Influence of pasture understoreys and tree management on soil moisture under a young New Zealand stand of Pinus radiata

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Soil moisture content (SMC) and throughfall were monitored along transects between rows of radiata pine (Pinus radiata D. Don) planted at 7 m row spacing and grown with ryegrass/clover or lucerne pastures or with no understorey. SMC was recorded to 1 or 2 m depths for years 2–6 or 4–6, respectively. The predominant rainfall from the south–south–west (SSW) direction created rain shadows (60% of open pasture) on the northern sunny side of the trees. The rain shadows increased as the trees grew but their position were modified by stand pruning and thinning in years 4 and 5. SMC in the top meter showed strong positional effects with the lowest SMC under the tree crowns and on the northern side. There were strong seasonal patterns driven by high spring and summer evapotranspiration, modified by rain shadows and stand age. For the no-understorey treatment, SMC remained high midway between tree rows until the trees were 4 years old and started utilizing this moisture. For the two pasture understoreys, the difference in SMC between the north and south sides decreased with time after trees were thinned and pruned. In the no-understorey there was little difference between the two aspects. Understorey evapotranspiration and fine rooting patterns also helped explain soil moisture use patterns. No difference was detected between two contrasting radiata pine tree types. The effective rooting depth of the ryegrass/clover understorey was <0.7 m while that of lucerne and trees were deeper. Alternative tree planting practices may reduce some of the negative effects of tree-understory moisture competition.