Nonstructural carbohydrates (NSCs) are important for the growth and survival of trees. Drought may lead to a decrease in tree growth and to NSC depletion, whereas increased soil moisture in otherwise dry ecosystems may increase growth and NSC concentrations. A long-term (13 yr) irrigation experiment was conducted in a Pinus sylvestris-dominated forest located at the dry margin of the species in southern Switzerland. We measured the relative leaf area, growth, NSCs, needle δ C, [N] and [P] in trees on control and irrigated plots. Irrigation resulted in higher growth rates and carbon isotope discrimination, but did not alter NSC levels. Growth and NSC decreased with decreasing leaf area in both treatments, but NSC did not correlate with leaf-level gas exchange indices, such as foliar δ C, [N] or [P]. A legacy effect was shown, as trees with initially low leaf area had limited ability to respond to prolonged irrigation. The NSC constancy across treatments provides evidence that carbohydrate storage may stay constant when climate changes are sufficiently slow to allow acclimation. Moreover, we speculate that total leaf area, rather than leaf gas exchange per unit leaf area, drives the variation in whole-tree carbohydrate dynamics in this system.

Drought alters carbon (C) allocation within trees, thereby impairing tree growth. Recovery of root and leaf functioning and prioritized C supply to sink tissues after drought may compensate for drought-induced reduction of assimilation and growth. It remains unclear if C allocation to sink tissues during and following drought is controlled by altered sink metabolic activities or by the availability of new assimilates. Understanding such mechanisms is required to predict forests’ resilience to a changing climate. We investigated the impact of drought and drought release on C allocation in a 100-y-old Scots pine forest. We applied 13CO2 pulse labeling to naturally dry control and long-term irrigated trees and tracked the fate of the label in above- and belowground C pools and fluxes. Allocation of new assimilates belowground was ca. 53% lower under nonirrigated conditions. A short rainfall event, which led to a temporary increase in the soil water content (SWC) in the topsoil, strongly increased the amounts of C transported belowground in the nonirrigated plots to values comparable to those in the irrigated plots. This switch in allocation patterns was congruent with a tipping point at around 15% SWC in the response of the respiratory activity of soil microbes. These results indicate that the metabolic sink activity in the rhizosphere and its modulation by soil moisture can drive C allocation within adult trees and ecosystems. Even a subtle increase in soil moisture can lead to a rapid recovery of belowground functions that in turn affects the direction of C transport in trees.

No abstract

No abstract