Effects of Medium- and Long-Term Soil Warming on Plant Photosynthesis in a Subarctic Grassland

High-latitude ecosystems are already experiencing accelerated warming, and predictions indicate that those areas will warm more than the global average in the coming decades. There is a lack of long-term manipulation experiments in Arctic and subarctic grasslands that can help with predictions on how these changes will affect those keystone ecosystems.

Here, we investigate the effects of medium-term (16 years) and long-term (>60 years) soil warming on leaf-level gas exchange measured in situ on Ranunculus acris L. (R. acris) growing at unmanaged grasslands at the ForHot soil warming infrastructure in southern Iceland. Measurements were done in plots having no additional warming (ambient) or with an increase in mean annual soil temperature of +8°C. Clamp-on measurements as well as response curves for both intracellular CO₂ (A/C_i) and light (A/I) were done, and non-linear modelling and the Farquhar model were used to estimate different physiological or photosynthetic traits. Finally, chemical analyses on the measured leaves were executed to gain further insights into apparent changes.

Our results showed little to no significant effect of prolonged soil warming on the characteristics of the A/C_i or A/I curve parameters, indicating a conservative response in C uptake per unit leaf area. Also, no significant effects were found for stomatal conductance (g_s), stable isotope ratio (δ¹³C) and leaf-N between the soil warming treatments, indicating that the expected indirect effects of the prolonged soil warming were not apparent. However, across the entire experiment (i.e., across all soil temperature plots), R. acris showed a strong positive response to leaf-N concentrations across almost all estimated traits. Indicating that variability in plant N status was still the primary indirect driver of photosynthetic capacity in these ambient and warmed subarctic grasslands, irrespective of soil warming or duration of warming.

Our findings suggest that R. acris already had high photosynthetic capacity in soils at ambient conditions, and it may therefore have allocated additional nutrients acquired in warmer soils to other growth-related processes rather than to enhancing the photosynthetic system at a leaf level.