Effects of wind induced soil moisture stress on the germination and early growth of Pinus banksiana Lamb. on three soil texture types

Abstract

A controlled environment wind tunnel was used to produce three wind speeds and hence three soil drying rates in order to study the effects of soil moisture stress on seeds and germinants of Pinus banksiana Lamb, planted in a sandy loam, loamy sand and sand. Osmotically primed and untreated seed were sown when the soil was saturated and germination was monitored daily. Germinants at five stages of ontogeny, the most advanced stage being the start of epicotyl growth, were placed into the wind tunnel with the soil saturated. When 50% of the germinants were dead and/or wilted, final length and dry weight of the shoots and roots were measured. All experiments were a split plot factorial design with wind speed as the main plots and the soil and seed or ontogeny stage treatments as the subplots. No seed germinated in the wind tunnel for any of the soil type and wind speed treatment combinations. Therefore, seed were transfered from the soil surface to Petri-dishes in a germination cabinet where germination was monitored daily. Compared to controls (primed and unprimed seed taken from cold storage), germination of unprimed seed decreased with each increase in wind speed, the magnitude of the decrease varying with soil type. A similar response was not noted for the primed seed; osmotic priming appeared to negate the effects of the physical environment. The exact cause could not be determined from these studies. The hypothesis that the unprimed seed did not imbibe some critical, minimum amount of water before drying and that something inhibited germination when the seed was rehydrated is presented and discussed. The fewer number of macropores and the more rapid drying rate of the loamy sand and sandy loam soil types was considered to be the cause of poorer establishment with increasing wind speed of the early ontogeny stages (stages up to the point the radicle has penetrated the soil surface and the seed is slightly elevated). The growth and survival of germinants once established was not affected by these factors. The data suggests that the drying rate of the soil and the inability of root growth to keep up with this drying was considered to be a major factor affecting growth and survival of the established germinants. Additionally, the transpiration rate of the germinants probably played a significant role in the survival time. The loamy sand and sandy loam soil types probably could not supply a sufficient quantity of water to the germinant to keep pace with the transpirational demand. The sandy soil was better for growth and survival than the finer soils probably because of the way water is held in the soil profile once the capillaries are broken. Germinants began to die at soil moisture potentials above the permanent wilting point (PWP) at the high wind speed. This supports the view that PWP is not a soil constant The increase in some growth parameters with increasing wind speed supports the view that increasing wind speed is not always detrimental to germinant growth. Application of the results to field conditions is discussed.