Wood Density and Hydraulic Properties of Ponderosa Pine From the Willamette Valley VS. the Cascade Mountains

The Willamette Valley (WV) race of ponderosa pine (Pinus ponderosa) is being widely planted for timber in the Willamette Valley, western Oregon, because it grows in habitats that are either too wet or too dry for Douglas-fir (Pseudotsuga menziesii). Compared to the eastern Cascade Mountains (CM), the WV has 3 to 5 times the annual precipitation and warmer temperatures year around. This study characterized the wood quality of the WV race (4 sites) and the CM (4 sites), and also compared the behavior of their wood for water transport for the living trees (1 site in the WV and 1 site in the CM). The average tree ages at the sites ranged from 30 to 83 years at breast height. Between rings 27 and 31, compared to the CM, the WV had denser wood (0.48 vs. 0.40 g/cm3), denser earlywood (0.41 vs. 0.36 g/cm3), and denser latewood (0.62 vs. 0.50 g/cm3), with no significant differences in mean latewood proportion (about 0.35) or mean growth ring width (about 2.5 mm). The pith-to-bark trend in density differed between regions. In the WV, total wood density, earlywood density, and latewood density increased with growth ring from the pith. In the CM, total wood density and latewood density decreased slightly with growth ring width, and earlywood density remained unchanged. An additional sample of younger trees (23 years at breast height) from a genetic trial in the WV in which the seed source was the CM, had low density wood in the first few rings (like the CM trees) but had a steady increase in wood density with growth ring number (like the WV trees). Specific conductivity (ks) of trunk wood was lower in the WV, consistent with its higher wood density and suggestive that the WV race is more drought-adapted than the CM populations. There was no decline in ks from outer to inner sapwood in the WV trees, but a large decline in the CM trees. In water transport experiments, at an applied air pressure of 3.0 MPa, the WV and CM trees had lost 19% and 32% of their ks, respectively, again suggesting that the WV trees are slightly more drought-adapted than are the CM trees. At the other applied air pressures tested (0.5, 2.0. 4.0, and 5.0 MPa), there were no significant differences in loss of conductivity between the two sites. Trunk wood from breast height had a 50% loss of ks at 3.3-3.6 MPa. The loss of relative water content (100% - RWC) was about the same in both sites, except at 4.0 MPa, in which the CM trees had a larger loss of RWC than the WV trees. More work is needed on physiology to better understand the wood density/water transport relations. Ponderosa pine may be more interesting to study than other species because the earlywood, which transports most of the water, shows substantial density differences between geographic regions

Effects of potassium fertilization and throughfall exclusion on the hydraulic redistribution of soil water in Eucalyptus grandis plantations

The transport of water from moist soil layers to dry through the roots of some species is an important process for plant survival during long dry periods. The objective of the present study was to evaluate if Eucalyptus grandis roots growing in a tropical region characterized by long dry periods passively move water from deep to shallow soil layers, which is known as “hydraulic redistribution”. The experiment was carried out at the Itatinga experimental station (SP, Brazil) that included four contrasting experimental plots resulting from the combination of two set of treatments: with/without potassium fertilization (+K/-K, respectively) and with/without throughfall exclusion (+W/-W, respectively). Sap flow was measured in superficial Eucalyptus coarse roots from the end of the 2014 dry season to the end of the 2015 rainy. We detected reverse sap flow (water in superficial roots going to the soil surface far from the trunks) all of the months, even during the rainy season, and in all the treatments, except in -K-W, where reverse flow started two months after the beginning of the rains (January). The lowest flow densities in superficial roots were observed in -K and/or -W, but reverse flow occurred in more roots or during more days per month than in treatments +K and +W

Upscaling key ecosystem functions across the conterminous United States by a water-centric ecosystem model

