Ashley Hart | ALES Graduate Seminar

Date(s) - 20/09/2021
1:00 pm - 2:00 pm

Event details:  A graduate exam seminar is a presentation of the student’s final research project for their degree.

This is an ALES MSc Final Exam Seminar by Ashley Hart.  This seminar is open to the general public to attend via Zoom link:

Thesis Topic:

Resource partitioning, allocation, and remobilization in trembling aspen saplings

Seminar Abstract:

The long-term survival of trees is highly dependent on their ability to acquire, utilize and store resources to endure local environmental conditions and stresses. Although external uptake and internal cycling of essential resources in deciduous trees is largely regulated by seasonal phenology, resource use and allocation may change under stress. This thesis focused on how trembling aspen (Populus tremuloides Michx.), a deciduous tree species widely distributed across boreal and temperate regions of North America, may share and/or store resources between and within organs when subjected to stress. To explore the remobilization of resources between source and sink organs during spring bud flush and after leaf area recovery following defoliation, I used aspen saplings which were isotopically labeled with C and N and then grafted. I also observed changes in non-structural carbohydrate (NSC) and N concentrations in the stems and roots to explore potential constraints on remobilization during leaf area recovery. In a second study, I used a split-pot experiment that subjected saplings to either heterogeneous (a root system that was partially exposed to drought conditions) or homogeneous (the whole root system droughted or well-watered) soil moisture conditions. I measured aboveground and belowground reserve and mass allocation in response to these treatments.

In the first study I found that C and N were remobilized from both stem and root reserve pools during spring budflush and following defoliation to support leaf growth. Leaves rapidly transitioned from heterotrophic to autotrophic, and distance from the source (root system) affected the reliance on C reserve remobilization, an effect that was even greater following defoliation. I found evidence of the supply-driven nature of N, as stem N reserve levels decreased following budburst and the distance effect on remobilization disappeared after defoliation. It remains unclear however, whether C and/or N limitation contributed to the incomplete recovery of leaf mass following defoliation.

In the second study I found that under heterogeneous soil moisture conditions, saplings maintained gas exchange and aboveground growth similar to well-watered saplings and evaded the leaf and fine root shedding observed in the homogeneous full drought treatment. For saplings subjected to the heterogeneous soil moisture conditions, the portion of the root system that was water limited had no root dieback and increased NSC reserve concentrations, while the portion that was not resource limited added new roots (30% increase). My results suggest differential allocation of mass or reserves between above- and belowground organs, but also within a root system when resource availability is spatially variable. While the mechanisms and processes involved in these allocation patterns are not clear, these responses could be interpreted as adaptations and acclimations to preserve the functionality of the entire sapling and suggests that different portions of plant organs might respond autonomously to local condition