their roles in Q. rubra by performing a systematic study of the phylogenetic relationships, molecular characteristics, motifs, chromosomal and subcellular locations, and cis-elements of QrPFKs. We identified 14 QrPFK genes in the genome of Q. rubra, followed by examining their expression in different tissues, including the roots, stems, and leaves. The phylogenetic tree divided the 14 QrPFK genes into two groups: 11 belonging to PFK and three belonging to PFP. The expression profiles of all 14 proteins were relatively the same in leaves but differed between stems and roots. Four genes (Qurub.02G189400.1, Qurub.02G189400.2, Qurub.09G134300.1, and Qurub.09G134300.2) were expressed at very low levels in both stems and roots, while two (Qurub.05G235500.1 and Qurub.05G235500.1) were expressed at low levels and the others showed relatively high expression in all tissues.

First report of Diplodia corticola causing dieback of white oak (Quercus alba) in Tennessee

Diplodia corticola is a fungal pathogen causing oak dieback in Quercus (oak) spp. in parts of North America, northern Africa, and Europe (Ferreira et al., 2021; Smahi et al., 2017; Tsopelas et al., 2018). In August 2021, a single mature white oak (Q. alba) exhibiting wilt symptoms, vascular discoloration, and interveinal chlorosis was observed in Cove Lake State Park in Campbell County, Tennessee, U.S.A. Small sections of phloem tissues were cut from the margins of discolored vasculature of a single wilt symptomatic branch with a sterile scalpel and surface sterilized following Parra et al. (2020). Surface sterilized wood chips were plated onto potato dextrose agar amended with antibiotics (PDA++) following Gazis et al. (2018). Three days after plating, we recovered a single fungal isolate from wood chips that when grown in ½ PDA resembled D. corticola, having irregular margins and white aerial mycelia that progressively turned greyish-black 15 days after sub-culturing (Alves et al., 2004). Total genomic DNA was extracted from the isolate following Gazis et al. (2018). The internal transcribed spacer (ITS) was then amplified using the ITS1 and ITS4 primers and the subsequent PCR product was sequenced. Resulting reads were assembled into a consensus sequence and identity was assigned using BLAST on the NCBI nucleotide database. The assembled sequence (accession OM716006) had a 100% identity match with D. corticola type culture CBS 112549 (accession NR_111152). To complete Koch's postulates and identify potential host range, 5 red oaks (Q. rubra; 2-3 yrs old; caliper 14.7 ± 2 mm) and 5 white oaks (Q. alba; 2-3 yrs old; caliper 22.8 ± 2.3 mm) were inoculated with D. corticola (isolate DC_2.5). Trees were inoculated 15 cm above the soil line in a greenhouse with a 3 mm diameter plug of a 10-day old culture of D. corticola grown on PDA following Sitz et al. (2017). As a negative control, 5 red and 5 white oaks were inoculated with a 3 mm diameter plug of PDA. For each species, trees were sampled when seepage was observed from D. corticola inoculated sites (15 days post-inoculation for red and white oaks). At time of sampling, bark adjacent to inoculation sites on each tree was removed and cankers were photographed. Using a sterile scalpel, four wood chips were cut from canker margins and placed onto PDA++. For all trees, canker areas were measured using ImageJ software (Rasband, 2012). Recovered isolate identities were confirmed by extracting total genomic DNA as described above (Gazis et al. 2018) and PCR amplification of the ITS, large ribosomal subunit (LSU), and elongation factor 1-α (ef1-α) following (Ferreira et al., 2021). Diplodia corticola was reisolated from wood chips of D. corticola inoculated red (5/5 trees) and white (5/5 trees) oaks and ITS (accession OM716954), LSU (accession OM716955), and ef1-α (accession OM752198) sequences matched D. corticola type culture 112549 ITS (100% identity), LSU (99.76%-100% identity; accession KF766323), and ef1-α (98%-98.9% identity; accession XM_020275852). All D. corticola inoculated trees exhibited seepage from inoculation sites with streaking present in vasculature. Cankers were significantly larger in D. corticola inoculated red (2.34 ± 1.36 cm; P=0.042) and white (2.96 ± 0.52 cm; P=0.00029) oaks compared to agar inoculated trees. To the best of our knowledge, this is the first report of D. corticola causing decline of oaks in Tennessee.