We developed a water-centric monthly scale simulation model (WaSSI-C) by integrating empirical water and carbon flux measurements from the FLUXNET network and an existing water supply and demand accounting model (WaSSI). The WaSSI-C model was evaluated with basin-scale evapotranspiration (ET), gross ecosystem productivity (GEP), and net ecosystem exchange (NEE) estimates by multiple independent methods across 2103 eight-digit Hydrologic Unit Code watersheds in the conterminous United States from 2001 to 2006. Our results indicate that WaSSI-C captured the spatial and temporal variability and the effects of large droughts on key ecosystem fluxes. Our modeled mean (±standard deviation in space) ET (556 ± 228 mm yr−1) compared well to Moderate Resolution Imaging Spectroradiometer (MODIS) based (527 ± 251 mm yr−1) and watershed water balance based ET (571 ± 242 mm yr−1). Our mean annual GEP estimates (1362 ± 688 g C m−2 yr−1) compared well (R2 = 0.83) to estimates (1194 ± 649 g C m−2 yr−1) by eddy flux-based EC-MOD model, but both methods led significantly higher (25–30%) values than the standard MODIS product (904 ± 467 g C m−2 yr−1). Among the 18 water resource regions, the southeast ranked the highest in terms of its water yield and carbon sequestration capacity. When all ecosystems were considered, the mean NEE (−353 ± 298 g C m−2 yr−1) predicted by this study was 60% higher than EC-MOD\u27s estimate (−220 ± 225 g C m−2 yr−1) in absolute magnitude, suggesting overall high uncertainty in quantifying NEE at a large scale. Our water-centric model offers a new tool for examining the trade-offs between regional water and carbon resources under a changing environment

Mechanisms for minimizing height-related stomatal conductance declines in tall vines

The ability to transport water through tall stems hydraulically limits stomatal conductance (g(s)), thereby constraining photosynthesis and growth. However, some plants are able to minimize this height-related decrease in g(s), regardless of path length. We hypothesized that kudzu (Pueraria lobata) prevents strong declines in g(s) with height through appreciable structural and hydraulic compensative alterations. We observed only a 12% decline in maximum g(s) along 15-m-long stems and were able to model this empirical trend. Increasing resistance with transport distance was not compensated by increasing sapwood-to-leaf-area ratio. Compensating for increasing leaf area by adjusting the driving force would require water potential reaching -1.9 MPa, far below the wilting point (-1.2 MPa). The negative effect of stem length was compensated for by decreasing petiole hydraulic resistance and by increasing stem sapwood area and water storage, with capacitive discharge representing 8-12% of the water flux. In addition, large lateral (petiole, leaves) relative to axial hydraulic resistance helped improve water flow distribution to top leaves. These results indicate that g(s) of distal leaves can be similar to that of basal leaves, provided that resistance is highest in petioles, and sufficient amounts of water storage can be used to subsidize the transpiration stream.Peer reviewe

Safety factors for xylem failure by implosion and air-seeding within roots, trunks and branches of young and old conifer trees

The cohesion-tension theory of water transport states that hydrogen bonds hold water molecules together and that they are pulled through the xylem under tension. This tension could cause transport failure in at least two ways: collapse of the conduit walls (implosion), or rupture of the water column through air-seeding. The objective of this research was to elucidate the functional significance of variations in tracheid anatomical features, earlywood to latewood ratios and wood densities with position in young and old Douglas-fir and ponderosa pine trees in terms of their consequences for the safety factors for tracheid implosion and air-seeding. For both species, wood density increased linearly with percent latewood for root, trunk and branch samples. However, the relationships between anatomy and hydraulic function in trunks differed from those in roots and branches. In roots and branches increased hydraulic efficiency was achieved at the cost of increased vulnerability to air-seeding. Mature wood of trunks had earlywood with wide tracheids that optimized water transport and had a high percentage of latewood that optimized structural support. Juvenile wood had higher resistance to air-seeding and cell wall implosion. The two safety factors followed similar axial trends from roots to terminal branches and were similar for both species studied and between juvenile and mature wood.Keywords: tracheid, cell wall, mature wood, juvenile wood, embolism, water transpor

