1. Midsummer Warming/Drought in the Boreal Forest

2. The inter- and intra-seasonal relationships between evaporation and rainfall, which are linked to summer air and soil temperature dynamics, that strongly influence nutrient availability and subsequently tree growth in boreal forests are unclear. Introduction

3. Study objective –  Determine the influence of summer rainfall on the growth of trees in both upland and floodplain locations in interior Alaska Hypothesis 1: tree growth in upland stands is strongly controlled by summer rainfall Hypothesis 2: tree growth in floodplain stands will show no relationship to summer rainfall due to the proximity of ground water related to river dynamics

4. Moisture stress and tree growth - Treatment

5. Past Work – Billings (1998, 2000) Sheltered soils had decreased profile concentrations and surface flux of CO 2. Sheltered soils had decreased CH 4 uptake rates on the floodplain and increased rates in the uplands. UplandFloodplain ControlDroughtControlDrought Surface CO 2 flux (g m -2 h -1 ) 0.1 – 0.950.1 – 0.530.2 – 1.450.1 – 0.55 Methane uptake (mg m -2 day -1 ) 0.1 – 0.50.3 – 1.00.1 – 0.6-0.1 – 0.2

6. Foliar N Concentrations – Yarie (1996) No significant differences for upland aspen, poplar and floodplain poplar Significant decrease for upland birch in 1990 Significant increase for upland white spruce in 1990 and a decrease in 1991 Significant decrease for floodplain white spruce in 1990 and 1991 No differences observed in 1994

7. Foliar P Concentrations – Yarie (1996) No significant differences for upland aspen, poplar and floodplain poplar Significant decrease for upland birch in 1990 Significant increase for upland white spruce in 1990 and decrease in 1991 Significant decrease for floodplain white spruce in 1991 No significant differences in 1994

8. S. Runck – work in progress Differences in current soil organic carbon at depth intervals of O horizon, 0-5 cm 5- 15 cm and 15-30 cm Differences in biomass of coarse and fine roots (same depth intervals) Decomposition – birch tongue depressors for 2 years

9. Upland Soil Moisture, Control and Drought Treatments

10. Floodplain Soil Moisture, Control and Drought Treatments

11. Annual Average Tree Growth Upland SpeciesTreatmentAvg BA Growth AspenControl4.86 Drought4.66 BirchControl1.47 Drought1.02 Balsam PoplarControl2.65 Drought2.13 White SpruceControl6.81 Drought6.35 Significant Reductions in Growth Birch – 1992, 1992 Balsam Poplar - 1992

12. Annual Average Tree Growth Floodplain Results SpeciesTreatmentBA Growth Balsam Poplar Control3.88 Drought1.73 White Spruce Control11.46 Drought4.43 Significant Reductions Balsam Poplar – 1992 White Spruce – 1991 - 2005

13. 5-yr tree growth Time Period 1 UP2 AspenBirchBalsam PoplarWhite Spruce ControlDroughtControlDroughtControlDroughtControlDrought 120.124.17.46.116.113.016.814.1 212.719.85.84.813.19.714.711.4 314.412.65.54.68.35.418.113.2 All Years43.043.315.711.833.224.748.437.8 1 - The time periods represent (1) 1989 - 1993, (2) 1993 - 1998, and (3) 1998 - 2003. Time Period 1 FP3 Balsam PoplarWhite Spruce ControlDroughtControlDrought 117.711.819.617.8 220.310.2 24.9*14.2* 316.26.5 23.5*12.1* All Years41.621.364.141.4 1 - The time periods represent (1) 1989 - 1993, (2) 1993 - 1998, and (3) 1998 - 2003.

14. Tree Growth and Soil Water Dynamics Upland Assumptions Rainfall reductions will have no effect on tree growth in upland locations Recharge of soil moisture from snowmelt is sufficient to maintain current levels of tree growth. Tree growth in upland locations is limited below the optimum level by several factors of which one is moisture supply

15. Tree growth and soil water dynamics Floodplain Assumptions Rainfall is a significant factor necessary to maintain current tree growth rates in floodplain locations Snow melt and ground water dynamics may be sufficient to maintain growth rates similar to upland sites Conductivity of the ground water may limit moisture uptake by the floodplain tree species

16. The next step to help clarify soil moisture dynamics Establish treatments that will limit aboveground soil moisture recharge from late fall rain and spring snowmelt events Summer rainfall will recharge soil moisture

17. Small Test Site

18. Questions ?