Differences in growth between shrubs and trees: How does the stature of woody plants influence their ability to thrive in cold regions?

Shrubs can be found far above or beyond cold tree limits. However, the mechanisms shrubs employ to thrive at sites not allowing the development of trees remain poorly understood. We hypothesize that shrubs are advantaged over trees thanks to: (i) their low stature reflected in a better thermal environment; (ii) differences in temperature thresholds of wood formation; and (iii) a shorter period of wood formation in the slender stems of shrubs with narrow cells compared to tree stems with large cells. We studied wood formation of Picea abies (trees) and Pinus mugo (shrubs) growing on the same site in the treeline ecotone of the Krkonoše Mts in the Czech Republic. We measured air temperature near tree (shrub) tops, stem temperature and soil temperature in the root zone. In addition, we determined the number of cells in individual phases of wood phenology. We then computed the duration of individual wood-phenology phases and temperature thresholds for the onset of wood formation. Our results show that in the growing season, shrubs experience higher amplitude of air and stem temperatures compared to trees. Mean growing season air and stem temperatures are similar between the two growth forms whereas mean soil temperatures are lower for shrubs because their dense canopies shade the ground. Temperature thresholds for wood formation are either similar (3 °C for soil temperature, onset of cell division) or greater by 1.2–2.6 K (onset of cell enlargement) for P. mugo shrubs than for P. abies trees, depending on the temperature metrics considered. Although we found ambiguous differences in the onset of wood formation, this was completed earlier in P. mugo than in P. abies, leading to a generally shorter growing period of shrubs (103 days) than trees (125 days). In conclusion, the main advantage of shrubs over trees resides in the earlier completion of wood formation and thus a shorter growing season. Trees with wide cells at stem base require more time for cell differentiation and maturation than shrubs with narrow cells. Other differences are either of lesser importance (the ambient thermal environment) or probably species-specific (temperature thresholds for wood formation).