Background

Root grafts are unions of roots, that can belong either to one or more individuals. When such unions include xylem, phloem and cambium tissues, grafts can be functional, this means they can exchange water and nutrients (Graham and Bormann, 1966). Natural root grafts have been thought to be of random occurrence with no major ecological implications, or as a risk due to the increased potential they pose for serving as vectors of pathogen transmission (Hessenburg & Hansen, 1986; Reynolds & Boomerang, 1982; Eis 1972).

Although Bormann (1966), and Bormann & Graham (1966) first suggested an adaptive advantage of grafting, this hypothesis had little impact at the time, and the only widely recognized advantage was an increased mechanical stability, particularly for forest trees growing in shallow or water logged soils (Keeley 1988, Lohele 1990). Recent studies in temperate forests, however, have been able to prove a positive impact of root grafting, and although it comes at a cost (e.g. reduced growth rates during graft formation), once a functional graft between two trees is fully formed, it can promote growth (Tarroux & DesRochers, 2011), increase survival rates to budworm outbreaks (Salomón et al. 2016), and support shaded trees through the translocation of carbohydrates (Fraser et al. 2006).

As our understanding of root-graft related interactions increases, the dynamics of tree network formation, and mechanisms of cooperation remain poorly understood. The limitations to research posed by a buried biomass can be tackled by focusing in swamp environments, where root systems are shallow and sediments soft and easy to work with (Vovides et al. 2016).

While root grafts in mangrove forests are considered of common occurrence, no studies have yet been conducted to understand their frequency nor the ecological implications they pose to forest community dynamics. Mangroves grow in water logged and salty environments, salt is an stress factor that limits water uptake, and it is possible that root graft networks contribute to the mitigation of salt stress by redistributing water at the forest scale.

Preliminary studies on mangrove root grafts

In 2015, our group started exploring anchoring root systems in the black mangrove (Avicennia germinans), from which a non-invasive methodology to detect and measure anchoring woody root systems was developed (Vovides et al. 2016). During this study we became aware of a high frequency of root grafts between black mangrove trees, and with the help of two students, we started mapping the networks on a few permanent plots as part of an exploratory study on the frequency of intraspecific root grafts.

Students who collaborated in the explorations 2017

Falk Schrewe, Technische Universität Dresden (Bachelor degree)

Thesis Title: Differences in the frequency of root grafting of Avicennia germinans L. in two neighbouring environments

David Götz, Technische Universität Dresden (Master Degree)

Thesis Title: Influence of root grafts on decreasing water-stress in black mangrove (Avicennia germinans)

References

Bormann (1966). The structure, function, and ecological significance of root grafts in Pinus strobus L. Ecological monographs 36: 1-26

Eis (1972) Root grafts and their silvicultural impli- cations. Canadian Journal of Forest Research, 2, 111-120.

Fraser et al. (2006) Carbohydrate transfer through root grafts to support shaded trees. Tree pysiology 26: 1019-23

Graham and Bormann (1966). Natural root grafts. The Botanical Review 32: 255-292

Hessenburg & Hansen (1986). Mechanisms of intertree transmission of Ceratocystis wageneri in young Douglas-fir. Canadian Journal of Forest Research 16: 1250-1254

Keeley (1988) population variation in root grafting and an hypothesys. Oikos 52: 364-366

Lohele (1990) Adaptive significance of root grafting in trees. Functional Ecology 4:268-271

Reynolds & Boomerang (1982) Estimating probability of intertree root contact in second-growth Douglas-fir. Canadian Journal of Forest Research, 12, 493-498.

Salomón et al. (2016) Natural root grafting in Picea mariana to cope with spruce budworm outbreaks. Canadian Journal of Forest Research. 46: 1059-1066

Tarroux & DesRochers (2011) Effect of natural root grafting on growth response of jack pine (Pinus banksiana; Pinaceae). American Journal of Botany 98: 967-74

Vovides et al. (2016) A simple and cost-effective method for cable root detection and extension measurement in estuary wetland forests. Estuarine Coastal and Shelf Science. 183: 117-122

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