Testing the ecophysiological basis for the control of monoterpene concentrations along canopy profiles in thinned and unthinned balsam fir stands

To determine which ecophysiological factors appear to control monoterpene c oncentrations in balsam fir foliage [Abies balsamea (L.) P. Mill.], the per centage of photosynthetically active radiation (%PAR), specific leaf area ( SLA), light-saturated photosynthesis (A(max)), and concentrations per unit leaf area of foliar nitrogen (N), total soluble sugars (TSS), starch and mo noterpenes were measured on current-year needles from three canopy levels ( upper, middle and lower) the year following a pre-commercial thinning. The thinning only modestly changed the light profile within the canopy. %PAR wa s negatively correlated with SLA (r(2)=0.62 in June, r(2)=0.53 in July and August) and positively correlated with foliar nitrogen concentrations (r(2) =0.51) within the crown profile. The positive relationship between N and A( max) was quite weak (r(2)=0.15), suggesting significant variations in non-p hotosynthetic N within the canopies. Total monoterpenes were positively cor related with both %PAR (r(2)=0.29) and A(max) (r(2)=0.27), and negatively c orrelated with SLA (r(2)=0.30). Contrary to that predicted by the carbon-nu trient balance hypothesis, total monoterpenes were negatively and only very weakly correlated with the starch/N ratio (r(2)=0.06) and were not signifi cantly correlated with either the TSS/N or the [TSS+starch]/N ratios. Monot erpenes were positively correlated with both N and TSS, although the relati onship varied with the phenological state of the folia,ae, i.e., monoterpen es were more highly correlated with TSS (r(2)=0.67) (immature foliage) in J une, and in July and August with N (r(2)=0.63) (mature foliage). Thus, it a ppears that monoterpene concentrations may be controlled primarily by carbo hydrate supply in the early growing season and later by enzymatic capacity. Data expressed on a dry weight basis showed a similar pattern.