Effects of CO2 Treatments on Functional Carbon Efficiencies and Growth of Forest Tree Seedlings: A Study of Four Early-Successional Deciduous Species

Abstract

Atmospheric CO2 levels have been increasing, and these changes may result in differential adaptive responses in both genera and species and highlight the need to increase carbon sequestration. Ecophysiological and morphological responses of four early-successional deciduous species were examined under ambient CO2 (aCO2, 400 ppm) and elevated CO2 (eCO2, 800 ppm) treatments. The four species, all of which are used in restoration, were Alnus viridis subsp. crispa (Ait.) Turrill (green alder), A. incana subsp. rugosa (Du Roi) R.T. Clausen (speckled alder), Betula populifolia (Marshall) (gray birch), and B. papyrifera (Marshall) (white birch); all are from the same phylogenetic family, Betulaceae. We examined biochemical efficiencies, gas exchange, chlorophyll fluorescence, chlorophyll concentrations, foliar nitrogen (N), and growth traits. A general linear model, analysis of variance, was used to analyze the functional carbon efficiency and growth differences, if any, among genera, species, and provenances (only for growth traits). The alders had greater biochemical efficiency traits than birches, and alders upregulated these traits, whereas birches mostly downregulated these traits in response to eCO2. In response to eCO2, assimilation either remained the same or was upregulated for alders but downregulated for birches. Stomatal conductance was downregulated for all four species in response to eCO2. Intrinsic water use efficiency was greater for alders than for birches. Alders exhibited a consistent upregulation of stem dry mass and height growth, whereas birches were somewhat lower in height and stem dry mass in response to eCO2. Foliar N played an important role in relation to ecophysiological traits and had significant effects relative to genus (alders > birches) and CO2 (aCO2 > eCO2), and a significant genus × CO2 interaction, with alders downregulating foliar N less than did birches. Covariate analysis examining carbon efficiency traits in relation to foliar N showed clear functional responses. Both species in both genera were consistent in their ecophysiological and morphological responses to CO2 treatments. There was supporting evidence that assimilation was sink-driven, which is related to a plant organ’s ability to continue to grow and incorporate assimilates. The alders used in this study are actinorhizal, and the additional available foliar N, paired with increased stem dry mass sink activity, appeared to be driving upregulation of the carbon efficiencies and growth in response to eCO2. Alders’ greater carbon efficiencies and carbon sequestration in impoverished soils demonstrate that alders, as opposed to birches, should be used to accelerate ecological restoration in a world of increasing atmospheric CO2.

Effects of CO2 and Soil Moisture Treatments on Morphological and Allometric Trait Variation in Coppiced Seedlings: A Study of Four Early-Successional Deciduous Species

“…Elevated CO 2 did not result in additional growth in birches, consistent with previous findings [19], and it was determined to be a result of (1) foliar N limitation and (2) a lack of sink activity, which are often observed limitations under eCO 2 [3,[22][23][24]. These factors have previously resulted in birch assimilation downregulation (A dr ) of −29%, but not in alders (+3%) [19]. Although from the same biological family, birches are not actinorhizal and thus cannot fix atmospheric N. Positive linear relationships between photosynthetic efficiency and foliar N have been found [25,26], and we found that foliar N was three times greater in alders than in birches (Figure 7).…”

“…Elevated CO 2 did not result in additional growth in birches, consistent with previous findings [19], and it was determined to be a result of (1) foliar N limitation and (2) a lack of sink activity, which are often observed limitations under eCO 2 [3,[22][23][24]. These factors have previously resulted in birch assimilation downregulation (A dr ) of −29%, but not in alders (+3%) [19].…”

“…Seed lots were taken from natural populations across three provinces of Canada, including New Brunswick, Nova Scotia, and Prince Edward Island. Please see [19] for a description of growing and cold-storing seedlings prior to planting in the experiment. Four replicates per provenance were randomly planted in each of eight chambers in individual fabric, root-control bags that measure 30 cm in diameter and 23 cm in height (Smart Pot PRO 5 Gallon, High Caliper Growing Systems, Oklahoma City, OK, USA) filled with a sand medium.…”

“…The soil texture properties were, on average, 1.8% clay, 7.8% silt, and 90.4% sand, with no significant nutrient or pH differences between blocks (Table 2). Please see [19] for a description of the experimental chambers used and environmental monitoring. The eight chambers were housed across two separate greenhouses (considered blocks) located at the Canadian Forest Service-Atlantic Forestry Center (CFS-AFC) in Fredericton, NB, Canada (45 • 52 ′ N, 66 • 31 ′ W).…”

Ecological Restoration in Eastern Canada Using Four Early-Successional Species on Severely Degraded Sites Using a Factorial of Site-Preparation Treatments: Growth and Biomass over Two Years

“…However, once established, it fully benefited alder's second-year growth. The increase in alder foliar N concentrations, as found by [52], allowed for far greater growth compared to birch as there was very little soil N (mean soil N = 0.12%). According to Ekblad [50], the N fixation by Alnus does not assist other nearby species in the uptake of N since the leaves are where the majority of N is allocated; during the fall, most foliar N is reallocated back into the stem and roots but there is residual N in senesced leaves.…”