The main objective of this study is to focus on the importance of the available water holding capacity (AWHC) of the soil for forest growth. The study area is the oak ecosystems of central mountainous Halkidiki, which can play a significant role in the context of multi-purpose forestry and the promotion of sustainability in natural environment. Increased values of soil depth correspond to increased AWHC. We have found AWHC to be 71 mm for a soil depth of 45 cm, 90 mm for a soil depth of 60 cm, 103 mm for a soil depth of 75 cm and 115 mm for a depth of 90 cm. Soil samples at the same depths were examined in three treatment plots, namely, control, thinning and clearcut. Forest floor increases the total AWHC of soil to a range of 4-8 mm, depending on the impact of various treatments on the mass of the organic matter. Total available water surplus (TAWS) (in the form of run off or deeper infiltration) appeared in the shallower soils (AWHC of 70-80 mm) for a duration of two more months compared to the deeper soils (AWHC 115 mm). In these deeper soils, 47 mm/year more soil-water is retained and is available for tree growth. AWHC values higher than 100 mm extend the tree growing period for two more months. The TAWS differences between AWHC values of 103 mm and 90 mm compared to the lowest value of 71 mm, was found to be 33 mm/year and 17 mm/year respectively. Similar results were found in all plots. The thinning up to 15 %, practiced by the Forest Service, provides one more month of TAWS (extending the period in which TAWS appeared) compared to the untreated plots. The treatment was a 50 % thinning of total basal area, which provided two more months of water surplus availability (TAWS), to enhance water yield. In general, the best site quality is characterized by higher values of AWHC which is correlated to deeper soils. This is apparently reflected in tree growth. The tallest trees of the best site found to be five meters higher than those growing in shallower soils. For all soil depths and AWHC values, intense thinning (50 %) compared to the control treatment, supply the trees with an additional 32 mm/year of soil moisture and less thinning can increase soil moisture by 18 mm/year, which help trees withstand hot and dry summers. The limiting growth factors in Mediterranean oak ecosystems are soil depth and the seasonal change of soil moisture, especially during the dry summer period, so increasing the AWHC, even marginally, may prove crucial for tree sustainment. The role of these forests is of outmost importance for providing multi –purpose services (e.g. promoting health-fresh air, water, recreation, stabilizes the microclimate, supports biodiversity, tourism, economy, and enhance storing carbon).