Nutrient cycling in the tropical dry forest: How do different tree species adapt to limited resources and how are nutrient and water uptake strategies connected?

Abstract

Tropical Dry Forests (TDF) experience one to two dry seasons per year in which hardly any precipitation occurs (D'Odorico & Porporato 2006). Depending on their adaption to drought stress, tree species in the TDF tend to develop different root systems. Deciduous species follow an exploiting strategy and usually have shallow roots, while evergreens tend to behave more conservatively and often develop deep roots in order to maintain a water supply during dry season (Sobrado 1991; Álvarez-Yépiz et al. 2017). Tree roots not only take up water, but also nutrients. Nutrient cycles are highly dependent on water availability, because many processes such as nutrient uptake, decomposition of litterfall or weathering of bedrock to release nutrients require water (Anaya et al. 2012; Lugo & Murphy 1986; George & Marschner 1996). Hence, variations in the nutrient concentration in different soil layers depending on the rooting depth of the different tree species are expected. Little research has been conducted on how nutrients cycle through TDF’s. There are two publications of which the first stresses the differences of nutrient cycling in TDF’s to Tropical Rain forests (Raulino et al. 2020) and the second compares macro- and micronutrient contents in leaves of N2-fixing and non-fixing legumes in TDF in Costa Rica (Alvarado et al. 2018). In both studies, certain tree species of the TDF were “responsible” for the cycling of certain nutrients. Nutrient cycling in TDF’s is still not fully understood and even though it is tightly bound to the hydrological cycle, there has been little research conducted which combines water and nutrient uptake in TDF’s. The objectives of the MSc thesis are: • to characterize nutrient availability and nutrient uptake patterns of four tree species in a Costa Rican TDF with a focus on micronutrients; and • to investigate how nutrient uptake relates to different water uptake strategies. The following hypotheses are to be tested: i. Due to the extreme conditions in dry forest, tree species are strongly adapted to the available resources and different species uses certain elements preferentially. ii. Deeper rooting dry forest species have access to other nutrients depending on its availability In order to answer these questions, an extended field campaign at the Estación Experimental Forestal Horizontes in Guanacaste, Costa Rica, was carried out. The goal of this campaign was to collect soil, leave and litterfall samples and analyze macro- and micronutrients in the laboratory. Samples were collected at two specific times: i) at the peak of the dry season and ii) in the beginning of main rainy season. Four species with different characteristics and water uptake strategies were selected and their leaves collected. Soil samples