Symbiotic ant traits produce differential host-plant carbon and water dynamics in a multi-species mutualism.

Cooperative interactions may frequently be reinforced by "partner fidelity feedback," in which high- or low-quality partners drive positive feedbacks with high or low benefits for the host, respectively. Benefits of plant-animal mutualisms for plants have been quantified almost universally in terms of growth or reproduction, but these are only two of many sinks to which a host-plant allocates its resources. By investigating how partners to host-plants impact two fundamental plant resources, carbon and water, we can better characterize plant-partner fidelity and understand how plant-partner mutualisms may be modulated by resource dynamics. In Laikipia, Kenya, four ant species compete for Acacia drepanolobium host-plants. These ants differ in multiple traits, from nectar consumption to host-plant protection. Using a 5-year ant removal experiment, we compared carbon fixation, leaf water status, and stem non-structural carbohydrate concentrations for adult ant-plants with and without ant partners. Removal treatments showed that the ants differentially mediate tree carbon and/or water resources. All three ant species known to be aggressive against herbivores were linked to benefits for host-plant resources, but only the two species that defend but do not prune the host, Crematogaster mimosae and Tetraponera penzigi, increased tree carbon fixation. Of these two species, only the nectivore C. mimosae increased tree simple sugars. Crematogaster nigriceps, which defends the tree but also castrates flowers and prunes meristems, was linked only to lower tree water stress approximated by pre-dawn leaf water potential. In contrast to those defensive ants, Crematogaster sjostedti, a poor defender that displaces other ants, was linked to lower tree carbon fixation. Comparing the effects of the four ant species across control trees suggests that differential ant occupancy drives substantial differences in carbon and water supply among host trees. Our results highlight that ant partners can positively or negatively impact carbon and/or water relations for their host-plant, and we discuss the likelihood that carbon- and water-related partner fidelity feedback loops occur across ant-plant mutualisms.

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.

Mutualism disruption by an invasive ant reduces carbon fixation for a foundational East African ant-plant.

Invasive ants shape assemblages and interactions of native species, but their effect on fundamental ecological processes is poorly understood. In East Africa, Pheidole megacephala ants have invaded monodominant stands of the ant-tree Acacia drepanolobium, extirpating native ant defenders and rendering trees vulnerable to canopy damage by vertebrate herbivores. We used experiments and observations to quantify direct and interactive effects of invasive ants and large herbivores on A. drepanolobium photosynthesis over a 2-year period. Trees that had been invaded for ≥ 5 years exhibited 69% lower whole-tree photosynthesis during key growing seasons, resulting from interaction between invasive ants and vertebrate herbivores that caused leaf- and canopy-level photosynthesis declines. We also surveyed trees shortly before and after invasion, finding that recent invasion induced only minor changes in leaf physiology. Our results from individual trees likely scale up, highlighting the potential of invasive species to alter ecosystem-level carbon fixation and other biogeochemical cycles.

The uncharacterized gene EVE contributes to vessel element dimensions in Populus.

The radiation of angiosperms led to the emergence of the vast majority of today's plant species and all our major food crops. Their extraordinary diversification occurred in conjunction with the evolution of a more efficient vascular system for the transport of water, composed of vessel elements. The physical dimensions of these water-conducting specialized cells have played a critical role in angiosperm evolution; they determine resistance to water flow, influence photosynthesis rate, and contribute to plant stature. However, the genetic factors that determine their dimensions are unclear. Here we show that a previously uncharacterized gene, ENLARGED VESSEL ELEMENT (EVE), contributes to the dimensions of vessel elements in Populus, impacting hydraulic conductivity. Our data suggest that EVE is localized in the plasma membrane and is involved in potassium uptake of differentiating xylem cells during vessel development. In plants, EVE first emerged in streptophyte algae, but expanded dramatically among vessel-containing angiosperms. The phylogeny, structure and composition of EVE indicates that it may have been involved in an ancient horizontal gene-transfer event.

Accounting for two-billion tons of stabilized soil carbon.

The pedosphere is the largest terrestrial reservoir of organic carbon, yet soil-carbon variability and its representation in Earth system models is a large source of uncertainty for carbon-cycle science and climate projections. Much of this uncertainty is attributed to local and regional-scale variability, and predicting this variation can be challenging if variable selection is based solely on a priori assumptions due to the scale-dependent nature of environmental determinants. Data mining can optimize predictive modeling by allowing machine-learning algorithms to learn from and discover complex patterns in large datasets that may have otherwise gone unnoticed, thus increasing the potential for knowledge discovery. In this analysis, we identify important, regional-scale determinants for top- and subsoil-carbon stabilization in production forestland across the southeastern US. Specifically, we apply recursive feature elimination to a large suite of socio-environmental data to strategically select a parsimonious, yet highly predictive covariate set. This is achieved by recursively considering smaller and smaller covariate sets-or features-by first training the estimator on the full set to obtain feature importance. The least important features are pruned, and the procedure is recursively repeated until a desired number of covariates is identified. We show that although carbon ranges from 0.3 to 8.2 kg m-2 in the topsoil (0 to 20 cm), and from 0.4 to 17.6 kg m-2 in the subsoil (20 to 100 cm), this variability is predictably distributed with precipitation, soil moisture, nitrogen and sand content, gamma ray emissions, mean annual minimum temperature, and elevation. From our spatial predictions, we estimate that 2.6 Pg of soil carbon is currently stabilized in the upper 100 cm of production forestland, which covers 34.7 million ha in the southeastern US.

