Weng, E. | Search bioRxiv

bioRxiv2025

Impact of sucrose sinks on phloem transport

The movement of photosynthates within plants is a focus in plant physiology, ecohydrology, and earth systems modeling. The phloem, one of the plants hydraulic systems, facilitates this transport. It is believed to be optimized for efficient photosynthates transport, notably sucrose. This has implications ranging from local impacts on plant survival during drought to ecosystem-scale effects on carbon and water cycling. Most models for phloem transport rely on the pressure-flow hypothesis, where sucrose is loaded in leaves, drawing water from the xylem through osmosis, generating pressure gradients for transport. Experimental challenges in measuring sugar fluxes have led to reliance on theoretical models, though discrepancies exist, especially for long-distance transport. Criticism of the pressure-flow hypothesis notes low hydraulic conductance in sieve tubes, possibly hindering sucrose transport in taller plants. This research explores osmotically driven flows through the development of a new one-dimensional numerical model that includes sources from photosynthesis and sinks towards the stem and roots. The model also incorporates a concentration-dependent viscosity and the xylem water potential. It shows that different allocation schemes of sucrose sinks towards the stem of the plant influence the speed at which sucrose is transported. These findings provide insight into how carbon allocation along the phloem may have evolved to enhance the efficiency of transporting soluble compounds in the phloem. HighlightsO_LISucrose sinks along the phloem influence mass flux beyond simple reaction-like consumption dynamics C_LIO_LIMass flux is modulated by the sucrose allocation profile along the tree stem C_LIO_LIObservations of xylem-phloem water exchange can provide insights into the physical sucrose sink profile C_LI

ecology↗

bioRxiv2022

Tree growth enhancement drives a persistent biomass gain in unmanaged temperate forests

While enhanced tree growth over the last decades has been reported in forests across the globe, it remains unclear whether it drives persistent biomass increases of the stands, particularly in mature forests. Enhanced tree growth and stand-level biomass are often linked with a simultaneous increase in density-driven mortality and a reduction in tree longevity. Identifying empirical evidence regarding the balance between these processes is challenging due to the confounding effects of stand history, management, and environmental changes. Here, we investigate the link between growth and biomass via the shift in the negative relationship between average tree size and stand density (tree number). We find increasing stand density for a given tree size in unmanaged closed-canopy forests in Switzerland over the past six decades and a positive relationship between growth and stand density - qualitatively consistent with simulations by a mechanistic, cohort-resolving ecosystem model (LM3-PPA). Model simulations show that, in the absence of other disturbances, enhanced growth persistently increases biomass stocks despite simultaneous decreases in carbon residence time and tree longevity, independent of assumptions about the drivers of tree mortality. However, the magnitude of simulated changes critically depends on the shape of the mortality parameterizations. Our analyses reconcile reports of growth-induced reductions of tree longevity with model predictions of persistent biomass increases, and with our finding of a trend towards denser forests in response to growth - also in mature stands.

ecology↗