Leaf life span spectrum of tropical woody seedlings: effects of light and ontogeny and consequences for survival

Abstract

Background and aims: Leaf life span is widely recognized as a key life history trait associated with herbivory resistance, but rigorous comparative data are rare for seedlings. The goal of this study was to examine how light environment affects leaf life span, and how ontogenetic development during the first year may influence leaf fracture toughness, lamina density and stem density that are relevant for herbivory resistance, leaf life span and seedling survival.

Methods: Data from three experiments encompassing 104 neotropical woody species were combined. Leaf life span, lamina and vein fracture toughness, leaf and stem tissue density and seedling survival were quantified for the first-year seedlings at standardized ontogenetic stages in shade houses and common gardens established in gaps and shaded understorey in a moist tropical forest in Panama. Mortality of naturally recruited seedlings till 1 year later was quantified in 800 1-m² plots from 1994 to 2011.

Key results: Median leaf life span ranged widely among species, always greater in shade (ranging from 151 to >1790 d in the understorey and shade houses) than in gaps (115-867 d), but with strong correlation between gaps and shade. Leaf and stem tissue density increased with seedling age, whereas leaf fracture toughness showed only a weak increase. All these traits were positively correlated with leaf life span. Leaf life span and stem density were negatively correlated with seedling mortality in shade, while gap mortality showed no correlation with these traits.

Conclusions: The wide spectrum of leaf life span and associated functional traits reflects variation in shade tolerance of first-year seedlings among coexisting trees, shrubs and lianas in this neotropical forest. High leaf tissue density is important in enhancing leaf toughness, a known physical defence, and leaf life span. Both seedling leaf life span and stem density should be considered as key functional traits that contribute to seedling survival in tropical forest understoreys.

Keywords: Fracture toughness; herbivory; leaf density; leaf life span; leaf toughness; light environment; ontogenetic changes; phenotypic plasticity; seedling survival; stem density; tropical lianas; tropical trees.

Fig. 1.

Scatter plots for (A) log₁₀(median leaf life span in days) in gaps and shade, and (B) log₁₀(median leaf life span in shade) and log₁₀(lamina fracture toughness, J m⁻²). Points represent species means. The ovals represent 95 % confidence ellipses. In (A) points above the diagonal indicate a greater leaf life span in shade than in gaps (paired t-test, P < 0·001). Free-standing woody species and lianas are as indicated in the key. Standard major axis regressions indicate that the slope was not significantly different from 1·0. See Table 2 for sample size and correlation coefficients.

Fig. 2.

Scatter plots for stem density (g cm⁻³) between ontogenetic stages of seedlings (first-leaf stage, 3-months later, 1-year later, and 2- to 3-year-old seedlings). Points represent species means. Free-standing woody species and lianas are as indicated in the key. The ovals represent 95 % confidence ellipses. Points above the diagonal indicate greater stem density at the later ontogenetic stage in each panel (paired t-test, P < 0·001 in all pair-wise comparisons). Standard major axis regressions indicate that the slope was not significantly different from 1·0, but the intercept was significantly greater than zero, except for first-leaf versus 3-months and 1-year versus 2–4 years.

Fig. 3.

Relationships of the mortality rate at early seedling stage (0–3 months) in the shade common gardens with (A) mortality rate of new recruits in the field for 1 year (n = 56 spp., r² = 0·213, P < 0·001) and with (B) mortality rate of 1- to 10-cm-diameter saplings (n = 47 spp., r² = 0·132, P = 0·012). Points represent free-standing species or lianas, as indicated in the key. The ovals represent 95 % confidence ellipses. The standard major axis regression slopes were significantly >0·0 and <1·0, meaning mortality reduction with ontogeny was greater for species with high early mortality.

Fig. 4.

Scatter plots of seedling mortality rates, (A, B) 0–3 months mortality % per day in shade common gardens and (C, D) mortality % per day of naturally recruited species in 800 1-m² plots till 1 year later, each plotted against (A, C) log₁₀(median leaf life span of shade-grown plants) and (B, D) log₁₀(stem density, g cm⁻³) at the first-leaf stage. Points represent free-standing species or lianas, as indicated in the key, and ovals represent 95 % confidence ellipses. See Table 2 for correlation statistics and Supplementary Data Table S4 for PIC correlations.