Decoupling structural and environmental determinants of sap velocity: Part I. Methodological development

This paper presents a novel theoretical framework for the study of individual tree sap flow that incorporates both spatial and temporal variability in sap velocities. In this formulation, the instantaneous sap velocity at any point in the radial profile of xylem tissue is defined as the product of a time-invariant sap velocity distribution and a time-varying term which is defined as a stem conductance. We hypothesize that the characteristic distribution of sap velocity is relatively uniform both within individual trees and between trees of the same species if location in the xylem tissue is expressed in normalized units relative to the total xylem depth (i.e. tree size). Experimental evidence confirms our hypothesis in the case of a population of sugar maples in a mixed deciduous forest and dwarf apple trees in an orchard, despite the fact that observations were drawn from a wide range of tree sizes and under varying soil moisture levels and atmospheric conditions that determines water demand. Furthermore, profiles of sap velocity and resulting integrals of total sap flow exhibit significant reduction in bias (by 30–40%) in comparison to prior methods used to extrapolate point observations of sap velocity. The method we describe exhibits the greatest improvement when only a small fraction of the total sapwood is measured, which is the typical scenario for most applications. While these results require further confirmation in order to be generalized, they nevertheless offer the basis to improve both the specific sampling strategies used to estimate whole-tree transpiration using sap velocity probes as well as methods employed to upscale water use of individual trees to larger scales for evaluation of landscape water balance.     

Decoupling structural and environmental determinants of sap velocity: Part II. Observational application

In this work we present experimental evidence in support of a new approach for investigating the dependence of sap velocity on atmospheric water demand and soil moisture supply. In this method, sap velocity is defined as the product of two components: the first describes the ‘shape’ of the radial profile of sap velocity, which is consistent through time and is likely linked to the species-specific anatomical and structural properties of the conducting xylem; the second, which we define as stem conductance, captures the time-dependent component of sap velocity that is mostly governed by shifts in atmospheric water demand and individual tree water supply. The heat pulse technique was used to estimate radial profiles of sap velocity and transpiration from a sample of 16 mature sugar maples (Acer saccharum) located along a topographic transect in a mixed deciduous forest. Our results demonstrate that: (1) stem conductance is strongly correlated with bulk air conditions (with confidence intervals for all the sampled trees greater than 99% and average R² of 0.43, 0.57, 0.54 for vapor pressure deficit (VPD), PPFD and net radiation, respectively) and atmospheric water demand (average R² equal to 0.73) on an hourly basis and that it is independent of tree size; (2) sensitivity of stem conductance to atmospheric water demand in sugar maples is also correlated to variation in local soil water availability (P-value = 0.014, R² = 0.43) which arises due to a mild topographic gradient (i.e. 20 m of relative relief along 140 m) and relatively shallow soil. Although the sampled trees were subjected to a wide range of atmospheric water demands and soil moistures, the response to changes in environmental conditions is entirely explainable by dynamics of stem conductance rather than the relative fraction of sap flow along the radial profile, as some of the previous studies reported. Overall, our results confirm our theoretical approach and the possibility of partitioning sap velocity variability between either xylem properties or changes in environmental conditions.     

Ecosystem-scale spatial heterogeneity of stable isotopes of soil nitrogen in African savannas

Soil ¹⁵N is a natural tracer of nitrogen (N) cycling. Its spatial distribution is a good indicator of processes that are critical to N cycling and of their controlling factors integrated both in time and space. The spatial distribution of soil δ¹⁵N and its underlying drivers at sub-kilometer scales are rarely investigated. This study utilizes two sites (dry vs. wet) from a megatransect in southern Africa encompassing locations with similar soil substrate but different rainfall and vegetation, to explore the effects of soil moisture and vegetation distribution on ecosystem-scale patterns of soil δ¹⁵N. A 300-m long transect was set up at each site and surface soil samples were randomly collected for analyses of δ¹⁵N, %N and nitrate content. At each soil sampling location the presence of grasses, woody plants, Acacia species (potential N fixer) as well as soil moisture levels were recorded. A spatial pattern of soil δ¹⁵N existed at the dry site, but not at the wet site. Woody cover distribution determined the soil δ¹⁵N spatial pattern at ecosystem-scale; however, the two Acacia species did not contribute to the spatial pattern of soil δ¹⁵N. Grass cover was negatively correlated with soil δ¹⁵N at both sites owing to the lower foliar δ¹⁵N values of grasses. Soil moisture did not play a role in the spatial pattern of soil δ¹⁵N at either site. These results suggest that vegetation distribution, directly, and water availability, indirectly, affect the spatial patterns of soil δ¹⁵N through their effects on woody plant and grass distributions.     

