Yale University School of Forestry & Environmental Studies FES519B - Methods of Ecosystem Analysis 1998

Data Analysis

    After using the ICP to determine the concentrations of several elements in the extract solution, we had to convert the raw data (in micrograms/ml) into actual concentrations, in mg/kg dry weight of sample. To do this, we imported our raw data into MS Excel to get something like this.

    To convert raw concentrations to mg/kg dry weight of sample, we multiplied the concentration in the sample bottles by 50 (because each bottle contained 50 ml of solution) and divided by the weight of the ash used to produce the solution. We then moved on to using our colleagues' allometry equations to estimate the total amount of each mineral element in an intact sassafras tree with dbh (diameter at breast height, 4.5 ft above ground) of 25 cm. We looked at Calcium, Magnesium, Phosphorus, and Potassium in the bole (trunk), bark, and branches.

    This page and this page show some of our results in graph form. Not complicated enough for you? Here's a table to mull over.

    We found that all four mineral elements tested were more abundant in the bark and branches than in the wood. We've drummed up several reasons for the difference:

• Bark is primarily composed of crushed phloem cells. The phloem is the part of the plant's vascular system that carries sugars created during photosynthesis, secondary compounds used to deter herbivory, and hormones, to the rest of the plant and thus is rich in elements used in the production of those compounds.
• The wood is composed primarily of dead xylem cells. Xylem cells are essentially hollow tubes made of cellulose, hemicellulose, and lignin. While xylem is the site of transport for all elements that pass through a plant's roots, these elements are used disproportionately by the parts of the plant that are actively growing (i.e. the apical meristems, flowers, fruits, and leaves), and are therefore more abundant in the bark and branches (which have a high bark/wood ratio) than in the wood.
• Xylem is far less dense than bark. As mentioned above, xylem cells are hollow while the bark contains crushed secondary phloem.
• Calcium is present in higher concentrations in the wood, bark, and branches than the other three elements because it is used to cement cell wall structure and does not transport well in the phloem. Therefore any calcium that has been taken up by the xylem and used to build cell walls is either lost to the ground in the leaves or remains in wood or bark cell walls. The other three elements are more "phloem-mobile" and can be recycled by the tree.
• Magnesium is the central molecule in chlorophyll and thus is used primarily in the leaves (and, in the case of Sassafras, the youngest branches.
• Potassium is also used primarily in the leaves. It is the element used to change osmotic potential in the guard cells around the stomates. In order to open the stomates, potassium is transported into the adjacent guard cells, which then fill up with water (due to the lower osmotic potential) and the stomates open. The reverse process closes the stomates.
• Phosphorous has a wide variety of physiological roles. It is major component of ADP and ATP, primary assimilators of energy produced by photosynthesis, respiration and fermentation. It is also used in proteins and in cell membrane construction. Again, it is more likely to be found in the actively respiring parts of the plant than in the xylem.