The role of bud protection and bark density in frost resistance of savanna trees

Abstract

Frost events occur with a significant frequency in savannas of the Southern Hemisphere, especially in the Cerrados of Brazil. One of the main strategies to deal with such events is to invest in thick and dense bark, which can insulate internal branch tissues and protect buds, essential to ensure resprouting if frost damage causes plant canopy die‐back. Such strategies may be fundamental to determine the persistence of savanna species in regions where low temperatures and frost events are recurrent. Here we describe bud protection and bark strategies of 53 woody species growing in typical savanna vegetation of central Brazil. In addition, we used an experimental approach exposing branches to 0 °C to measure temperature variation in internal branch tissue and test its relationship to bud protection and bark properties. We found that the majority of species (69%) showed medium to high bud protection against extreme temperatures; however, the degree of bud protection was not clearly related to bark properties, such as bark thickness and density. Bark density is a fundamental trait in determining protection against low temperatures (0 °C), since species with low bark density showed lower temperature variation in their internal branch tissues, independently of the bud protection degree. Bark properties and bud protection are two different (albeit related) strategies for the protection and persistence of savanna trees under extreme environmental temperatures and can explain ecological observations related to savanna tree responses after frost events.

Advanced materials design based on waste wood and bark

“…Trees in cold regions need to protect the living cells against intracellular ice formation. In very cold regions, this protection is mainly biochemical [ 69 , 70 ] while a physically protective role of the bark is possible in regions with more random and shorter frost events, as seen in savannahs [ 71 ]. Trees growing in fire-prone regions often possess fire-resistant barks which protect the underlying meristematic tissues against high temperatures [ 72 ], at the same time protection against mechanical damage may play an important role, as recently shown for giant sequoias growing in regions with regular rockfalls [ 73 ].…”

Stored water in the inner bark and sapwood: Atypical patterns of daily discharge and recharge, radial osmotic gradient and freezing resistance

“…Bark is involved in mechanical, defenses and physiological functions but a less known role of bark in the plant functioning it is the insulation capacity to mitigate the damaging effects of air subzero temperatures on more internal tissues (Arco Molina et al, 2016;Burke et al, 1976). The capacity of thermal insulation of the bark is derived from its morphology and composition (De Antonio et al, 2020;Jönsson et al, 2001). A thick layer of bark with a large amount of water with low thermal conductivity and high heat capacity surrounding the cambium and parenchyma cells within the sapwood can help to prevent freezing in more sensitive tissues.…”

Editor’s Highlight: Adaptive responses of tropical savanna trees to frost

“…Resprouting from aboveground buds depends on the preservation of the vascular tissues even if the plant undergoes top-kill during a frost event. In this regard, the structural properties of the bark are instrumental in protecting and insulating the vascular tissues during frost episodes for plants that resprout epicormically after top-kill (Bond and Midgley 2001;Clarke et al 2013;De Antonio et al 2020). The inner and the outer bark have different ecological functions and respond differently to fire, water stress and frost.…”

“…The inner and the outer bark have different ecological functions and respond differently to fire, water stress and frost. Water storage and transport are related to properties of the inner bark, which also affects thermal insulation of the vascular tissues, while variations in the outer bark, particularly outer bark thickness is strongly related to fire resistance (Rosell 2016;De Antonio et al 2020;Loram-Lourenço et al 2020). Deciduousness during the winter months, a trait that has been selected in woody clades that successfully colonized freezing-prone environments (Zanne et al 2014), would also reduce the risk of tree death by drought or fire in savannas subjected to cold and dry winters.…”