Metronidazole neurotoxicosis in two cats

Two cats were presented for neurological dysfunction from suspected metronidazole toxicity. One cat was receiving 111 mg/kg body weight per day of metronidazole for 9 weeks. After 9 weeks, the dose was increased to 222 mg/kg body weight per day, and 2 days later the cat began to experience progressive neurological signs that culminated in generalized seizures. The second cat was receiving metronidazole at a total dose of 58 mg/kg body weight per day for 6 months. This cat experienced acute onset of ataxia and alteration in mentation. Laboratory evaluations in both cases were without significant findings. The neurological signs in both cats resolved within days of initiating supportive therapy and withdrawal of the drug. This report describes the two cases and discusses the etiology of metronidazole neurotoxicosis.     

Intra- and interobserver measurement variability of tibial plateau slope from lateral radiographs in dogs

Measurement of the tibial plateau slope from lateral hind-limb radiographs is a preoperative requirement when performing tibial plateau leveling osteotomy (TPLO) for repair of the cruciate-deficient stifle in dogs. Two measurements of the tibial plateau slope in 312 stifles of 156 dogs were taken from lateral radiographs by each of three observers with varying degrees of experience in the measurement method. Intraobserver variability was +/-3.4 degrees, and interobserver variability was +/-4.8 degrees. No significant differences were identified for the intraobserver measurements; however, in evaluating interobserver variability, a significant difference was found between the inexperienced observer and the two experienced observers.     

Simulated productivity of heterogeneous patches in Southern African savanna landscapes using a canopy productivity model

A daily model of terrestrial productivity is used to simulate the annual productivity of heterogeneous vegetation structure at three savanna/woodland sites along a large moisture gradient in southern Africa. The horizontal distributions of vegetation structural parameters are derived from the three-dimensional canopy structure generated from detailed field observations of the vegetation at each site. Rainfall and daily climatic data are used to drive the model, resulting in a spatially explicit estimate of vegetation productivity in 100 m² patches over an area 810,000 m² (8,100 patches per site). Production is resolved into tree and grass components for each subplot. The model simulates the relative contribution of trees and grasses to net primary productivity (NPP) along the rainfall gradient. These simulated production estimates agree with previously published estimates of productivity in southern African savannas. Water-use efficiency of each site is directly related to the structural composition of the site and the differing water-use efficiencies for tree and grass functional types. To assess the role of spatial scale in governing estimates of vegetation productivity in heterogeneous landscapes, spatial aggregation is performed on the canopy mosaic at the northern-most (wettest) site for 625 m², 2500 m² and 5625 m² resolutions. These simulations result in similar overall patterns of average NPP for both trees and grasses, but drastically reduced distributions of productivity due to reduced structural heterogeneity. In particular, the aggregation of the detailed spatial mosaic to coarser resolutions is seen to eliminate information regarding demographic processes such as regeneration and mortality, and the dependence of grass productivity on over-story density. These results indicate that models of system productivity in savanna/woodland ecosystems must retain high spatial resolution to adequately characterize multi-year structural responses and to accurately represent the contribution of grass biomass to overall ecosystem production.This revised version was published online in May 2005 with corrections to the Cover Date.     

Mineral nutrition during establishment of golden delicious ‘smoothee’ apples on dwarfing rootstocks and interstems

This study was conducted to determine the influence of 4 interstems (EM.27 EMLA, Mark, M.9 EMLA, and EM.26 EMLA) and 8 rootstocks (EM.27 EMLA, Mark, M.9 EMLA, EM.26 EMLA, M.7A, MM. 106 EMLA, MM. 111 EMLA, and seedling) with and without interstems on foliar element concentrations [nitrogen (N,) phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), manganese (Mn), iron (Fe), zinc (Zn), boron (B)] of the Golden Delicious ‘Smoothee’ (Malus domestica, Borkh). The trees were planted in 1990 and the experiment was conducted until 1996. Soil pH was low (pH=5.9) before planting but liming raised the pH to 6.5 by the 4th year after planting. Soil P was adequate, K and Mg were high, and Ca was low based on local recommendations for apples. The year by year variation in foliar element concentrations was much higher than rootstock and interstem effects. Differences among interstems and rootstocks were important as foliar element concentrations approached those of deficiency or toxicity. In this study, K decreased to deficiency concentrations by the end of the experiment except for seedling rootstocks, which slightly increased. Foliar Ca was deficient for all interstems and rootstocks at the start of the experiment, but increased extensively for M.9 EMLA and EM.26 EMLA rootstocks across years. Foliar Mn increased to nearly toxic concentrations (300 μg g^‐1) in EM.27 EMLA and Mark rootstocks, whereas the other rootstocks did not. No deficiency or toxicity symptoms were noted for any elements during this study. These results indicate that a single range of foliar nutrient concentrations can be used as an aid for determining fertilization rates for the apple rootstocks and interstems used in this study. However, individual rootstocks vary in the rate at which they approach toxicity and deficiency concentrations, which needs to be known to prevent mineral nutritional related problems in commercial apple orchards.