Stef Weijers

Centraal Bureau voor de Statistiek | CBS

An improved knowledge of long‐term climatic variations over the Altai‐Dzungarian region will increase our understanding of the current climate and help to predict the effects of global warming on future water availability in this region. We sampled 77 Larix sibirica Ledeb. trees at upper and lower treelines in the southern Mongolian Altai, and reco...

Woody encroachment is increasingly threatening savanna ecosystems, but it remains unclear how this is driven by different land tenures and management systems. In South Africa, communal land is mainly managed under continuous grazing, while commercial land is under rotational grazing. We hypothesize that woody encroachment has increased since the en...

Evergreen dwarf shrubs respond swiftly to warming in the cool and dry High Arctic, but their response in the warmer Low Arctic, where they are expected to be outcompeted by taller species under future warming, remains to be clarified. Here, 12,528 annual growth increments, covering 122 years (1893–2014), were measured of 764 branches from 25 indivi...

The ongoing warming of the Earth’s atmosphere is projected to cause a northward shift of species’ distributions, as they track their climatic optimum. In the rapidly warming Arctic, this has already led to an increase of shrubs in tundra ecosystems. While this northern expansion of woody biomass has been studied relatively extensively over the last...

The North Atlantic Oscillation (NAO) determines wind speed and direction, seasonal heat, moisture transport, storm tracks, cloudiness and sea-ice cover through atmospheric mass balance shifts between the Arctic and the subtropical Atlantic. The NAO is characterized by the typical, yet insufficiently understood, seesaw pattern of warmer winter and s...

1. Evergreen dwarf shrubs respond swiftly to warming in the cool and dry High Arctic, but their response in the warmer Low Arctic, where they are expected to be outcompeted by taller species under future warming, remains to be clarified. 2. Here, 12,528 annual growth increments, covering 122 years (1893-2014), were measured of 764 branches from 25...

Effects of climate change on shrub growth in high mountains: With the rapid warming of the alpine zone shrub growth has increased and shrubs have expanded. Satellite proxies for vegetation biomass have shown greening trends in recent decades over large parts of the tundra biome, which have been linked to shrub growth chronologies, and summer temper...

The majority of variation in six traits critical to the growth, survival and reproduction of plant species is thought to be organised along just two dimensions, corresponding to strategies of plant size and resource acquisition. However, it is unknown whether global plant trait relationships extend to climatic extremes, and if these interspecific r...

Aim Plant functional groups are widely used in community ecology and earth system modelling to describe trait variation within and across plant communities. However, this approach rests on the assumption that functional groups explain a large proportion of trait variation among species. We test whether four commonly used plant functional groups rep...

The High Arctic region has experienced marked climate fluctuations within the past decades strongly affecting tundra shrub growth. However, the spatial variability in dwarf shrub growth responses in this remote region remains largely unknown. This study characterizes temperature sensitivity of radial growth of two willow dwarf shrub species from tw...

Motivation: The Tundra Trait Team (TTT) database includes field‐based measurements of key traits related to plant form and function at multiple sites across the tundra biome. This dataset can be used to address theoretical questions about plant strategy and trade‐offs, trait–environment relationships and environmental filtering, and trait variation...

The tundra is warming more rapidly than any other biome on Earth, and the potential ramifications are far-reaching because of global feedback effects between vegetation and climate. A better understanding of how environmental factors shape plant structure and function is crucial for predicting the consequences of environmental change for ecosystem...

Warming may lead to a cover increase of tundra shrubs and a north-and upward shift of tree-lines. The latter may be inhibited by a densification of shrub stands. However, the climatic drivers of near-treeline shrub growth are relatively unexplored, especially that of shrub species from different functional groups growing intertwined, in competition...

The Arctic and alpine biome is rapidly warming, which might be causing an encroachment of relatively tall woody shrub vegetation into tundra ecosystems, which will probably result in an overall positive feedback to climate warming. This encroachment is, however, believed to remain limited to the relatively warm parts of the biome, where taller shru...

The arctic-alpine biome is warming rapidly, resulting in a gradual replacement of low statured species by taller woody species in many tundra ecosystems. In northwest North America, the remotely sensed normalized difference vegetation index (NDVI), suggests an increase in productivity of the arctic and alpine tundra and a decrease in productivity o...

