Performance of tropical forest seedlings under shade and drought: an interspecific trade-off in demographic responses

Abstract

Seedlings in moist tropical forests must cope with deep shade and seasonal drought. However, the interspecific relationship between seedling performance in shade and drought remains unsettled. We quantified spatiotemporal variation in shade and drought in the seasonal moist tropical forest on Barro Colorado Island (BCI), Panama, and estimated responses of naturally regenerating seedlings as the slope of the relationship between performance and shade or drought intensity. Our performance metrics were relative height growth and first-year survival. We investigated the relationship between shade and drought responses for up to 63 species. There was an interspecific trade-off in species responses to shade versus species responses to dry season intensity; species that performed worse in the shade did not suffer during severe dry seasons and vice versa. This trade-off emerged in part from the absence of species that performed particularly well or poorly in both drought and shade. If drought stress in tropical forests increases with climate change and as solar radiation is higher during droughts, the trade-off may reinforce a shift towards species that resist drought but perform poorly in the shade by releasing them from deep shade.

Figure 1

Relationship between observed and fitted relative growth rate (RGR, upper panels) and survival rate (lower panels) and shade (a,d), spatial drought (b,e) and inter-annual drought (c,f) of the abundant treelet Faramea occidentalis. Growth decreased significantly in deeper shade (a) and survival decreased significantly in drier sites (e, spatial drought) and years (f, inter-annual drought). Large dots represent mean observed growth or survival for ten shade or drought classes, each containing 10% of the individuals of the species (only six classes in (f), due to high abundance in one year). Lines show fitted growth and survival with increasing shade (a,d, orange), spatial drought (b,e, green) and inter-annual drought (c,f, blue), at mean values of the other independent variables. Solid and dotted lines indicate significant and non-significant responses, respectively. Lines whose colour differs from the large dots within each panel represent 1 SD increase in shade (orange), spatial drought (green) or inter-annual drought (blue). Figure S1 presents responses to shade and drought for all analysed species.

Figure 2

Relationships between species responses to shade and inter-annual drought (i.e. dry season severity) for growth (a), survival (d), or growth versus survival (b,c). Solid and dashed lines indicate significant (p < 0.05) and marginally significant (0.05 ≤ p < 0.10) relationships, respectively. Negative relationships indicate a trade-off between shade and drought responses. Correlations are weighted by the uncertainty in species tolerances (smaller dots have higher uncertainty and lower weight, see equation (5) in text). Colours identify species with insignificant (grey) or significant responses to shade (orange), inter-annual drought (blue) or both (red).

Figure 3

Relationships between the fast–slow continuum and responses to shade (a,b) and dry season severity (i.e. inter-annual drought) (c,d) for growth (left) and survival (right). The position of species along the continuum was quantified by a weighted PCA of demographic rates (growth, survival, number of sapling recruits) of trees ≥1 cm dbh recorded in the BCI 50-ha plot. Low and high scores correspond to species with fast and slow demographic strategies, respectively. Colours identify species with insignificant (grey) or significant responses to shade (orange) or inter-annual drought (blue). Relationships were consistent when the fast–slow continuum was calculated using seedling performance and/or seed number additionally (see Supplementary Table S1.2).