The relative surface ratio for lenticels related to stomatal  leaf area makes them quite uneffective for O2 and CO2 but water exchabge and minor effects on defoliated samples are expected.

a review with old refshttp://jxb.oxfordjournals.org/content/57/11/2535.full.pdf

13C composition of plant tissue is primarily a function of CO2 fixation (C3 v. C4 pathway).  However, if a plant is water stressed d13C values in plant tissue and endproducts such as sugars or lipids become more positive since closure of stomata results in CO2 fixation now drawing from a limited CO2 reservoir (isotopic fractionation) in contrast to using CO2 from an unlimited reservoir (= atmosphere) when the stomata are open.Plant tissue d13C should not be confused with Farquhar's Delta(d13C) term as proxy for water stress as this term becomes smaller with increasing water stress (or decreasing WUE).

Other, secondary factors influencing 13C composition are temperature and availability of nutrients (minerals) since these impact on biochemical activity.  Deconvoluting these factors from precipitation (as in precipitation level or water stress) will not be easy unless you have (monthly) data for all these to build an input (CO2) - output (tissue C) model that translates these factors into fluxes and turn-over rates.

18O composition of plant tissue is a more complex issue since here we are looking at three precursor pools and not just one as for 13C.  These precursor pools are water, CO2 and atmospheric O2.  The latter is of particular importance when looking at d18O values of lignin or basically any compound in the shikimate pathway.

In analogy to Farquhar's Delta(d13C) term (using d13C of atmospheric CO2 as reference point) a Delta(d18O) term has been defined (using d18O of source water as reference point).  This term becomes more positive as WUE increases.

Due to the 2 amu difference between 16O and 18O isotopic fractionation during biochemical reactions has a much more pronounced impact on plant tissue (plant product) d18O values (cf. H-J Schmidt's papers).  Biochemical reactions (= enzyme activity) are temperature dependent which will make it challenging to correlate plant tissue d18O simply to (monthly) precipitation.

In short, there are plent of processes or factors impacting on d13C and d18O that will make it difficult to determine a (significant) correlation to monthly precipiation levels.  For the reasons mentioned by Guido Bongi already It is quite unlikely lenticles will have a significant effect on d13C and d18O values.

The main drivers for mass discrimination and hence isotopic fractionation are physico-chemical processes such as evaporation/condensation (thermodynamic isotope effect) or chemical processes involving breaking / formation of chemical bonds (kinetic isotope effect).

I think there are many processes or factors affecting d13C and d18O that make it difficult to determine a correlation with the monthly sinificative precipiation levels. but it is possible as you have shown that you can be even distinguish between the 2 species of trees and branches in the young, and both are defoliated of 4 to 6 months in the year.

The isotopic fractionation During biochemical reactions: has a much more pronounced impact is cellulose plant product d18O gains. Biochemical reactions (= enzyme activity) are temperature dependent qui Will make it challenging to correlate plant tissue d18O simply to (monthly) precipitation.