1 Fixed Nitrogen Log Nitrogen Dinitrogen TempMoist Log Substrate Trees Log Oxygen Carbon Dioxide N2 Fixation Respiration Diffusion Figure 5.1. A diagram of the direct and indirect influences on nitrogen fixation included in our model.

2 Figure 5.2. The relationship of tissue radii and log radius.

3 a b Figure 5.3. The effect of temperature on (a) nitrogen fixation and (b) respiration rate.

4 EVAPORATION THRU FALL STORE1 STORE2 STORE3 PRECIP LEACHATERUNOFF OVERFLOW LAYER 1 OUTER LAYER 3 INNER LAYER 2 DIFFUSION Figure 5.4. A diagram of the method used to model log moisture content.

5 Figure 5.5. The relationship of (a) precipitation and throughfall, and (b) wood density and throughfall runoff. a b

6 Figure 5.6. The relationship of the moisture diffusion rate and (a) wood density and (b) temperature. a b

7 Figure 5.7. The relationship of wood density and the maximum log moisture content.

8 Figure 5.8. The effect of wood moisture on (a) nitrogen fixation and (b) respiration rate. a b

9 Figure 5.9. The effect of oxygen concentration on (a) nitrogen fixation and (b) respiration rate. a b

10 Figure 5.10. The relationship of the oxygen diffusion rate and (a) the fraction of pore space in air and (b) wood density. a b

11 Figure 5.11. The effect of altering several parameters related to (a) generating log moisture content and (b) oxygen diffusion on the annual amount of nitrogen fixed(NFIX, nmol·g -1 ·d -1 ). a b

12 Figure 5.12. A comparison of relative respiration rates from field data and the model.

13 Figure 5.13. Predicted versus observed plots for comparing data from the model and the field, respectively, for (a) respiration (b) moisture content and (c) oxygen concentration. b a c r 2 = 0.35 p = 0.01 r 2 = 0.60 p < 0.01 r 2 = 0.48 p < 0.01

14 Figure 5.14. The influence of log diameter on average log temperature.

15 Figure 5.15. A comparison of wood moisture content from (a) a model and (b) field data. a b

16 Figure 5.16. A comparison of log oxygen concentration from (a) model and (b) field data. a b

17 Figure 5.17. Cumulative nitrogen inputs to Pacific Northwest forests over 500 years of secondary succession.