Limited evidence that larger acorns buffer Quercus rubra seedlings from density-dependent biotic stressors

PREMISE

Plant performance and functional traits vary considerably within species, particularly in response to environmental variation. Plant responses may reflect life-history trade-offs, such as between resource acquisition and resource conservation. Larger seeds may buffer young plants from the negative effects of environmental variation, such as limitations in nutrients or water. However, whether seed size plays a similar role in how plants respond to variation in their biotic environment, including competition and soil microbial communities, remains poorly understood.

METHODS

We used a greenhouse experiment to test the interactive effects of intraspecific competition, the origin of the soil microbial community, and seed size on performance and functional traits in Quercus rubra L. seedlings.

RESULTS

Intraspecific variation in seedling traits weakly aligned with a resource acquisition-conservation trade-off. Across the wide range of initial acorn mass, competition decreased seedling biomass by about 35%. Competition directly decreased the root mass ratio and indirectly increased specific leaf area and specific root length, via the negative effects on total biomass. In contrast, soil microbial communities had minor effects on seedlings, and we found no differences between plants receiving soil originating from a conspecific adult and plants receiving soil originating from a heterospecific adult.

CONCLUSIONS

Competition is a more important driver of intraspecific variation in young Quercus rubra seedling performance and traits, both directly and by delaying ontogenetic development, than soil microbial communities. Seed size is an important predictor of seedling biomass, but a larger seed does not necessarily buffer seedlings from the effects of competition.

Red oak seedlings as indicators of deer browse pressure: Gauging the outcome of different white-tailed deer management approaches

After decades of high deer populations, North American forests have lost much of their previous biodiversity. Any landscape-level recovery requires substantial reductions in deer herds, but modern societies and wildlife management agencies appear unable to devise appropriate solutions to this chronic ecological and human health crisis. We evaluated the effectiveness of fertility control and hunting in reducing deer impacts at Cornell University. We estimated spring deer populations and planted Quercus rubra seedlings to assess deer browse pressure, rodent attack, and other factors compromising seedling performance. Oak seedlings protected in cages grew well, but deer annually browsed ≥60% of unprotected seedlings. Despite female sterilization rates of >90%, the deer population remained stable. Neither sterilization nor recreational hunting reduced deer browse rates and neither appears able to achieve reductions in deer populations or their impacts. We eliminated deer sterilization and recreational hunting in a core management area in favor of allowing volunteer archers to shoot deer over bait, including at night. This resulted in a substantial reduction in the deer population and a linear decline in browse rates as a function of spring deer abundance. Public trust stewardship of North American landscapes will require a fundamental overhaul in deer management to provide for a brighter future, and oak seedlings may be a promising metric to assess success. These changes will require intense public debate and may require new approaches such as regulated commercial hunting, natural dispersal, or intentional release of important deer predators (e.g., wolves and mountain lions). Such drastic changes in deer management will be highly controversial, and at present, likely difficult to implement in North America. However, the future of our forest ecosystems and their associated biodiversity will depend on evidence to guide change in landscape management and stewardship.

Nitrogen deposition and climate change effects on tree species composition and ecosystem services for a forest cohort

