Differences in soil moisture in two Middle Eastern oak forests: Comparing the effects of trees and soil composition

摘要

The incidence of large rain events in Mediterranean ecosystems vary among years. Summer aridity is interpreted as a resetting event, eliminating previous soil-moisture dynamics. The dynamics of soil moisture and retention are critical to tree survival, particularly in dry regions. This study examines the long-term soil water content (theta(V)) dynamics in two distinct locations within the forest, under the canopy and forest clearing, within two diverse oak forests: subhumid mixed oak forests (MG) and semiarid monospecific oak woodlands (YE). Plots were established at small-scale catchments and soil water contents were measured during 2010-2013, at three depths in the two different locations. Cumulative rainfall was used as an independent proxy for theta(V) analysis. A novel bell-bilogistic mathematical model of wetting, saturation, and drying arms was developed. We aimed to study the theta(V) distribution differences between soil profiles giving the large climatic gradient between the two forested sub basins, the differences in vegetation traits along with soil attributes. We further aimed at determining the role of an individual tree in regulating soil-moisture dynamics. We hypothesized the occurrence of distinct responses between sites in all soil-moisture indices with higher theta(V) at the wetter site. We tested the hypothesis that seasonal cumulative rainfall dictates the variations in soil-moisture regimes throughout contiguous years. Annual rainfall was higher than long-term average throughout the study. Soil profiles under the canopies at both sites were consistently wetter. Infiltration and depletion constants were higher at MG whereas maximum soil moisture was higher at YE. Homogenous recharge patterns were seen at MG although YE evinced more variation. Oaks had no effect on recharge at MG compared with the forest clearing. Soil properties primarily affected the wetting arm whereas vegetation composition regulated the drying arm. Mixed-stands characterized by ever-green and deciduous species may maintain favourable soil-moisture conditions, in comparison with other mixed stand morphologies. The increasing role of slacking forces in infiltration process may alter the interaction between trees and herbaceous vegetation.