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 concentrations in balsam fir foliage [Abies balsamea (L.) P. Mill.], the percentage 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 monoterpenes 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 was 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-photosynthetic N within the canopies. Total monoterpenes were positively correlated with both %PAR (r 2=0.29) and A max (r 2=0.27), and negatively correlated with SLA (r 2=0.30). Contrary to that predicted by the carbon-nutrient balance hypothesis, total monoterpenes were negatively and only very weakly correlated with the starch/N ratio (r 2=0.06) and were not significantly correlated with either the TSS/N or the [TSS+starch]/N ratios. Monoterpenes were positively correlated with both N and TSS, although the relationship varied with the phenological state of the foliage, i.e., monoterpenes were more highly correlated with TSS (r 2=0.67) (immature foliage) in June, and in July and August with N (r 2=0.63) (mature foliage). Thus, it appears that monoterpene concentrations may be controlled primarily by carbohydrate supply in the early growing season and later by enzymatic capacity. Data expressed on a dry weight basis showed a similar pattern.