Root grafts matter for inter-tree water exchange – a quantification of water translocation between root grafted mangrove trees using field data and model-based indications

Abstract

Background and Aims Trees interconnected through functional root grafts can exchange resources, but the effect of exchange on trees remains under debate. A mechanistic understanding of resources exchange via functional root grafts will help understand their ecological implications for tree water exchange for individual trees, groups of trees, and forest stands. Methods To identify the main patterns qualitatively describing the movement of sap between grafted trees, we reviewed available literature on root grafting in woody plants that focus on tree allometry and resource translocation via root grafts. We then extended the BETTINA model, which simulates mangrove (Avicennia germinans) tree growth on the individual tree scale, in order to synthesize the available empirical information. Using allometric data from a field study in mangrove stands, we simulated potential water exchange and analyzed movement patterns between grafted trees. Key Results In the simulations, relative water exchange ranged between -9.17 and 20.3 %, and was driven by gradients of water potential, i.e. differences in tree size and water availability. Moreover, the exchange of water through root grafts alters the water balance of trees and their feedback with the soil: grafted trees that receive water from their neighbors reduce their water uptake. Conclusions Our individual-tree modelling study is a first theoretical attempt to quantify root graft-mediated water exchange between trees. Our findings indicate that functional root grafts represent a vector of hydraulic redistribution, helping to maintain the water balance of grafted trees. This non-invasive approach can serve as a fundament for designing empirical studies to better understand the role of grafted root interaction networks on a broader scale.

Root‐centric β diversity reveals functional homogeneity while phylogenetic heterogeneity in a subtropical forest

“…The low functional turnover suggests that once some species are lost under environmental turbulence, the ecological roles they play can be compensated by other species, maintaining root network functioning (Fetzer et al, 2015). Therefore, a combined root network may increase the stability of soil structures, sustain nutrient cycling, and better cope with environmental stresses through nutrient sharing among species (Freschet et al, 2021; Wimmler et al, 2022). However, as functionally similar species might respond similarly to environmental changes, root networks with high functional redundancy may be less resilient to abiotic and biotic stress (e.g., drought or the outbreak of root pests; Wang et al, 2021).…”

Does natural root grafting make trees better competitors?

“…Negative and positive interactions reported between grafted trees support the idea that relationships between grafted trees are non‐neutral and can be summarized as: 1) facilitation: moderation of biotic and abiotic stress, enrichment of resources or increased access to resources, or 2) parasitism: resources are translocated from the dominant to suppressed trees improving their survival at the expense of the growth of dominant trees. In the case of parasitism, it would also be conceivable that larger trees would take advantage of smaller trees, as larger members of a root complex may be able to, at the expenses of the less vigorous trees, establish gradients that cause water and nutrients absorbed by the communal root system to move primarily to them (Bormann 1966, Wimmler et al 2022). Larger trees, with their larger crowns, will transpire more than suppressed trees, creating a stronger sink for water (Bader and Leuzinger 2019, Wimmler et al 2022).…”

“…In the case of parasitism, it would also be conceivable that larger trees would take advantage of smaller trees, as larger members of a root complex may be able to, at the expenses of the less vigorous trees, establish gradients that cause water and nutrients absorbed by the communal root system to move primarily to them (Bormann 1966, Wimmler et al 2022). Larger trees, with their larger crowns, will transpire more than suppressed trees, creating a stronger sink for water (Bader and Leuzinger 2019, Wimmler et al 2022). However, we don't know if grafted trees are differently affected by root grafting according to their size within the union.…”