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

Growth of tree (Pinus sylvestris) and shrub (Amelanchier ovalis) species is constrained by drought with higher shrub sensitivity in dry sites

We lack understanding of how variable is radial growth of coexisting tree and shrub species, and how growth is constrained by drought depending on site aridity. Here, we compared the radial growth of two widespread and coexisting species, a winter deciduous shrub (Amelanchier ovalis Medik.) and an evergreen conifer tree (Pinus sylvestris L.). We sampled four sites in Northeastern Spain subjected to different aridity levels and used dendrochronological methods to quantify growth patterns and responses to climate variables. The growth of the two species varied between regions, being lower in the driest sites. The first-order autocorrelation (growth persistence) was higher in more mesic sites but without clear differences between species. Tree and shrub growth negatively responded to elevated summer temperatures and positively to spring-summer precipitation and wet conditions. However, negative growth responses of the shrub to drought were only observed in the two driest sites in contrast to widespread responses of the tree. Abrupt growth reductions were common in the drier sites, but resilience indices show that the two species rapidly recovered pre-drought growth levels. The lower growth synchrony of the shrub as compared to the tree can be due to the multistemmed architecture, fast growth and low stature of the shrub. Besides, the high dependency of the shrub growth on summer rainfall can explain why drought limitations were only apparent in the two driest sites. In any case, results point out to the dendrochronological potential of shrubs, which is particularly relevant giving its ability to inhabit woodlands and treeless regions under harsh climatic conditions. Nevertheless, further research is required to elucidate the capacity of shrub species to tolerate drought, as well as to understand how shrubs thrive in water- and cold-limited environments.

Patterns, timing, and environmental drivers of growth in two coexisting green-stemmed Mediterranean alpine shrubs species

The Mediterranean alpine is one of the most vulnerable ecosystems under future environmental change. Yet, patterns, timing and environmental controls of plant growth are poorly investigated. We aimed at an improved understanding of growth processes, as well as stem swelling and shrinking patterns, by examining two common coexisting green-stemmed shrub species. Using dendrometers to measure daily stem diameter changes, we separated these changes into water-related shrinking and swelling and irreversible growth. Implementing correlation analysis, linear mixed effects models, and partial least squares regression on time series of stem diameter changes, with corresponding soil temperature and moisture data as environmental predictors, we found species-specific growth patterns related to different drought-adaptive strategies. We show that the winter-cold-adapted species Cytisus galianoi uses a drought tolerance strategy combined with a high ecological plasticity, and is, thus, able to gain competitive advantages under future climate warming. In contrast, Genista versicolor is restricted to a narrower ecological niche using a winter-cold escape and drought avoidance strategy, which might be of disadvantage in a changing climate. Pregrowth environmental conditions were more relevant than conditions during growth, controlling the species' resource availability. Thus, studies focusing on current driver constellations of growth may fail to predict a species' ecological niche and its potential future performance.

Climate effect on the growth and hydraulic traits of two shrubs from the top of a Mediterranean mountain

Increasing mean global temperatures in conjunction with increases in the frequency and severity of drought events affect plant growth and physiology, particularly in more arid and mountainous ecosystems. Thus, it is imperative to understand the response of plant growth to climatic oscillations in these regions. This study used dendrochronological and wood anatomical traits of two shrub species growing over 1500 m.a.s.l. in the Serra da Estrela (Portugal), Juniperus communis and Cytisus oromediterraneus, to analyze their response to temperature and water availability parameters. Results showed an increase in shrub growth related to the increase over time of the mean minimum and maximum monthly temperature in Serra da Estrela. Warming seems to promote shrub growth because it lengthens the growing season, although J. communis responds mainly to spring maximum temperature while C. oromediterraneus is influenced by fall maximum temperature. Hydraulic traits of J. communis and C. oromediterraneus were negatively influenced by winter drought. Additionally, there were species-specific differences in response to changes in water availability. J. communis radial growth was significantly affected by spring drought conditions, while C. oromediterraneus radial growth was significantly affected by spring precipitation. C. oromediterraneus hydraulic traits were also significantly affected by drought conditions from the previous spring and fall. This study shed light on specific differences in the response to climate between two co-occurring shrub species in the top of an understudied Mediterranean mountain, which could have implications in the future distribution of woody species within this region.