Abstract. Our ability to predict whether elevated atmospheric [CO.sub.2] will alter the cycling of C and N in terrestrial ecosystems requires understanding a complex set of feedback mechanisms initiated by changes in C and N acquisition by plants and the degree to which changes in resource acquisition (C and N) alter plant growth and allocation. To gain further insight into these dynamics, we grew six genotypes of Populus tremuloides Michx. that differ in autumnal senescence (early vs. late) under experimental atmospheric [CO.sub.2] (35.7 and 70.7 Pa) and soil-N availability (low and high) treatments. Atmospheric [CO.sub.2] concentrations were manipulated with open-top ...

<0.5 mm) [(**)]   (0.011)  (0.035) 
 
Biomass 
 Total above-2.76(a)  3.05(a) 
  ground [(**)] (0.207)  (0.387) 
 Total below-1.85(a)  2.28(a) 
  ground [(**)] (0.156)  (0.244) 
 Total [(**)]4.61(a)  5.33(a) 
(0.355)  (0.619) 
 
   Interaction means 
(n=5) 
 
 High soil N 
Plant   Ambient  Elevated 
components([dagger])   [CO.sub.2]   [CO.sub.2] 
 
Leaves [(**)]2.51(b)  3.44(c) 
(0.104)  (0.092) 
Stems [(**)] 6.70(b)  8.96(c) 
(0.269)  (0.572) 
Roots([sections]) 
 Coarse (diameter3.57(b)  5.07(c)><0.5 mm) [(**)]   (0.038)  (0.046) 
 
Biomass 
 Total above-9.20(b) 12.40(c) 
  ground [(**)] (0.307)  (0.602) 
 Total below-4.29(b)  6.21(c) 
  ground [(**)] (0.102)  (0.298) 
 Total [(**)]   13.50(b) 18.61(c) 
(0.374)  (0.882) 
 
  Main-effect means 
   (n=10) 
 
   Soil N 
   Change 
Plant  ([double dagger]) 
components([dagger]) Low  High (%)(%) 
 
Leaves [(**)]   1.072.98178(**) 
   (0.104) (0.239) 
Stems [(**)]1.847.83326(**) 
   (0.202) (0.680) 
 
Roots([sections]) 
 Coarse (diameter   1.734.32149(**)><0.5 mm) [(**)]  (0.026) (0.097) 
 
Biomass 
 Total above-   2.91   10.8 272(**) 
  ground [(**)](0.301) (0.878) 
 Total below-   2.065.25154(**) 
  ground [(**)](0.218) (0.498) 
 Total [(**)]   4.97   16.05223(**) 
   (0.505) (1.3640) 
 
  Main-effect means 
   (n=10) 
 
  [CO.sub.2] 
 
 Change 
Plant([double dagger]) 
components([dagger])   AmbientElevated  (%) 
 
Leaves [(**)]1.76   2.28  30(**) 
(0.255)(0.395) 
Stems [(**)] 4.22   5.44  29(**) 
(0.838)(1.208) 
 
Roots([sections]) 
 Coarse (diameter2.55   3.50  37(**)>

<0.5 mm) [NS] 
 
Biomass 
 Total above-8 980(a) 7 564(a) 
  ground [(*)](480)(292) 
 Total below-6 119(a) 5 941(a) 
  ground [(*)](342)(685) 
 Total [(*)] 7 841(a) 6 876(a) 
  (355)(451) 
 
  Interaction means 
(n=5) 
 
High soil N 
 
Plant  Ambient  Elevated 
components([double dagger])   [CO.sub.2]   [CO.sub.2] 
 
Leaves [NS] 23 520   19 886 
  (585)(532) 
Stems [NS]   4 9544 598 
  (164)(316) 
 
Roots([sections]) 
 Coarse (diameter9 0385 852><0.5 mm) [NS] 
 