The composition of forests in the northeastern United States and the ecosystem services they provide to future generations will depend on several factors. In this paper, we isolate the effects of two environmental drivers, nitrogen (N) deposition and climate (temperature and precipitation) change, through an analysis of a single cohort of 24 dominant tree species. We assembled a tree database using data from U.S. Forest Service Forest Inventory and Analysis monitoring plots. Applying observed species-specific growth and survival responses, we simulated how forest stands in a 19-state study area would change from 2005 to 2100 under 12 different future N deposition-climate scenarios. We then estimated implications for three selected forest ecosystem services: merchantable timber, aboveground carbon sequestration, and tree diversity. Total tree biomass (for 24 species combined) was positively associated with both increased N deposition and temperatures; however, due to differences in the direction and magnitude of species-specific responses, forest composition varied across scenarios. For example, red maple (Acer rubrum) trees gained biomass under scenarios with more N deposition and more climate change, whereas biomass of yellow birch (Betula alleghaniensis) and red pine (Pinus resinosa) was negatively affected. Projections for ecosystem services also varied across scenarios. Carbon sequestration, which is positively associated with biomass accumulation, increased with N deposition and increasing climate change. Total timber values also increased with overall biomass; however, scenarios with increasing climate change tended to favor species with lower merchantable value, whereas more N deposition favored species with higher merchantable value. Tree species diversity was projected to decrease with greater changes in climate (warmer temperatures), especially in the northwestern, central, and southeastern portions of the study area. In contrast, the effects of N deposition on diversity varied greatly in magnitude and direction across the study area. This study highlights species-specific and regional effects of N deposition and climate change in northeastern U.S. forests, which can inform management decision for air quality and forests in the region, as well as climate policy. It also provides a foundation for future studies that may incorporate other important factors such as multiple cohorts, sulfur deposition, insects, and diseases.

Invasive exotic shrub modifies a classic animal-habitat relationship and alters patterns of vertebrate seed predation

Recent evidence suggests that invasive exotic plants can provide novel habitats that alter animal behavior. However, it remains unclear whether classic animal-habitat associations that influence the spatial distribution of plant-animal interactions, such as small mammal use of downed woody debris, persist in invaded habitats. We removed an invasive exotic shrub (buckthorn, Rhamnus cathartica) from 7 of 15 plots in Wisconsin. In each plot, we deployed 200 tagged Quercus rubra seeds in November 2014. After five months, tags were recovered to track spatial patterns of small mammal seed predation. Most recovered tags were associated with consumed seeds (95%); live-trapping, ancillary camera-trapping, and previous behavioral studies suggest that white-footed mice (Peromyscus leucopus) were responsible for most seed predation. In habitats without R. cathartica, most seed predation occurred near woody debris. In habitats with R. cathartica, small mammals rarely consumed seeds near woody debris, and seed predation occurred farther from the plot center and was less spatially clustered. Our results illustrate that invasive exotic shrubs can disrupt an otherwise common animal-habitat relationship. Failing to account for changes in habitat use may diminish our ability to predict animal distributions and outcomes of species interactions in novel habitats created by invasive exotic plants.

Back to America: tracking the origin of European introduced populations of Quercus rubra L

Quercus rubra has been introduced in Europe since the end of the 17th century. It is widely distributed today across this continent and considered invasive in some countries. Here, we investigated the distribution of genetic diversity of both native and introduced populations with the aim of tracing the origin of introduced populations. A large sampling of 883 individuals from 73 native and 38 European locations were genotyped at 69 SNPs. In the natural range, we found a continuous geographic gradient of variation with a predominant latitudinal component. We explored the existence of ancestral populations by performing Bayesian clustering analysis and found support for two or three ancestral genetic clusters. Approximate Bayesian Computations analyses based on these two or three clusters support recent extensive secondary contacts between them, suggesting that present-day continuous genetic variation resulted from recent admixture. In the introduced range, one main genetic cluster was not recovered in Europe, suggesting that source populations were preferentially located in the northern part of the natural distribution. However, our results cannot refute the introduction of populations from the southern states that did not survive in Europe.

