Effects of seasonal drought on gap and understorey seedlings in a Bolivian moist forest

Abstract

In tropical moist forests, length of the dry period may have a profound influence on leaf dynamics, plant growth and survival. To evaluate the role of light and water availability on seedling performance, a 1-y experiment was carried out in a tropical moist forest in the Bolivian Amazon in which seedlings of three tree species (Brosimum lactescens, Cedrela odorata and Schizolobium amazonicum) were planted in gaps and the understorey. Variation in length of the dry period was simulated by subjecting part of the seedlings to a water treatment at the end of the dry period. Gaps and understorey had a similar soil moisture content, which varied between 39% in the wet season and 16% in the dry season. Height and leaf growth rates were higher in gap compared to understorey plants, and in the wet compared to the dry season. A high growth during the wet season provided gap plants with a decisive size advantage over understorey plants during the dry season. Their larger root system allowed gap plants to explore a larger surface area and deeper soil layers for water. Consequently, gap plants of Cedrela experienced a shorter deciduous period (22 d) compared to understorey plants (61 d). Watering at the end of the dry season cued the flushing of new leaves by Cedrela, although it did not lead to a higher plant growth.