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A diversity of Belowground Bud Banks (BBB) for resprouting

January 15th, 2018 Leave a comment Go to comments

Many plants are able to survive recurrent disturbance by resprouting from a bud bank. In fire prone ecosystems, plants must protect their buds from fire heat or perish. One way to protect them is by growing a thick insulating bark or sink the buds in the stem [1,2]. Another way is to locate the buds below ground, as soil is an excellent heat insulator (belowground bud bank or BBB). In fire-prone ecosystems, there is a diversity of ways by which plants successfully conceal their buds below ground that enable them to survive and resprout vigorously after fire [3]. There are at least six locations where belowground buds are stored [3]: roots, root crown, rhizomes, woody burls, fleshy swellings and belowground caudexes. These support many morphologically distinct organs (figure below). Considering their history and function, these organs may be divided into three groups:

(a) Those that originated in the early history of plants and that currently are widespread; they act as a resprouting source after a range of disturbances, not just fire. These include bud-bearing roots and root crowns.

(b) Those that also originated early and have spread mainly among ferns and monocots; they are often tied to seasonal stresses and have been highly successful under recurrent fire regimes. Theses include non-woody rhizomes and a wide range of fleshy underground swellings. They are characteristic of the geophyte growth forms occurring in many ecosystems, often tied to seasonal stresses; they have been highly successful under recurrent fire regimes.

(c) And those that originated later in history and are strongly tied to fire-prone ecosystems. These are woody rhizomes, lignotubers and xylopodia. They are characteristic of the geoxyle growth form.

Recognizing the diversity of BBBs is the starting point for understanding the many evolutionary pathways available for responding to severe recurrent disturbances.

Figure: Stylized diagrams of 16 belowground bud bank structures that enable plants to resprout postfire (highlighted in red). Broken horizontal line indicates position of soil surface. Structures characterized by woody tissues, in pink; fleshy tissues, in blue; and neither woody nor fleshy, in orange (usually highly sclerified primary tissues, fibrous or ‘wiry’). Shoots highlighted in apple green: stems with leaves, branched; leaves only, unbranched. Roots highlighted in olive green: triangular-shaped roots indicate a primary system, those arising directly from the bud-storing structures are adventitious. Drawings from [3]. From top left to bottom right:

· Xylopodium (in red) joined to tuberous root (in blue); Lignotuber; Root Crown; Woody Rhizome, here arising from a burl
· Bud-bearing lateral Root arising (here) from a burl (the root is not necessarily woody); Taproot Tuber; Bulb; Corm, with previous year’s corm still present
· Stem Tuber; Non-woody Fleshy Rhizome; Rhizophore (note buds are only supported by the oldest rhizophores); Adventitious Root Tuber
· Non-woody fibrous Rhizome with a monopodial arrangement leading to expansive clone; Non-woody fibrous Rhizome with sympodial arrangement leading to a caespitose habit; Stolons that produce new ramets postfire (note that it is not a BBB); Belowground Caudex

For details and a full description of each structure, see reference [3].

References
[1] Pausas J.G. 2017. Bark thickness and fire regime: another twist. New Phytologist 213: 13-15. [doi | wiley | pdf | post1, post2]

[2] Pausas J.G. & Keeley J.E. 2017. Epicormic resprouting in fire-prone ecosystems. Trends in Plant Science 22: 1008-1015. [doi | pdf | post ]

[3] Pausas J.G., Lamont B.B., Paula S., Appezzato-da-Glória B., Fidelis A. 2018. Unearthing belowground bud banks in fire-prone ecosystems. New Phytologist  [doi | pdf | suppl | BBB database]

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