Production efficiency of loblolly pine and sweetgum in response to four years of intensive management

Development of DNA methylation-based epigenetic age predictors in loblolly pine (Pinus taeda)

Biological aging is connected to life history variation across ecological scales and informs a basic understanding of age-related declines in organismal function. Altered DNA methylation dynamics are a conserved aspect of biological aging and have recently been modeled to predict chronological age among vertebrate species. In addition to their utility in estimating individual age, differences between chronological and predicted ages arise due to acceleration or deceleration of epigenetic aging, and these discrepancies are linked to disease risk and multiple life history traits. Although evidence suggests that patterns of DNA methylation can describe aging in plants, predictions with epigenetic clocks have yet to be performed. Here, we resolve the DNA methylome across CpG, CHG, and CHH-methylation contexts in the loblolly pine tree (Pinus taeda) and construct epigenetic clocks capable of predicting ages in this species within 6% of its maximum lifespan. Although patterns of CHH-methylation showed little association with age, both CpG and CHG-methylation contexts were strongly associated with aging, largely becoming hypomethylated with age. Among age-associated loci were those in close proximity to malate dehydrogenase, NADH dehydrogenase, and 18S and 26S ribosomal RNA genes. This study reports one of the first epigenetic clocks in plants and demonstrates the universality of age-associated DNA methylation dynamics which can inform conservation and management practices, as well as our ecological and evolutionary understanding of biological aging in plants.

Spatial variability in tree-ring carbon isotope discrimination in response to local drought across the entire loblolly pine natural range

Considering the temporal responses of carbon isotope discrimination (Δ13C) to local water availability in the spatial analysis of Δ13C is essential for evaluating the contribution of environmental and genetic facets of plant Δ13C. Using tree-ring Δ13C from years with contrasting water availability at 76 locations across the natural range of loblolly pine, we decomposed site-level Δ13C signals to maximum Δ13C in well-watered conditions (Δ13Cmax) and isotopic drought sensitivity (m) as a change in Δ13C per unit change of Palmer's Drought Severity Index (PDSI). Site water status, especially the tree lifetime average PDSI, was the primary factor affecting Δ13Cmax. The strong spatial correlation exhibited by m was related to both genetic and environmental factors. The long-term average water availability during the period relevant to trees as indicated by lifetime average PDSI correlated with Δ13Cmax, suggesting acclimation in tree gas-exchange traits, independent of incident water availability. The positive correlation between lifetime average PDSI and m indicated that loblolly pines were more sensitive to drought at mesic than xeric sites. The m was found to relate to a plant's stomatal control and may be employed as a genetic indicator of efficient water use strategies. Partitioning Δ13C to Δ13Cmax and m provided a new angle for understanding sources of variation in plant Δ13C, with several fundamental and applied implications.

Physiological Mechanisms of Foliage Recovery after Spring or Fall Crown Scorch in Young Longleaf Pine (Pinus palustris Mill.)

We hypothesized that physiological and morphological responses to prescribed fire support the post-scorch foliage recovery and growth of young longleaf pine. Two studies conducted in central Louisiana identified three means of foliage regrowth after fire that included an increase in the gas exchange rate of surviving foliage for 3 to 4 months after fire. Saplings also exhibited crown developmental responses to repeated fire that reduced the risk of future crown scorch. Starch reserves were a source of carbon for post-scorch foliage regrowth when fire was applied in the early growing season. However, the annual dynamics of starch accumulation and mobilization restricted its effectiveness for foliage regrowth when fire was applied late in the growing season. As such, post-scorch foliage regrowth became increasingly dependent on photosynthesis as the growing season progressed. Additionally, the loss of foliage by fire late in the growing season interrupted annual starch dynamics and created a starch void between the time of late growing season fire and mid-summer of the next year. The occurrence of drought during both studies revealed barriers to foliage reestablishment and normal stem growth among large saplings. In study 1, spring water deficit at the time of May fire was associated with high crown scorch and poor foliage and stem growth among large saplings. We attribute this lag in stem growth to three factors: little surviving foliage mass, low fascicle gas exchange rates, and poor post-scorch foliage recovery. In study 2, May fire during a short window of favorable burning conditions in the tenth month of a 20-month drought also reduced stem growth among large saplings but this growth loss was not due to poor post-scorch foliage recovery. Application of this information to prescribed fire guidelines will benefit young longleaf pine responses to fire and advance efforts to restore longleaf pine ecosystems.

