: On growth patterns and mechanisms in arctic-alpine shrubs

Abstract

Arctic-alpine ecosystems are considered hot-spots of environmental change, with rapidly warming conditions causing massive alterations in vegetational structure. These changes and their environmental controls are highly complex and variable across spatial and temporal scales. Yet, despite their numerous implications for the global climate system, the underlying physiological processes and mechanisms at the individual plant scale are still little explored. Using hourly recordings of shrub stem diameter change provided by dendrometers, paired with on-site environmental conditions, enabled us to shed light on these processes. In this way, growth patterns in three widely distributed shrub species were assessed and linked to thermal and hygric conditions. We started our analysis with a close examination of one evergreen species under extreme environmental conditions, followed by a comparison of evergreen and deciduous species, and, finally, a comparative look at growth patterns across local micro-habitats. The results revealed distinct growth strategies, closely linked to species-specific water-use dynamics and cambial rhythms. Within the heterogenous alpine landscape these conditions were mainly attributed to the variation in local micro-habitats, defined by fine-scale topography and consequent variation in snow conditions and exposure. Thus, the overall growth success was mainly controlled by complex seasonal dynamics of soil moisture availability, snow conditions, and associated freeze–thaw cycles. It was therefore in many cases decoupled from governing regional climate signals. At the same time, exceedingly high summer temperatures were limiting shrub growth during the main growing season, resulting in more or less pronounced bimodal growth patterns, indicating potential growth limitation with on-going summer warming. While shrubs are currently able to maximize their growth success through a high level of adaptation to local micro-site conditions, their continued growth under rapidly changing environmental conditions is uncertain. However, our results suggest a high level of heterogeneity across spatial and temporal scales. Thus, broad-scale vegetational shifts can not be explained by a singular driver or uniform response pattern. Instead, fine-scale physiological processes and on-site near-ground environmental conditions have to be incorporated into our understanding of these changes.

Complex environmental control of growth in a dominant Mediterranean‐alpine shrub species

“…Thus, our results confirm recent findings of bimodal growth patterns in alpine shrub species (Albrecht et al, 2023(Albrecht et al, , 2024Dobbert, Albrecht, et al, 2022;Olano et al, 2013). In contrast to lowland species, in which these bimodal patterns likely arise from favourable warm and humid conditions during spring and autumn, but reduced growth rates in summer, and short winter dormancy (Cherubini et al, 2003), we found additional winter stem contraction when frozen ground occurred in our alpine species (Figure 3), similar to arctic-alpine shrubs, where such processes have been interpreted as a mechanism to protect the plant from embolism under frost droughts (Dobbert et al, 2022b). At the same time, the observed overall low stem water deficit during winter (Figure 4b) suggests an et al, 2024;Steppe et al, 2015).…”

“…To monitor radial stem diameter variations of our species, we used high-precision point dendrometers (type DD-RO; Ecomatik, Dachau/Germany) designed to measure changes in stem diameter of particularly small stems (0-20 mm diameter) and capable of generating data with unprecedented temporal resolution, providing deep insights into seasonal growth patterns and mechanisms of arctic-alpine and Mediterranean-alpine shrubs (Albrecht et al, 2023(Albrecht et al, , 2024Dobbert et al, 2021aDobbert et al, , 2021bDobbert, Albrecht, et al, 2022;Dobbert et al, 2022b). The dendrometers had sensors with a temperature coefficient of <0.2 μm/K, recorded data at 1-min intervals and were mounted on a T-shaped aluminium bar that we carefully attached to the main stem of the selected specimens horizontally above the ground surface.…”

“…To minimise the influence of hygroscopic swelling and shrinking of the bark (Zweifel & Häsler, 2001), we removed the outermost layers of dead periderm (0.5 mm) at the contact point of the dendrometer (cf. Dobbert et al, 2022b). We took care to place dendrometers as close as possible to the specimens' root collar, which is thought to integrate the growth of the whole plant (Bär et al, 2007;Ropars et al, 2017), although the extent to which changes in radial stem diameter vary within individual specimens and between multiple stems of the same specimen has not been fully explored (Dobbert et al, 2021b).…”

“…Thus, growth of alpine shrubs from different biomes has been shown to be not only influenced by the regional climate (e.g. Bär et al., 2008; Gazol & Camarero, 2012a; Li et al., 2013) but also by the local environment which has been addressed as the major driver of shrub growth (Dobbert et al., 2022a; Gazol & Camarero, 2012b). In this context, plastic xylogenesis has been described as a competitive advantage under the Mediterranean climate regime (cf.…”