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Harvard Forest Research Project 2023

• Title: Management of Non-Structural Carbohydrates in Canopy Trees; Bullard Fellowship
• Principal investigator: Maciej Zwieniecki (mzwienie@ucdavis.edu)
• Institution: University of California (All Campuses)
• Primary contact: Maciej Zwieniecki (mzwienie@ucdavis.edu)
• Team members:
• Abstract:

  Research:
  Recent climatic shifts in concert with anthropogenic pressures and their interactions are changing the forest landscape; accelerating fragmentation, causing range and age shifts, and inciting widespread tree mortality (Pugh et al. 2019). Looming changes in climate regimes are expected to further magnify the effects of biotic and abiotic disturbances consequently increasing the incidence of forest die-off events. Such events can fundamentally alter forest compositional and functional diversity and reverberate across ecosystems profoundly hampering or exacerbating climate change. Therefore, understanding the mechanisms that promote forest resilience and survival is critical.
  Burgeoning evidence points to a strong link between non-structural carbohydrates (NSC) and trees’ capacity to cope with and recover from stress (McDowell 2011). NSC are primarily composed of starch and soluble carbohydrates and are a plant’s main carbon and energy source and as such are fundamental to plant function. Plants utilize NSC to sustain vital processes such as growth, metabolism, defense, and reproduction. In particular, stored carbohydrates buffer periods of asynchrony, when photosynthetic carbon cannot meet demand, which regularly occurs diurnally (Tixier et al. 2018) but can also occur seasonally, in response to abiotic/biotic stress or disturbance (Tixier et al. 2019). Deciduous trees, for example, depend exclusively on NSC reserves to not only meet the maintenance costs of prolonged dormancy but to support bloom and leaf flush before net-positive leaf photosynthetic activity. During drought, transpirational water loss is limited by stomatal closure at the expense of reduced photosynthetic assimilation consequently trees must draw upon their NSC stores to satisfy continued metabolic demands. Reserves are also essential in reconciling this supply/demand disparity within the canopy shade where photosynthetic activity is often constrained by light availability. Thus, to survive and persist plants must maintain a delicate balance between budgeting for immediate and future needs all the while maintaining enough to combat abrupt disturbances.
  In a forest ecosystem, individuals interact within a complex framework comprised of co-existing species representing diverse life-history strategies and age-cohorts, that spans the vertical strata of the canopy, from the ground layer to crown closure. Furthermore, conditions such as light quality and quantity, water availability, and microclimatic gradients can vary vastly and continuously within the vertical canopy structure (Nakamura et al. 2017). Therefore, to survive and dominate within the complex forest structure, plants must optimize the formation and utilization of NSC reserves not only in response to climatic variation but must also contend with local competition for resources. Thus, one can expect that each species may represent a different evolved strategy in reserve management that reflects their habit, position in forest canopy, and age etc. Some species may assume a conservative approach preferring slow growth, marked by the maintenance of high levels of NSC reserves, others might be risk takers where low NSC levels allow for fast growth in a competitive environment. Despite the importance of NSC stores for growth, survival, and persistence little is known how NSC reserves are managed by different species within the structure of the forest canopy and how climatic conditions alter these strategies. To our knowledge there are no studies that track long-term NSC management strategies in relation to plant age (juvenile-mature plant), ecosystem placement (canopy position), or geographical location (within species climatic range). Understanding how diverse species regulate and allocate carbohydrates can help elucidate species susceptibility to disturbances and how these factors inform community structure.
  Moreover, plant NSC management may be dynamic not only across space but also across time. In temperate species, typical yearly patterns of NSC storage are sigmoidal with minimum reserves being encountered in the summer (June-July; Northern hemisphere) and maximum prior to senescence (October)(Tixier et al. 2020). Studies have shown inter-species variation in both the total amount of stored NSC and in the magnitude of change along the year possibly reflecting differential carbohydrate management strategies. For instance, large swings in NSC concentrations potentially representing risky strategies while less pronounced swings more conservative ones (Fig. 1; Davidson et al. 2021; Furze et al. 2019). In the forest, many trees start their life under canopy cover – some waiting for local disturbance to enter the canopy while other species spend their entire life under the canopy cover. While in ’purgatory’, shade intolerant species might undertake the risky strategy of allocating all available resources for growth, storing only the necessary levels required to survive dormancy. This strategy would promote entering the high-light environment of the canopy as fast as possible increasing competitiveness albeit potentially risking exposure to unusual abiotic/biotic stressors that could result in death. Nominal levels of NSC can jeopardize survival by reducing the capacity to recover, for instance, from water stress (Secchi, Pagliarani, and Zwieniecki 2017), or altogether delimit recovery in the case of infestation (Barker Plotkin, Blumstein, and Laflower 2021). Penetrating the high-light canopy, however, can facilitate a switch to a more conservative strategy (redirecting higher % of photosynthates to NSC storage). In contrast, shade tolerant species may accumulate NSC reserves while actively suspending growth. Such conservative strategies might reduce their growth-related competitiveness but increase resilience to stress.
  Resolving how NSC reserves are managed demands a fine scale of temporal and spatial resolution which entails broad and extensive sampling and analysis. However, the significant time and prohibitive costs associated with running NSC analyses pose some of the greatest limitations for carbohydrate studies. Jessica Orozco, a PhD student in my lab, has developed and began implementing a high-throughput protocol that circumvents the current limitations. Using near-infrared (NIR) technology and machine learning we have cut the cost and time down to a fraction of what has until now been necessary. This advancement has significantly increased our capacity to analyze several thousand samples per month at minimal cost. Such capacity provides the potential to study novel complex questions related to NSC storage in perennial plants that require analysis of hundreds if not thousands of samples. Currently in my lab we apply this method to study orchard ecosystems (http://zlab-carb-observatory.herokuapp.com). However, I would like to take the opportunity of the Bullard Fellowship to expand my research program to study natural ecosystems and build a foundation for the development of a larger future study.