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The commonly observed negative relationship between stomatal density (SD) and atmospheric CO2 has led to SD being proposed as an indicator of atmospheric CO2 concentration. The use of SD as a proxy for CO2 , however, has been hampered by an insufficient understanding of the intraspecific variation of this trait. We hypothesized that SD in Pinus sylvestris, a widely distributed conifer, varies geographically and that this variation is determined by major climatic variables. By sampling needles from naturally growing trees along a latitudinal range of 32.25°, equivalent to 13.7°C gradient of mean annual temperature (MAT) across Europe, we found that SD decreased from the warmest southern sites to the coldest sites in the north at a rate of 4 stomata per mm2 for each 1°C, with MAT explaining 44% of the variation. Additionally, samples from a provenance trial exhibited a positive relationship between SD and the MAT of the original localities, suggesting that high SD is an adaptation to warm temperature. Our study revealed one of the strongest intraspecific relationships between SD and climate in any woody species, supporting the utility of SD as a temperature, rather than direct CO2 , proxy. In addition, our results predict the response of SD to climate warming.

Occupancy of cold habitats by evergreen species requires structural modification of photosynthetic organs for stress resistance and longevity. Such modifications have been described at interspecific level, while intraspecific variation has been underexplored. To identify structural and anatomical traits that may be adaptive in cold environments, we studied intraspecific variability of needles of Scots pine (Pinus sylvestris L.), a wide-ranging tree, along a 1,900 km temperate-boreal transect in Europe. Needles from 20 sites representing mean minimum winter temperatures between -4.0 and -19.9°C and mean annual temperatures between 8.3 and -1.7°C were sampled for measurements of leaf mass per area (LMA, g/m2), leaf density (LD, g/cm3) and 30 other morpho-anatomical traits. Needles from cold sites lived longer, were shorter, showed higher LMA and LD, had narrower and more collapse-resistant tracheids, thicker epidermal cells with thicker cell walls and wider resin ducts occupying larger fraction of needle volume in comparison to needles from warmer sites. Along the steep climatic gradient, needles presented a coordinated phenotypic spectrum of external and internal traits that are largely interpretable in functional, adaptive terms. This intraspecific pattern of covarying traits provides insight into the adaptive syndrome associated with stress tolerance and extended needle longevity under cold conditions of high latitudes. A plain language summary is available for this article. (Less)

Plant functional traits may be altered as plants adapt to various environmental constraints. Cold, low fertility growing conditions are often associated with root adjustments to increase acquisition of limiting nutrient resources, but they may also result in construction of roots with reduced uptake potential but higher tissue persistence. It is ultimately unclear whether plants produce fine roots of different structure in response to decreasing temperatures and whether these changes represent a trade-off between root function or potential root persistence. We assessed patterns of root construction based on various root morphological, biochemical and defense traits including root diameter, specific root length (SRL), root tissue density (RTD), C:N ratio, phenolic compounds, and number of phellem layers across up to 10 root orders in diverse populations of Scots pine along a 2000-km climatic gradient in Europe. Our results showed that different root traits are related to mean annual temperature (MAT) and expressed a pattern of higher root diameter and lower SRL and RTD in northern sites with lower MAT. Among absorptive roots, we observed a gradual decline in chemical defenses (phenolic compounds) with decreasing MAT. In contrast, decreasing MAT resulted in an increase of structural protection (number of phellem layers) in transport fine roots. This indicated that absorptive roots with high capacity for nutrient uptake, and transport roots with low uptake capacity, were characterized by distinct and contrasting trade-offs. Our observations suggest that diminishing structural and chemical investments into the more distal, absorptive roots in colder climates is consistent with building roots of higher absorptive capacity. At the same time, roots that play a more prominent role in transport of nutrients and water within the root system saw an increase in structural investment, which can increase persistence and reduce long-term costs associated with their frequent replacement.