Non-linear mixed model analysis of photosynthetic-light response of sugar maple to calcium and nitrogen addition in the Catskills Mountains, NY

Background/Question/Methods

Health of sugar maple, an important species within the northeastern US forests, is declining in the Catskills, NY. This decline has been attributed partially to soil calcium (Ca) loss by acidic precipitation. Acid deposition can also affect soil nitrogen (N), which is essential for photosynthesis, and is usually a limiting factor in forest ecosystem. However, atmospheric nitrogen deposition may cause N-saturation, which can exacerbates Ca loss. Photosynthesis is a key eco-physiological process that governs plant growth and health. We quantified photosynthetic-light response of sugar maple seedlings and mature trees under a 2² factorial arrangement of N and Ca soil amendments during summers for two years. Response variables estimated were maximum net photosynthesis (Anmax), quantum efficiency (Aqe), light compensation point (LCP), and optimum light. Such estimations involve non-linear systems and repeated-measures requiring complex statistical analyses. To model non-linear nature of the photosynthetic-light responses we used one of the eight parameterizations of the Mitscherlich law as: An= Anmax [ 1-e^{(-Aqe(PAR-LCP)}]. We also used a segmented ‘linear-plateau’ non-linear model to estimate optimum PAR. We measured soil and foliar nutrients. We used the SAS NLIN, NLMIXED, DISCRIM, and MIXED procedures for data analyses.

Results/Conclusions

Ca addition increased soil Ca. N addition increased soil total N but decreased soil Ca. Ca addition increased foliar N, but N addition had no major effect on foliar chemistry. In seedlings, calcium addition increased Anmax (P < 0.04). In mature trees, Ca × N interactive effect on Anmax was significant (P < 0.03). Comparison of simple effect means showed that Ca+ N addition increased Anmax (P < 0.01) compared to any other three treatment combinations. N addition decreased Aqe (P < 0.01).