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.

Fig 1. Performance of Ghanaian tree seedlings in terms of (a) survival and (b) relative biomass growth rate (RGR) in response to drought and shade.

Shade treatments consisted of low light (5% of full sunlight, black bars) and high light (20% of full sunlight, grey bars). The water treatment consisted of a wet treatment (plants continuously watered for nine weeks) and dry treatment (water was withheld from seedlings for nine weeks). Means and standard error of the means are shown. Bars accompanied by a different letter are significantly different at P < 0.05 (ANOVA, Post hoc LSD test). n = 10 species.

Fig 2. Seedling responses to drought and shade in terms of allocation (left panels), tissue morphology (middle panels) and whole-plant efficiency (right panels) for leaf- (top panels), stem- (middle panels) and root (lower panels) tissues.

Shade consisted of low light (5% of full sunlight, black bars) and high light (20% of full sunlight, grey bars). The drought treatment consisted of a wet treatment (plants continuously watered) and dry treatment (water was withheld from plants for nine weeks). (a) leaf mass fraction (LMF), (b) specific leaf area (SLA), (c) leaf area ratio (LAR), (d) stem mass fraction (SMF), (e) specific stem length (SSL), (f) stem length per unit plant mass (SLPM), (g) root mass fraction (RMF), (h) specific root length (SRL), and (i) root length per unit plant mass (RLPM). Means and standard errors are shown. Bars accompanied by a different letter are significantly different (ANCOVA, Turkey’s test P<0.05). n = 10 species. For droughted plants no bars are shown for SLA and LAR, because the leaves were desiccated, and their leaf area could not be measured precisely.

Fig 3. Seedling leaf physiological traits: (a) stomatal conductance (g_s), (b) midday leaf water potential (ψ_mid) of plants from which water was withheld and plants that received daily watering for 9 weeks in 5% of full sunlight (black bars) and 20% of full sunlight (grey bars).

Bars are means and error bars are standard errors of the means.

Fig 4. Relationship between survival under stressful conditions (drought and high light) and (a) mean plasticity of nine traits, (b) Leaf mass fraction (LMF) plasticity.

Relationship between relative growth rate under optimal conditions (continuous watering and 20% of full sunlight) and (c) mean plasticity and (d) root mass fraction plasticity. Plasticity for each trait was calculated as maximum minus minimum mean trait values divided by maximum mean trait values across four treatment combinations. For each species, mean plasticity was calculated as the average plasticity of 9 traits. Regression line and coefficient of determination are shown. Ns = not significant, * = p ≤ 0.05, ** = p ≤ 0.01, n = 10 species.