Long-term trends in fruit production in a tropical forest at Ngogo, Kibale National Park, Uganda

Abstract

Fruit production in tropical forests varies considerably in space and time, with important implications for frugivorous consumers. Characterizing temporal variation in forest productivity is thus critical for understanding adaptations of tropical forest frugivores, yet long-term phenology data from the tropics, in particular from African forests, are still scarce. Similarly, as the abiotic factors driving phenology in the tropics are predicted to change with a warming climate, studies documenting the relationship between climatic variables and fruit production are increasingly important. Here we present data from 19 years of monitoring the phenology of 20 tree species at Ngogo in Kibale National Park, Uganda. Our aims were to characterize short- and long-term trends in productivity and to understand the abiotic factors driving temporal variability in fruit production. Short-term (month-to-month) variability in fruiting was relatively low at Ngogo, and overall fruit production increased significantly through the first half of the study. Among the abiotic variables we expected to influence phenology patterns (including rainfall, solar irradiance, and average temperature), only average temperature was a significant predictor of monthly fruit production. We discuss these findings as they relate to the resource base of the frugivorous vertebrate community inhabiting Ngogo.

Keywords: abiotic factors; frugivores; fruit production; temporal variation.

Figure 1 –

Histograms (on diagonals) of univariate frequency distributions and bivariate correlation plots (in panels below diagonal) for each of the measured abiotic predictor variables.

Figure 2 –

Boxplot of monthly proportion of trees bearing fruit. Each box ranges from 25^th to 75^th percentiles, with medians indicated by black lines within boxes. Whiskers on each extreme of each plot represent values within 1.5x the interquartile range.

Figure 3 –

Time series of monthly proportion of trees bearing ripe fruit between Feb 1998 and Nov 2017

Figure 4 –

Value of the inter-annual (upper figure) and intra-annual (lower figure) smooth functions. Solid lines are model averaged smooth function estimates, and dashed lines indicate the 95% credible region.

Figure 5 –

Inter-annual smooth term functions with 95% credible regions for each of the 20 most important chimpanzee food resources included on the phenology trail. Interior plot shading corresponds to the log-likelihood value of the fitted model, with darker shades indicating stronger fit (species with unshaded plots had smooth terms with P values > 0.05)