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Introduction: Populus and its hybrids are attractive bioenergy crops and the southeastern United States has broad ability to supply bioenergy markets with woody biomass. Breeding and hybridization have led to superior eastern cottonwood (Populus deltoides W. Bartram ex Marshall) and hybrid poplars adapted to a wide variety of site types not suited for agricultural production. In order to maximize productivity and minimize inputs, genotypes need to efficiently use available site resources and tolerate environmental stresses. In addition, we need to determine plasticity of traits and their coordination across sites to select traits that will broadly characterize genotypes. Therefore, our study objectives were to determine (1) which leaf traits were correlated with growth, (2) if traits and genotypes exhibited significant plasticity across sites, and (3) how traits were coordinated within and across sites and Populus taxa. Methods: We measured trees at two sites in northeastern Mississippi, United States: one upland and one alluvial terrace site. Genotypes included eastern cottonwoods as well as F1 crosses of eastern cottonwood and P. maximowiczii (Henry), P. nigra (L.) and P. trichocarpa (Torr. & Gray). Results: We found that sites differed in which leaf traits were correlated with productivity; with water use efficiency specifically being positively correlated with growth at an alluvial terrace site, but negatively correlated with growth at an upland site. Tree height growth, leaf isotope composition (δ13C and δ15N), as well as leaf mass per area (LMA) exhibited the least plasticity across sites, while physiological gas exchange parameters and leaf nitrogen concentration exhibited the highest plasticity. Broadly across taxa, leaf carbon isotope ratios were correlated with intrinsic water use efficiency, and stomatal conductance was positively correlated with photosynthetic nitrogen use efficiency across sites, while leaf nitrogen isotope ratios exhibited contrasting relationships with leaf nitrogen concentration. Discussion: Overall, these results allow us to refine selections of productive genotypes based on site conditions and site-specific relationships with physiological parameters to better match Populus taxa with sites and landowner objectives. [ABSTRACT FROM AUTHOR]

As trees grow taller, the energetic cost of moving water to the leaves becomes higher and could begin to limit carbon gain and subsequent growth. The hydraulic limitation hypothesis states that as trees grow taller, the path length and therefore frictional resistance of water flow increases, leading to stomatal closure, reduced photosynthesis and decreased height growth in tall trees. Although this hypothesis is supported by the physical laws governing water movement in trees, its validation has been complicated by the complex structure of most tree species. Therefore, this study tested the hydraulic limitation hypothesis in Washingtonia robusta (H. Wendl.), a palm that, while growing to tall heights, is still structurally simple enough to act as a model organism for testing. There were no discernable relationships between tree height and stomatal conductance, stomatal densities, guard cell lengths, leaf dry mass per unit area (LMA) or sap flux, suggesting that these key aspects of hydraulic limitation are not reduced in taller palms. Taller palms did, however, have higher maximum daily photosynthetic assimilation rates, lower minimum leaf water potentials that occurred earlier in the day and fewer, smaller leaves than did shorter palms. Leaf epidermal cells were also smaller in taller palms compared with shorter ones. These findings are consistent with hydraulic compensation in that tall palms may be overcoming the increased path length resistance through smaller, more efficient leaves and lower leaf water potentials than shorter palms.

