TREE LINE SHIFTS AND CLIMATE CHANGE: VARIATION IN ABUNDANCE AND
Seminar on Climate Change
The world is experiencing an increase in temperature, where the global mean temperature of this
increase is around 0.6°C (Houghton et al., 2001). Mountain regions and high latitudes, such as Antarctica
and the arctic present the greatest amount of increase during the last century.
Climatic conditions shape the ranges and
distribution of species and thus the composition of
biomes (Pretice et al., 1992). Therefore changes in
climate are supposed to affect sensitive species and
ecosystems. Alpine and subalpine ecosystems are
one of the most sensitive environments to climate
change, since they are highly dependent on
temperature. Temperatures in these areas define
the point where trees cannot survive anymore.
Climate change can affect mountain ecosystems by
shifting the tree line or by affecting the upper limit of the pan life.
Tree line referred to the boundary of the forest that is usually form by immature trees and old dwarf
trees scattered above a very ragged line (Grace et al., 2002). Usually for tropical and subtropical areas
the tree line can be found at the summer isotherm as low as 3 to 6°C (Korner, 1998), meanwhile in
temperate ecosystems this line can be found at temperature near to 7°C. So an increase in the
temperature of the area could lead to a change in the presence of the tree line.
Climate change can also affect pan life, which means the life existing in extreme areas such as the top of
a mountain or very cold areas. The increase in the temperature could lead to a displacement of this
habitat with its final disappearance.
The purpose of studying the effects of climate changes on mountain or extreme ecosystems is that the
advancing of tree line or the increase in the density of these forests could have implications in the global
carbon cycle by the possible increase of the terrestrial sink, could have implications on biodiversity in
the alpine ecotone by the possible disappearance of rare species and by the disruption of the alpine and
subalpine communities. Finally, the shift of tree line could lead to a change in the montage landscapes
and the livelihood of the habitants of these areas.
Now we should ask why the increase of temperature can affect tree line? For that we should be aware
that there are 4 major environmental aspects plants respond to. The first one is the increase of
temperature, the second one the rise of CO2 concentration and the increasing of deposition of nitrogen;
all this aspects affect the photosynthetic rate. A more indirect variable is the decrease of solar radiation
as a result of the increase of cloudiness or aerosols in the atmosphere.
So the possible effects of climate warming could lead to longer growing season; and since growing
season in this area is very short this could have big implications on the annual assimilation of CO2. The
increase of temperature could increase the rate of cell division and therefore the use of glucose which
will cause more carbon availability to built tissues, the increase of soil organic matter decomposition as
ecological aspects. And on the other hand it could lead at the disappearance of former snowfields and
meadows that therefore could be colonized by plants as spatial consequences, while behavioral
consequences could include the change in mortality patterns, affecting the frequency and severity of
fires, and the susceptibility of vegetation to insects and diseases because warmer temperature make
less resistant trees to attacks. Another behavioral effect consists in changes in recruitment, where
warmer temperatures lead to the spread of the seedling, and if the warming is permanent the change
on the tree line. Finally, we could also see changes in forest structure where from krumholz the
vegetation could actually shift to trees (Grace et al., 2002).
The possible effects of climate warming depend on the type of species, what reproduction strategies
and reproduction rate that specie has, the genetic variability existing which makes species more
resistant, the physiological plasticity of the vegetation that would make easier the adaptation to new
conditions in the environment and finally the inter and intra-specific competition, where probably the
strongest species would survive.
Actual changes of climate warming on tree lines are review by Walther et al. in different continents.
Increases in temperature suggest the vertical advance of the tree line by 400-700 m over the century.
But we should be aware that this shift in the tree line can be temporal since the establishment of
vegetation needs a permanent increase of the temperature through the years, not only an isolated
Also effects on mountain ecosystem and extreme ecosystems can be seen on the increase of tree
density near the tree line. Vale and Vale (1994) and Lenoir (2009) show that tree density has change in
the meadows located in Yosemite National Park, cause by the increase of temperature, even though
tree line has not change.
Other studies on France (Lenoir, 2009) show a
change in biodiversity by the change of the
species altitude range. They measured 171
plant species for 2 periods, 1905-1985 and
1986-2005, along 0 to 2600 meters above sea
level. The estimated that there was a change
in the optimum elevation of 29 m per decade,
and that this shift is higher in mountain
regions and for the grass species. Finally, the
paper concluded that the increase of
temperatures bring changes on the spatial
core of distributional range of plant species
and changes in the distributional margins.
Walther et al. paper explored the effect of changes in temperature in the distribution of plant
communities with emphasis on sensitive species and ecosystem focusing on tree line shifts and the
upper limit of plants. Different hypothesis are discussed, whether the vegetation will move to upper
altitudes, will stay the same but changes on the ecological processes will occur or whether species will
be lost. Clearly there's still no agreement in the true effects of an increased in temperature but it seems
that vegetation will move in altitude displacing alpine flora to higher altitudes, and if they are already on
the top of the mountain making the alpine flora to disappear replacing it with trees. This kind of shift
could lead to high changes on the environment, therefore could imply the lost of habitats, species and
changes in biogeochemical processes therefore energy and gases cycles. The complexity of ecosystems
makes the prediction of changes by climate change to be uncertain; models should include several
additional variables, specially related to the possible change in biogeochemical processes that are
probably the main drivers in tree line shifts.
