Midsummer Warming/Drought in the Boreal Forest. The inter- and intra-seasonal relationships between...
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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 ?