Using δ13C and δ18O to analyze loblolly pine (Pinus taeda L.) response to experimental drought and fertilization

Drought frequency and intensity are projected to increase throughout the southeastern USA, the natural range of loblolly pine (Pinus taeda L.), and are expected to have major ecological and economic implications. We analyzed the carbon and oxygen isotopic compositions in tree ring cellulose of loblolly pine in a factorial drought (~30% throughfall reduction) and fertilization experiment, supplemented with trunk sap flow, allometry and microclimate data. We then simulated leaf temperature and applied a multi-dimensional sensitivity analysis to interpret the changes in the oxygen isotope data. This analysis found that the observed changes in tree ring cellulose could only be accounted for by inferring a change in the isotopic composition of the source water, indicating that the drought treatment increased the uptake of stored moisture from earlier precipitation events. The drought treatment also increased intrinsic water-use efficiency, but had no effect on growth, indicating that photosynthesis remained relatively unaffected despite 19% decrease in canopy conductance. In contrast, fertilization increased growth, but had no effect on the isotopic composition of tree ring cellulose, indicating that the fertilizer gains in biomass were attributable to greater leaf area and not to changes in leaf-level gas exchange. The multi-dimensional sensitivity analysis explored model behavior under different scenarios, highlighting the importance of explicit consideration of leaf temperature in the oxygen isotope discrimination (Δ18Oc) simulation and is expected to expand the inference space of the Δ18Oc models for plant ecophysiological studies.

Fine root responses to temporal nutrient heterogeneity and competition in seedlings of two tree species with different rooting strategies

There is little direct evidence for effects of soil heterogeneity and root plasticity on the competitive interactions among plants. In this study, we experimentally examined the impacts of temporal nutrient heterogeneity on root growth and interactions between two plant species with very different rooting strategies: Liquidambar styraciflua (sweet gum), which shows high root plasticity in response to soil nutrient heterogeneity, and Pinus taeda (loblolly pine), a species with less plastic roots. Seedlings of the two species were grown in sandboxes in inter‐ and intraspecific combinations. Nutrients were applied in a patch either in a stable (slow‐release) or in a variable (pulse) manner. Plant aboveground biomass, fine root mass, root allocation between nutrient patch and outside the patch, and root vertical distribution were measured. L. styraciflua grew more aboveground (40% and 27% in stable and variable nutrient treatment, respectively) and fine roots (41% and 8% in stable and variable nutrient treatment, respectively) when competing with P. taeda than when competing with a conspecific individual, but the growth of P. taeda was not changed by competition from L. styraciflua. Temporal variation in patch nutrient level had little effect on the species’ competitive interactions. The more flexible L. styraciflua changed its vertical distribution of fine roots in response to competition from P. taeda, growing more roots in deeper soil layers compared to its roots in conspecific competition, leading to niche differentiation between the species, while the fine root distribution of P. taeda remained unchanged across all treatments. Synthesis. L. styraciflua showed greater flexibility in root growth by changing its root vertical distribution and occupying space of not occupied by P. taeda. This flexibility gave L. styraciflua an advantage in interspecific competition.

Influence of nitrogen and phosphorous on the growth and root morphology of Acer mono

Nitrogen and phosphorous are critical determinants of plant growth and productivity, and both plant growth and root morphology are important parameters for evaluating the effects of supplied nutrients. Previous work has shown that the growth of Acer mono seedlings is retarded under nursery conditions; we applied different levels of N (0, 5, 10, and 15 g plant^-1) and P (0, 4, 6 and 8 g plant^-1) fertilizer to investigate the effects of fertilization on the growth and root morphology of four-year-old seedlings in the field. Our results indicated that both N and P application significantly affected plant height, root collar diameter, chlorophyll content, and root morphology. Among the nutrient levels, 10 g N and 8 g P were found to yield maximum growth, and the maximum values of plant height, root collar diameter, chlorophyll content, and root morphology were obtained when 10 g N and 8 g P were used together. Therefore, the present study demonstrates that optimum levels of N and P can be used to improve seedling health and growth during the nursery period.

