Below-ground respiratory responses of sugar maple and red maple saplings to atmospheric CO2 enrichment and elevated air temperature

The research described in this paper represents a part of a much broader re search project with the general objective of describing the effects of elev ated [CO2] and temperature on tree growth, physiological processes, and eco system-level processes. The specific objective of this research was to exam ine the below-ground respiratory responses of sugar maple (Acer saccharum M arsh.) and red maple (Acer rubrum L.) seedlings to elevated atmospheric [CO 2] and temperature. Red maple and sugar maple seedlings were planted in the ground in each of 12 open-top chambers and exposed from 1994 through 1997 to ambient air or air enriched with 30 Pa CO2, in combination with ambient or elevated (+4 degrees C) air temperatures. Carbon dioxide efflux was meas ured around the base of the seedlings and from root-exclusion zones at inte rvals during 1995 and 1996 and early 1997. The CO2 efflux rates averaged 0. 4 mu mol CO2 m(-2) s(-1) in the root-exclusion zones and 0.75 mu mol CO2 m( -2) s-1 around the base of the seedlings. Mineral soil respiration in root- exclusion zones averaged 12% higher in the high temperature treatments than at ambient temperature, but was not affected by CO2 treatments. The fracti on of total efflux attributable to root + rhizosphere respiration ranged fr om 14 to 61% in measurements made around red maple plants, and from 35 to 6 2% around sugar maple plants. Root respiration rates ranged from 0 to 0.94 mu mol CO2 s(-1) m(-2) of soil surface in red maple and from 0 to 1.02 in s ugar maple. In both 1995 and 1996 root respiration rates of red maple were highest in high-CO2 treatments and lowest in high temperature treatments. S pecific red maple root respiration rates of excised roots from near the soi l surface in 1996 were also highest under CO2 enrichment and lowest in high temperature treatments. In sugar maple the highest rates of CO2 efflux wer e from around the base of plants exposed to both high temperature and high- CO2, even though specific respiration rates were lowest for this species un der the high temperature and CO2 enrichment regime. In both species, patter ns of response to treatments were similar in root respiration and root mass , indicating that the root respiration responses were due in part to differ ences in root mass. The results underscore the need for separating the proc esses occurring in the roots from those in the forest floor and mineral soi l in order to increase our understanding of the effects of global climate c hange on carbon sequestration and cycling in the below-ground systems of fo rests.