Rapid climate warming has resulted in shrub expansion, mainly of erect deciduous shrubs in the Low Arctic, but the more extreme, sparsely vegetated, cold and dry High Arctic, is generally considered to remain resistant to such shrub expansion in the next decades. Dwarf shrub dendrochronology may reveal climatological causes of past changes in growt...

Rapid climate warming in the tundra biome has been linked to increasing shrub dominance. Shrub expansion can modify climate by altering surface albedo, energy and water balance, and permafrost yet the drivers of shrub growth remain poorly understood. Dendroecological data consisting of multi-decadal time series of annual shrub growth provide an und...

Deeper winter snow is hypothesized to favor shrub growth and may partly explain the shrub expansion observed in many parts of the arctic during the last decades, potentially triggering biophysical feedbacks including regional warming and permafrost thawing. We experimentally tested the effects of winter snow depth on shrub growth and ecophysiology...

Arctic and alpine ecosystems are among the most sensitive ecosystems regarding climate change, as the species which make up these ecosystems are adapted to the specific and harsh climatic conditions at high latitudes and altitudes. Increasing growing season temperatures may lead to a general northward and upward shift of arctic and alpine ecozones....

The reliability of Cassiope tetragona as temperature proxy might be restricted by influence on growth of precipitation and amount of Photosynthetically Active Radiation (PAR). Carbon-13 discrimination (Δ) in C3-plants is generally influenced by temperature and precipitation and can therefore potentially record important additional climatic informat...

As trees are obviously absent in cold environments such as at high elevations and in the high arctic, dendrochronology has previously largely been limited by the tree-line. In recent years, however, dendrochronology and –ecology of (dwarf) shrubs has emerged as a promising new scientific field, yet largely unexplored. Retrospective growth analyses...

Annual shoot length of the circumarctic dwarf shrub Cassiope tetragona has proved to be a reliable proxy for past and ongoing climate change in the Arctic. This is based on its strong linear relationship with monthly climate parameters. Monthly means are, however, coarse units for prediction of growth in marginal regions with short growing seasons....

The dwarf shrub Cassiope tetragona (Arctic bell-heather) is increasingly used for arctic climate reconstructions, the reliability of which depends on the existence of a linear climate-growth relationship. This relationship was examined over a high-arctic to sub-arctic temperature gradient and under multi-year artificial warming at a high-arctic sit...

Global change is expected to lead to range shifts of plant species. The ecological mechanisms underpinning these shifts are currently not well understood. Here, we compared ecological responses possibly underlying southern range contraction and northern range expansion of Empetrum nigrum, a key species in northern heathlands, which may be related t...

Recent research using repeat photography, long-term ecological monitoring and dendrochronology has documented shrub expansion in arctic, high-latitude and alpine tundra ecosystems. Here, we (1) synthesize these findings, (2) present a conceptual framework that identifies mechanisms and constraints on shrub increase, (3) explore causes, feedbacks an...

Part of Focus on Dynamics of Arctic and Sub-Arctic Vegetation Recent research using repeat photography, long-term ecological monitoring and dendrochronology has documented shrub expansion in arctic, high-latitude and alpine tundra ecosystems. Here, we (1) synthesize these findings, (2) present a conceptual framework that identifies mechanisms and c...

The instrumental Arctic climate record is both temporally and spatially limited. Therefore, there is a need for reliable climate proxies to increase knowledge of past and future Arctic climate change. Annual shoot length increase of the circumarctic dwarf shrub species Cassiope tetragona represents such a new climate proxy. We measured annual shoot...

Annual growth of the polar evergreen shrub Cassiope tetragona on Svalbard was evaluated as a proxy for Arctic summer temperatures. Transfer functions were derived from temperature-growth correlations of shoots and from a temperature-growth response, obtained from experimental warming using open top chambers (OTC) in high Arctic tundra vegetation at...

We study the (sub)arctic evergreen shrubs Cassiope tetragona, Empetrum nigrum and the deciduous Salix polaris in response to global warming in the present time and in the past. Summer temperatures in the arctic climate zone of Svalbard vary from 2-7 °C. Based on number of leaves, length and weight of leaves, leaf scar distances and distances betwee...