Tree growth in a changing world: Factors mediating the effects of CO2 on tree ring growth and Water Use Efficiency

Background/Question/Methods

Terrestrial ecosystems encounter an increasingly changing world characterized not only by increased CO₂, but also concurrent land-use changes, climate changes, and vegetation shifts. Therefore, to truly evaluate the effect of rising CO₂ on tree growth, we need to account for other factors influencing tree growth, such as tree age/size class, climate, and vegetation structure. The last century in Midwestern North America typifies these changes; while increases in CO₂ have likely increased tree intrinsic Water Use Efficiency (iWUE), extensive land-use created a landscape full of regenerating forests, fire suppression converted open savanna and woodlands to closed forests, and climate shifted from a relatively dry in the early 20^th century to remarkably wet after 1950. In the context of other anthropogenic changes, how much of a role has CO₂ played in changing vegetation, and in changing the response of vegetation to climate? To answer this, we developed an experimental design accounting for age/size, stand structure, and climate. Specifically, we compare the climate sensitivity of both tree ring growth and isotope-derived iWUE between trees in the past (pre-1950) and modern trees (post-1950) of the same age/size class, in the same ecosystem type (either savanna or forests), and in years of similar drought conditions.

Results/Conclusions

After standardizing for tree age/size class, stand structure, and climate, we find that modern trees are less sensitive to drought compared to trees in the past across the region, even in years of high drought. This is consistent with changes in drought sensitivity due to CO₂-mediated increases in iWUE. Modern trees are less sensitive to drought than past trees in both closed forest and open savanna systems, suggesting that the effects of CO₂ may not be dependent on stand structure. However, differences in drought responses between modern and past trees are greater in both drier climates and edaphic site conditions, suggesting that the positive impacts of CO₂ and wetter moisture conditions after 1950 are greater in drier regions. We find that changes in drought sensitivity are dependent on many factors: age, site conditions, and moisture availability. Rising CO₂ is just one of many changes impacting terrestrial systems today, and the observed impacts of CO₂ or iWUE may be masked or exacerbated by other concurrent changes. Therefore, a more careful accounting of the factors that affecting both tree growth and iWUE is necessary to better constrain which changes in growth or growth sensitivity may be due to increases in CO₂.