WP1 Plant Growth and Allocation (Leader: Dr G. Phoenix + Ineson, Press, Williams)

The aim of WP1 is to understand how plant growth and C allocation drive C dynamics at the community level and how this relates to plant N status and site fertility.It therefore focuses on testing H1 (plant-soil interactions) and will also provide valuable data for answering H3a (temporal transitions) and H4a (plant respiration).

To address H1, we will undertake replicate peak biomass harvests, above and below ground (van Wijk et al. 2003) to quantify C and N stocks within key component pools of shoot, foliage, roots and soil to determine the relationship between C allocation to components for C sequestration (foliage)and to components for N uptake (roots). This will also allow us to determine the relationship between LAI, total foliar N and soil N and C (H1) and - where a clear relationship is apparent - allow below ground plant and soil C stocks to be estimated from LAI measurements. By undertaking these measurements in key communities and across transitional boundaries, we will be able to quantify confidence intervals for these relationships. Further, the extensive data on biomass allocation across multiple sites will be invaluable for improving model parameterization.

To address H3a, we will undertake weekly measurements of vegetative bud-burst phenology and shoot growth rates (Phoenix et al, 2001, 2003), leaf/tiller production (Stenström & Jónsdóttir, 1997) and leaf-level gas exchange of dominant species along with measurements of root production and turnover (minirhizotrons). This will be undertaken within each community throughout the snow-free period. These measurements will allow us to identify the contribution of each species within a community to vegetation C dynamics (chamber measurements, WP3) throughout the growing season and (importantly) during the key transitional periods of spring and autumn (H3).

H4 will be addressed by using ¹³C pulse labelling within key communities to track the fate of recently fixed ¹³C, and (with the biomass harvest data) determine NPP. We have recently developed 13CO2 labelling techniques (see Ostle et al., 2003) that allow extrapolation of short-term flux measurements in the field to long-term C stock changes (e.g. Rangel-castro et al., 2004). The technique involves short-term in situ pulse-labelling with ¹³CO₂, with subsequent monitoring of the ‘return’ of the ¹³C in respired ¹³CO₂ using a dedicated mobile mass spectrometer. Mass balance calculations then enable the percentage return of the C recently fixed through photosynthesis to be unambiguously calculated, together with any ‘priming’ effects. Our recent development of the field labelling system enables the deployment of the labelling approach at remote sites. In conjunction with data collected in WP3, this work will allow us to determine whether autotrophic respiration is a fixed fraction of gross C fixation over weekly timescales (H4a) and whether more new C is turned over at less fertile sites (H4b). Additionally, subsequent soil and root sampling enables tracking the fate of the ¹³C (or depleted ¹⁴C) label into both physical and biological fractions (e.g. Staddon et al., 2003; Treonis et al., 2004).

Deliverables
A. Datasets on above and below ground C & N stocks and LAI across multiple communities of differing fertility at Abisko.
B. Repeat of A for Kevo. (A and B will be combined into a paper in Glob. Ch. Biol. addressing H2).
C. Temporally detailed data on phenology, growth and leaf-level gas exchange across multiple sites and within transitional boundaries at Abisko.
D. Repeat of C for Kevo. (C and D will be combined into a paper in J. Ecol, addressing H3a).
E. First assessment of root growth and turnover in contrasting arctic communities, response to soil thawing and relationship to physiological activity above ground at Abisko.
F. Repeat of E for Kevo. (A paper in Funct. Ecol).
G. Fate/allocation of recently fixed C quantified (13C) at Abikso. Comparisons made across communities and related to vegetation data from deliverable-B.
H. A repeat of G for Kevo. (A paper in New Phyt. Addressing H4).

Tasks
1. Prepare and test equipment in UK: rhizotron root scanner, IRGAs, 13C mobile lab.
2. Vegetation surveys and selection of key communities and boundary/transition zones.
3. Insertion of minirhizotron tubes; shoot tagging for phenology/growth monitoring.
4. Peak biomass harvests, C and N pool, soil moisture and LAI measurements in sub-set of communities to be monitored in all years. Abisko and Kevo 2006, 2007, 2008.
5. Data analysis and interpretation, manuscript preparation (in all years outside field season).
6. Weekly phenology, growth, leaf gas exchange, soil moisture and rhizotron measurements.
7. Peak biomass vegetation surveys, above and below ground harvests, quantification of C and N pools, LAI, soil N.
8. 13C labelling and tracer study.
Overall, we will work in Abisko in 2007 and Kevo in 2008, though peak biomass harvests, C and N pool and LAI measurements will be undertaken in a sub-set of communities in a location’s “inactive” year to assess inter-annual differences.