Effects of fog and ENSO in tree line: Parque Nacional Fray Jorge
The olivillo (Aetoxicum punctatum) forests are located in Parque Nacional Fray Jorge in the coastal
mountains of Chile, in the middle of the semiarid ecosystem. They are remnants temperate rainforests
that are dependent on water inputs of oceanic fog to survive. Fog is dependent on the sea level
temperature, the height of the inversion and the structure of the forest. So events such as climate
warming and ENSO could affect the survival of this ancient forests, Gutierrez et al. (2008) explored the
effect of this climate event on the growth of the trees and on the size of the remnant patches. They
came to realize that variable such as precipitation and ENSO frequency did not really affect the size of
the patches or the growth of the trees. Their study consisted on the use of dendrochronology to see
how periods with different climatic condition affect tree growth by the measuring of tree rings; in this
case they relate tree growth to the amount of rainfall in the area. So it is hypothesized that lower levels
of rainfall should derived in lower growth rates. They get to the conclusion that precipitation did not
have a higher correlation with the increment in growth or with the reduction of the patches; the same
was concluded with ENSO where no particular pattern were recognize. So the exploration of the effect
of fog in the patches needed to be explored, but the available data to do it was scarce for the area.
So finally the question arises, does fog is going to be affected by climate warming?, probably yes since
an increase in temperature would lead to changes in sea temperature and who knows what other
variable that could be affecting the fogging process. Also, at the same time the decrease in the sizes of
these patches would change the capacity of water capture by fog, since the structure of the forest is
being change. And as a final point, does fog affect climate in a global way or is more a local dependent
process, therefore it should only be consider in regional climate models?.
As a first point it was stated that the effects of climate change in mountain areas should be explore
according to the robustness and capacity of adaptability of plant species. Also other variables besides
temperature should be explore, they should include what will be the effect on ecological processes
when climate change occurs since local factors probably would have a big effect on this areas. Changes
in other variables that affect trees should be explored to name temperature as the only variable
affecting tree line shift.
What is the meaning of “episodic” changes shifting with regard of time? The episodic change in tree line
refers to the effect occurring for a short period of time cause by an increase of temperature in just one
season which leads to the expansion of the tree line, but the probability of survival of the new tree line
is very low, since temperature would return to the usual condition for the next season. These episodic
changes in the tree line could lead to the fragmentation of the ecosystem temporarily or permanently,
impacting the existing alpine ecosystem, making some species even to disappear.
As potential positive effects, the decrease of albedo and the increase of roughness in the mountains
areas by the colonization of meadows, therefore increasing the carbon terrestrial sink. Also it was
mentioned that a shift on the tree line could lead to the increase of the habitat of lowland species that
would be beneficial for the increase on forest productivity by also the extension of the growing season.
To human well being the increase of temperature with the consequence of the tree line shift could lead
to provide more available territory to new populations, increasing food supply and energy. Also there
will be more land available for agriculture or human settlements in areas where previously, because of
temperature could not be habited or were not productive lands.
When referring to the second paper written by Gutierrez et al. the conclusion were unanimous, the
exploration of fog behavior with climate change could be fundamental to some ecosystems and that this
variable should be included in global or regional models. From my point of view the inclusion of fog
should be done in only regional models, since their effect is mainly local. Climate variability could lead to
the lost of several different species and even ecosystems, local effects of climate should be explore, and
the importance of the combination of regional and local models takes big importance.
The difficulty of incorporating fog into climate models comes from the existence of good data that help
us with the prediction of how the fog behavior would change with climate change. So the elaboration of
an accurate model becomes lowly probable. However, efforts should be made since fog is related to
surface energy budgets and the production of water vapor, one of the main GHG’s that affect climate
Fog made us realize the importance of studying the effects of climate change in forests other than
tropical or subtropical, that temperate forests also maintain ecosystems with high levels of carbon. So
the inclusion of climatic variables that are determinant to them is of great importance to increase the
accuracy of predictions all over the world.
Finally in reference to dendrochronology, the technique was seen as helpful for exploring ENSO
influence on ecosystems for even more than 100 years. This technique works for different working
scales than ice cores or boreholes, but could give us very useful information and also data that is easy to
get and with the proper software resources could provide us with very accurate information about
Grace J., Berninger F., Nagy L. 2002. Impacts of climate change on tree line. Annals of Botany 90, 537-
Gutierrez A,G., Barbosa O., Christie D.A., Del-Val E., Ewing H.A., Jones C.G., Marquet P.A., Weathers K.C.,
Armesto J.J., 2008. Regeneration patterns and persistence of the fog dependent Fray Jorge forest in
semiarid Chile during the past two centuries. Global Change Biology 14, 161-176.
Lenoir J., Gegout J.C., Marquet P.A., de Ruffray P., Brisse H. 2008. A significant upward shift in plant
species optimum elevation during the 20th century. Science 320, 1768.
Walther G-R., Beibner S., Pott R., Climate change and high mountain vegetation shifts. In Mountain
Ecosystems, studies in tree line ecology, regional tree lines in the Americas. 21 p.