Understory variation and the influence of biotic and abiotic environmental factors in Nothofagus antarctica forests of Tierra del Fuego (Argentina).

LENCINAS, MV; BOTTAN, L; BENITEZ, J; CHAVES, JE; ARAVENA ACUÑA, MC; RODRÍGUEZ SOUILLA, J; CELLINI, JM; BARRERA, MD; PERI, PL; MARTÍNEZ PASTUR, GJ

Conferencia; INTERNATIONAL SCIENTIFIC CONFERENCE Forest science for people and societal challenges. The 90th "Marin Drăcea" INCDS Anniversary; 2023

The understory vegetation, as an integral component of native forests, plays a fundamental role in nutrient and energy recycling, as habitat for biodiversity, and for the stability and resilience of the ecosystem against impacts. It is also a shelter for arboreal regeneration, and a source of resources for productive activities, e.g., forage for cattle in silvopastoral systems. In a specific forest type, the understory characteristics vary in their natural distribution area, being influenced by changes in the structure of the canopy, soil, climate and other environmental factors of site and landscape, under natural or human modified conditions. Vascular plant assemblage in the understory of Nothofagus antarctica forests are among the most diverse of Tierra del Fuego (Argentina). However, there is a lack of information on how it varies, and how biotic and abiotic environmental factors influence its structure. Therefore, the aim of this work was to analyze the variation in understory structure and the effect of some biotic and abiotic environmental factors on richness, cover and biomass, in order to detect the most influential variables that could be modulated through management. We analyzed 137 stands of N. antarctica forests, evenly distributed in their natural range on Tierra del Fuego, including forests with different ages under natural dynamics and with different degrees of disturbance (harvesting, livestock use, fire). For each stand, we evaluated (i) specific richness (n° of species) by floristic survey; (ii) ground cover (%) by point intercept method; and (iii) dry matter biomass (kg DM/ha) by manual collection in 0.25 m2 plots and weighed after oven-drying to constant weight. Different environmental characteristics were also surveyed, including biotic (forest structure, net productivity) and abiotic (edaphic, climatic) variables. For forest structure, dominant height (m), basal area (m2/ha), density (individuals/ha), stand density index (%), mean square diameter (cm), total volume with bark (m3/ha), crown cover (%), and relative leaf area index were obtained. Among the edaphic characteristics, we surveyed apparent soil density, soil moisture, content of carbon, nitrogen and phosphorus content (kg/m2/30 cm depth), organic matter (%), and pH. Climatic variables included the total/incident radiation rate (%), plus 11 temperature variables measured in °C (annual average, daily amplitude, isothermality, seasonality, maximum of the warmest month, minimum of the coldest month, annual amplitude, mean of the wettest quartile, mean of the driest quartile, mean of the warmest month, mean of the coldest month) and 8 precipitation variables measured in mm/year (annual, of the rainiest month, of the driest month, seasonality, of the rainiest quartile, of the driest quartile, of the warmest quartile, of the coldest quartile). Other variables were the topographic elevation, the distance to rivers (m), the global aridity index, the global potential evapotranspiration, and the net primary productivity. Data were analyzed by univariate and multivariate analyses, including correlations, generalized linear models, non-metric multidimensional scaling and multi-response permutational proceeding. Forest stands under natural dynamic presented variations in the understory structure (from 6 to 43 species; 38-300% cover, 60-6400 kg DM/ha biomass) depending on stand age (young stands with the lowest values). Disturbances heterogenized the understory, both by species incorporation and loss, leading also to changes in cover and biomass. Regarding the most influential variables, basal area, mean square diameter, apparent soil density, soil nitrogen content and elevation were important in modelling richness; while phosphorus content and elevation influenced understory cover. Basal area, mean square diameter and soil nitrogen content were used to modelling understory biomass. Some of these variables can be modified by human practices (e.g., basal area and mean square diameter by harvesting), thus the understory structure of N. antarctica forests can be managed for livestock use and/or conservation. However, the permeability to foreign species and the susceptibility to species loss when affected by strong disturbances indicate the convenience of excluding some patches from management, for the preservation of more diverse natural structures within the matrix of managed forests.