Pinus halepensis plantations are widespread throughout semiarid-subhumid landscapes of the Mediterranean. Recently, drought-induced decline has often been reported raising concerns with regard to the future of these man-made ecosystems. The study was set out to investigate thinning as a means to alleviate water stress and improve performance of mature P. halepensis plantations experiencing prolonged drought. The study was conducted in a 40-year-old P. halepensis forest in the Jerusalem Mountains of Israel (rainfall: 550 mm year -1 ). Declining stands (stand basal area increment, BAI stand & -3 % year -1 ) were treated by thinning: (1) intense thinning-tree density, BA stand , and leaf area index (LAI) reduced by 81, 68, and 59 %, respectively; (2) moderate-56, 48, and 26 %, respectively;(3) control (*560 tree ha -1 ). Plots of 70 9 70 m were used in four replicates. Individual tree-to stand-level variables were monitored during 4 years through stem and leaf area metrics alongside with predawn shoot water potential (w PD ) and tree mortality. Thinning ameliorated drought stress, reduced mortality, and improved individual tree growth (w PD = -1.7, -1.8, and -2.0 MPa; mortality = 0.2, 2, and 5 % year -1 ; BAI tree = 3.4, 2.0, and 1.4 % year -1 in intense, moderate, and control treatments, respectively). Thinning effects became more pronounced with time. LAI and individual tree leaf area (LA tree ) fluctuated with association to annual rainfall. Higher LA tree caused by thinning reflected a ''selection effect'' while increased leaf area efficiency (BAI per unit LA tree ) was attributed to a ''release effect'' of thinning.
Natural regeneration that occurs in the understory of Mediterranean pine monocultures provides the basis for the transition of these simply structured systems toward a more complex and sustainable state. However, the course and consequences of this process, and its relationships with environmental and silvicultural variables, are still inadequately understood. We investigated the relationship between rainfall amount and understory woody vegetation (UWV) structure in mature (40-50 year) Pinus halepensis plantations in the Mediterranean zone of Israel, where rainfall ranges from 280 mm/year in the south to 900 mm/year in the north. We measured abundance, diversity and species composition on south-and northfacing slopes, in forest sites distributed along the rainfall gradient. UWV abundance, as measured by cover percentage and height, increased with rainfall amount along the entire gradient (2-113% and 0.1-3.4 m, respectively), more rapidly on north-facing slopes. Species composition varied along the rainfall gradient, with ranges of species occurrences corresponding to those in unforested habitats. The relationship between rainfall and UWV species richness was positive throughout most of the rainfall gradient, possibly with a shift in pattern at the highest rainfall levels. UWV richness increased sharply with increasing abundance, up to a certain point with no further increase in richness as abundance increased further. We concluded that UWV structure in the studied forest environment and climatic range is strongly determined by rainfall and suggested that the design and management of Mediterranean forests should focus more on optimizing water availability for the various components.
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