Biomass 
 Total above-   10 025(b) 8 877(b) 
  ground [(*)](404)(513) 
 Total below-   10 618(b) 7 480(c) 
  ground [(*)](318)(453) 
 Total [(*)]10 216(b) 8 411(c) 
  (247)(465) 
 
   Main-effect means 
(n=10) 
 
Soil N 
PlantChange 
components([double dagger]) Low  High  (%) 
 
Leaves [NS]   15 43221 703 41(**) 
(830)   (1 006) 
Stems [NS] 4 057 4 776 18(**) 
(271) (252) 
 
Roots([sections]) 
 Coarse (diameter  4 148 7 445 79(**)><0.5 mm) [NS](611)   (1 127) 
 
Biomass 
 Total above-  8 272 9 451 14(*) 
  ground [(*)]  (501) (521) 
 Total below-  6 030 9 049 50(**) 
  ground [(*)]  (512) (827) 
 Total [(*)]   7 358 9 313 26(**) 
(455) (551) 
 
   Main-effect means 
(n=10) 
 
[CO.sub.2] 
 
Plant  Change 
components([double dagger])   Ambient   Elevated(%) 
 
Leaves [NS]   20 272 16 863-17(**) 
  (1 605)(1 485) 
Stems [NS] 4 588  4 245 -7([dagger]) 
(271)  (328) 
 
Roots([sections]) 
 Coarse (diameter  6 519  5 074 -22(**)>

<0.5 mm) [(*)]   (0.21)   (0.58) 
Biomass 
 Total above-  24.4(a)  22.9(a) 
  ground [(**)](0.68)   (2.86) 
 Total below-  11.2 13.1 
  ground [(NS)](1.22)   (1.73) 
 Total [(**)]  35.6(a)  36.0(a) 
   (1.68)   (4.22) 
 
 Interaction means 
   (n=5) 
 
 High soil N 
 
Plant  Ambient  Elevated 
components([double dagger])   [CO.sub.2]   [CO.sub.2] 
 
Leaves [(**)]  58.9(b)   68.4(c) 
   (2.43)(2.21) 
Stems [(**)]   33.0(b)   40.7(c) 
   (0.91)(2.22) 
Roots([sections]) 
 Coarse (diameter  31.4  29.0><0.5 mm) [(*)]   (0.79)(0.74) 
Biomass 
 Total above-  92.0(b)  109.0(c) 
  ground [(**)](2.91)(3.73) 
 Total below-  44.7  45.7 
  ground [(NS)](2.44)(1.59) 
 Total [(**)] 136.7(b)  154.8(c) 
   (2.69)(4.15) 
 
  Main-effect means 
   (n=10) 
 
   Soil N 
 
Plant Change 
components([double dagger])LowHigh  (%) 
 
Leaves [(**)]  16.4  63.7 288(**) 
   (1.08)(2.21) 
Stems [(**)]7.3  36.9 408(**) 
   (0.37)(1.70) 
Roots([sections]) 
 Coarse (diameter   7.0  30.2 333(**)><0.5 mm) [(*)]   (0.29)(0.75) 
Biomass 
 Total above-  23.7 100.5 325(**) 
  ground [(**)](1.41)(3.62) 
 Total below-  12.1  45.2 273(**) 
  ground [(NS)](1.05)(1.38) 
 Total [(**)]  35.8 145.7 307(**) 
   (2.14)(3.81) 
 
  Main-effect means 
   (n=10) 
 
[CO.sub.2] 
 
Plant Change 
components([double dagger])   Ambient   Elevated   (%) 
 
Leaves [(**)]  38.1   42.0 10([dagger]) 
   (7.05) (8.92) 
Stems [(**)]   20.1   24.019(*) 
   (4.33) (5.67) 
Roots([sections]) 
 Coarse (diameter  18.7   18.4-2(NS)>

<0.5 mm) [(NS)](0.30)   (0.93) 
Biomass 
 Aboveground [(*)]60.0(a)  57.0(a) 
  (0.75)   (1.21) 
 Belowground [(*)]40.0(a)  43.0(a) 
  (0.75)   (1.21) 
 
