What influences the season length of perennial herbs?

During the year, perennial herbs must overcome changes in temperature, precipitation, and light regimes. They must change their active season to match these changes. That is why they have limited time to create leaves, accumulate enough nutrients, and store them in safety beneath the ground before the frost comes. The length of the active season is primarily determined by two factors – the beginning of the growth in spring and senescence (aging and dying of the aboveground parts).

In countries that are in the temperate zone, the frost is the plants' biggest enemy. Especially in spring when frostbite can ruin the whole season for the plant. Therefore, it is important to properly time the beginning and the end of the season to maximize the yield and prevent damage.

Different plant species have different life strategies. Some of them are trying to avoid spring struggle for the light and pollinators, so they start their season during the time when they can be damaged by some delayed frost.  In arid regions, some of the perennial herbs can start to grow earlier so that they avoid lack of moisture during the summer.

In the autumn, they try to avoid the loss of nutrients from the aboveground parts of the plant (stem, leaves), therefore there is the reabsorption of them. They know that autumn is coming because the temperature is decreasing, and the days are getting shorter. But even here we can find differences between the species – some of them can extend the length of their activity to the late fall, because they weren’t active because of the drought in summer, or there was too strong competition for the light.

In the current work, botanists were studying if the length of the season is primarily decided by the spring growth or by the aging and senescing in the autumn. Furthermore, the length of the season differs between the individual groups of the plants whether according to habitats where they grow or according to their characteristics. They were measuring the size of the perennial angiosperm herbs growing in Central Europe during the year. A total of 231 plants growing in the Botanical Garden of the Faculty of Science, Charles University, was measured.

Fig. 1.: Measuring in the Botanical Garden of the Faculty of Science, Charles University. Photo: Tomáš Koubek

Fig. 2.: Measuring in the Botanical Garden of the Faculty of Science, Charles University. Photo: Tomáš Koubek

For the measured plants, the seasonal range was from 73 days to 337 days, eight of the species even overwintered (i.e. they did not retract completely from the surface to the underground structure). Plants that reproduce clonally (they create individuals that are the same as the mother plant) exhibited longer periods of activity. Clonal reproducing plants use rhizomes and stolons for reproduction.  It seems that these species need more time to accumulate enough nutrients to create reproductive structures.

Fig. 3.: Wand loosestrife (Lythrum virgatum). Colourful rings mark measured herbs. Photo: Tomáš Koubek

Fig. 4.: Ashy cinquefoil (Potentilla incana). Photo: Tomáš Koubek

It was found that the beginning and the end of the season are independent of each other (more or less). Differences between the species in the length of the active season are generated especially because of processes in the autumn. The length of the season was primarily determined by the period of the fastest growth in spring and the period of senescence. Herbs grow later in the spring if they are from damp and open sites.

Actual global change is affecting the phenology of plants (periodic changes in the life cycles of plants). That leads to shifts in the beginnings of the seasons and changes in their length. The common start of the growth in the spring is likely the reason why could the length of the season be more affected by the temperature changes and precipitation in the spring than at the end of the growth season. Changes in the quantity and timing of the summer precipitation could be responsible for the intensification of the first senescence wave.

The strong influence of the availability of light and water on the length of the active season suggests that changes in phenological events and the length of the season in individual species will not be the same for every plant, but that they will differ in their habitats.

Most of the studies about plant reactions to climate change are focused on spring phases. Thanks to this study we now know that the key interspecific difference in the length of the season is given by the autumn, not by the spring. It showed that it is important to pay greater attention to the events that are happening in the autumn.

Eliška Rychtecká

Tomáš Koubek, Tereza Mašková, Tomáš Herben, Determinants of interspecific variation in season length of perennial herbs, Annals of Botany, Volume 132, Issue 2, 25 July 2023, Pages 281–291, https://doi.org/10.1093/aob/mcad088