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Satellite images reveal northwestern Siberia as a green and vegetated region. A new study by Bjerknes scientists looks into variations in plant productivity in recent decades. Background map: NASA

Varying climate response of arctic and subarctic vegetation

Warm summers have made the Siberian tundra greener. A new study shows that higher temperatures do not always equate to greening. Rainfall may be a limiting factor for some forest types. 

Written by Martin Miles and Victoria Miles from the Bjerknes Centre and NORCE and NERSC, respectively.

A Bjerknes Centre collaboration has produced a new publication on the climate response of arctic and subarctic vegetation in Siberia. Earlier satellite studies using normalized difference vegetation index (NDVI) reported changes in high-latitude bioproductivity in recent decades, including widespread “greening” of tundra, an apparent response to warming in the Arctic.

However, when researchers at the Nansen Center looked in finer detail across a vast expanse of northern West Siberia, it was revealed just how spatially heterogeneous (patchy) the NDVI trends really are. There are contrasting trends not only between bioclimatic zones but also within them, due to differences between forest types.

Siberia
Complex patterns of vegetation productivity trends in northwest Siberia, 2000-2016. (a) Positive trends (“greening”) appear mostly in the unforested (tundra) and deciduous needleaf (larch) in the forest-tundra biome. (b) Negative trends (“browning”) appear mostly in the northern and middle taiga regions. Ill.: Miles et al., 2019.

So, in this new collaborative study, we wanted to see whether – despite the noisy appearance –  there are any general relationships between climate and vegetation productivity. We analysed maximum NDVI – a well recognised proxy of vegetation productivity – and climate variables across four bioclimatic zones (tundra, forest–tundra, middle taiga and northern taiga) across northern West Siberia, further stratified into eight forest–land cover types. We isolated three significant relationships, with positive correlations between (1) tundra NDVI and June-July temperature, (2) middle taiga NDVI and July precipitation, and (c) deciduous needle-leaf (larch) NDVI and July temperature.

Our study period happened to end with 2016, a record warm summer with temperatures several degrees above normal across most of the region, while it was wetter than average in the taiga region and drier than average in the tundra region. As a response to these favorable conditions, vegetation productivity was high (“greening”) across all biomes. However the tundra was not as green as would be expected from the general temperature relationship. Meanwhile the boreal forest thrived under warm and wet conditions. These results highlight the large spatial-temporal variability in vegetation growth responses to climate change in northern areas. The response is determined by the limiting factors of plant growth inherent for a particular biome, suggesting that it is important to model future precipitation as well as temperature.

Climate and vegetation response
Climate and vegetation response during the record warm summer 2016 in northern West Siberia. Seasonal composites of June–July anomalies in (a) surface air temperature (SAT, °C), (b) precipitation (mm per day) and (c) NDVImax for 2016, showing positive anomalies (green) across most of the region. (d) Co-variability between tundra NDVImax and June–July SAT anomalies across the tundra region, 2000–2016. Ill.: Miles et al., 2019. 

Reference

Miles, M. W., Miles, V, and Esau I. (2019). Varying climate response across the tundra, forest–tundra and boreal forest biomes in northern West Siberia. Environmental Research Letters 14, doi:10.1088/1748-9326/ab2364.