Genetic improvement and hybridization in the Populus genus have led to the development of genotypes exhibiting fast growth, high rooting ability and disease resistance. However, while large biomass production is important for bioenergy crops, efficient use of resources including water is also important in sites lacking irrigation and for maintaining ecosystem water availability. In addition, comparison of water use strategies across a range of growth rates and genetic variability can elucidate whether certain strategies are shared among the fastest growing and/or most water use efficient genotypes. We estimated tree water use throughout the second growing season via sapflow sensors of 48 genotypes from five Populus taxa; P. deltoides W. Bartram ex Marshall × P. deltoides (D × D), P. deltoides × P. maximowiczii A. Henry (D × M), P. deltoides × P. nigra L. (D × N), P. deltoides × P. trichocarpa Torr. & Gray (D × T) and P. trichocarpa × P. deltoides (T × D) and calculated average canopy stomatal conductance (GS). We regressed GS and atmospheric vapor pressure deficit (VPD) wherein the slope of the relationship represents stomatal sensitivity to VPD. At the end of the second growing season, trees were harvested, and their dry woody biomass was used to calculate whole tree water use efficiency (WUET). We found that D × D and D × M genotypes exhibited differing water use strategies with D × D genotypes exhibiting high stomatal sensitivity while retaining leaves while D × M genotypes lost leaf area throughout the growing season but exhibited low stomatal sensitivity. Across measured taxa, biomass growth was positively correlated with WUET, and genotypes representing each measured taxa except D × N and T × D had high 2‐year dry biomass of above 6 kg/tree. Overall, these data can be used to select Populus genotypes that combine high biomass growth with stomatal sensitivity and WUET to limit the negative impacts of bioenergy plantations on ecosystem water resources. Short rotation woody crops including poplars provide renewable, carbon‐neutral bioenergy. To meet a greater demand for energy, large‐scale plantations are needed; however, fast‐growing trees may deplete resources including water or grow poorly on sites with limited available water or lacking irrigation. We compared the growth and water use of 48 varieties of poplar to identify varieties that efficiently used water and tolerated drought while maintaining high growth. This research will help landowners match the optimal poplar varieties with their site, breed better poplar varieties that use water more efficiently and refine estimates of biomass productivity in the southeastern US. [ABSTRACT FROM AUTHOR]

Abstract: Little is known of wood properties in trees that were initially suppressed and subsequently released from suppression. The purpose of this study was to assess differences in growth ring [...]

The southeastern United States contains extensive loblolly pine (Pinus taeda L.) plantations at risk from bark beetle damage that can change ecosystem biogeochemical cycling. Functional changes in tree physiology have the potential to occur before visual evidence of mortality making them difficult to incorporate into ecosystem models. Therefore, we girdled loblolly pines to simulate bark beetle damage and measured physiological processes including sapflow rates, photosynthesis, litterfall, and needle nitrogen concentrations to determine the physiological changes occurring in trees undergoing mortality. We found that the girdling treatment took 5 months to significantly reduce sapflow rates but visual crown mortality occurred more than one year after girdling. Girdled pines had approximately 2.5 times lower water use than control pines and exhibited greater susceptibility to atmospheric water stress. Girdled and control pines had similar needle nitrogen concentrations and photosynthetic rates measured during the mortality year. However, more litterfall with higher nitrogen concentrations occurred in the mortality year than in the previous year, resulting in redistribution of carbon and nitrogen in the ecosystem. Overall, these data allow for better quantification of the effects of background disturbance levels and individual tree mortality on water, carbon, and nitrogen cycling within a loblolly pine ecosystem. [ABSTRACT FROM AUTHOR]

Hurricane Katrina caused large losses of bottomland hardwood forests on the Gulf Coastal Plain. Heavy‐seeded species such as oaks (Quercus) generally require direct planting for restoration after such losses. However, evaluating the performance of various oak planting stocks using biometric data alone can be challenging due to their slow juvenile growth and belowground carbon allocation. Our study goals were to evaluate physiological parameters including photosynthesis, stomatal conductance, and water‐use efficiency (WUE) and their correlation with annual height growth to determine differences in functional performance and drought tolerance between seedling types and whether physiology can predict height growth. Monthly growing season gas exchange measurements were made on two oak species (Quercus texana and Quercus shumardii) and three planting stocks (bare root, conventional containerized, large containerized [LC]) planted on two sites in coastal Mississippi. We found that photosynthesis decreased throughout the growing season while stomatal conductances increased leading to decreasing WUEs in all seedling types. Physiological parameters differed across planting stocks but not species. Particularly, LC seedlings exhibited greater WUEs and sensitivity to vapor pressure deficit (VPD) suggesting increased moisture stress compared with other planting stocks. Across seedling types, photosynthesis, stomatal sensitivity to VPD, and seasonal changes in intrinsic WUE measured in year one of the study were significantly correlated with year two, but not year one height growth, suggesting belowground allocation of carbon during the first growing season. In total, these results highlight the use of physiological measurements in selecting successful planting stocks for various site conditions. [ABSTRACT FROM AUTHOR]