Forest floor and soil properties in different development stages of Oriental beech forests

Highlights

• The development stages of Oriental beech forests affect forest floor nutrients.

• The decay stage exhibits a more abundant soil biota and more nutrients.

• The population of the soil fauna was more abundant in fall than in summer.

• Old-growth Oriental beech forests give rise to increased soil fertility and biological activity.

Abstract

Native to Eurasia, Eastern Europe, and Western Asia, Oriental beech (Fagus orientalis Lipsky) stands extend from the north-western part of Turkey, the eastern part of the Caucasus Mountains in Georgia and Russia to the Alborz Mountains in Iran. The effect of different development stages of beech stands on the forest floor and soil characteristics are so far almost unknown. For this purpose, the Langa forest that is located in northern Iran was investigated to detect the effect of forest development on major traits of soil biology and related soil characteristics. For this aim, 45 forest floors and soil samples (15 samples per each development stage i.e. initial, optimal, and decay stage) were analysed. In addition, the water content, temperature, and biological dynamics were monitored over the different development stages in summer and fall. Among the considered independent factors, the development stage of beech had the largest explanatory power for the variations of the soil water content (34.7%), total earthworm density and biomass (29.6% and 32.3%, respectively), Acarina (30.3%), nematode (64.3%), protozoa (57.7%), bacteria, (62.8%), fungi (59.5%), basal respiration (53.0%), substrate-induced respiration (53.8%), microbial biomass carbon (30.4%), microbial biomass nitrogen (36.0%), microbial biomass phosphorous (59.1%) and net nitrification rate (19.1%). Soil temperature (42.5%), collembola density (18.2%), net ammonification rate (29.9%), and net N mineralization rate (2.1%) were predominantly determined by the seasons. With the ageing of the forest stands, soil fertility, biota abundance, and soil function were enhanced. To figure out the most vital soil features affecting C and N sequestrations, a stepwise regression model was applied. Carbon sequestration correlated positively to N, the C/N ratio, and negatively to porosity, and the nematode abundance. In addition, nitrogen sequestration correlated positively to C, the sand content, and fine root biomass and negatively to the C/N ratio, soil porosity, and aggregate stability. Although all forest stages of the Oriental beech have an important ecological role and their place in the evolutionary process, the old-growth (i.e. decay) stage is especially important for the soil functions exhibiting the highest fertility and biological relevance. The understanding of ecological changes within different development stages can be helpful in forest management and the evaluation of silvicultural methods.

Introduction

The Oriental beech (Fagus orientalis Lipsky) forests are native to Eurasia, Eastern Europe, and Western Asia. This type of forest covers large areas and extends from Bulgaria's Strandja mountains over the north-western part of Turkey to the eastern part of the Caucasus Mountains in Georgia and Russia until the Alborz Mountains in northern Iran. Oriental beech forests are among the most important and biologically richest stands and cover about one-fifth of Hyrcanian forests (Moslehi et al., 2019). The Hyrcanian ecosystem in the northern regions of Iran consists of dense forests along the southern slope of the Caspian Sea and the northern slope of the Alborz Mountains. The climate of this region is temperate with moderate winters and includes a large number of broad-leaved tree species that remained from the Tertiary period. This tree species can be found in the plain, together with other broad-leaved trees. The upper distribution restriction of this species is caused by climate, soil properties, and topography. At higher altitudes, the Oriental beech prefers to grow at warm and sunny slopes (Sagheb-Talebi et al., 2004). Oriental beech forests in northern Iran are classified into three development stages (initial, optimal, and decay) based on the number and volume of living trees and deadwood, diameter categories, canopy cover and also tree regeneration (Korpel, 1995; Akhavan et al., 2012). The stand structure at the initial stage tends to have more than one layer and relatively large variability of trees ages, while at the optimal stage, the stand structure is much more even-aged due to the close canopy cover, and scarce regeneration. Due to poor sunlight conditions in this stage, only deadwood trees with small dimensions can be found under the dominant canopy cover. At the older or ‘decay’ stage, the volume and density of tress increase. During the latest phase of this stage, regeneration is starting beneath canopy gaps. Young trees start to growth and the number of old mature trees decrease; thus, the forest slowly turns back to the initial stage, and this cycle repeats (Korpel, 1995; Emborg et al., 2000; Sagheb-Talebi et al., 2005; Akhavan et al., 2012).

Above-ground and below-ground organic matter contributes to the C-pool. These have a substantial effect on the soil physicochemical and biological traits (Gartzia-Bengoetxea et al., 2016; Schwarz et al., 2015). Forest floor (O-horizon including the L, F, and H layers; Kooch and Bayranvand, 2019) quality and the relative proportion of carbon and nitrogen vary among different stages of tree species (Parsapour et al., 2018a, Parsapour et al., 2018b) and are a basic stimulant for soil processes. The positive effect of soil biota on the plant litter decomposition is well-known. In forests, the soil biota is an indispensable player in the process of organic matter and nutrient decomposition (Yang and Chen, 2009). In addition, their activity helps to improve the physicochemical and biological properties of forest soils (Sarlo, 2006; Wu et al., 2019). Furthermore, C and N sequestration finally depends on soil characteristics (Bellassen et al., 2010). In forest ecosystems, soil organic matter and the forest floor have important functions for the provision of energy and substrates for the soil biota (Bongiorno et al., 2019; Kooch et al., 2020a). Soil organic matter and the forest floor, thus, influence soil microbial activities and nutrient cycles (Hansen et al., 2009). Soil biota activity is influenced by forest floor quality. Therefore, forest floor quality and soil biota interactions determine the release or fixation rates of organic matter and nutrients in soils (Gartner and Cardon, 2004). Although soil and forest floor quality may vary in different forest types, comparisons of these properties for various stand stages have rarely been done (Zhao et al., 2016). Most of the studies were done in the context of afforestation.

So far, no information is available about the effects of development stages of Oriental beech stands on forest floor composition and the topsoil. Therefore, this study aims at evaluating the properties of forest floor and soils at different development stages (i.e. initial, optimal, and decay) of Oriental beech forests to answer the following three questions: (1) Do forest floor nutrients change with stand developments?, (2) will soil properties be improved at beech stands in the decay stage?, (3) in which season are the activities of soil biota most enhanced? Following these questions, our hypotheses were: (i) the content of forest floor quality will be higher at the older stage, (ii) under older stands soil functions improve compared to younger forest stands and (iii) the activity of soil biota communities differs between different seasons. Moister and cooler conditions in fall should provide an optimal setting for soil biota's activity in fall.