Spatially-explicit modeling of multi-scale drivers of aboveground forest biomass and water yield in watersheds of the Southeastern United States.

Understanding ecosystem processes and the influence of regional scale drivers can provide useful information for managing forest ecosystems. Examining more local scale drivers of forest biomass and water yield can also provide insights for identifying and better understanding the effects of climate change and management on forests. We used diverse multi-scale datasets, functional models and Geographically Weighted Regression (GWR) to model ecosystem processes at the watershed scale and to interpret the influence of ecological drivers across the Southeastern United States (SE US). Aboveground forest biomass (AGB) was determined from available geospatial datasets and water yield was estimated using the Water Supply and Stress Index (WaSSI) model at the watershed level. Our geostatistical model examined the spatial variation in these relationships between ecosystem processes, climate, biophysical, and forest management variables at the watershed level across the SE US. Ecological and management drivers at the watershed level were analyzed locally to identify whether drivers contribute positively or negatively to aboveground forest biomass and water yield ecosystem processes and thus identifying potential synergies and tradeoffs across the SE US region. Although AGB and water yield drivers varied geographically across the study area, they were generally significantly influenced by climate (rainfall and temperature), land-cover factor1 (Water and barren), land-cover factor2 (wetland and forest), organic matter content high, rock depth, available water content, stand age, elevation, and LAI drivers. These drivers were positively or negatively associated with biomass or water yield which significantly contributes to ecosystem interactions or tradeoff/synergies. Our study introduced a spatially-explicit modelling framework to analyze the effect of ecosystem drivers on forest ecosystem structure, function and provision of services. This integrated model approach facilitates multi-scale analyses of drivers and interactions at the local to regional scale.

Pinus taeda forest growth predictions in the 21st century vary with site mean annual temperature and site quality.

Climate projections from 20 downscaled global climate models (GCMs) were used with the 3-PG model to predict the future productivity and water use of planted loblolly pine (Pinus taeda) growing across the southeastern United States. Predictions were made using Representative Concentration Pathways (RCP) 4.5 and 8.5. These represent scenarios in which total radiative forcing stabilizes before 2100 (RCP 4.5) or continues increasing throughout the century (RCP 8.5). Thirty-six sites evenly distributed across the native range of the species were used in the analysis. These sites represent a range in current mean annual temperature (14.9-21.6°C) and precipitation (1,120-1,680 mm/year). The site index of each site, which is a measure of growth potential, was varied to represent different levels of management. The 3-PG model predicted that aboveground biomass growth and net primary productivity will increase by 10%-40% in many parts of the region in the future. At cooler sites, the relative growth increase was greater than at warmer sites. By running the model with the baseline [CO2 ] or the anticipated elevated [CO2 ], the effect of CO2 on growth was separated from that of other climate factors. The growth increase at warmer sites was due almost entirely to elevated [CO2 ]. The growth increase at cooler sites was due to a combination of elevated [CO2 ] and increased air temperature. Low site index stands had a greater relative increase in growth under the climate change scenarios than those with a high site index. Water use increased in proportion to increases in leaf area and productivity but precipitation was still adequate, based on the downscaled GCM climate projections. We conclude that an increase in productivity can be expected for a large majority of the planted loblolly pine stands in the southeastern United States during this century.

Ecosystem carbon density and allocation across a chronosequence of longleaf pine forests.

