From seedling to sapling: tree species responses to spatial and temporal understory light heterogeneity in disturbed tropical montane forests

Abstract

Difficulties in measuring small-scale light heterogeneity in forest understories is reflected in the classification of species, conventionally based on their responses to gaps or shade, rarely present in uniform conditions in space and time. Spatial and temporal heterogeneity expressed as a contrast between direct vertical light and light coming from the surroundings allows for a better assessment of the understory light environment and thus of the effect of its variation on species performance. Using repeated measures, we evaluated the effect of vertical light, contrast, and distance to the gap center on the performance of transplanted seedlings of Alnus acuminata, Cornus excelsa, Liquidambar styraciflua, Quercus laurina and Persea americana. Highlands of Chiapas between 999 and 2005 in eight plots of pine-oak forest, each one with a small gap in the middle. Illumination was evaluated with hemispherical photographs at the beginning and the end of the study. We measured stem slenderness and survival after one, two, four, and six years following transplantation. Distance had an effect on survival of Alnus and Liquidambar. Vertical light only had an effect on Liquidambar slenderness. Contrast explained slenderness of Liquidambar, Cornus and Quercus, and survival of Liquidambar, Quercus and Persea conditional on plant age. Initial light contrast predicted the final condition well, which vertical light did not. Contrast between two gradients of the same resource produced a remarkable improvement in the predictive power of light over the performance of the studied species describing the small variations of light in forest understory.

Keywords

• disturbance
• light contrast
• shade tolerance
• sunflecks
• temporal elasticity