Role of photosynthesis and stomatal conductance on the long-term rising of intrinsic water use efficiency in dominant trees in three old-growth forests in Bosnia-Herzegovina and Montenegro

Old-growth forests have an important role in maintaining animal and plant diversity, are important carbon (C) reservoirs and are privileged sites to study long-term plant physiological responses, long-term forest dynamics and climate change impact on forest ecosystems. Several studies have highlighted how old-living trees undergo age-related declines with hydraulic limitations and reduction in photosynthesis, though some recent works have suggested that such a decline is not always observed. Our study aims at understanding the role of atmospheric CO2 increase on tree C uptake and stomatal conductance (g(s)) in old-living trees by analysing the long-term patterns of tree growth and intrinsic water use efficiency (iWUE) in three old-growth forests in the Balkans (Bosnia-Herzegovina and Montenegro), using dendrochronology and isotopic analysis. We hypothesised a long-term increase in iWUE in the studied old-growth forests, mostly related to enhanced photosynthesis rather than reduced stomatal conductance. Tree cores were sampled from dominant silver fir (Abies alba Mill.) trees in each forest. Tree-ring widths were measured and basal area increments (BAI) were assessed for each sampled tree and, from the six longest chronologies, five decades were chosen for cellulose extraction, its Isotopic analysis (delta C-13, delta O-1(8)), iWUE and leaf water O-18 evaporative enrichment above the source water (delta O-18(L)) determination. We observed a continuous and significant increase in iWUE from 1800 to 2010 in the sampled dominant trees at all the three old-growth forests. Our BAI data and our estimates of delta O-18(L) across the study period support the idea that enhanced photosynthesis rather than reduced stomatal conductance is the major driver of the measured iWUE increase. Thus, our results support some recent findings challenging the hypothesis that iWUE in forests is primarily the result of a CO2 -induced reduction in stomatal conductance as well as the so called hydraulic limitation hypothesis.

Automated summary

Old-growth forests play an important role in maintaining animal and plant diversity, acting as carbon reservoirs, and studying long-term plant physiological responses, forest dynamics, and climate change impact on ecosystems. A study in the Balkans analyzed the long-term patterns of tree growth and intrinsic water use efficiency in old-growth forests, showing a continuous and significant increase in iWUE from 1800 to 2010. The results challenge the hypothesis that iWUE in forests is primarily a result of CO2-induced reduction in stomatal conductance.