Environmental variation shifts the relationship between trees and scatterhoarders along the continuum from mutualism to antagonism

The conditional mutualism between scatterhoarders and trees varies on a continuum from mutualism to antagonism and can change across time and space, and among species. We examined 4 tree species (red oak [Quercus rubra], white oak [Quercus alba], American chestnut [Castanea dentata] and hybrid chestnut [C. dentata × Castanea mollissima) across 5 sites and 3 years to quantify the variability in this conditional mutualism. We used a published model to compare the rates of seed emergence with and without burial to the probability that seeds will be cached and left uneaten by scatterhoarders to quantify variation in the conditional mutualism that can be explained by environmental variation among sites, years, species, and seed provenance within species. All species tested had increased emergence when buried. However, comparing benefits of burial to the probability of caching by scatterhoarders indicated a mutualism in red oak, while white oak was nearly always antagonistic. Chestnut was variable around the boundary between mutualism and antagonism, indicating a high degree of context dependence in the relationship with scatterhoarders. We found that different seed provenances did not vary in their potential for mutualism. Temperature did not explain microsite differences in seed emergence in any of the species tested. In hybrid chestnut only, emergence on the surface declined with soil moisture in the fall. By quantifying the variation in the conditional mutualism that was not caused by changes in scatterhoarder behavior, we show that environmental conditions and seed traits are an important and underappreciated component of the variation in the relationship between trees and scatterhoarders.

Expression of Genes Associated with Nickel Resistance in Red Oak (Quercus rubra) Populations from a Metal Contaminated Region

Although many studies have reported mechanisms of resistance to metals in herbaceous species, there is very little information on metal coping strategy in hardwood species such as Quercus rubra. The main objective of this study was to determine the expression of genes associated with nickel resistance in red oak (Q. rubra) populations from metal contaminated and uncontaminated sites in the Northern Ontario. Six genes associated with nickel resistances in model and non-model plants were targeted. Differential expressions of these genes were observed in Q. rubra from all the sites, but association between metal contamination and gene expression was not established. This suggests that the bioavailable amounts of metals found in metal contaminated soils in mining sites in northern Ontario and likely in many mining regions around the world cannot trigger a genetic response in higher plant species.

Evidence for environment-dependent introgression of adaptive genes between two red oak species with different drought adaptations

PREMISE OF THE STUDY

Introgression of adaptive alleles between hybridizing species is likely an important mechanism to generate new genetic variation for adaptation to rapidly changing environmental conditions. Oaks provide a model for the study of adaptive gene introgression because environmental selection maintains high interspecific differentiation at a few outlier loci and species-specific adaptations despite recurrent interspecific gene flow. Previously, we identified a CONSTANS-like gene under strong divergent selection between drought-tolerant Quercus ellipsoidalis and drought-averse Quercus rubra. Neighboring (parapatric) populations of both species were fixed for alternate alleles, Q. ellipsoidalis for allele 138 and Q. rubra for allele 141.

METHODS

In this study, we analyzed introgression of these outlier alleles in sympatric and in parapatric Q. rubra/Q. ellipsoidalis populations.

KEY RESULTS

We found evidence that environmental selection affected the level and direction of outlier allele introgression. Thus, in the face of symmetric interspecific gene flow, outlier allele introgression was asymmetric and introgression of allele 138 into Q. rubra was consistently higher than introgression of allele 141 into Q. ellipsoidalis in sympatric populations, but the opposite pattern was found in parapatric populations. Furthermore, the rate of introgression of outlier alleles between species in sympatric stands was related to soil quality (i.e., soil water holding capacity, nutrient availability).

CONCLUSIONS

This pattern suggests that introgression of adaptive genes between these two red oak species is strongly affected by environmental selection and is an important mechanism for species' adaptation to changing environmental conditions in a changing climate.

High-quality genetic mapping with ddRADseq in the non-model tree Quercus rubra

Background

Restriction site associated DNA sequencing (RADseq) has the potential to be a broadly applicable, low-cost approach for high-quality genetic linkage mapping in forest trees lacking a reference genome. The statistical inference of linear order must be as accurate as possible for the correct ordering of sequence scaffolds and contigs to chromosomal locations. Accurate maps also facilitate the discovery of chromosome segments containing allelic variants conferring resistance to the biotic and abiotic stresses that threaten forest trees worldwide. We used ddRADseq for genetic mapping in the tree Quercus rubra, with an approach optimized to produce a high-quality map. Our study design also enabled us to model the results we would have obtained with less depth of coverage.

