How Trees Use Stored Nutrients to Regrow Leaves After Damage

Greg Howard
29th January, 2024

How Trees Use Stored Nutrients to Regrow Leaves After Damage
Forests face increasing stress from disturbances like drought and defoliation, impacting their ability to function and survive. A key factor in how trees respond to these stresses is their reserves of carbon (C) and nitrogen (N) – essentially, the stored energy and building blocks they use when immediate resources are limited. Understanding how trees manage and allocate these reserves is crucial for predicting forest health in a changing climate. Researchers at the University of Alberta[1] investigated how aspen trees regulate and distribute these vital resources, focusing on whether the distance between where reserves are stored and where they are needed influences the process. The study centered on understanding how aspen trees utilize stored C and N during spring growth and after experiencing defoliation – the loss of leaves. Trees draw on reserves to fuel new leaf growth in the spring, and to recover after damage. However, it’s unclear exactly where these reserves come from, and whether trees prioritize certain tissues when allocating them. Previous research has highlighted the importance of both hydraulic failure and carbon starvation in tree mortality during drought[2], and demonstrated that even seemingly recoverable trees can experience long-term consequences from carbon stress[3]. This new study builds on this understanding by examining the mechanics of reserve allocation. To track the movement of C and N, the researchers used stable isotopes – non-radioactive forms of these elements that can be traced within a plant. They created aspen saplings with specifically labeled reserves in either the roots or stems using these isotopes (13C for carbon, 15N for nitrogen). These labeled parts were then grafted onto either labeled or unlabeled rootstocks, allowing the researchers to follow the path of C and N from source to sink. A ‘sink’ is a tissue needing resources (like growing leaves), and a ‘source’ is where the resources are stored (like roots or stems). The study revealed that during spring leaf flush, reserves from the roots were preferentially allocated to leaves closer to the root source – lower canopy leaves received more than upper canopy leaves. This suggests that distance plays a role in how efficiently reserves are delivered. However, the proportion of root-derived carbon allocated to leaves decreased as the spring progressed, indicating that trees increasingly relied on newly produced carbon through photosynthesis as the season advanced. Following defoliation, the trees re-flushed leaves, but these new leaves showed a different pattern of reserve use. While they imported the same proportion of nitrogen from the roots as the initial spring leaves, they imported significantly less carbon. This suggests that after defoliation, aspen trees prioritize current photosynthesis over relying on stored carbon for regrowth. This is supported by the observation that re-flushed canopies recovered only about 31% of their initial leaf mass. Importantly, the roots still held substantial starch reserves, indicating the tree was actively maintaining these reserves even while limiting carbon allocation to the canopy. This finding is significant because it demonstrates that aspen trees don’t simply draw down all available reserves during stress. Instead, they appear to prioritize maintaining root reserves, potentially for long-term survival, even if it means slower canopy recovery. This aligns with observations that multi-year consequences of drought can be important for forest responses[3], and suggests that the ability to store and protect reserves is a critical factor in a tree’s resilience. Furthermore, the study highlights the complex interplay between carbon and nitrogen allocation, and how this can shift depending on the type of stress experienced. The study also touches upon the seasonal cycling of nitrogen within trees, a process where nitrogen is moved from leaves to stems for storage during the dormant season[4]. While this study didn’t directly investigate the mechanisms of nitrogen transport, it reinforces the importance of understanding how trees manage this crucial nutrient, particularly in the context of disturbances like defoliation.

BiochemEcologyPlant Science

References

Main Study

1) Tracing carbon and nitrogen reserve remobilization during spring leaf flush and growth following defoliation.

Published 28th January, 2024

https://doi.org/10.1093/treephys/tpae015

Related Studies

2) A multi-species synthesis of physiological mechanisms in drought-induced tree mortality.

https://doi.org/10.1038/s41559-017-0248-x

3) Infestation and hydraulic consequences of induced carbon starvation.

https://doi.org/10.1104/pp.112.198424

4) Seasonal nitrogen cycling in temperate trees: Transport and regulatory mechanisms are key missing links.

https://doi.org/10.1016/j.plantsci.2018.02.021


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