Warming may extend tree growing seasons and compensate for reduced carbon uptake during dry periods

1. Warming and drought alter plant phenology, photosynthesis and growth
with important consequences for the global carbon cycle and the earth’s
climate. Yet, few studies have attempted to tease apart their effects on
tree phenology, particularly leaf senescence, and on source and sink
activity. 2. We experimentally assessed the single and combined effects of
warming and reduced soil moisture on the phenology (leaf-out and
senescence date, growing season length) and aboveground sink (height and
diameter growth, leaf area and Huber values) and source activity (net
photosynthesis, photosynthetic efficiency, chlorophyll concentration and
total carbon (C) uptake) of two tree species with distinct strategies to
deal with drought: European beech and pubescent oak. 3. Warming advanced
leaf-out, irrespective of soil moisture levels, particularly in oak and to
a lower extent in beech, leading to a prolonged growing season in oak but
not beech. No impacts of warming on senescence timing were found for both
species. Reduced moisture had little impact on the phenology of both
species. Warming-induced advances in phenology and higher photosynthetic
efficiency increased the annual C uptake for oak and compensated for the
reduced photosynthetic activity in the presence of reduced moisture.
Conversely, for beech, source activity, including yearly C uptake, was
lower in all treatments than the control, indicating no compensation of
the C budget by phenological shifts. 4. Synthesis. Our results demonstrate
that a warming-driven earlier activity and higher photosynthetic
efficiency compensates for reduced photosynthesis during hot and dry
periods, but only for pubescent oak, which is a rather drought tolerant
species. Current predictions of warming-induced mitigation effects through
extended C uptake seem incorrect for beech. --