Differential root responses to experimental drought in seedlings from five dominant andean species highlight the importance of ecosystem functional diversity

Abstract

ABSTRACT : Seedling establishment is a critical stage for forest turnover and is susceptible to abiotic conditions such as drought events, which are becoming more frequent and severe combined with a drier atmosphere. Seedlings can respond differently to water limitation, depending on their life strategies and specific traits. The root system's response to water limitation is particularly relevant, as it is critical in water uptake, nutrition, and plant growth. Tropical Andean forests are among the most vulnerable ecosystems to climate change because of their limited geographic range and dependence on humidity conditions. However, knowledge about the effects of drought on Andean tree species is limited. In this study, we aimed to assess the root systems' responses to drought conditions during the seedling establishment of five Andean tree species (Quercus humboldtii, Croton magdalenensis, Erythrina edulis, Meriania nobilis, and Clusia sp.). We subjected seedlings to four soil water levels in the plant-available water (PAW) range: (1) watered to field capacity (control), (2) watered to 80% of PAW, (3) watered to 50% of PAW, and (4) total soil moisture exclusion. Seedlings in the soil moisture exclusion treatment started at field capacity and were allowed to dry down during the experiment, while 80% and 50% of PAW treatments included three successive dry–rewetting cycles. We measured various root morpho-functional traits, relative growth rate, above- vs. below-ground biomass allocation, predawn water potential, and recorded seedling mortality. Our results indicate that drought conditions reduced the species' growth rate and produced species-specific root responses more evident in the plants subjected to successive stress. E. edulis and M. nobilis were vulnerable species with high mortality percentages (23% and 80%, respectively). These species were associated with root acquisitive resource strategy with higher specific root length, thinner lateral roots, and high taproot branching density. C. magdalenensis, in turn, was a drought-resistant species also related to an acquisitive resource strategy. In contrast, Clusia sp. and Q. humboldtii were drought-resistant species related to a conservative resource strategy with thicker lateral roots, low specific root length, and higher root dry matter content. The results suggest that Andean species have the potential to respond to drought conditions by adjusting their intrinsic morpho-functional root traits. These responses depend on their resource acquisition strategies and highlight the potential impacts of climate change on seedling establishment and the forest composition due to species vulnerability to drought. Our findings could also aid in selecting species resistant to drought with root traits associated with conservative strategies for forest restoration projects.