Contrasting Hydraulic Strategies in Two Tropical Lianas and Their Host Trees

• Premise of the Study: Tropical liana abundance has been increasing over the past 40 yr, which has been associated with reduced rainfall. The proposed mechanism allowing lianas to thrive in dry conditions is deeper root systems than co-occurring trees, although we know very little about the fundamental hydraulic physiology of lianas. • Methods: To test the hypothesis that two abundant liana species would physiologically outperform their host tree under reduced water availability, we measured rooting depth, hydraulic properties, plant water status, and leaf gas exchange during the dry season in a seasonally dry tropical forest. We also used a model to compare water use by one of the liana species and the host tree during drought. • Key Results: All species measured were shallowly rooted. The liana species were more vulnerable to embolism than host trees and experienced water potentials that were predicted to result in substantial hydraulic losses in both leaves and stems. Water potentials measured in host trees were not negative enough to result in signifi cant hydraulic losses. Model results predicted the liana to have greater gas exchange than its host tree during drought and nondrought conditions. • Conclusions: The host tree species had a more conservative strategy for maintenance of the soil-to-leaf hydraulic pathway than the lianas it supported. The two liana species experienced embolism in stems and leaves, based on vulnerability curves and water potentials. These emboli were presumably repaired before the next morning. However, in the host tree species, reduced stomatal conductance prevented leaf or stem embolism.Keywords: Anacardium excelsum, Prionostemma aspera, Transpiration, Gas exchange, Embolism, Water relations, Drought stress, Trichostigma octandru

Monthly land cover-specific evapotranspiration models derived from global eddy flux measurements and remote sensing data

Evapotranspiration (ET) is arguably the most uncertain ecohydrologic variable for quantifying watershed water budgets. Although numerous ET and hydrological models exist, accurately predicting the effects of global change on water use and availability remains challenging because of model deficiency and/or a lack of input parameters. The objective of this study was to create a new set of monthly ET models that can better quantify landscape-level ET with readily available meteorological and biophysical information. We integrated eddy covariance flux measurements from over 200 sites, multiple year remote sensing products from the Moderate Resolution Imaging Spectroradiometer (MODIS), and statistical modelling. Through examining the key biophysical controls on ET by land cover type (i.e. shrubland, cropland, deciduous forest, evergreen forest, mixed forest, grassland, and savannas), we created unique ET regression models for each land cover type using different combinations of biophysical independent factors. Leaf area index and net radiation explained most of the variability of observed ET for shrubland, cropland, grassland, savannas, and evergreen forest ecosystems. In contrast, potential ET (PET) as estimated by the temperaturebased Hamon method was most useful for estimating monthly ET for deciduous and mixed forests. The more data-demanding PET method, FAO reference ET model, had similar power as the simpler Hamon PET method for estimating actual ET. We developed three sets of monthly ET models by land cover type for different practical applications with different data availability. Our models may be used to improve water balance estimates for large basins or regions with mixed land cover types

Maximum height in a conifer is associated with conflicting requirements for xylem design

Despite renewed interest in the nature of limitations on maximum tree height, the mechanisms governing ultimate and species-specific height limits are not yet understood, but likely involve water transport dynamics. Tall trees experience increased risk of xylem embolism from air-seeding because tension in their water column increases with height due to path-length resistance and gravity. We used morphological measurements to estimate the hydraulic properties of the bordered pits between tracheids in Douglas-fir trees along a height gradient of 85 m. With increasing height, the xylem structural modifications that satisfied hydraulic requirements for avoidance of runaway embolism imposed increasing constraints on water transport efficiency. In the branches and trunks, the pit aperture diameter of tracheids decreases steadily with height, whereas torus diameter remains relatively constant. The resulting increase in the ratio of torus to pit aperture diameter allows the pits to withstand higher tensions before air-seeding, but at the cost of reduced pit aperture conductance. Extrapolations of vertical trends for trunks and branches show that water transport across pits will approach zero at a height of 109 m and 138 m, respectively, which is consistent with historic height records of 100 - 127 m for this species. Likewise, the twig water potential corresponding to the threshold for runaway embolism would be attained at a height of about 107 m. Our results suggest that the ultimate height of Douglas-fir trees may be limited in part by the conflicting requirements for water transport and water column safety.Keywords: hydraulic architecture, embolism resistance, ecological wood anatomy, bordered pit, tree height, air-seeding pressur