Effects of artificial defoliation on growth and biomass accumulation in short-rotation sweetgum (Liquidambar styraciflua) in North Carolina

Sweetgum, Liquidambar styraciflua L. (Hamamelidales: Hamamelidaceae), is a species of interest for short-rotation plantation forestry in the southeastern United States. Despite its high levels of resistance to many native insects and pathogens, the species is susceptible to generalist defoliators during outbreak epidemics. The objective of this field study was to evaluate the potential impact of defoliation on sweetgum growth and productivity within the context of an operational plantation. Over three growing seasons, trees were subjected to artificial defoliation treatments of various intensity (control = 0% defoliation; low intensity = 33% defoliation; moderate intensity = 67% defoliation; high intensity = 99% defoliation) and frequency (not defoliated; defoliated once in April of the first growing season; defoliated twice, once in April of the first growing season and again in April of the second growing season). The responses of stem height, stem diameter, stem volume, crown volume, total biomass accumulation, and branch growth were measured in November of each growing season. At the end of the first growing season, when trees had received single defoliations, significant reductions in all growth traits followed the most severe (99%) defoliation treatment only. After the second and third growing seasons, when trees had received one or two defoliations of varying intensity, stem diameter and volume and total tree biomass were reduced significantly by 67 and 99% defoliation, while reductions in stem height and crown volume followed the 99% treatment only. All growth traits other than crown volume were reduced significantly by two defoliations but not one defoliation. Results indicate that sweetgum is highly resilient to single defoliations of low, moderate, and high intensity. However, during the three-year period of the study, repeated high-intensity defoliation caused significant reductions in growth and productivity that could have lasting impacts on yield throughout a harvest rotation.

Interactive effects of nocturnal transpiration and climate change on the root hydraulic redistribution and carbon and water budgets of southern United States pine plantations

Deep root water uptake and hydraulic redistribution (HR) have been shown to play a major role in forest ecosystems during drought, but little is known about the impact of climate change, fertilization and soil characteristics on HR and its consequences on water and carbon fluxes. Using data from three mid-rotation loblolly pine plantations, and simulations with the process-based model MuSICA, this study indicated that HR can mitigate the effects of soil drying and had important implications for carbon uptake potential and net ecosystem exchange (NEE), especially when N fertilization is considered. At the coastal site (C), characterized by deep organic soil, HR increased dry season tree transpiration (T) by up to 40%, and such an increase affected NEE through major changes in gross primary productivity (GPP). Deep-rooted trees did not necessarily translate into a large volume of HR unless soil texture allowed large water potential gradients to occur, as was the case at the sandy site (S). At the Piedmont site (P) characterized by a shallow clay-loam soil, HR was low but not negligible, representing up to 10% of T. In the absence of HR, it was predicted that at the C, S and P sites, annual GPP would have been diminished by 19, 7 and 9%, respectively. Under future climate conditions HR was predicted to be reduced by up to 25% at the C site, reducing the resilience of trees to precipitation deficits. The effect of HR on T and GPP was predicted to diminish under future conditions by 12 and 6% at the C and P sites, respectively. Under future conditions, T was predicted to stay the same at the P site, but to be marginally reduced at the C site and slightly increased at the S site. Future conditions and N fertilization would decrease T by 25% at the C site, by 15% at the P site and by 8% at the S site. At the C and S sites, GPP was estimated to increase by 18% and by >70% under future conditions, respectively, with little effect of N fertilization. At the P site, future conditions would stimulate GPP by only 12%, but future conditions plus N fertilization would increase GPP by 24%. As a consequence, in all sites, water use efficiency was predicted to improve dramatically with future conditions. Modeling the effect of reduced annual precipitation indicated that limited water availability would decrease all carbon fluxes, including NEE and respiration. Our simulations highlight the interactive effects of nutrients and elevated CO(2), and showed that the effect of N fertilization would be greater under future climate conditions.

Leaf-level gas-exchange uniformity and photosynthetic capacity among loblolly pine (Pinus taeda L.) genotypes of contrasting inherent genetic variation

