Update 2011: Nitrogen & Water Modeling
Our overall goal is to develop a management model to monitor and predict nutrient demand and nutrient status in pecan trees, along with the interaction of nutrient and water stress on nut yield.
Physiological Response to Reduced Water
Our specific objective is to develop an optimal schedule of irrigation and nitrogen fertilization that will maximize yield when irrigation water is reduced by a specified fraction of normal, and N application efficiency is increased. We are developing a Complex Photosynthesis Pecan Tree Model to develop N and water use efficiency functions needed by the simpler Pecan Grow Model.
If trees are given reduced water supplies, many physiological acclimations occur. The first response of the tree is a reduction in stomatal conductance (gs) . This cuts leaf transpiration almost in proportion. The reduction in gs also cuts leaf photosynthesis, but considerably less than proportionally because the stomatal resistance is a much smaller part of the total pathway resistance for incoming CO2. Consequently, WUE rises.
Highlights of Current Work & Initial Results (2008-10)
Our previous measurements of water use efficiency under nonwater-stress conditions (2007) have verified both the complex photosynthesis model and the simple pecan plant grow model.
The complex photosynthesis model was calibrated against two dry down irrigation cycles imposed on a pecan orchard near Las Cruces, NM, to verify the model under moisture stress, and against selected pecan trees in the same orchard showing nitrogen (N) and water stress conditions. The complex photosynthesis model was then run under moisture and N stress to develop the WUE function vs plant water potential and leaf N level used in the whole pecan plant model:
Outreach Webpage for Model
See the webpage that we have developed to provide an indepth presentation & discussion of the model:
- The photosynthesis pecan model’s relative change in transpiration occurs linearly as leaf N decreases
- Modeled WUE also decrease linearly with a decrease in relative N because the leaf temperature rises when photosynthesis capacity is lowered due to N stress conditions in the leaves.
- When water is not limiting, a decrease in transpiration caused only by N stress also caused leaf temperature to rise and causes a decrease in WUE. The decrease in WUE was minimal and was not incorporated into the Pecan grow model.
The Pecan model was verified against experiments conducted in other climate environments & years, reported in the literature:
The measured relative decrease in growth related linearly to relative transpiration from the experiment by Sparks and Baker (1975) agrees with the model simulation of pecans under both N and water stress until the N level becomes < 1.66% N at which time the relative transpiration decreases as a non linear function.
The N stress function was incorporated into the pecan plant model that was tested against a separate water and N stress experiments in OK (Smith et al 1985) and TX (Rohla et al 2007). The climate data for the Oklahoma and Texas studies were acquired from National Climate Data Center. The pecan trees were not stressed for N or water, but both the TX and OK orchards were stressed for potassium (K) and a K stress function that reduces ET and yield due to limiting K availability in the leaves was added to the model.
Oklahoma experiments where no irrigation or N was applied to the orchard show the problem of alternating bearing on yield when trying to compare the first years simulation to measured data. It is necessary in a pecan orchard to start the model simulation several years prio to the measurenments in order to have a correct simulation comparion between modeled and measured yield data where the affect of alternate bearing can be observed in the orchard.