Winter crop growth at low temperature may hold the answer for alpine treeline formation

Abstract

Background: Undisturbed high elevation treelines follow a common growing season isotherm, irrespective of latitude. Small stature plants thrive at much higher elevations because they grow in a favourable microclimate near the ground, whereas trees are aerodynamically coupled to free atmospheric circulation, hence the uniform treeline elevation in a given region. Aims: I argue that the treeline results from tree architecture and not from a tree-specific inferior physiology. At tissue level, all cold adapted higher plants (including winter crops) face similar temperature related limitations and are constrained by similar thermal thresholds. Methods: In order to explore this hypothesis, winter rape and winter oat were grown in the field and the daily rate of leaf expansion was measured during early winter, with temperatures similar to those at treeline at the beginning of the growing season (monthly mean temperature of 5 °C). Results: Leaves of these winter crops only grew when the degree hours above 5 °C for a given day exceeded zero. Rape and oats showed very similar responses. The data support a common temperature threshold for tissue formation in winter crops and treeline trees. Conclusions: A literature survey revealed lower temperature thresholds (close to freezing point) in arctic and Antarctic algae. It appears that the formation of complex, partly lignified tissue does not happen below 5 °C, whereas such temperatures exert little constraints on photosynthesis. The study illustrates that tissue growth in winter crops can help understand growth of treeline trees, given that both exhibit similar minimum temperature requirements for meristem functioning.

Treelines Will be Understood Once the Functional Difference Between a Tree and a Shrub Is

“…There is now broad evidence that plant tissue cannot be built at temperatures close to 0°C, and there is very slow (if any) growth activity up to ?5°C, both in cambial and apical meristems, above and below the ground, matching a long known threshold for growth in winter crops as well as trees (for reviews, see Rossi et al 2007;Körner 2008). This means, most nights during the growing season (which may last 12 months in the tropics) are too cold for any length growth of shoots or thickness growth of stems, and also many hours during the day will permit only marginal growth.…”

Hierarchical regression models for dendroclimatic standardization and climate reconstruction

“…The lack of fit is also seen in Figure 2, where M1 exhibits the most egregious failure of the normality test and the highest MSE for individual tree-ring series (as compared to M0 and M2). We also note that some of the models in Schofield et al (2016) produced implausibly cold growing season temperatures (<4°C) (Körner and Paulsen, 2004;Körner, 2008) in their reconstructions for the Abisko site, which we also found when using M1 for the Abisko data, but which was resolved using M0 (see supporting information). This further suggests that the logarithmic transformation imposes an artificial cold bias in the reconstruction.…”

Topoclimatological case-study of Alpine pastures near the Albula Pass in the eastern Swiss Alps

“…During five months (November-March), the monthly mean air temperature was below freezing, and during seven months (October-April) it was below 5 • C, a general threshold for plant growth (Körner, 2008). During the winter months (December-February), the mean daily maxima remained below 0…”