Results

Our sequencing design produced a high sequencing depth in the parents (248×) and a moderate sequencing depth (15×) in the progeny. The digital normalization method of generating a de novo reference and the SAMtools SNP variant caller yielded the most SNP calls (78,725). The major drivers of map inflation were multiple SNPs located within the same sequence (77% of SNPs called). The highest quality map was generated with a low level of missing data (5%) and a genome-wide threshold of 0.025 for deviation from Mendelian expectation. The final map included 849 SNP markers (1.8% of the 78,725 SNPs called). Downsampling the individual FASTQ files to model lower depth of coverage revealed that sequencing the progeny using 96 samples per lane would have yielded too few SNP markers to generate a map, even if we had sequenced the parents at depth 248×.

Conclusions

The ddRADseq technology produced enough high-quality SNP markers to make a moderately dense, high-quality map. The success of this project was due to high depth of coverage of the parents, moderate depth of coverage of the progeny, a good framework map, an optimized bioinformatics pipeline, and rigorous premapping filters. The ddRADseq approach is useful for the construction of high-quality genetic maps in organisms lacking a reference genome if the parents and progeny are sequenced at sufficient depth. Technical improvements in reduced representation sequencing (RRS) approaches are needed to reduce the amount of missing data.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3765-8) contains supplementary material, which is available to authorized users.

Climate Influences the Content and Chemical Composition of Foliar Tannins in Green and Senesced Tissues of Quercus rubra

Environmental stresses not only influence production of plant metabolites but could also modify their resorption during leaf senescence. The production-resorption dynamics of polyphenolic tannins, a class of defense compound whose ecological role extends beyond tissue senescence, could amplify the influence of climate on ecosystem processes. We studied the quantity, chemical composition, and tissue-association of tannins in green and freshly-senesced leaves of Quercus rubra exposed to different temperature (Warming and No Warming) and precipitation treatments (Dry, Ambient, Wet) at the Boston-Area Climate Experiment (BACE) in Massachusetts, USA. Climate influenced not only the quantity of tannins, but also their molecular composition and cell-wall associations. Irrespective of climatic treatments, tannin composition in Q. rubra was dominated by condensed tannins (CTs, proanthocyanidins). When exposed to Dry and Ambient^*Warm conditions, Q. rubra produced higher quantities of tannins that were less polymerized. In contrast, under favorable conditions (Wet), tannins were produced in lower quantities, but the CTs were more polymerized. Further, even as the overall tissue tannin content declined, the content of hydrolysable tannins (HTs) increased under Wet treatments. The molecular composition of tannins influenced their content in senesced litter. Compared to the green leaves, the content of HTs decreased in senesced leaves across treatments, whereas the CT content was similar between green and senesced leaves in Wet treatments that produced more polymerized tannins. The content of total tannins in senesced leaves was higher in Warming treatments under both dry and ambient precipitation treatments. Our results suggest that, though climate directly influenced the production of tannins in green tissues (and similar patterns were observed in the senesced tissue), the influence of climate on tannin content of senesced tissue was partly mediated by the effect on the chemical composition of tannins. These different climatic impacts on leaves over the course of a growing season may alter forest dynamics, not only in decomposition and nutrient cycling dynamics, but also in herbivory dynamics.

Tree-ring δ13C and δ18O, leaf δ13C and wood and leaf N status demonstrate tree growth strategies and predict susceptibility to disturbance