  Interaction means 
   (n=5) 
 
 High soil N 
 
Plant   Ambient  Elevated 
components([dagger])   [CO.sub.2]   [CO.sub.2] 
 
Leaves [(NS)] 18.6 18.6 
  (0.80)   (0.86) 
Stems [(NS)]  49.5 48.0 
  (0.66)   (0.98) 
Roots 
 Coarse (diameter 26.5 27.2><0.5 mm) [(NS)](0.22)   (0.29) 
Biomass 
 Aboveground [(*)]68.2(b)  66.6(c) 
  (0.61)   (0.45) 
 Belowground [(*)]31.8(b)  33.4(c) 
  (0.61)   (0.45) 
 
   Main-effect means 
(n=10) 
 
Soil N 
 
Plant   Change 
components([dagger])Low High  (%) 
 
Leaves [(NS)]  21.6 18.6-14(**) 
0.52 0.55 
Stems [(NS)]   36.9 48.8 32(**) 
0.71 0.61 
Roots 
 Coarse (diameter  34.6 26.8-23(**)><0.5 mm) [(NS)]  0.46 0.23 
Biomass 
 Aboveground [(*)] 58.5 67.4 15(**) 
0.83 0.44 
 Belowground [(*)] 41.5 32.6-21(**) 
0.83 0.44 
 
   Main-effect means 
(n=10) 
 
   [CO.sub.2] 
 
Plant   Change 
components([dagger])   Ambient   Elevated(%) 
 
Leaves [(NS)]   20.4   19.8  -3(NS) 
 0.83   0.60 
Stems [(NS)]43.6   42.0  -4(*) 
 2.05   2.10 
Roots 
 Coarse (diameter   29.9   31.6   6(**)>

<0.5 mm) [(NS)]   (0.17)   (1.64) 
 
Biomass 
  Aboveground [(*)]  68.9(a)  63.6(a) 
 (2.23)   (2.52) 
  Belowground [(*)]  31.1(a)  36.4(a) 
 (2.23)   (2.52) 
 
   Interaction means 
(n=5) 
 
 High soil N 
 
Plant   Ambient   Change 
components([dagger])   [CO.sub.2]   [CO.sub.2] 
 
Leaves [(*)] 43.1(b)  44.2(ab) 
 (1.68)   (1.02) 
Stems [(NS)] 24.2 26.2 
 (0.26)   (0.86) 
 
Roots 
 Coarse (diameter23.0(b)  18.7(ac)><0.5 mm) [(NS)]   (0.63)   (0.60) 
 
Biomass 
  Aboveground [(*)]  67.3(ab) 70.4(b) 
 (1.70)   (0.97) 
  Belowground [(*)]  32.7(ab) 29.6(b) 
 (1.70)   (0.97) 
 
   Main-effect means 
(n=10) 
 
Soil N 
 
Plant  Change 
components([dagger]) Low  High  (%) 
 
Leaves [(*)] 45.8 43.7   -5(NS) 
 (1.41)   (0.94) 
Stems [(NS)] 20.5 25.2   23(**) 
 (0.73)   (0.54) 
 
Roots 
 Coarse (diameter19.2 20.89(NS)><0.5 mm) [(NS)]   (0.78)   (0.45) 
 
Biomass 
  Aboveground [(*)]  66.3 68.94(NS) 
 (1.73)   (1.06) 
  Belowground [(*)]  33.7 31.1   -8(NS) 
 (1.73)   (1.06) 
 
   Main-effect means 
(n=10) 
 
 [CO.sub.2] 
 
Plant Change 
components([dagger])AmbientElevated(%) 
 
Leaves [(*)]  45.9   43.6  -5(NS) 
  (1.35) (1.00) 
Stems [(NS)]  22.3   23.4   5(NS) 
  (0.76) (1.18) 
 
Roots 
 Coarse (diameter 19.9   20.1   1(NS)>