forests as carbon sinks and their response to drought
Carbon sink strength of
a Mediterranean cork oak understorey: how do semi-deciduous and evergreen
shrubs face summer drought?
January 2018 Supervisor: Alexander
region covers only 2.3% of the world land surface, it has a significant
contribution in giving carbon feedback to the atmosphere (Ramabal, 2001). Drought
plays a significant role in limiting the ecosystem carbon and water balance in Mediterranean
region (Reichstein et al., 2002). Thus, it makes sense to study about the
carbon sink strength and drought response of Mediterranean forests. There are perspectives
which make this topic scientifically relevant.
Ecosystem regulates the climate
(Reichstein et al., 2002), we can predict climate change effects by examining
whether this ecosystem is capable of acting as a carbon sink or not (Cox et
Detail understanding of soil and
vegetation response to change in water, carbon and light availability occurring
in forests is not possible without measurement in the understory (Correria et
The studies on effects
of droughts on ecosystem functions and stability are mainly concentrated on
boreal tropical and temperate forests which only represent moderate level of
droughts (Schultz, 1995). This presents a gap in the study of ecosystem
functions with respect to the effect of drought on Mediterranean region which
represents temperate steppe and deserts and tropical biomes with summer or
winter drought (Melillo et al., 1993).
The understory shrubs
in the study area have a significant contribution to ecosystem carbon
How do semi-deciduous and evergreen
shrubs exploit environmental resources during summer drought?
What is the contribution of the understorey
shrubby layer to ecosystem carbon assimilation?
understorey species taken in this study was: a) Semi-deciduous: Cistus
salviifolius and Cistus crispus and b) Evergreen: Ulex
ecosystem productivity data was taken
from an eddy-covariance ?ux tower (22m) installed above the tree canopies. Dynamic
chambers (40cm × 40cm and 54cm high) were used to measure carbon ?uxes
from dominant understory shrubs and ef?ux from the soil. Net co2 exchange
(Fnee) and transpiration (Fet) was measured in light chambers whereas plant and soil respiration (Freco) was measured
in dark chambers. Various calculations like Water use efficiency (WUE),
Light use efficiency (LUE) and Carbon use efficiency (CUE) from the data from
The leaf area of the
semi deciduous species was seemed to decrease as a response to the start of the
summer drought. The leaf area reduction caused overall photosynthesis to reduce
in all those deciduous species. But, the noticeable difference was seen in the
evapotranspiration and above ground respiration during the period of late summer
and late autumn.
deciduous understorey species: Cistus salviifolius which had shallow root could use the light
availability more efficiently. But in response to water loss and high above
ground respiration rates, its stomatal control was comparatively poor. Thus, Cistus
salviifolius utilized the water as an opportunist and could rehydrate the
leaf immediately and suddenly increase photosynthesis. Hence, we can conclude
that Cistus salviifolius was less efficient in using water and carbon
when compared to another semi deciduous species Cistus crispus.
But, as the future is predicted to have extended dry summers and higher
probability of rain pulse events, this opportunistic growth response may be
disadvantageous to shallow rooted shrubs.
Ulex airensis which
had deep root had very low rate of photosynthesis. But, it maintained maintained
higher leaf water potentials and the transpiration rate was low for the whole
period of summer drought. A summer rain
pulse showed that deep-rooted shrubs (U. airensis) were not
responsive to water availability and only recovered photosynthesis when there
was water available on the soil.
Together these shrubs
contribution in the GPP (ecosystem carbon uptake) was 17%on the average. Among
the three species Cistus salviifolius made a high contribution of 14% whereas contribution of other two
shrubs was only 2% each. This contribution could be also the result of the
variation of biomass between three species.
Observed reduction in
ability of plant to recover after autumn rains may be because of increase in
the rate of transpiration and respiration during the drought which was followed
by rain pulse.
The highly competitive
sclerophyll shrubs that naturally colonized this region showed different
strategies to overcome summer drought. The contribution of these shrubs to
total ecosystem CO2 uptake during summer and autumn recovery
was 17%. This high contribution implies that shrub density management decisions
should consider a carbon balance perspective. This research has an implication
for further scientific work in this region which should emphasize on the
ecophysiological responses of these understory shrubs to exploit the resources
in different condition a functional groups. Nevertheless, we cannot ignore the
fire risk associated with these shrubs which could hamper the long term
sustainable carbon sequestration associated with this region.
Strength: The measurement of
understory response to understand ecosystem processes was a very strong
methodology which supplemented the recordings from the eddy- covariance flux
towers. It was strength of this study to include the response generated by the
unexpected summer rain pulse and derive a result on the ability of species to
use the unexpected water and its effect on the whole carbon and water balance
Weakness: The data obtained from
the chambers has the possibility to be biased because the sampling used to
locate the sample plot was random sampling. In a heterogeneous forest this
might seem to either overestimate or underestimate the data. The study only selected two representative
days from early summer, summer end and mid-autumn which has chance that the
study cannot conclude results about the whole drought period.
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understorey: how do semi?deciduous
and evergreen shrubs face summer drought?. Journal of vegetation
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