Photosynthesis, respiration, and carbon flux patterns of bracken fern in a mixed hardwood forest understory.

Abstract

In order to better understand ecosystem carbon flux patterns, the flux of individual ecosystem components must be studied. This project attempted to model and quantify photosynthesis rates, respiration rates, and carbon flux for bracken fern (Pteridum aquilinum) in the understory of an Aspen-dominated mixed hardwood forest. Bracken's contribution to net ecosystem exchange (NEE) during the month of July was also evaluated. Thirty-six m2 plots were constructed for use during both physiology and carbon flux research. To study fern physiology, plots were grouped into three distinct light regines, and average photosynthetic response curves were constructed for each. Respiration was measured for all light regimes, and a regression equation was fitted to the data. In order to study fern carbon flux, the light environments established during work on fern physiology were used. Light incident on the forest canopy was first described as a funciton of time. The ratio of understory light intensity to canopy light intensity was then found as a function of time. Assimilation was described as a function of light intensity, and respiration as a function of temperature. A regression equation relating frond length and width to frond area was generated to determine total plot frond area. Maximum assimilation (Amax) occurred at approximately 1000 umol/m2/s. Calculated Amax values were significantly different between regines, with the high light regime assimilating at almost twice the rate of the low light regime. Respiration rates remained between 0.36 umol CO2/m2/s at 18 C and 1.11 umol CO2/m2/s at 35C. Bracken's net assimilation for the month of July was estimated to be 250.84 kg C/m2/s, and its contribution to NEE was estimated at approximately 15.75%. The accumulated error associated with this model is uncertain; however, measured understory light intensities and total bracken respiration are thought to be underestimates, while bracken assimilation is thought to be an underestimate.