Midsummer Warming/Drought in the Boreal Forest

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

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

Moisture stress and tree growth - Treatment

Past Work – Billings (1998, 2000)

Sheltered soils had decreased profile concentrations and surface flux of CO2.

Sheltered soils had decreased CH4 uptake rates on the floodplain and increased rates in the uplands.

Upland Floodplain

Control Drought Control Drought

Surface CO2 flux (g m-2 h-1)

0.1 – 0.95 0.1 – 0.53 0.2 – 1.45 0.1 – 0.55

Methane uptake (mg m-2 day-1)

0.1 – 0.5 0.3 – 1.0 0.1 – 0.6 -0.1 – 0.2

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

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

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

Upland Soil Moisture, Control and Drought Treatments

Floodplain Soil Moisture, Control and Drought Treatments

Annual Average Tree GrowthUpland

Species Treatment Avg BA Growth

Aspen Control 4.86

Drought 4.66

Birch Control 1.47

Drought 1.02

Balsam Poplar Control 2.65

Drought 2.13

White Spruce Control 6.81

Drought 6.35

Significant Reductions inGrowth

Birch – 1992, 1992Balsam Poplar - 1992

Annual Average Tree GrowthFloodplain Results

Species Treatment BA Growth

Balsam Poplar

Control 3.88

Drought 1.73

White Spruce

Control 11.46

Drought 4.43

Significant Reductions

Balsam Poplar – 1992White Spruce – 1991 - 2005

5-yr tree growth

Time Period1

UP2

Aspen Birch Balsam Poplar White Spruce

Control Drought Control Drought Control Drought Control Drought

1 20.1 24.1 7.4 6.1 16.1 13.0 16.8 14.1

2 12.7 19.8 5.8 4.8 13.1 9.7 14.7 11.4

3 14.4 12.6 5.5 4.6 8.3 5.4 18.1 13.2

All Years 43.0 43.3 15.7 11.8 33.2 24.7 48.4 37.8

1 - The time periods represent (1) 1989 - 1993, (2) 1993 - 1998, and (3) 1998 - 2003.

Time Period1

FP3

Balsam Poplar White Spruce

Control Drought Control Drought 

1 17.7 11.8 19.6 17.8

2 20.3 10.2 24.9* 14.2*

3 16.2 6.5 23.5* 12.1*

All Years 41.6 21.3 64.1 41.4

1 - The time periods represent (1) 1989 - 1993, (2) 1993 - 1998, and (3) 1998 - 2003.

Tree Growth and Soil Water DynamicsUpland 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

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

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

Small Test Site

Questions ?