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In order to be better managers of their irrigation water usage, nurseries need  simple, accurate method of determining tree water requirements on a day to day or at least a week to week basis.  This knowledge does not exist for landscape trees, yet  is required to develop Best Management Practices for the irrigation of landscape trees.  Since the adoption of soil-less substrates in the late 1960's, overall production of landscape trees has gradually shifted from in-ground to container production for trees up to 6-inches(15cm) in caliper.  The rate of this shift to container production has accelerated in the United States over the past decade.

In container production systems, irrigation management is the key for economically viable tree growth and water conservation.  When properly irrigated, container-grown trees can grow as fast or faster than trees in the ground (Beeson and Keller, 2003).  However, as trees approach marketable size for the container in which they are grown, they are often under-irrigated, reducing growth rates and vigor.  Reduced vigor leads to its own problems once trees are transplanted into a landscape.

In landscapes, improper irrigation is the principal cause of transplant failure.  In soils with  a high clay content or a subsoil layer impervious to water, over-irrigation often occurs.  In porous and/or deep soils, under irrigation is the principal cause of tree death.  Thus, knowledge of tree water requirements, based on an easily measured unit of tree size , and portable to different climates and locations, is necessary for water conservation during tree production, and for increasing tree survival after transplanting.

The goal of this project is to determine precisely how much water trees need in a given situation and the simplest way to determine this.  Adoption of the results from this study would conserve the most water possible while maintaining economically viable tree growth.  Adoption of these recommendations will become more economically advantageous as the cost and availability of water, along with the necessity of water conservation, increases.

With the requirements of portability among locations and ease of scaling to a tree in question, we set out to measure daily tree transpiration using suspension lysimeters.  Suspension lysimeters are, essentially, in  large scales on which trees are semi-permanently resting.  By weighing a tree throughout each 24 hour day, one can calculate the tree's daily water loss.  The study  will be a 5 to 6 year project; 3 tree species are being grown from liner stage (new seedling or recently rooted cutting) to a 5-inch (15cm) caliper, in suspension lysimeters.  Each lysimeter is monitored by computer and is being irrigated accordingly based on their daily water usage.  Because of the methodology, the results should also be readily applicable to in-ground tree production and in landscapes.

Our weighing (or floating) lysimeter (pic 1 & 2) measures  weight based on downward tension applied to a measuring device called a load cell(3).  This load cell then sends a voltage reading to the computer which is compared to a known range of values and is calculated into weight.    These readings are taken every half hour. Trees are irrigated twice daily during active growth and once daily in the winter,  based on the amount of weight loss, being water loss, due  mainly to  transpiration.  We also record weather data every half hour that includes temperature, humidity, rainfall, wind speed & direction, solar radiation, and reference evapotranspiration(4).

The three species studied in this initial research are the Live Oak (Quercus Virginiana Mill.;9) for Florida and much of the Southeast, the 'Nellie R. Stevens' Holly (Ilex cornuta x I. aquifolium;10) as a more cold tolerant evergreen species, and the 'Florida Flame' maple( Acer rubrum cv. Florida Flame;8), a deciduous tree,  representing a very wide geographical distribution of maples.  All the trees  were grown from liners initially potted into #7(26.6L) containers.  The lysimeter trees' containers were painted with a root pruning compound on the inside (Spin Out, Griffin Corp, Valdosta, GA, USA) and covered with aluminum foil on the outside to reduce soil temperatures(5).  Lids were placed on the pots to minimize evaporation from the soil and to limit rainfall accumulation in the containers.  At appropriate times the trees were, and will continue to be, potted up into larger containers, eventually resulting in the tree growing in #300 (1140 L) containers.

Lysimeter trees are surrounded with comparable trees to simulate production conditions(6 & 7).  Every three weeks the trees are measured for height, canopy widths, and trunk caliper.  They are fertilized accordingly with controlled-release prills as a top dressing and irrigated with micro-irrigation spray stakes in each pot.  They are also pruned and staked as needed to produce a quality tree.

From the data collected, we will develop models that relate tree size and microclimate data to tree actual transpiration (ET).  We anticipate this can be used to:
    
     -lead to more precise irrigation of trees during production, both in container and  in-ground

     -set irrigation requirements for landscape tree establishment periods

     -justify irrigation rates for Consumptive Use Permits

          
     -aid Water Management Districts in the CUP determination process 

AET or Actual Evapotranspiration is, briefly, a measure of the amount of water loss from the plant due to environmental factors such as temperature, humidity, wind, rainfall, and solar radiation.  Below are graphs that represent the data that has been collected thus far in our experiment.  To use the graphs you must first know 1) the average caliper of the tree or block of trees you intend to irrigate and 2) the potential or reference evapotranspiration (ETo), from the previous day.   If you do not have a weather station at your site that calculates evapotranspiration in Florida  it can be accessed at the Florida Automated Weather Network, or FAWN, website using http://fawn.ifas.ufl.edu/tools/et/et.asp . (In other states, similar networks have been established.  Interested persons should contact their local Agricultural Extension Agent (if in the United States) or regional agricultural official to learn of the availability in your location.  Where state or regional networks are available, you will need to select an appropriate site complimentary to your location.)   Here will be a weather summary, from the various FAWN sites located throughout the state, with ET recorded in inches.  Using this value and the caliper of your tree, refer to the charts below to find a recommended irrigation value.  Adjustments to these values may be necessary based on the efficiency of your irrigation system. 

     First, find your tree's caliper on the bottom of the graph.  Next, go vertically until you find the line corresponding to your ET value on the right hand side of the graph.  Now follow directly  horizontal (do not follow the curve) to the left side of the graph.  This is your recommended irrigation value for the day.

The goal of this project is to determine precisely how much water trees need in a given situation and the simplest way to determine this.  Adoption of the results for this study therefore would conserve the most water possible while maintaining economically viable growth.  Adoption of these recommendations will be economically advantageous as the cost and availability of water, along with the necessity of water conservation, increases.  The information derived from this project would also provide a vehicle to accurately assess an operation's CUP request and place this process on a sound scientific basis.

 Maple

                                    Caliper

       0-1"                         1-2"                        2-3"         

Oak 

      0-1"                            1-2"                          2-3"    

Holly

      0-1"                       1-2"

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