Vegetation plays a fundamental role in soil conservation, so it is common to consider an
increase in vegetation cover as one of the techniques to mitigate the effects of
desertification in Mediterranean forest environments. There are two factors limiting the
establishment and growth of seedlings in dry environments: (i) an excessive radiation
and, (ii) the limited availability of water during the summer drought. During an
afforestation plan, soil preparation is always necessary to reduce sapling mortality. The
goal of this study was to analyze the effect of various organic amendments on soil
according to chemical and hydrological properties, and to assess the effects of these
parameters on an afforestal proposal under Mediterranean climate conditions.
Five amendments were applied in an experimental set of plots: straw mulching (SM);
mulch with chipped branches of Aleppo Pine (PM); TerraCotten hydroabsobent
polymers (HP); sewage sludge (RU); sheep manure (SH) and control (C). Plots were
afforested following the same spatial pattern, and amendments were mixed with the soil
at the rate 10 Mg ha -1 .
Under bare soil conditions (C), most of mortalities occurred during the summer period
of the first year. A substantial positive effect of SM, PM and HP on the survival rates
have been clearly observed. Conversely, when the soil was amended with SH, the
survival rate quickly decreased or remained more or less constant regarding to C.
In this study, the lack of differences on chemical properties indicates that there may
exist other reasons to justify the differences that were found in the pattern of vegetation.
However, regarding to the hydrological properties some differences have been found.
In C, soils were registered below the wilting point during 4 months a year, and
therefore, in the area of water unusable by plants. These months were coinciding with
the summer Mediterranean drought and can justify the high mortality found on plants.
Conversely, in SM, PM and HP, soil moisture remained below the wilting point less
period than C and, the plant available water was also higher. In these treatments, the
survival sapling rates measured were the highest. SH showed water holding capacity
slightly more limited than C. For this treatment, the survival sapling rates measured
were the lowest.
In conclusion, from a land management standpoint, the PM, SM and HP have been
proved as a significant method to reduce sapling mortality rates during the
Mediterranean summer drought.