Performance of the treeline FACE
The design and setup of an experimental system to expose native plants in the alpine treeline ecotone to an elevated atmospheric CO2 concentration was challenging because of the difficult access (no roads creating CO2 transport problems), and the steep and uneven terrain. These constraints limited the construction of large sized field installations and made the use of a traditional free-air CO2 enrichment (FACE) system (Hendrey et al., 1993, 1999) impractical. FACE rings typically have a diameter of > 20 m and use around 2 tons CO2 per ring and per day. While considerably reducing CO2 consumption, the use of tall open top chambers (OTCs) would have posed other problems, technical difficulties for field installation, large changes in microclimate to the frequent wind occurrence, and high radiation being the most obvious ones. Hence, we customized the pure CO2-release technology (Miglietta et al., 2001; Okada et al., 2001; S. Roberts, pers. comm.) for the peculiar situation on a mountain slope. The CO2 control system described here was able to maintain [CO2] within ±20% of the target value for 90% of the exposure time (1-min means), similar to the > 91% and 90% in the pure CO2 FACE systems described, respectively, by Miglietta et al. (2001) and Okada et al. (2001), and to the 92% in the ‘traditionally designed’ Duke forest FACE (Hendrey et al., 1999). One minute average [CO2] within ±10% of the target value were recorded for 63% of the exposure time in this study which is similar to the 60% reported for the rice FACE (Okada et al., 2001), but somewhat lower than the 69% for the Duke forest FACE (Hendrey et al., 1999) and the 75% for the POPFACE (Miglietta et al., 2001). It should be emphasized that our 1-min averages actually derived from four 1-s measurements every 15 s, and therefore rather underestimate the proportion of exposure time within a certain limit of target compared to the estimates of other FACE systems based on averages of continuous readings.
Spatial distribution of [CO2] was rather homogenous within the exposed volume of air. Continuous measurements at the periphery and in the center of the CO2 enriched plots at mid-height of the tree canopies showed rarely differences exceeding c. 60 ppm CO2. Extensive [CO2] measurements during the initial phase of system setup and in a prototype system constructed in Basel before the actual experiment started, indicated decreasing [CO2] from the bottom to the top of the plot (data not shown). This [CO2] gradient was commonly less than 100 ppm with [CO2]c. 50 ppm higher than the target at the bottom and c. 50 ppm lower than the target at the top of the plot, resulting in more than 80% of the total exposed volume being within ±10% of the target. Spatial variability of [CO2] in our system was lower than in other pure CO2 fumigation systems (Miglietta et al., 2001; Okada et al., 2001; Pepin & Körner, 2002). This is explained by the much smaller plot size chosen here and by the use of a vertical array of small diameter laser-drilled CO2 emitting holes of a high density (an average of 215 holes per 1.4 m3 volume of air). The four independently acting CO2 control points within such a small volume of air, enabled us to achieve a highly fine-tuned CO2 control and regulation. The spatially close arrangement of [CO2] control points used in the CO2 regulation algorithm was the reason why measurements of wind speed and direction were not needed for a reliable feedback algorithm, which may be the most evident difference to other FACE systems that need controlling for [CO2] over an area of up to 650 times the size of our plots.
Short-term variability in [CO2] and excursions of very high [CO2] (> 1000 ppm) are likely to occur more frequently in any FACE using pure CO2 injection than in FACE operated with premixed air using blowers (Pinter et al., 2000). Such fluctuations in [CO2] can affect physiological processes in plants (Cardon et al., 1995). However, we recorded [CO2] higher than 1000 ppm less than 0.1% of the time and [CO2] deviating more than ±20% of the target value lasted seldomly longer than a few seconds. It takes at least one minute to induce changes in stomatal conductance in most plants and even longer in trees (Ellsworth et al., 1995; Hendrey et al., 1997; Saxe et al., 1998). Therefore, it is unlikely that short-term variations in [CO2] observed here had any significant effect on plant responses to elevated [CO2].
Using pure CO2 emitted through fine tubing with very small diameter laser-drilled holes under high pressure provided a useful alternative type of FACE system for the CO2 enrichment of vegetation in the treeline ecotone. The fine tubing, originally designed for irrigation purposes and first used for atmospheric CO2 enrichment by Steven Roberts from San Diego State University (pers. comm.) is flexible in its application and might be the only alternative for in situ CO2 enrichment in some ‘difficult’ environments. It has even been modified for use in 35 m tall old-growth forest canopies (web-FACE, Pepin & Körner, 2002), until recently not believed to be possibly studied in CO2 research.
