Recovery following defoliation involves shifts in allocation that favour storage and reproduction over radial growth in black oak

Abstract

Summary Carbon allocation to growth, storage, reproduction and the effects of stress and disturbances on these processes remain poorly understood in trees. Non‐structural carbohydrates (NSC) are an important component of the carbon storage pool and are commonly used to assess whether tree growth under various conditions is carbon limited. Carbon limitation is generally assumed to result in reduced NSC levels and allocation, but changes in allocation priorities that favour NSC storage could also limit carbon available for growth. We defoliated six mature oak trees in June 2010 and explored how defoliation affects the relationship between radial growth and net NSC storage, estimated by the seasonal increase in NSC concentration, for several years. We interpret our results in the context of a conceptual model of allocation – developed here – to help understand how allocation changed following defoliation. We also monitored flower and acorn production in 2011, as changes in reproduction could also affect allocation to growth and storage. Defoliation reduced radial growth for 3 years. Stemwood starch levels and storage were also generally reduced, although not as consistently as growth. In contrast, root collar NSC storage was significantly greater in defoliated trees, allowing below‐ground stores to recover to control levels by November 2011. Defoliation had mixed effects on reproduction in 2011: production of second‐year acorns (initiated before defoliation) was not significantly affected, but there was complete cessation of flower production. Defoliation also changed the relationship between NSC allocation and growth in the 2 years following defoliation, indicating a shift in allocation favouring storage. According to our conceptual model, the positive relationship between growth and NSC allocation indicates that the allocation shifts occurred under carbon‐limiting conditions. We, therefore, propose that growth can be carbon limited by two different means: (i) a decrease in plant carbon availability and (ii) allocation shifts that reduce the priority of growth relative to other processes such as storage. Prioritizing storage over both growth and reproduction may be a strategy to protect against further disturbance or stress following storage depletion. Synthesis. Recovery following defoliation can involve substantial shifts in how trees allocate carbon between different functions, with carbohydrate storage and already initiated reproduction cycles being favoured relative to growth and new reproductive cycles.