Understanding how tree growth strategies may influence tree susceptibility to disturbance is an important goal, especially given projected increases in diverse ecological disturbances this century. We use growth responses of tree rings to climate, relationships between tree-ring stable isotopic signatures of carbon (δ(13)C) and oxygen (δ(18)O), wood nitrogen concentration [N], and contemporary leaf [N] and δ(13)C values to assess potential historic drivers of tree photosynthesis in dying and apparently healthy co-occurring northern red oak (Quercus rubra L. (Fagaceae)) during a region-wide oak decline event in Arkansas, USA. Bole growth of both healthy and dying trees responded negatively to drought severity (Palmer Drought Severity Index) and temperature; healthy trees exhibited a positive, but small, response to growing season precipitation. Contrary to expectations, tree-ring δ(13)C did not increase with drought severity. A significantly positive relationship between tree-ring δ(13)C and δ(18)O was evident in dying trees (P < 0.05) but not in healthy trees. Healthy trees' wood exhibited lower [N] than that of dying trees throughout most of their lives (P < 0.05), and we observed a significant, positive relationship (P < 0.05) in healthy trees between contemporary leaf δ(13)C and leaf N (by mass), but not in dying trees. Our work provides evidence that for plants in which strong relationships between δ(13)C and δ(18)O are not evident, δ(13)C may be governed by plant N status. The data further imply that historic photosynthesis in healthy trees was linked to N status and, perhaps, C sink strength to a greater extent than in dying trees, in which tree-ring stable isotopes suggest that historic photosynthesis was governed primarily by stomatal regulation. This, in turn, suggests that assessing the relative dominance of photosynthetic capacity vs stomatal regulation as drivers of trees' C accrual may be a feasible means of predicting tree responses to some disturbance events. Our work demonstrates that a dual isotope, tree-ring approach can be integrated with tree N status to begin to unravel a fundamental question in forest ecology: why do some trees die during a disturbance, while other conspecifics with apparently similar access to resources remain healthy?

Factors affecting stress tolerance in recalcitrant embryonic axes from seeds of four Quercus (Fagaceae) species native to the USA or China

BACKGROUND AND AIMS

Quercus species are often considered 'foundation' components of several temperate and/or subtropical forest ecosystems. However, the populations of some species are declining and there is considerable urgency to develop ex situ conservation strategies. In this study, the storage physiology of seeds within Quercus was explored in order to determine factors that affect survival during cryopreservation and to provide a quantitative assessment of seed recalcitrance to support future studies of this complex trait.

METHODS

Water relations and survival of excised axes in response to water loss and cryo-exposure were compared for four Quercus species from subtropical China (Q. franchetii, Q. schottkyana) and temperate USA (Q. gambelii, Q. rubra).

KEY RESULTS

Seed tissues initially had high water contents and water potentials. Desiccation tolerance of the embryonic axis was not correlated with the post-shedding rainfall patterns where the samples originated. Instead, higher desiccation tolerance was observed in samples growing in areas with colder winters. Survival following cryo-exposure correlated with desiccation tolerance. Among species, plumule tissues were more sensitive than radicles to excision, desiccation and cryo-exposure, and this led to a higher proportion of abnormally developing embryos during recovery following stress.

CONCLUSIONS

Quercus species adapted to arid and semi-humid climates still produce recalcitrant seeds. The ability to avoid freezing rather than drought may be a more important selection factor to increase desiccation tolerance. Cryopreservation of recalcitrant germplasm from temperate species is currently feasible, whilst additional protective treatments are needed for ex situ conservation of Quercus from tropical and subtropical areas.

Evidence for selection on a CONSTANS-like gene between two red oak species

BACKGROUND AND AIMS

Hybridizing species such as oaks may provide a model to study the role of selection in speciation with gene flow. Discrete species' identities and different adaptations are maintained among closely related oak species despite recurrent gene flow. This is probably due to ecologically mediated selection at a few key genes or genomic regions. Neutrality tests can be applied to identify so-called outlier loci, which demonstrate locus-specific signatures of divergent selection and are candidate genes for further study.

METHODS

Thirty-six genic microsatellite markers, some with putative functions in flowering time and drought tolerance, and eight non-genic microsatellite markers were screened in two population pairs (n = 160) of the interfertile species Quercus rubra and Q. ellipsoidalis, which are characterized by contrasting adaptations to drought. Putative outliers were then tested in additional population pairs from two different geographic regions (n = 159) to support further their potential role in adaptive divergence.

KEY RESULTS

A marker located in the coding sequence of a putative CONSTANS-like (COL) gene was repeatedly identified as under strong divergent selection across all three geographically disjunct population pairs. COL genes are involved in the photoperiodic control of growth and development and are implicated in the regulation of flowering time.

