Leaf adaptations of evergreen and deciduous trees of semi‐arid and humid savannas on three continents

Abstract

Summary1. Drought stress selects for a suite of plant traits at root, stem and leaf level. Two strategies are proposed for trees growing in seasonally water-stressed environments: drought tolerance and drought avoidance. These are respectively associated with evergreen phenology, where plants retain their leaves throughout the year, and deciduous phenology, where plants drop their leaves during dry seasons. Evergreen species are thought to have leaf traits supporting lower photosynthesis and transpiration rates, in order to conserve water during dry periods. 2.We evaluated 18 morphological, chemical and physiological leaf traits of 51 abundant savanna tree species that differed in leaf habit (deciduous and evergreen), selected from two climate types (semi-arid and humid) in three continents (Australia, Africa and South America) (annual rainfall range: 500-1550 mm), and grown in a common garden experiment. We hypothesised that evergreen species have more conservative water use and differ more across climate types than deciduous species because evergreen species are forced to endure extended water deficits during dry seasons. 3.Trait shifts between semi-arid and humid savannas did not differ between evergreen and deciduous species. 4. Evergreen species had similar assimilation rates but lower photosynthetic water-use efficiency (PWUE) than deciduous species, possibly to extend their leaf lifespans by protecting their photosynthetic machinery from overheating through evaporative cooling. 5.Species of humid and semi-arid environments did not differ with respect to assimilation rate or PWUE, but semi-arid species did have smaller leaf sizes and greater leaf potassium and phosphorus concentrations. These traits may enable semi-arid species to maximize growth during episodes of favourable moisture availability.6. Species from the three continents differed in their leaf traits. These probably reflect the greater proportion of evergreen species in Australia as compared to the other continents and generally infertile soils in the South American sampling sites compared to the wider fertility range in the African sites.7. Synthesis: Water stress in savannas does not select for more conservative water use, but may select for rapid adjustment to prevailing water conditions and for heat avoidance mechanisms.

Potassium: a neglected nutrient in global change

“…However, the RE of K remains considerable and could be expected to be higher in dry ecosystems, compensating for the low capacity for K uptake in dry conditions (in comparison with plants at wetter sites as reported in some studies; see, e.g. Fayyaz et al, 2013;Tomlinson et al, 2013). Moreover, the capacity of plants to retain K in arid environments could be improved by lower leaching (Singer, 1989).…”

“…On the other hand, some studies positively related the capacity of plants to take up K to soil water availability (Fernandez et al, 2011;Ge et al, 2012). Tomlinson et al (2013), while comparing leaf traits of different plant species growing in wet and arid environments, observed that leaves of species adapted to arid sites are small with high K concentrations. Furthermore, it was observed that tree species at the driest sites, such as in mediterranean evergreen and dry tropical forests, have a higher capacity to change their seasonal internal allocation of K, with a higher allocation of K to leaves during summer (the driest season) than the species at wetter sites (Milla et al, 2005;Rivas-Ubach et al, 2012;Sardans et al, 2012a).…”

“…This effect is related to the role K plays in enhancing photosynthetic CO2 fixation, improving the transport of photosynthates into sink organs and inhibiting the transfer of photosynthetic electrons to O2, thereby reducing ROS production (Cakmak, 2005). Many field studies have thus observed correlations between K concentrations in plants and soils and mechanisms of drought avoidance (Nandwal et al, 1998;Arquero et al, 2006;Teixeira et al, 2008;Fayyaz et al, 2013;Tomlinson et al, 2013). Many physiological and metabolomic studies in garden and potted plants (Babita et al, 2010;Cuéllar et al, 2010) and in crops (Römheld & Kirkby, 2010;Waraich et al, 2011;Wang et al, 2013), and some studies across environmental gradients under field conditions (Kadar & Foldesi, 2001), have observed a strong link between K concentrations and the resistance and adaptation of plants to drought.…”

Assessing Landsat Fractional Ground-Cover Time Series Across Australia’s Arid Rangelands: Separating Grazing Impacts From Climate Variability

“…This is both a process of growth, senescence and accumulation of dry plant material and a dominance of the vegetation fraction. The later a result of plant adaptations of arid / semi-arid plant species that aim to lower photosynthesis and transpiration rates, to conserve water (Tomlinson et al, 2013). The first change point is identified as the upper growth point or peak of the NPV response.…”

“…An example LFGC image of Australia in figure 2 next illustrates each fractions colour representation. Arid vegetation has adapted to reduce water loss through reducing respiration and photosynthesis (Tomlinson et al, 2013). reported that traditional methods are limited in their ability to separate less photosynthetic vegetation from that of background soil.…”