| We compare assimilation and respiration rates, and water use strategies in
four divergent ecosystems located in cold-continental central Siberia and in
semi-arid southern Africa. These seemingly unrelated systems have in common
a harsh and highly seasonal environment with a very sharp transition between
the dormant and the active season, with vegetation facing dry air and soil
conditions for at least part of the year. Moreover, the northern high
latitudes and the semi-arid tropics will likely experience changes in key
environmental parameters (e.g., air temperature and precipitation) in the
future; indeed, in some regions marked climate trends have already been
observed over the last decade or so.
The magnitude of instantaneous or daily assimilation and respiration rates,
derived from one to two years of eddy covariance measurements in each of the
four ecosystems, was not related to the growth environment. For instance,
respiration rates were clearly highest in the two deciduous systems included
in the analysis (a Mopane woodland in northern Botswana and a Downy birch
forest in Siberia; >300 mmol m−2 d−1), while assimilation rates
in the Mopane woodland were relatively similar to a Siberian Scots pine
canopy for a large part of the active season (ca. 420 mmol m−2 d−1).
Acknowledging the limited number of ecosystems compared
here, these data nevertheless demonstrate that factors like vegetation type,
canopy phenology or ecosystem age can override larger-scale climate
differences in terms of their effects on carbon assimilation and respiration
rates.
By far the highest rates of assimilation were observed in Downy birch, an
early successional species. These were achieved at a rather conservative
water use, as indicated by relatively low levels of λ, the marginal
water cost of plant carbon gain. Surprisingly, the Mopane woodland growing
in the semi-arid environment had significantly higher values of λ.
However, its water use strategy included a very plastic response to
intermittently dry periods, and values of λ were much more
conservative overall during a rainy season with low precipitation and high
air saturation deficits. Our comparison demonstrates that forest ecosystems
can respond very dynamically in terms of water use strategy, both on
interannual and much shorter time scales. But it remains to be evaluated
whether and in which ecosystems this plasticity is mainly due to a
short-term stomatal response, or alternatively goes hand in hand with
changes in canopy photosynthetic capacity. |