CONCLUSIONS

The location of the polymorphism in the Quercus COL gene and given the potential role of COL genes in adaptive divergence and reproductive isolation makes this a promising candidate speciation gene. Further investigation of the phenological characteristics of both species and flowering time pathway genes is suggested in order to elucidate the importance of phenology genes for the maintenance of species integrity. Next-generation sequencing in multiple population pairs in combination with high-density genetic linkage maps could reveal the genome-wide distribution of outlier genes and their potential role in reproductive isolation between these species.

Effects of Invasive Winter Moth Defoliation on Tree Radial Growth in Eastern Massachusetts, USA

Winter moth, Operophtera brumata L. (Lepidoptera: Geometridae), has been defoliating hardwood trees in eastern Massachusetts since the 1990s. Native to Europe, winter moth has also been detected in Rhode Island, Connecticut, eastern Long Island (NY), New Hampshire, and Maine. Individual tree impacts of winter moth defoliation in New England are currently unknown. Using dendroecological techniques, this study related annual radial growth of individual host (Quercus spp. and Acer spp.) trees to detailed defoliation estimates. Winter moth defoliation was associated with up to a 47% reduction in annual radial growth of Quercus trees. Latewood production of Quercus was reduced by up to 67% in the same year as defoliation, while earlywood production was reduced by up to 24% in the year following defoliation. Winter moth defoliation was not a strong predictor of radial growth in Acer species. This study is the first to document impacts of novel invasions of winter moth into New England.

Experimentally reduced root-microbe interactions reveal limited plasticity in functional root traits in Acer and Quercus

BACKGROUND AND AIMS

Interactions between roots and soil microbes are critical components of below-ground ecology. It is essential to quantify the magnitude of root trait variation both among and within species, including variation due to plasticity. In addition to contextualizing the magnitude of plasticity relative to differences between species, studies of plasticity can ascertain if plasticity is predictable and whether an environmental factor elicits changes in traits that are functionally advantageous.

METHODS

To compare functional traits and trait plasticities in fine root tissues with natural and reduced levels of colonization by microbial symbionts, trimmed and surface-sterilized root segments of 2-year-old Acer rubrum and Quercus rubra seedlings were manipulated. Segments were then replanted into satellite pots filled with control or heat-treated soil, both originally derived from a natural forest. Mycorrhizal colonization was near zero in roots grown in heat-treated soil; roots grown in control soil matched the higher colonization levels observed in unmanipulated root samples collected from field locations.

KEY RESULTS

Between-treatment comparisons revealed negligible plasticity for root diameter, branching intensity and nitrogen concentration across both species. Roots from treated soils had decreased tissue density (approx. 10-20 %) and increased specific root length (approx. 10-30 %). In contrast, species differences were significant and greater than treatment effects in traits other than tissue density. Interspecific trait differences were also significant in field samples, which generally resembled greenhouse samples.

CONCLUSIONS

The combination of experimental and field approaches was useful for contextualizing trait plasticity in comparison with inter- and intra-specific trait variation. Findings that root traits are largely species dependent, with the exception of root tissue density, are discussed in the context of current literature on root trait variation, interactions with symbionts and recent progress in standardization of methods for quantifying root traits.

Interpopulation variation in allelopathic traits informs restoration of invaded landscapes

Invasive species can show substantial genetic variation in ecologically important traits, across ranges as well within the introduced range. If these traits affect competition with native species, then management may benefit from considering the genetic landscape of the invader. Across their introduced range, Alliaria petiolata populations vary in their investment in allelopathic traits according to invasion history, which could lead to gradients of impact on native species. Red oak (Quercus rubra) seedlings were transplanted into eight A. petiolata-invaded sites that varied in their invasion history and allelochemical concentrations. At each site, an invader removal treatment was crossed with experimental inoculations of native soil biota, to test whether the benefits of these restoration actions differed across invader populations. Q. rubra seedlings grew faster in invader populations with a longer invasion history and lower allelochemical concentrations. Invader removal and soil inoculation interacted to determine seedling growth, with the benefits of soil inoculation increasing in younger and more highly allelopathic invader populations. A greenhouse experiment using soils collected from experimentally inoculated field plots found similar patterns. These results suggest that the impact of this invader varies across landscapes and that knowledge of this variation could improve the efficacy and efficiency of restoration activities.

