John D. Marshall

This article comments on: Lack of photosynthetic or stomatal regulation after 9 years of elevated [CO2] and 4 years of soil warming in two conifer species at the alpine treeline

The effects of the past century's increase in atmospheric CO₂ concentration ([CO₂]) have been recorded in the stable carbon isotope composition (δ¹³C) of the annual growth rings of trees. The isotope record frequently shows increases in photosynthetic CO₂ uptake relative to stomatal conductance, which estimates the CO₂ concentration gradient across the stomata (c_a − c_i). This variable, which is one control over the net photosynthetic rate, has been suggested as a homeostatic gas-exchange set point that is easy to estimate from δ¹³C and [CO₂]. However, in high-latitude conifer forests, the literature is mixed; some studies show increases in (c_a − c_i) and others show homeostasis. Here we present leaf and tree-ring δ¹³C data from a controlled experiment that tested factorial combinations of elevated [CO₂] (365 and 700 ∝mol mol⁻¹) and fertilization on mature Norway spruce (Picea abies (L.) Karst.) trees in northern Sweden. We found first that the leaf carbon pool was contaminated by the current photosynthate in the older leaf cohorts. This is the reverse of the common observation that older photosynthate reserves can be used to produce new tissue; here the older tissue contains recent photosynthate. We found that the tree-ring data lack such contamination and in any case they better integrate over the canopy and the growing season than do leaves. In the second and third years of treatment, elevated [CO₂] alone increased (c_a − c_i) by 38%; when combined with fertilization, it increased (c_a − c_i) by 60%. The results of this study support the idea that annual rings provide a clearer isotopic signal than do foliage age-classes. The tree-ring data show that inferred (c_a − c_i) depends not only on [CO₂], but also on mineral-nutrient status. The differences in (c_a − c_i) are sufficiently large to account for the treatment-induced increase in wood-volume production in these stands.

The stable oxygen isotope ratio (δ¹⁸O) of tree-ring cellulose is jointly determined by the δ¹⁸O of xylem water, the δ¹⁸O of atmospheric water vapor, the humidity of the atmosphere and perhaps by species-specific differences in leaf structure and function. Atmospheric humidity and the δ¹⁸O of water vapor vary seasonally and annually, but if the canopy atmosphere is well mixed, atmospheric characteristics should be uniform among co-occurring trees. In contrast, xylem water δ¹⁸O is determined by the δ¹⁸O of water being drawn from the soil, which varies with depth. If co-occurring trees draw water from different soil depths, this soil-water δ¹⁸O signal would be manifest as differences in δ¹⁸O among the trees. We examined the variation in tree ring δ¹⁸O, over eight decades during the 20th Century, among three species co-occurring in natural forest stands of the northern Rocky Mountains in the USA. We sampled 10 Douglas-firs (Pseudotsuga menziesii (Mirb.) Franco var. glauca), 10 ponderosa pines (Pinus ponderosa Laws.) and seven western white pines (Pinus monticola Dougl.). As expected, variation in atmospheric conditions was recorded in the δ¹⁸O of the cellulose produced in a given year, but observed climatic correlations with δ¹⁸O were weak. Significant correlations with June climate data included: daily maximum temperature (r = 0.29), daily minimum temperature (r = −0.25), mean temperature (r = 0.20), mean daily precipitation (r = −0.54), vapor pressure deficit (r = 0.32) and solar radiation (r = 0.44). Lagged effects were observed in Douglas-fir and western white pine. In these species, the δ¹⁸O of a given annual ring was correlated with the δ¹⁸O of the previous ring. Ponderosa pine showed no significant autocorrelation. Although the species means were correlated among years (r = 0.67 to 0.76), ponderosa pine was consistently enriched in δ¹⁸O relative to the other species; differences were close to 2%‰ and they are steadily increasing. Relative to the mean for the three species, ponderosa pine is becoming steadily more enriched (−1.0%‰). In contrast, Douglas-fir is being steadily depleted and western pine is intermediate, with an enrichment of 0.5%‰. Because all trees were exposed to the same atmospheric conditions, the differences in δ¹⁸O observed between species are likely due either to differences in the depth of water extraction or leaf function. If the former, presumably ponderosa pine has steadily taken up more water from near the soil surface and Douglas-fir has shifted uptake to a greater depth. If the latter, we suggest the pronounced changes in leaf-water δ¹⁸O are a result of changes in leaf structure and function with tree size and age.