Adaptive Traits of Savanna Vegetation

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Savanna plants display a suite of traits to cope with seasonal drought, low water and nutrient availability, and the impacts of regular fire and herbivory. Adaptive traits which aid in the survival of fire for woody plants include thick insulating bark, high wood moisture content, and significant resprouting capacity. Resprouting can occur via lignotubers and from other underground and stem basal tissues following the death of aerial stems. This enables recovery with minimal developmental costs. Vegetative reproduction from roots, rhizomes, or stolons is dominant in much of the savanna biome. Adaptations to low nutrient availability include root mycorrhizal associations, particularly of ectomychorrhizae. Savanna trees can rapidly translocate sequestered nutrients from the leaves to other tissues (e.g., bark) prior to leaf fall. Woody savanna plants often have thorns that restrict grazing, as well as chemical features, such as tannins making leaves less palatable. Savanna grasses also display morphological features, such as serrated edges, and chemical features, including tannins and silica bodies, to restrict grazing.

The herbaceous grass layer is dominated by grasses with a C4 photosynthetic pathway. This pathway enables high photosynthetic rates at high temperatures and irra-diance and low water availability. Most savanna trees and shrubs have the C3 photosynthetic pathway that has a higher efficiency under low light when compared to the C4 pathway, a characteristic which facilitates recruitment and establishment under shaded tree canopies. The growth of savanna plants tends to occur mostly during the wet season with senescence or dormancy in the fire prone dry season, a trait that facilitates persistence in unfavorable conditions. Annual herbaceous species persist via a soil seed bank, whereas aboveground parts of perennial herbaceous species die during dry periods, with dormant, regenerative buds protected within belowground rhizomes or by cataphylls. Some annual grass species use hygroscopically active awns and pointed calluses on their seed to work them into the soil, also protecting them from fire. Perennial herbaceous species require wet season rains to produce their first green shoots as carbohydrate storage from the previous wet season is limited. Rainfall stimulates germination of annual herbaceous and grass species and the early wet season is a period of rapid growth. Most herbaceous species flower in the wet season, although in contrast, many woody species flower in the dry season.

Woody species have evolved physiological and morphological mechanisms to either tolerate (evergreen habit) or avoid (deciduous habit) prolonged periods of water stress. Deep-rooting woody plants (usually evergreen) are able to access water resources throughout the year and provides them with their full photosynthetic capacity when favorable conditions occur. Deciduous species rehydrate stems prior to onset of wet season rains, which is then followed by leaf expansion to maximize photosynthetic activity during the wet season. Deciduousness and evergreeness represent extremes of physiological adaptations to survive the seasonal savanna climate. Evergreen species invest more resources in longer lived leaves, whereas deciduous species tend to support shorter-lived leaves with high leaf photosynthetic capacity. Deciduous species need to acquire enough nutrient and photosynthate to ensure persistence and reproduction during the wet season, whereas evergreen species tend to have slower growth rates but persist throughout the seasonal cycle. Evergreeness also allows opportunistic acquisition of resources when soil nutrients are severely limiting and the cost of producing new leaves to respond to change in soil moisture is prohibitive.

Although this section has described broad seasonal growth patterns, it is important to note that the world's savanna plants include a high diversity of species and life forms, with many distinct phenological patterns. All periods of the climatic cycle is favorable to certain vegetative or flowering phenophases in at least one group of species.

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