Non-structural carbohydrates and leaf ecophysiology in tropical and temperate forests

Increasing atmospheric CO2 concentration has led to an increment in forest biomass because of both an inhibition in photorespiration rates, and an increase in photosynthesis main substrate, atmospheric CO2. Whether this trend can be sustained by biomes, thus helping to regulate anthropogenic CO2 emissions, or whether this trend will reach a threshold as soil nutrient and soil moisture limitations become controlling factors that limit growth, remains unclear. Non-structural carbohydrates (NSC) have proven to play a major role in downregulating leaf gas exchange by intervening in the Calvin-Benson-Bassham cycle. When sink activity in the plant becomes limited, an increase in NSC concentration in the phloem occurs at a sink level, thus jeopardising the source-sink gradient needed for carbohydrates mass-flow across the phloem. Photoassimilate translocation is hampered, thus accumulating in the blade, and downregulating plant leaf gas exchange. Empirical evidence of NSC downregulation is needed to be integrated with Dynamic Global Vegetation Models in order to improve land-atmosphere exchange predications under climate change scenarios.
The aim of this thesis is to improve the understanding of the role of NSCs on ecosystem productivity. From empirical observations at a leaf level, to modelling at a plot level, this thesis explores leaf NSC concentration across species, sites and time, and how does it affect photosynthesis. I collected data from contrasting sites: a tropical drought gradient in Ghana, encompassing a dry deciduous forest, a moist deciduous transition forest, and a wet evergreen forest, and an ancient temperate woodland in the UK.
In the first empirical chapter of this thesis (Chapter 3), I explore the effect of NSC on net photosynthesis at a leaf level. I added sucrose to leaves and studied the effect on photosynthesis, before and after treatment, in six species (three tropical evergreen species, and three temperate deciduous European species). I also correlated the observed decrease in photosynthesis with key leaf traits related to fast translocation of resources from the leaf to the plant, such as investment in leaf vein infrastructure, petiole diameter, and specific leaf area. These key traits showed a negative relation with photosynthetic downregulation mediated by NSC, meaning that species on the fast side of the leaf economics spectrum would benefit more from increasing atmospheric CO2 concentrations.
In the second empirical chapter (Chapter 4), I develop an open source tool to improve the time of analysis of pressure-volume curves, used to determine leaf hydraulic traits. The tool also presents an add-on which allows to uniform the choice between different users when discerning points between the turgor loss region and the linear region. When emulating different criteria for the selection of the points belonging to each one of these two regions, derived leaf hydraulic traits remain fairly constant. Altogether, the tool reduces the time of curves analysis from 10-15 minutes, to less than 1 minute, and standardises user’s choice when determining points belonging to the turgor loss region or linear region of the curve.
Chapter 5 looks at the seasonal distribution of leaf osmotic potential at full turgor (a hydraulic trait derived from the open source tool for pressure-volume curves presented in Chapter 4) across a tropical drought gradient in Ghana. Leaf osmotic adjustment, defined as the capacity of the leaf to accumulate solutes in order to lower its water potential when facing water stress without compromising cell turgor, is a key acclimation mechanism to maintain physiological functioning under low soil moisture. In aerial organs, such as leaves, osmotic adjustment is heavily dependent on the accumulation of NSC. Whether NSC accumulation in the leaf is the result of a passive process resulting from the lack of sink demand given water stress, or is an active mechanism of the plant to cope with water shortage, has been debated. I studied seasonal osmotic adjustment across three contrasting sites across the gradient. Osmotic adjustment was correlated with key leaf traits that provide for fast NSC translocation and avoid photosynthetic downregulation mediated by photoassimilates (from results obtained from Chapter 3). Finally, I found that leaf photosynthesis at light saturation was correlated with leaf osmotic potential at full turgor. The evidence found was not enough to sustain the hypothesis that increase in leaf NSC concentration observed during the drought season is solely associated with a decrease in sink activity. The accumulation of solutes in the leaf remains as an active strategy of plant acclimation to water stress.
In the last empirical chapter of this thesis (Chapter 6), I try to predict leaf NSC concentration using leaf reflectance in the 400 to 2500 nm spectrum obtained with a field leaf spectrophotometer, thus leading to an opportunity to monitor leaf NSC dynamics in vivo in the field. Using several leaves from four temperate European deciduous species, I looked at correlating leaf optical footprint with leaf total NSC, and its fractions (monosaccharides, sucrose, soluble and non-soluble fractions). The relationships found were poor, except for Fagus sylvatica. Previous research that found a positive relationship between leaf NSC concentration, and leaf optical footprint obtained with a leaf field spectrophotometer, might be confounded by the strong correlations between leaf NSC concentration and other, more directly detectable, leaf traits such as leaf nitrogen concentration and specific leaf area.
This thesis demonstrates the importance of the role of NSC in leaf and tree physiology. The upscaling of these mechanisms would pave the way to a new generation of Dynamic Global Vegetation Models, with a better understanding of the feedbacks between increasing CO2 concentration and terrestrial ecosystem productivity.

Keywords: Non-structural carbohydrates
Climate Change
Ecophysiology
Leaf gas exchange
Leaf hydraulics
Leaf spectroscopy
Advisor: Malhi, Y