Carbon assimilation and mortality during drought in tree species with different stomatal sensitivities

Background/Question/Methods

With drought an increasing concern in many areas due to climate change, it becomes critical to understand how ecosystem composition and ecosystem-climate feedbacks change with drought. Yet we do not fully understand the relationship between drought, tree carbon assimilation, and tree mortality. Tree species have different strategies for surviving drought. For instance, some trees close their stomata more quickly than others in response to drought, leading to greater decreases in carbon assimilation rates. Our question was how much these differences in stomatal sensitivity affect carbon assimilation rates and whether decreases in carbon assimilation were correlated with plant mortality.

We grew four species of deciduous trees of varying stomatal sensitivies together in large containers. Cornus florida and Fagus grandifolia have less sensitive stomata, while Liquidambar styraciflua and Liriodendron tulipifera have more sensitive stomata. Under control conditions, we used a Licor LI-6400 to measure the response of carbon assimilation to varying CO₂ concentrations and irradiance for five individuals per species. These measurements were used to parameterize the Farquhar-von Caemmerer photosynthesis model. These models were used in combination with measures of stomatal conductance and light throughout a gradual drydown treatment over a two-month period to calculate the response of carbon assimilation to drought.

Results/Conclusions

The species with stomata most sensitive to water potential, L. tulipifera and L. styraciflua, had the largest decreases in assimilation rates over the drought treatment (over 10 μmol CO₂/m²/s). Their relative decreases in assimilation rates were also high (over 50%). These species suffered the  greatest mortality (both over 80%).

C. florida and F. grandifolia, the species with less sensitive stomata, had lower absolute decreases in assimilation rates (approximately 8 μmol CO₂/m²/s and 4 μmol CO₂/m²/s, respectively). C. florida, however, had the largest percent decrease in assimilation (63%) while F. grandifolia had the lowest percent decrease (27%). These species had lower mortality rates (both under 60%) than the species with more sensitive stomata.

The results of this study indicate that stomatal sensitivity has strong effects on decreases in assimilation rates with drought and indicate an association between higher absolute decreases in assimilation rates and greater mortality. This association suggests a predictive function for carbon assimilation rates in determining tree species sensitivity to drought mortality. Further research is needed to determine if this predictive ability holds for a wider range of species and whether decreases carbon assimilation may also have a causative role in determining plant mortality.