Forests can partially offset greenhouse gas emissions and contribute to climate change mitigation, mainly through increases in live biomass. We quantified carbon (C) density in 20 managed longleaf pine (Pinus palustris Mill.) forests ranging in age from 5 to 118 years located across the southeastern United States and estimated above- and belowground C trajectories. Ecosystem C stock (all pools including soil C) and aboveground live tree C increased nonlinearly with stand age and the modeled asymptotic maxima were 168 Mg C/ha and 80 Mg C/ha, respectively. Accumulation of ecosystem C with stand age was driven mainly by increases in aboveground live tree C, which ranged from <1 Mg C/ha to 74 Mg C/ha and comprised <1% to 39% of ecosystem C. Live root C (sum of below-stump C, ground penetrating radar measurement of lateral root C, and live fine root C) increased with stand age and represented 4-22% of ecosystem C. Soil C was related to site index, but not to stand age, and made up 39-92% of ecosystem C. Live understory C, forest floor C, downed dead wood C, and standing dead wood C were small fractions of ecosystem C in these frequently burned stands. Stand age and site index accounted for 76% of the variation in ecosystem C among stands. The mean root-to-shoot ratio calculated as the average across all stands (excluding the grass-stage stand) was 0.54 (standard deviation of 0.19) and higher than reports for other conifers. Long-term accumulation of live tree C, combined with the larger role of belowground accumulation of lateral root C than in other forest types, indicates a role of longleaf pine forests in providing disturbance-resistant C storage that can balance the more rapid C accumulation and C removal associated with more intensively managed forests. Although other managed southern pine systems sequester more C over the short-term, we suggest that longleaf pine forests can play a meaningful role in regional forest C management.

Thermal optimality of net ecosystem exchange of carbon dioxide and underlying mechanisms.

• It is well established that individual organisms can acclimate and adapt to temperature to optimize their functioning. However, thermal optimization of ecosystems, as an assemblage of organisms, has not been examined at broad spatial and temporal scales. • Here, we compiled data from 169 globally distributed sites of eddy covariance and quantified the temperature response functions of net ecosystem exchange (NEE), an ecosystem-level property, to determine whether NEE shows thermal optimality and to explore the underlying mechanisms. • We found that the temperature response of NEE followed a peak curve, with the optimum temperature (corresponding to the maximum magnitude of NEE) being positively correlated with annual mean temperature over years and across sites. Shifts of the optimum temperature of NEE were mostly a result of temperature acclimation of gross primary productivity (upward shift of optimum temperature) rather than changes in the temperature sensitivity of ecosystem respiration. • Ecosystem-level thermal optimality is a newly revealed ecosystem property, presumably reflecting associated evolutionary adaptation of organisms within ecosystems, and has the potential to significantly regulate ecosystem-climate change feedbacks. The thermal optimality of NEE has implications for understanding fundamental properties of ecosystems in changing environments and benchmarking global models.

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.

Quantitative genetic analysis of biomass and wood chemistry of Populus under different nitrogen levels.

The genetic control of carbon allocation and partitioning in woody perennial plants is poorly understood despite its importance for carbon sequestration, biofuels and other wood-based industries. It is also unclear how environmental cues, such as nitrogen availability, impact the genes that regulate growth, biomass allocation and wood composition in trees. We phenotyped 396 clonally replicated genotypes of an interspecific pseudo-backcross pedigree of Populus for wood composition and biomass traits in above- and below-ground organs. The loci that regulate growth, carbon allocation and partitioning under two nitrogen conditions were identified, defining the contribution of environmental cues to their genetic control. Sixty-three quantitative trait loci were identified for the 20 traits analyzed. The majority of quantitative trait loci are specific to one of the two nitrogen treatments, demonstrating significant nitrogen-dependent genetic control. A highly significant genetic correlation was observed between plant growth and lignin/cellulose composition, and quantitative trait loci co-localization identified the genomic position of potential pleiotropic regulators. Pleiotropic loci linking higher growth rates to wood with less lignin are excellent targets to engineer tree germplasm improved for pulp, paper and cellulosic ethanol production. The causative genes are being identified with a genetical genomics approach.

Clonal variation in crown structure, absorbed photosynthetically active radiation and growth of loblolly pine and slash pine.

Crown structure, absorbed photosynthetically active radiation (APAR) and growth were analyzed in 300 replicated loblolly (Pinus taeda L.) and slash pine (Pinus elliottii Engelm. var. elliotti) clones to: (1) quantify genetic variation in crown structural traits, growth and APAR at the species, family and clonal levels; and (2) estimate within-family genetic and environmental influences on measured variables. Species and family-within-species differences were found in some growth traits, crown size, leaf area, APAR and branch angle. Loblolly pine developed larger crowns, exposed more leaf area with an acute angle, and intercepted more radiation than slash pine. Significant differences among clones within-family were found for stem volume and crown architecture. Loblolly pine and slash pine within-family, individual-tree broad-sense heritabilities ranged from 0.00 to 0.41 for growth and crown structural traits and most were between 0.10 and 0.25 when estimated from a combined analysis across families. Genetic correlations of crown size, leaf area and APAR with volume increment generally ranged from 0.60 to 0.75. This knowledge of the genetic interactions among growth and crown structural traits improves our understanding of how crown morphology affects light interception and stand development, and ultimately how these attributes can be incorporated in the selection of families or clones for the development of new crop tree ideotypes.

Short-term physiological and developmental responses to nitrogen availability in hybrid poplar.

