The Effects of Drought and Shade on the Performance, Morphology and Physiology of Ghanaian Tree Species

Abstract

In tropical forests light and water availability are the most important factors for seedling growth and survival but an increasing frequency of drought may affect tree regeneration. One central question is whether drought and shade have interactive effects on seedling growth and survival. Here, we present results of a greenhouse experiment, in which seedlings of 10 Ghanaian tree species were exposed to combinations of strong seasonal drought (continuous watering versus withholding water for nine weeks) and shade (5% irradiance versus 20% irradiance). We evaluated the effects of drought and shade on seedling survival and growth and plasticity of 11 underlying traits related to biomass allocation, morphology and physiology. Seedling survival under dry conditions was higher in shade than in high light, thus providing support for the “facilitation hypothesis” that shade enhances plant performance through improved microclimatic conditions, and rejecting the trade-off hypothesis that drought should have stronger impact in shade because of reduced root investment. Shaded plants had low biomass fraction in roots, in line with the trade-off hypothesis, but they compensated for this with a higher specific root length (i.e., root length per unit root mass), resulting in a similar root length per plant mass and, hence, similar water uptake capacity as high-light plants. The majority (60%) of traits studied responded independently to drought and shade, indicating that within species shade- and drought tolerances are not in trade-off, but largely uncoupled. When individual species responses were analysed, then for most of the traits only one to three species showed significant interactive effects between drought and shade. The uncoupled response of most species to drought and shade should provide ample opportunity for niche differentiation and species coexistence under a range of water and light conditions. Overall our greenhouse results suggest that, in the absence of root competition shaded tropical forest tree seedlings may be able to survive prolonged drought.

Light acts as a stressor and influences abiotic and biotic stress responses in plants

“…This would imply that under these conditions, plants have a higher water usage; however, in response to drought, stomata are rapidly closed through an increased sensitivity to ABA (Gonzalez et al, 2012). Under shade conditions, the survival of Ghanaian trees is higher than in high light suggesting that shade enhances plant performance under drought stress conditions (Amissah, Mohren, Kyereh, & Poorter, 2015). The activity of osmoprotectants and antioxidants was also increased under shade conditions suggesting that pre‐treatment with shade might help to overcome the drastic effects of drought in soybeans in dense cropping systems (Asghar et al, 2020).…”

Changes in the Shadow: The Shifting Role of Shaded Leaves in Global Carbon and Water Cycles Under Climate Change

“…Radiation, ambient air temperature, and transpiration together determine leaf temperature, which in turn affects the leaf photosynthetic capacity through altered carboxylation rate. Systemic change in leaf temperature can significantly affect the C assimilation and transpiration (Amissah et al, 2015;Martins et al, 2014;Quero et al, 2006). Systemic change in leaf temperature can significantly affect the C assimilation and transpiration (Amissah et al, 2015;Martins et al, 2014;Quero et al, 2006).…”

“…The sunlit portion of a canopy receives both direct and diffuse radiation, while shaded leaves receive only diffuse radiation for photosynthesis; therefore, the temperature of shaded leaves is usually a few degrees lower than that of sunlit leaves (Spayd et al, 2002). Systemic change in leaf temperature can significantly affect the C assimilation and transpiration (Amissah et al, 2015;Martins et al, 2014;Quero et al, 2006). For example, shading a canopy from the Sun was found to be effective for photoinhibition mitigation and water use efficiency (WUE) improvement (Alarcon et al, 2006;Montanaro et al, 2009;Sofo et al, 2009).…”

Effects of Drought, Pest Pressure and Light Availability on Seedling Establishment and Growth: Their Role for Distribution of Tree Species across a Tropical Rainfall Gradient

“…Higher light requirements of dry forest species have been hypothesized as a consequence of a trade-off between shade and drought tolerance [ 18 , 20 ], based mainly on a trade-off between biomass allocation to roots, which would confer drought tolerance, and allocation to leaves, which confers shade tolerance. However, there is no conclusive support for a trade-off between drought and shade tolerance in tropical forest plants [ 8 , 21 – 23 ], as traits conferring drought or shade tolerance are complex, not necessarily related and can be uncoupled. Higher light requirements of dry forest species have also been hypothesized due to their evolution in higher light environments in dry forests [ 15 , 20 ].…”