Environmental changes in biodiversity hotspot ecosystems of
South Ecuador: RESPonse and feedback effECTs (FOR2730)

Abstract:

Tropical dry forests are an intricate ecosystem with special adaptations to periods of drought. Arbuscular mycorrhizal fungi (AMF) are essential for plant survival in all terrestrial ecosystems but might be of even greater importance in dry forests as plant growth is limited due to nutrient and water deficiency during the dry season. Tropical dry forests in Ecuador are highly endangered, but studies about AMF communities are scarce. We investigated the AMF community of a premontane semi-deciduous dry forest in South Ecuador during the dry season. We estimated AMF diversity, distribution, and composition of the study site based on operational taxonomic units (OTUs) and compared the results to those from the tropical montane rainforest and páramo in South Ecuador. OTU delimitation was based on part of the small ribosomal subunit obtained by cloning and Sanger sequencing. Nearly all OTUs were Glomeraceae. The four frequent OTUs were Glomus, and comparison with the MaarjAM database revealed these to be globally distributed with a wide range of ecological adaptations. Several OTUs are shared with virtual taxa from dry forests in Africa. Ordination analysis of AMF communities from the tropical dry and montane rainforests in South Ecuador revealed a unique AMF community in the dry forest with only few overlapping OTUs. Most OTUs that were found in both dry and rainforests and on the two continents were globally distributed Glomus.

Abstract:

Lowered temperatures may reduce the root water uptake of tropical trees at high elevations through several mechanisms; however, field studies to test their relevance are lacking. We measured sap flux density (J) in small-diameter tree roots across a 2000-m elevation transect in a tropical mountain forest for quantifying the effects of temperature, VPD and soil moisture on root water flow and uptake at different elevations. Recently developed miniature heat balance-sap flow gauges were applied to roots of about 10mm in diameter in mountain forest stands at 1050, 1890 and 3060m a.s.l. in the Ecuadorian Andes and the measured flow was related to anatomical properties of the root xylem. Between 1050 and 3060 m, mean J decreased to about a third. VPD was the most influential environmental factor controlling J at 1050 and 1890 m, while Ta was the key determinant at 3060 m. Large vessels were absent in the root xylem of high-elevation trees which resulted in a 10-fold decrease of theoretical hydraulic conductivity (kh theor) between 1050 and 3060 m. We conclude that both physical limitations (reduced VPD, increased viscosity of water) and biological constraints (large decrease of kh theor) result in a significantly reduced J and root water uptake of the trees in high-elevation tropical forests.

Abstract:

It is generally assumed that tree growth in tropical low-elevation forests is primarily limited by phosphorus while nitrogen limitation is more prominent in tropical montane forests where temperature is lower and the soils are poorly developed. We tested this hypothesis in mountain rainforests of South Ecuador by investigating the growth response of tree fine roots to N, P and K fertilization in ingrowth cores exposed at 1050 m (pre-montane) and 3060 m (upper montane) elevation. Root growth into unfertilized ingrowth cores (control treatment) was about 10 times slower at 3060 m than at 1050 m. At 1050 m, root growth was stimulated not only by P, but also by N and K. In contrast, N was the only element to promote root growth at 3060 m. The N concentration in fine root biomass dropped to nearly a third between 1050 and 3060 m, those of P, K, Ca and Mg decreased as well, but to a lesser degree. According to a 15NO3 15NH4 tracer study along the slope, tree fine roots accumulated nitrate and ammonium in root biomass at similar rates between 1050 and 3060 m, despite lower temperatures higher upslope.We conclude that the nature of nutrient limitation of tree fine root growth changes with elevation from an apparent co-limitation by P together with N and K at 1050 m to predominant N limitation at 3060 m, which is also reflected by low foliar N concentrations. Increasing N limitation may have caused the high fine root biomass and root/shoot ratio in the high elevation forest, while the capability of the roots to acquire mineral N apparently was not affected by lower temperatures at high elevations.