Nitrogen fertilization induces dramatic changes in the growth and development of plants, including forest trees. In this study we examined short-term responses of hybrid poplar, Populus balsamifera ssp. trichocarpa x deltoides, to N fertilization. Glasshouse-grown saplings subjected to limiting, intermediate, and luxuriant levels of ammonium nitrate over a 28 d time course demonstrated rapid changes to whole-plant architecture and biomass accumulation. Nitrogen-associated shifts in allocation occurred in temporally distinct stages. Nitrogen availability modulated parameters that affect carbon gain, including light-saturated net photosynthesis and leaf area. These parameters were affected by N-induced changes to leaf maturation and senescence. Leaf area was also affected by N-induced sylleptic branch development. Genes encoding vegetative storage proteins and starch biosynthetic enzymes exhibited contrasting patterns of expression under differential N availability. A gene encoding a previously uncharacterized putative pectin methylesterase inhibitor displayed expression patterns comparable to the starch biosynthetic genes. The results of this study illustrate the phenotypic plasticity that P. balsamifera ssp. trichocarpa x deltoides exhibits in response to differential N availability.

Integrating within-crown variation in net photosynthesis in loblolly and slash pine families.

We examined photosynthetic characteristics of two fast- and two slow-growing half-sib families of both loblolly pine (Pinus taeda L.) and slash pine (Pinus elliottii var. elliottii Engelm.) on two sites in northern Florida to: (1) quantify variation in light-saturated net photosynthesis (Amax) associated with vertical crown position and foliage age; (2) quantify the amount and distribution of leaf area by foliage age class; and (3) determine whether photosynthetic indices, ranging from leaf-level through whole-crown Amax, were related to growth differences among species and families. In both species, leaf-level Amax was higher in more recently formed foliage both within the same year (where Amax in the third flush averaged 10 to 30% higher than Amax in the first flush) and between years (where Amax in current-year foliage averaged 20 to 40% higher than Amax in 1-year-old foliage). When expressed on a leaf area basis, Amax of current-year foliage was higher in slash pine than in loblolly pine, but Amax expressed on a mass basis did not differ between species. Loblolly pine had higher whole-tree leaf area than slash pine, whereas whole-tree Amax did not differ between species. When the mean values for fast-growing families were compared with the mean values for slow-growing families, there were no differences in leaf-level characteristics, whereas at the whole-tree level, fast-growing families had higher leaf area and whole-tree Amax than slow-growing families in both species. When comparisons were made among the individual fast- and slow-growing families, however, results were more variable. In both species, stem volume growth was strongly correlated with whole-tree Amax, with most of the strength of the correlation deriving from the relationship between volume growth and tree leaf area.

Dressing for success: choosing garbing standards for the laboratory animal facility.

The animal research literature devotes much attention to the function of clothing and personal protective equipment in preventing transmission of zoonotic diseases from animal to human, but has said comparatively little about the potential for transmission of disease from human to animal. The authors consider appropriate garbing standards for personnel charged with various tasks within the animal facility, emphasizing the protection of animals from potential health threats.

Boundary layer conductance, leaf temperature and transpiration of Abies amabilis branches.

We used three methods to measure boundary layer conductance to heat transfer (g(bH)) and water vapor transfer (g(bV)) in foliated branches of Abies amabilis Dougl. ex J. Forbes, a subalpine forest tree that produces clumped shoot morphology on sun-formed branches. Boundary layer conductances estimated in the field from energy balance measurements increased linearly from approximately 10 mm s(-1) at low wind speeds (< 0.1 m s(-1)) to over 150 mm s(-1) at wind speeds of 2.0 m s(-1). Boundary layer conductances measured on shoot models in a wind tunnel were consistently higher than field measurements. The difference between wind tunnel values and field measurements was attributable to variation in path length between the two experimental environments. Boundary layer conductance estimated by subtracting stomatal resistance (r(sV)) measured with a porometer from the total branch vapor phase resistance were unusually small. Sensitivity analysis demonstrated that this method is not suitable for coniferous foliage or when stomatal conductance (g(sV)) is small compared with g(bV). Analysis of the relative magnitudes of g(sV) and g(bV) revealed that, under most conditions, A. amabilis branches are well coupled (i.e., g(sV) is the dominant controller of transpiration). The boundary layer conductance to heat transfer is small enough that leaf temperature can become substantially higher than air temperature when radiation is high and wind speed is low. Over a two-month period, the maximum difference between leaf and air temperatures exceeded 6 degrees C. Leaf temperature exceeded air temperature by more than 2 degrees C on 10% of the daylight hours during this period. Consideration of both the photosynthetic temperature response of A. amabilis foliage as well as the summer air temperature conditions in its habitat suggests that these elevated leaf temperatures do not have a significant impact on carbon gain during the growing season.