Other Abstract | Since the limit of the length of climate record, it is not easy to understand the
mechanism of the climate change. The paleo-climate proxies become the most important
method of current climate research to overcome this shortness. Tree ring data have been
widely used to reconstruct the variability of many climate factors (e.g., temperature,
precipitation), due to their annual resolution, wide spatial distribution, and the possibility
of using simple linear models of climate–tree growth relationships that seem to be easily
verified and calibrated. The stable relationship between tree ring and climate factors is
the fundamental base of dendroclimatology. The rapid global temperature increasing has
caused lot of environmental problems and climatic disasters during last half of 20th
century. More and more investigations displayed that trees growing in high latitude of
northern hemisphere and circle-polar area lost their sensitivity to growth season
temperature that was the main limit factor of tree growth in those area. The relationship
between tree growth and climate factors has been reduced, and even changed. In China,
tree ring also plays an important role in past climate researches. What will happen on tree
have been widely used in China during last half century? In order to investigate this
question, here we study the relationship between tree ring-climate during last half 20th
century in Qilian Mts area where is really sensitive to climate change by using the
regionally dominating tree Qinghai spruce (Picea crassifolia).
In total, we used 342 Qinghai spruces trees which were sampled from 11spatial sites
at upper tree line and 7 different elevation sites along two different slopes. Basing on this
dataset, we discussed the relationship between tree growth and climate factor. The
correlation relationships between every single tree and main climate factors have changed
over time. Within a site, trees had consistent correlations with monthly precipitation, but
the relationships decreased over time. The relationships between trees and monthly
temperature, especially in growth season, had obvious response divergence, which means
within those sites, the consistent relationship between trees and monthly temperature has
been broken. Trees showed different response to monthly temperature as negative
responder and positive responder.
According to different response between trees and monthly temperature, trees of
each site could be clustered into different responder groups as positive responder and
negative responder. The chronologies based on different response groups show same
response divergence with monthly temperature especially in growth season. Most sites
have this response divergence. The response divergences mainly show in STD
chronologies which include mainly low frequency signal. There is almost none response
divergence in RES chronologies which indicate high frequency signal. The percentage of
positive responder within site has a spatial difference. In east Qilian Mts, more sites have
high percentage of positive responder. This percentage deceases at western part of
mountains.
On contrary of the most conclusions about tree growth-climate response around the
world, Qinghai spruces in Qilian Mts show increasing sensitivity to monthly temperature.
As global warming, the relationship between spruce and monthly temperature especially
in growth season become intensive. The correlation relationships, no matter negative
correlation and positive correlation, were getting stronger over time. However, if we
ignore the response divergence within site, the sensitivity between Qinghai Spruce and
monthly temperature will be underestimated.
Comparing the growth divergence within site, it shows that about 71% Qinghai
Spruce had growth reduce, only 29% trees have an increasing growth trend during last
thirty years. Considering the regional hydrothermal and topographical condition, it is
clear to see that trees growing at site that have enough moisture tender to have positive
correlation with monthly temperature, but trees growing at sites in drought area show
more negative response with monthly temperature. Temperature induced drought stress
affect most trees growth than before since circumstance warming. The different response
of trees under different hydrothermal and topographical condition indicate that the
increasing warming during last half century might be harmful for Qinghai spruce,
especially at and near tree line.
The instable relationship between Qinghai spruce and temperature and response
divergence phenomena exist at most sites in Qilian Mts. It happened around 1980. At
spatial scale, trees growing at east Qilian Mts. had increasing positive correlation with
temperature and the ratio of response divergence also increased, trees from west Qilian
Mts. had stronger negative correlation with temperature and low ratio of response
divergence. Sites at the high elevation have the high ratio of response divergence. If we
do not consider the instable relationship between trees and climate factor and ignore the
response divergence within site, the dendroclimatological analysis might be wrong.
Therefore, in future, it is necessary to consider the relationships between tree growth and
climate factors, especially during the end of last century.
At the end of last half century, Qinghai spruce was limited mainly by the
temperature induced drought stress, which also might be reason for the changing
response between trees and monthly temperature. There is no obvious affect on response
divergence from detrending methods and tree age etc.. The standing changing and forest
disturbance have strong influence on response divergence. The threshold between tree
growth and temperature might be the one of the potential reason of response divergence. |
Edit Comment