Drought tolerance of alternative tree species

The two trial plots are located in northern Bavaria. The Lower Franconian area, in the Main Valley near Großostheim (125 m  a.s.l.), is characterised by lower precipitation and higher temperatures as well as sandy soil (drifting sand, > 2 m deep). The Upper Franconian trial area near Schmellenhof (Figure 1) is at 495 m above sea level and has a rather cool and humid climate and a strongly alternating wet and dry, clayey-loamy soil with waterlogging layers at a depth of approx. 0.5-0.8 m (stagnosol).

During the vegetation periods of 2020 and 2021, the trunk thickness growth of five individual specimens of each of the seven tree species was recorded every half an hour, and data on the microclimate and on soil moisture at two depths were collected. In several measurement campaigns in the field, the leaf water potential was determined, and the hydraulic conductivity of the xylem was determined in the laboratory.

High-resolution measurements of trunk diameter fluctuations show the immediate response of trees to changes in microclimatic conditions. They provide indications of the water status and the (net) growth of trees (Figure 2).

2020 was characterised by the occurrence of prolonged dry phases during the growing season. A dry phase in August in combination with high temperatures resulted in lower overall trunk growth for most species at both sites than in June and July. After heavy rainfall, all tree species took in sufficient soil water and were able to saturate their stem tissues, visible in the swelling of the trunks. In 2021, there were no prolonged dry periods, and no reactions in trunk diameter due to drought stress could be detected. The trunk growth was thus greater in 2021 than in 2020. An interesting observation was made at the beginning of the measurements in 2020, when there was an obvious shrinking of the trunk diameter of all species during a severe late frost in the middle of May.

There were differences in trunk growth between the two sites. The silver lime in Großostheim reacted sensitively to high temperatures and soil dryness, for example. There it had the lowest growth of all species, whereas it grew quite well in Schmellenhof (Figure 3). The biggest diameter growth of all species studied was shown by the western hemlock in both years in Schmellenhof, with an increment of 6.9 mm in diameter in 2020, and 8.8 mm in 2021. For the other species, trunk diameter growth was between 3 and 5 mm and did not differ significantly between the two sites.

The duration of the growth period for 2021 was determined using the effective increases in thickness determined from the dendrometer data. The measurements began at the end of April 2020 in Schmellenhof and at the beginning of May in Großostheim, when relatively warm weather meant that the trunk thickness growth had already started. Figure 3 summarises the calculated values of the annual trunk growth increment and the duration of the growth period for all species for the year 2021. On both sites, the growing season started earlier for conifers (13 to 21 April) than for deciduous trees (9 to 13 May). An exception was western red cedar, which did not show any gain in thickness until 26 April in Großostheim and 3 May in Schmellenhof. Western hemlock was the tree species with the longest period of trunk diameter growth (168 days in Schmellenhof), silver lime the species with the shortest period of growth (81 days in Großostheim).

The xylem water potential (tensile stress in the xylem vessels) fluctuates daily between a (more) relaxed state before sunrise and a more or less tense state (more negative water potential) at midday. This is due to the increased demand for water through transpiration with a limited uptake capacity of the roots. The warmer and drier the ambient air and the less water is available from the soil, the more negative the water potential values are in the leaves and in the xylem. However, plants can limit the increasing tensile stress by closing the stomata.

This conservative strategy of trees, to protect themselves from embolism through early closure of the stomata (isohydry) is at the expense of growth, as it hinders the intake of carbon dioxide for photosynthesis. Late closure of the stomata at lower water potentials (anisohydry) risks the embolism of the conduits, but maintains photosynthesis for longer. The transitions between the strategies are fluid; anisohydric species are considered to be more drought-tolerant. From the minimum achieved water potential Pmin of a species and the determined water potential at which embolism formation starts (P12), a hydraulic safety margin and an indication of the drought sensitivity of a species (Figure 6) are obtained.