How does spatial heterogeneity affect inter‐ and intraspecific growth patterns in tundra shrubs?

Abstract

Arctic and alpine ecosystems are strongly affected by rapidly changing environmental conditions, resulting in profound vegetation shifts, which are highly heterogeneous and hard to predict, yet have strong global impacts. Shrubs have been identified as a key driver of these shifts. In this study, we aim to improve the understanding of how such broad‐scale vegetation changes are locally impacted by inter‐ and intraspecific plasticity and topographically driven heterogeneity in microsite conditions. We assessed continuous stem diameter variation of three dominant tundra shrub species at daily resolution during 5 years, using high‐precision dendrometers, thus bridging the gap between classical dendroecology and plant physiology. From this data, we identified distinct growth patterns which we linked to microsite environmental drivers. The observed patterns appeared highly variable depending on site and species, strongly influenced by characteristics of the individual plant. As the main driver of this variability, we identified fine‐scale topographic complexity, causing the sampled specimens to adjust locally by developing distinct growth strategies. We found these strategies strongly related to snow‐cover variation and associated freezing and thawing. Predicted changes in winter conditions and associated snow regimes will therefore have strong effects on shrub growth and community structure, yet, these effects are highly complex and not uniform in direction. Synthesis. The ability to adapt in a heterogeneous environment appeared highly differentiated between species and closely connected to intraspecific plasticity. Here, we identified spatial variability related to local topography as a main indicator for potential future redistribution and niche shifts in response to environmental change.

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

“…For the tundra biome, it is generally assumed that radial growth of woody plants is mainly restricted to a short phase of active xylogenesis during the warmest, snow-free parts of the year from June to August 14 , with growth onset in spring triggered by gradually rising temperatures or temperature thresholds 11 , 33 . Here, we found an earlier onset of radial stem expansion in all three tundra species, closely linked to the rise of available soil moisture associated with thawing soils or snow melt.…”

“… 31 , our results did not indicate any influences of carbon depletion, limiting such growth processes at the start of the growing season, but late frost events might limit early season cambial activity in the tundra biome 46 . Here, shrubs have developed important adaptive mechanisms for survival during the extreme winter conditions characterizing the region, including a phase of active cell water reduction to avoid cell damage at the exposed ridge positions, where a protective snow cover is missing 14 , 17 . Accordingly, radial stem growth ceased during autumn in our species, and we found the timing of this growth cessation closely linked to the drop in soil moisture associated with soil freezing.…”

“…This link to temperature and the conditions of the free atmosphere is thought to be less pronounced in shrubs, compared to trees, mainly due to their lower stature 12 . At the same time, recent studies emphasize the local impacts of other drivers on plant growth and distribution, including the duration and spatial distribution of snow cover 13 , 14 and associated soil moisture regimes 7 , 15 . Potential shifts in the amount and seasonality of precipitation might therefore additionally affect growth of woody plants in these areas.…”

“…To monitor intra-annual and spatial patterns in radial stem expansion and contraction, xylem phenology and development, as well as their relations to the environment, high-precision dendrometers have proven to be effective tools for trees 33 , 35 , and, more recently, for shrubs 14 , 36 . Capable of recording changes at a high temporal resolution, they reveal insights at the relevant time scales of action, likely to bridge the obvious gap between plant physiology (i.e., xylem formation) and its outcome, overall radial stem growth 37 .…”

Wood Anatomical Traits Respond to Climate but More Individualistically as Compared to Radial Growth: Analyze Trees, Not Means

“…This modeling approach is an excellent tool to represent the hierarchical sampling design and has found wide application in ecological studies in recent years as an excellent tool to decompose the trait variation within grouped data (e.g., [41,42]).…”