Variation in leaf-level gas exchange among widely planted genetically improved loblolly pine (Pinus taeda L.) genotypes could impact stand-level water use, carbon assimilation, biomass production, C allocation, ecosystem sustainability and biogeochemical cycling under changing environmental conditions. We examined uniformity in leaf-level light-saturated photosynthesis (A(sat)), stomatal conductance (g(s)), and intrinsic water-use efficiency (A(sat)/g(s) or δ) among nine loblolly pine genotypes (selected individuals): three clones, three full-sib families and three half-sib families, during the early years of stand development (first 3 years), with each genetic group possessing varying amounts of inherent genetic variation. We also compared light- and CO(2)-response parameters between genotypes and examined the relationship between genotype productivity, gas exchange and photosynthetic capacity. Within full-sib, half-sib and clonal genotypes, the coefficient of variation (CV) for gas exchange showed no consistent pattern; the CV for g(s) and δ was similar within clonal (44.3-46.9 and 35.5-38.6%) and half-sib (41.0-49.3 and 36.8-40.9%) genotypes, while full-sibs showed somewhat higher CVs (46.9-56.0 and 40.1-45.4%). In contrast, the CVs for A(sat) were generally higher within clones. With the exception of δ, differences in gas exchange among genotypes were generally insignificant. Tree volume showed a significant positive correlation with A(sat) and δ, but the relationship varied by season. Individual-tree volume and genotype volume were positively correlated with needle dark respiration (R(d)). Our results suggest that uniformity in leaf-level physiological rates is not consistently related to the amount of genetic variation within a given genotype, and δ, A(sat) and R(d) were the leaf-level physiological parameters that were most consistently related to individual-tree and genotype productivity. An enhanced understanding of molecular and environmental factors that influence physiological variation within and between loblolly pine genotypes may improve assessments of genotype growth potential and sensitivity to global climate change.

Water availability and genetic effects on water relations of loblolly pine (Pinus taeda) stands

The effect of water availability on water relations of 11-year-old loblolly pine stands was studied over two growing seasons in material from two contrasting seed sources. Increasing soil water availability via irrigation increased transpiration rate, and maximum daily transpiration rate on irrigated plots was similar for both seasons, reaching values of 4.3 mm day(-)(1). Irrigation also changed soil water extraction patterns. In the rain-fed control plots, 73% of the average daily transpiration was extracted from the upper 0.75 m of the soil profile. Under irrigated conditions, 92% of transpired water was extracted from the upper 0.75 m of soil, with 79% of transpired water coming from the upper 0.35 m of the profile; only 10% of total transpiration in this treatment was extracted from the soil below 1 m. There was an irrigation x seed source interaction in the response of canopy conductance to water vapor (G(C)) to vapor pressure deficit (D). Under water-limited conditions, trees from the South Carolina seed source (SC) had stronger stomatal control than trees from the Florida seed source (FL), but this difference was not present when water was not limiting. The transpiration-induced water potential gradient from roots to shoots (DeltaPsi) was relatively constant across treatments (P = 0.52) and seed sources (P = 0.72), averaging 0.75 MPa. This reflects strong stomatal control that maintains relatively constant DeltaPsi but at the same time allows leaf water potential (Psi(l)) to fluctuate dramatically in synchrony with soil water potential (Psi(s)). The two seed sources evaluated also showed differences in foliar N and delta(13)C, possibly reflecting differences in adaptation to ambient humidity and water availability regimes in their respective ranges. These differences among seed sources under different water availability scenarios may be informative to natural resource managers and breeders as they design tree improvement and genetic deployment programs for future climate scenarios. For example, the increased stomatal control of SC under decreased soil moisture availability may make this taxon a more conservative deployment choice than FL under future, drier climate scenarios but perhaps at the risk of lower productivity.

Interactive effects of fertilization and throughfall exclusion on the physiological responses and whole-tree carbon uptake of mature loblolly pine

Few studies have examined the combined effects of nutrition and water exclusion on the canopy physiology of mature loblolly pine (Pinus taeda L.). Understanding the impacts of forest management on plantation productivity requires extensive research on the relationship between silvicultural treatments and environmental constraints to growth. We studied the physiological responses of 18-year-old loblolly pine trees exposed to a combination of fertilization (fertilizer or no fertilizer) and throughfall (normal throughfall or throughfall exclusion). Gas exchange variables were measured in the upper and lower crown between 0900 and 1700 h from May to November in 1999. Needle fall was collected to estimate foliage mass and leaf area. Summer drought and throughfall exclusion significantly decreased predawn xylem pressure potential. Needle-level photosynthesis, transpiration, and stomatal conductance declined during the drought and were significantly lower in the throughfall exclusion treatment. Throughfall exclusion also reduced annual foliage mass and daily whole-crown photosynthesis and transpiration. In the normal throughfall treatment, fertilization had no effect on needle-level physiology, but increased annual foliage mass and whole-crown photosynthesis by 26% and 41%, respectively. With the exclusion of throughfall, however, annual foliage mass and daily whole-crown photosynthesis exhibited little response to fertilization. We conclude that greater nutrient availability enhances the carbon uptake of mature loblolly pine trees by stimulating foliage production, but the positive effects of fertilization on leaf area and carbon fixation are limited by low water availability.Key words: foliage mass, photosynthesis, Pinus taeda, seasonal trend, transpiration, xylem pressure potential.