11:15 〜 11:30
[MIS01-09] Dark conifer forests dieback at mid-to-highlands in the southern Siberia’s mountains is not caused by drought stress
キーワード：forest ecosystems, annual moisture index, SPEI, die-back, dark-needled species
Foresters and ecologists of the world register dieback of dark conifer (shade-tolerant, water-loving Pinus sibirica, Abies sibirica and Picea obovata) forests across the boreal zone since the 1960-x. This issue has become urgent since the 1970s across the southern Siberia’s mountains (west-east: Altai, Kuznetsky Alatau, West and East Sayans and southern Transbaikalia) in the most productive dark conifer forests. The damage was expressed through: necroses of branches with generative organs that cause microsporogenesis and pollen development irregularities followed by low pollen and seed vitality; chloroses and premature needle falls; the occurrence of stem rot; the linear and radial increment decrease etc.
To date, there are several hypotheses to explain dieback of dark conifers within the 50-56° N latitudes across Northern Eurasia associated with:
1. climate aridization supposedly taking place under global warming;
2. anthropogenic environmental pollution (various gas emissions, acid rainfall).;
3. phytopathogens (Armillaria mellea, Heterobasidion annosum, etc;
4. invasive insect pests;
5. increased hard ultraviolet radiation (UV-- caused by the troposphere ozone depletion and additionally amplified by the increased ozone concentration in the near-surface air layer).
Our goal was to concentrate only on one Hypothesis #1 and accept/reject that: 1. Has the climate across the southern Siberia’s Mts been drying for the last 60 years; 2. If the current climate is drying then whether it may cause dieback of dark conifer trees as referred in the literature. Finally, would be climate change predicted from the CMIP5 projections (AR5) dry enough to cause dieback hotspots within the range of climax dark conifer forests by the end of the 21st century.
To achieve our goals we investigated climate change trends of annual precipitation, summer monthly temperatures and aridity/moisture indices (annual moisture index, AMI, and standardized precipitation evapotranspiration index, SPEI) for 1961-2019; and modeled dark conifer forest distributions and their shifts in a changing climate using our mountain bioclimatic vegetation model.
Our examinations of damaged trees in dark conifer forests along four mountain transects in each mountain system across the southern Siberia’s Mts showed that most dieback was found in the mid-elevation taiga at 700-900 m and up to 1400 m in highland taiga. To identify whether those events were associated with climate droughts AMI, SAMI (standardized AMI) and water balance (precipitation minus evapotranspiration), SPEI, were calculated for nine weather stations located in various ecosystem types (Fig. 1) and interpolated then for the lower border of decline/dieback occurrences. Climate trends for 1961-2019 demonstrated that moisture conditions have not changed much at these dieback sites and remained sufficient for dark conifer forests. Given the AMI limit for the lower/southern border of dark conifer forests was 2.25, AMI varied between 0.8-0.2 at 800-1200 m that indicated very wet conditions. Modeled moisture conditions would still remain satisfactory for dark conifers at the 2080s even for the extreme dry scenario RCP 8.5. Given the SPEI values <-1.5 were characteristic of extreme droughts, such moisture conditions occurred only 3-4 times for 1961-2019 (Fig. 1). Trees are known to survive occasional 1-2 drought years and then successfully recover. To state on a drought event for an ecosystem based only on SPEI is not ecologically sufficient. Dark conifers grow under positive annual water balance > 150 mm and true steppe – under negative. However, SPEI in these ecosystems varied within the same range ± 2 (Fig. 1).
We conclude that Siberian pine and fir dieback observed mainly at the mid-to-high elevations were caused by reasons other than the drought stress as cited in the literature (Kharuk et al. 2020 et seq.). Other hypotheses are to be investigated to explain the phenomena.
Supported by RFBR grant 20-05-00540
To date, there are several hypotheses to explain dieback of dark conifers within the 50-56° N latitudes across Northern Eurasia associated with:
1. climate aridization supposedly taking place under global warming;
2. anthropogenic environmental pollution (various gas emissions, acid rainfall).;
3. phytopathogens (Armillaria mellea, Heterobasidion annosum, etc;
4. invasive insect pests;
5. increased hard ultraviolet radiation (UV-- caused by the troposphere ozone depletion and additionally amplified by the increased ozone concentration in the near-surface air layer).
Our goal was to concentrate only on one Hypothesis #1 and accept/reject that: 1. Has the climate across the southern Siberia’s Mts been drying for the last 60 years; 2. If the current climate is drying then whether it may cause dieback of dark conifer trees as referred in the literature. Finally, would be climate change predicted from the CMIP5 projections (AR5) dry enough to cause dieback hotspots within the range of climax dark conifer forests by the end of the 21st century.
To achieve our goals we investigated climate change trends of annual precipitation, summer monthly temperatures and aridity/moisture indices (annual moisture index, AMI, and standardized precipitation evapotranspiration index, SPEI) for 1961-2019; and modeled dark conifer forest distributions and their shifts in a changing climate using our mountain bioclimatic vegetation model.
Our examinations of damaged trees in dark conifer forests along four mountain transects in each mountain system across the southern Siberia’s Mts showed that most dieback was found in the mid-elevation taiga at 700-900 m and up to 1400 m in highland taiga. To identify whether those events were associated with climate droughts AMI, SAMI (standardized AMI) and water balance (precipitation minus evapotranspiration), SPEI, were calculated for nine weather stations located in various ecosystem types (Fig. 1) and interpolated then for the lower border of decline/dieback occurrences. Climate trends for 1961-2019 demonstrated that moisture conditions have not changed much at these dieback sites and remained sufficient for dark conifer forests. Given the AMI limit for the lower/southern border of dark conifer forests was 2.25, AMI varied between 0.8-0.2 at 800-1200 m that indicated very wet conditions. Modeled moisture conditions would still remain satisfactory for dark conifers at the 2080s even for the extreme dry scenario RCP 8.5. Given the SPEI values <-1.5 were characteristic of extreme droughts, such moisture conditions occurred only 3-4 times for 1961-2019 (Fig. 1). Trees are known to survive occasional 1-2 drought years and then successfully recover. To state on a drought event for an ecosystem based only on SPEI is not ecologically sufficient. Dark conifers grow under positive annual water balance > 150 mm and true steppe – under negative. However, SPEI in these ecosystems varied within the same range ± 2 (Fig. 1).
We conclude that Siberian pine and fir dieback observed mainly at the mid-to-high elevations were caused by reasons other than the drought stress as cited in the literature (Kharuk et al. 2020 et seq.). Other hypotheses are to be investigated to explain the phenomena.
Supported by RFBR grant 20-05-00540