I have a dataset or co-occuring trees sampled from a stand. Predawn water potential varies during the dry season. Since we assume that predawn water potential depends is measured at low (or 0) transpiration rates I tend to argue that the differences are due to differences in rooting depths. However, I wonder have connections of predawn water potential and rooting depths been explored in a systematic way?
Hi Frank,
Theoretically, if some trees are rooted in deeper/wetter soil than their neighbors, you would expect to see relatively higher predawn water potentials, provided that the plant is in equilibrium with the soil. It would be necessary to verify soil moisture profiles. In practice, such an approach tends to be problematic. You might also consider the use of stable isotopes. Here is a paper you may find helpful.
http://treephys.oxfordjournals.org/content/19/11/717.short
root depth is specific to plant genus for branching pattern and soil permeability to oxygen and soil humidity for rooting density but Nitrogen is a strong attractor. Would therefore add to >Georgianne comment an experiment of soil excavation using trenches and soil sieving for root density profile, as in http://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCEQFjAA&url=http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS037811279700340X&ei=AgotVN2fIpPuaNajgtgL&usg=AFQjCNGK5oJ_ncxtibkjzuEhyxfrtEHyMg&sig2=_Me9T_nnJdH7E_vwtNRuSw&bvm=bv.76477589,d.bGQ
Of course if you got only a dataset I would not suggests root depth approach. A tree with limited stomatal response like oak can retain more water at the same root depth of another one with broader leaves and high traspiration like poplar. Another way is counting the intregral of potential by time for different tree classes and seeking for variance between classes. You may also check specific conductivity issues as in Tyree literature for a general intro.
Yes I am aware of the isotopic methods and I have been doing cores and trenches. I fear that these are not really praticable for a large number of species in a subtropical forest.
My problem is also that I am working on a ready dataset where I found that the predawn water potentials are quite different. On the other hand it could be fun to go back there and do isotopes.
sucker or epicormic potential variations between 2 consecutive nights is the result of daily transpiration rate divided by frictional resistance (Tyree). IIf I catch your point we presume that root hydraulic resistance is the same across Spp, and thus the potential is a measure of soil water content. Root density varies with distance from base in all direction X and Y and root depth is not a clear value The only sound way is classify Spp in dought sensible and resistant and obtain the stress duration for the 2 classes. This time can be plotted in a thematic map and gives a sort of drought index
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