Changes in drought response strategies with ontogeny in Quercus rubra: implications for scaling from seedlings to mature trees

We investigated scaling of physiological parameters between age classes of Quercus rubra by combining in situ field measurements with an experimental approach. In the in situ field study, we investigated changes in drought response with age in seedlings, juveniles, and mature trees of Q. rubra. Throughout the particularly dry summer of 1995 and the unusually wet summer of 1996 in New England, we measured water potential of leaves (Ψ_Leaf) and gas exchange of plants at three sites at the Harvard Forest in Petersham, Massachusetts. In order to determine what fraction of the measured differences in gas exchange between seedlings and mature trees was due to environment versus ontogeny, an experiment was conducted in which seedlings were grown under light and soil moisture regimes simulating the environment of mature trees. The photosynthetic capacity of mature trees was three-fold greater than that of seedlings during the wet year, and six-fold greater during the drought year. The seedling experiment demonstrated that the difference in photosynthetic capacity between seedlings and mature trees is comprised equally of an environmental component (50%) and an ontogenetic component (50%) in the absence of water limitation. Photosynthesis was depressed more severely in seedlings than in mature trees in the drought year relative to the wet year, while juveniles showed an intermediate response. Throughout the drought, the predawn leaf water potential (Ψ_PD) of seedlings became increasingly negative (-0.4 to -1.6 MPa), while that of mature trees became only slightly more negative (-0.2 to -0.5 MPa). Again, juveniles showed an intermediate response (-0.25 to -0.8 MPa). During the wet summer of 1996, however, there was no difference in Ψ_PD between seedlings, juveniles and mature trees. During the dry summer of 1995, seedlings were more responsive to a major rain event than mature trees in terms of Ψ_Leaf , suggesting that the two age classes depend on different water sources. In all age classes, instantaneous measurements of intrinsic water use efficiency (WUE_i), defined as C assimilation rate divided by stomatal conductance, increased as the drought progressed, and all age classes had higher WUE_i during the drought year than in the wet year. Mature trees, however, showed a greater ability to increase their WUE_i in response to drought. Integrated measurements of WUE from C isotope discrimination (Δ) of leaves indicated higher WUE in mature trees than juveniles and seedlings. Differences between years, however, could not be distinguished, probably due to the strong bias in C isotope fractionation at the time of leaf production, which occurred prior to the onset of drought conditions in 1995. From this study, we arrive at two main conclusions.

The role of microclimate in determining the sensitivity of Quercus rubra L. to ozone

Microclimate has been suggested as a factor to consider when scaling ozone responses of forest trees from the seedling to tree level. To test whether microclimate influences the response of seedlings to ozone. 1-yr-old Ouercus rubra L, seedlings were grown in the shaded upper or lower canopy of 31-yr-old trees in open-top tree chambers, or in more exposed open-top seedling chambers, and received subambient (cumulative SUM.M06* dose of (0 ppm-h), ambient (cumulative SUM06 dose of l9 ppm-h) or twice ambient (cumulative SUMU6 dose of 135 ppm-h) exposures to ozone for one growing season. Seedlings exposed to warmer temperatures and greater PAR within the seedling chambers had greater biomass, total leaf nitrogen, second foliar flush net photosynthesis, leaf conductance and dark respiration than seedlings grown in either location within the tree canopies. However, microclimate did not influence the responses to ozone. Net photosynthesis, leaf conductance and apparent quantum yield of first flush leaves of seedlings exposed to the same leaf microclimate as trees were reduced between 16% and 20% by ozone treatment. Ozone induced larger reductions in the net photosynthesis, leaf conductance and apparent quantum yield of tree leaves. These results indicate that, although microclimate influenced seedling growth and physiology, microclimate did not play a dominant role in determining the sensitivity of Q. rubra seedlings to ozone.