Greenhouse-grown Scots pine, spruce, pedunculate oak and beech seedlings were subjected to different levels of water stress to test the impact of drought on their photosynthetic activity and level of soil water uptake.
During the experiment, Scots pine and spruce seedlings limited their water uptake and stopped photosynthesis at still relatively high soil water levels, while oak and beech seedlings consumed soil water more rapidly but continued to photosynthesize up to a severe drought stress level. The rapid juvenile growth of hardwoods is a strategy that enables them to be more competitive with herbaceous, shrubby and arborescent competition. In the experiment, however, it led to greater water consumption and earlier drought suffering than the more water-efficient softwood seedlings.
The study shows that the four studied species adopt different soil water use strategies at the juvenile stage :
- Scots pine saves water while continuing to photosynthesize down to fairly low soil water levels, giving its seedlings a better chance of survival in the event of drought.
- Oak is the biggest consumer of water, but stops photosynthesizing only under severe drought conditions. Its high water consumption could be attributed to maintaining its high transpiration (stomata opening and photosynthesis) under moderate drought conditions.
- Beech shows an intermediate behavior, consuming less water than oak but stopping photosynthesis faster when the soil dries out.
Softwoods have opted for a strategy of avoiding water stress, while hardwoods have shown a certain tolerance to drought, opting for an efficient use of available water. The behavior of oak and beech would make them the best candidates in the context of climate change, but Scots pine also has its advantages. These results now need to be confirmed under natural conditions and over longer periods of time.