Alpine shrub growth follows bimodal seasonal patterns across biomes – unexpected environmental controls

Abstract

Under climate change, cold-adapted alpine ecosystems are turning into hotspots of warming. However, the complexity of driving forces of growth, associated biomass gain and carbon storage of alpine shrubs is poorly understood. We monitored alpine growth mechanisms of six common shrub species across contrasting biomes, Mediterranean and tundra, using 257 dendrometers, recording stem diameter variability at high temporal resolution. Linking shrub growth to on-site environmental conditions, we modelled intra-annual growth patterns based on distributed lag non-linear models implemented with generalized additive models. We found pronounced bimodal growth patterns across biomes, and counterintuitively, within the cold-adapted biome, moisture, and within the drought-adapted biome, temperature was crucial, with unexpected consequences. In a warmer world, the Mediterranean alpine might experience strong vegetation shifts, biomass gain and greening, while the alpine tundra might see less changes in vegetation patterns, minor modifications of biomass stocks and rather browning.

Patterns, timing, and environmental drivers of secondary growth in two physiologically distinct Mediterranean alpine shrub species

“…The observed bimodality in cambial activity can be interpreted as a response to the separation of favorable conditions before and after summer drought (de Luis et al 2007, Camarero et al 2010. It has also been suggested to be present in Mediterranean alpine perennial forbs (Olano et al 2013) and was previously observed in the Mediterranean alpine shrubs studied here (Dobbert et al 2022a).…”

“…As such, resource acquisition is rather optimized during phases when the traits and growth strategies of the species allow for physiological activity at high gain and low costs, resulting in species-specific windows of maximum growth (and heterotrophic growth in A. granatensis). This is of major importance for understanding the adaptive capacity in alpine environments under future climate variability (Olano et al 2013, Kramp et al 2022, Dobbert et al 2022a and contributes to the question of when and how woody plants grow (Jevšenak et al 2022), the knowledge of which is crucial for assessing vegetation evolution across biomes.…”

“…Although predicted winter warming could enhance shrub growth by reducing thermal constraints and, consequently, lengthening the growing season (Castagneri et al 2018), warmer growing seasons coupled with increased aridification (Giorgi and Lionello 2008) might limit growth in these regions (Gazol and Camarero 2012). Both could lead to shifts in growth patterns of species, as growth may continue well into the winter months (Dobbert et al 2022a), while phases of reduced cambium activity during the summer drought might lengthen (Tumajer et al 2021). Moreover, changes may modify the distributional range of species (Bravo et al 2008, Dobbert et al 2022a because shifts in biomass productivity, dispersal, and germination rates have been shown to result in an upward shift in distributions of species and vegetation belts (Grabherr 2003, Sanz-Elorza et al 2003, Körner 2004 and, as previous studies suggested, a shift toward vegetation types currently found in drier and warmer conditions in the Mediterranean mountains (Gritti et al 2006).…”

“…The approach most commonly used by plants to cope with cold stress is cold acclimation, which allows plants to survive freezing by accumulating cryoprotective polypeptides and osmolytes (Ritonga and Chen 2020). Water availability is one of the most limiting factors for plant growth (Battipaglia et al 2014, Szymczak et al 2020, and in addition to summer drought, water shortage during periods with frozen ground is crucial (Dobbert et al 2022a). Plants employ different strategies to cope with drought (Chaves et al 2003, Guo et al 2017.…”

Growth form and leaf habit drive contrasting effects of Arctic amplification in long‐lived woody species

“…Under the ongoing Arctic and low-Arctic warming trends and future projections (IPCC, 2021), tundra shrub will also likely become less temperature-limited and more moisture limited, especially if the warming is coupled with insufficient water availability (Weijers, 2022;Weijers et al, 2017). Recent investigations performed through direct growth measurements with dendrometers, highlighting a bimodal response to climate (Dobbert, Albrecht, et al, 2022;, or adopting satellite-derived vegetation indices (Myers-Smith et al, 2020;Phoenix & Bjerke, 2016), seem to confirm this scenario. However, our results suggest that Icelandic long-lived woody species are currently experiencing a pervasive boost in secondary growth, with tundra shrub rapidly growing and most likely expanding its distribution.…”