The continued capacity of terrestrial forest ecosystems to sequester carbon (C) emissions is critical to mitigating future environmental change. How increases in atmospheric CO2 and changes in climate interact with reductions in acid pollution to influence the physiology and growth of ecologically important tree species is yet unknown. To address this, we examined long-term tree growth and physiology reconstructed from C isotope chronologies of red oak (Quercus rubra) and tulip poplar (Liriodendron tulipifera), two of the most abundant and ecologically important tree species in the northeastern U.S. We collected increment cores from twenty trees of each species from two reference watersheds (WS10 and WS13) at the Fernow Experimental Forest in Parsons, WV from which we developed chronologies of basal area increment, seasonally-integrated photosynthesis (A), and intrinsic water use efficiency (iWUE) for the mature growth phase (1940-2015) of the trees. We examined the relationships between growth and underlying physiology derived from ∆13C, as well as their relationship to environmental factors. We then used generalized linear mixed models to quantitatively investigate the contribution, and potential interactions, of each environmental factor (atmospheric CO2, acid pollution, climatic water balance, and temperature) with changes in growth and reconstructed physiology these tree species.
Results/Conclusions
Across all trees in both watersheds (WS10 & WS13), relative basal area (rBA) increased 160% from 6.28 to 16.34 cm2 yr-1 cm-2 from 1940-2015. However, red oak rBA increased 0.056 cm2 cm-2 faster each year than for tulip poplar regardless of watershed (ANCOVA, F = 26.45, p < 0.01). The increases in growth were accompanied by a 39% increase in seasonally-integrated photosynthesis derived from ∆13C from 9.51 to 13.24 µmol CO2 m-2 s-1 from 1940 to 2015 across all tree species at both sites (R2 = 0.43, p < 0.01). Generalized linear mixed models indicated main effects of atmospheric CO2 and U.S. national emissions of NOx being the most important drivers of seasonally-integrated photosynthesis over the 1940-2015 time period, resulting in a simultaneous influence of +0.053 µmol CO2 m-2 s-1 and -0.002 µmol CO2 m-2 s-1 in A, respectively. iWUE increased by 26.5% across the chronology and was positively related to increased rBA (R2 = 0.27, p < 0.01), but showed no relationship to climatic water balance. Together, these results point to the complex environmental influences over the growth and physiology of ecologically important tree species, and the need to incorporate acid pollution into process-based models.