Growth of alpine treeline conifers in a CO2 enriched atmosphere
The physiological responses to elevated [CO2] observed at the needle and branch level in the two tree species studied, are among the most consistent plant responses to atmospheric CO2 enrichment (Ceulemans & Mousseau, 1994; Poorter et al., 1997; Norby et al., 1999; Körner, 2000). The mean enhancement in photosynthesis of 51% (on a needle area basis) in response to elevated [CO2] measured in needles developed after the initiation of the CO2 treatment, compares well with the mean stimulation of 66% calculated from a number of studies with trees growing in the field (Norby et al., 1999), and even better if only conifers are considered (53%, Norby et al., 1999). The CO2 responses in stomatal conductance (gs) reported in the literature are not consistent and range from no differences to comparatively large reductions (Curtis & Wang, 1998; Norby et al., 1999). A frequently confirmed pattern, however, is that conifers show less and often not significant reductions in gs than do deciduous tree species (Saxe et al., 1998). In comparison to recent studies with different conifer species showing no or only moderate (up to −14%) CO2 effects on gs (Dixon et al., 1995; Tissue et al., 1997; Wang & Kellomäki, 1997; Ellsworth, 1999), we found slightly greater and significant responses to elevated [CO2] in the present study. However, the CO2 effect on gs can change somewhat over the course of the season (Egli et al., 1998) which was not assessed here. Moreover, larch as the more responsive of the two studied species functionally compares better with broadleaf deciduous species than with conifers.
The data compilation by Norby et al. (1999) showed an average decrease in leaf nitrogen concentration of 11% in conifers that is a little more than was found here. Lower leaf [N] in larch and pine trees observed here, was exclusively due to a dilution effect of higher nonstructural carbohydrate (NSC) concentrations under elevated CO2. If expressed on either a leaf area basis or on NSC-free needle dry mass the CO2 effect on needle [N] disappears (data not shown). These data suggest that CO2 is unlikely to have had any effect on N allocation during that first year of exposure.
Both tree species showed immediate growth responses upon CO2 enrichment in the first growing season. While such fast responses to increasing CO2 are commonly found when starting with small seedlings, particularly when they are not limited by other resources (Ceulemans & Mousseau, 1994; Norby et al., 1999), it was a rather unexpected result in our study with comparatively old trees, growing in the densely vegetated treeline ecotone. Trees competing for resources other than CO2 and rooting in soils of naturally low fertility showed no or only moderate CO2 induced growth stimulation in several previous studies (Norby et al., 1992; Hättenschwiler & Körner, 1998; Spinnler et al., 2002). Moreover, current-year shoot elongation in trees with a determinate shoot growth pattern, such as pine and larch, is believed to be largely determined by previous year's carbon balance and bud formation. For that reason, CO2 effects on shoot growth – if any – were expected to occur in the second year of growth under treatment conditions at the earliest. The up to 23% increase in length growth of current-year shoots exposed to elevated CO2, thus, is considered a particularly strong response to CO2 enrichment in the trees studied here.
Higher rates of photosynthetic carbon assimilation, increased accumulation of nonstructural carbohydrates in leaves and branches, and increased shoot growth, all are strong evidence for a significantly improved carbon balance in larch and pine trees growing in a CO2-enriched atmosphere at treeline. With regard to our first hypothesis (tree growth at treeline is carbon limited), these results suggest that tree growth at the upper alpine treeline might indeed be limited by carbon availability and that rising atmospheric [CO2] can stimulate tree growth in the treeline ecotone. However, this is a preliminary conclusion based on first-year data and needs verifying in the coming years with an extended assessment of growth including measurements of stem diameter increment and root growth. A long-term perspective is generally important in field experiments due to the variability of climatic factors affecting the CO2 response (Körner, 2000), and particularly in studies involving long-lived plants such as trees, because diminishing CO2 responses over time are likely (Körner, 1995; Loehle, 1995; Hättenschwiler et al., 1997; Oren et al., 2001). The first-year data reported here form an important baseline for a multiyear evaluation of the responses of these trees to atmospheric CO2 enrichment, as necessary for the testing of hypotheses about causes of treeline formation and possible consequences for treeline dynamics in response to global change.
There is little evidence in support of our second hypothesis (tree species differ in their response to CO2) so far, because larch and pine trees were similarly affected by elevated CO2. Nonetheless, the significantly greater CO2 effect on photosynthesis per unit of needle dry mass in larch compared to pine may be indicative of future stronger responses of this deciduous conifer in the long run. Needless to say that this remains speculative because CO2 effects on rates of photosynthesis and growth rarely correlate (Curtis et al., 1996), and sustained CO2 stimulation of leaf-level photosynthesis at times may have no detectable influence on growth in the same plants (Hättenschwiler & Körner, 1996; Egli et al., 1998). On the other hand, minute not yet identified differences in the CO2 responses between species might gain in importance over time, particularly if biotic interactions such as those with understory species, herbivores or mycorrhizae come into play.