- Top of page
- Materials and Methods
Efforts to increase carbon sequestration have focused renewed interest on understanding how forest management affects forest carbon gain over timescales of decades. Two of the most common forest management tools are thinning and fertilization, and yet details of the physiological mechanisms responsible for carbon gain responses to management are often lacking, particularly in relation to long-term responses over decades of forest management. One of the best sources of information on long-term growth dynamics in response to management comes from long-term growth and yield plots with associated controls (Allen et al., 1990; Stegemoeller & Chappell, 1990; Brix, 1993; Balster & Marshall, 2000; Jokela et al., 2004). However, with relatively few exceptions, data collected in these long-term trials are focused on diameter and volume growth with other parameters only being measured infrequently for specific shorter-term studies. Very few of these trials have any long-term physiological data, and even those data span less than a decade (Brix, 1993). Therefore, better understanding of the underlying physiological mechanisms behind growth increases is critical for predicting how forest carbon gain might respond to management under changing climate conditions.
Some past physiological responses to forest management can be obtained from these long-term experiments by analyzing the stable isotopes in tree-ring cellulose which record physiological and environmental processes at the time the ring was formed (McCarroll & Loader, 2004; Barbour, 2007). The carbon isotope ratio of plant tissue (δ13Cplant) reflects gas-exchange processes by the plant at the time the carbon was fixed, and is often used as an index of intrinsic water-use efficiency defined as the ratio of photosynthesis to stomatal conductance (A /gs, Farquhar et al., 1989b; see Materials and Methods section for details). The δ13C in cellulose has been particularly useful for understanding responses to past management actions. For example, McDowell et al. (2003) noted that carbon isotope discrimination in tree-ring cellulose (Δ13Ccell) and growth increased for dominant old-growth ponderosa pine trees after understory thinning. They concluded that an increase of water resource and gs was responsible for the increased growth, and that this resource increase lasted over 15 yr. McDowell et al. (2006) also observed in ponderosa pine (Pinus ponderosa) that Δ13Ccell decreased with increasing residual basal area of thinned stands from 5 to 12 yr after thinning. However, Martín-Benito et al. (2010) found no change in Δ13Ccell after thinning in European black pine (Pinus nigra) stand, while other studies noted an increase in Δ13Ccell with increasing basal area (Warren et al., 2001; Powers et al., 2010). Brooks & Coulombe (2009) found that while tree growth increased for over 20 yr in a Douglas-fir fertilization trial in the Wind River Experimental Forest, Δ13Ccell decreased by 1.5‰ for only 4 yr after fertilization regardless of the amount of fertilizer applied. They speculated that the decrease was the result of increased leaf nitrogen increasing photosynthesis, while the later growth increases were attributed to gains in tree leaf area. Other fertilization trials have not shown such a decrease in Δ13Ccell (Balster et al., 2009), and interpreted their growth increases as increases in leaf area, not leaf nitrogen content.
To understand long-term carbon uptake dynamics, it would be useful to easily separate the effects of photosynthesis and stomatal conductance within A /gs as indicated by δ13C. Several studies have indicated how the oxygen isotope ratio of plant tissue (δ18Oplant) might be useful in separating the effects of A from gs in δ13Cplant, because the δ18Oplant values are only influenced by water cycle (including gs) and not by A (Scheidegger et al., 2000; Grams et al., 2007). The δ18Oplant is influenced by the isotopic composition of soil water and atmospheric water, and evaporative enrichment of leaf water (Roden et al., 2000; Barbour, 2007). In many long-term forest management experiments, treated and control plots are collocated, so δ18O of soil water and atmospheric water vapor should be similar. Thus, differences in δ18Oplant between treatments should result from differences in the evaporative enrichment of leaf water, which is influenced by gs, relative humidity (RH), and leaf temperature (Tleaf, assuming air temperature is the same between plots). Brooks & Coulombe (2009) used δ18Ocell to interpret changes in gs because other sources of potential change to δ18Ocell were either the same between treatment plots (source water and vapor δ18O) or were ruled out (RH and Tleaf; see Discussion section for more details). They found that latewood δ18Ocell values increased with fertilization and the duration of the increase above controls was longer for higher amounts of fertilization. They speculated that the increase of tree leaf area outpaced the increase in roots to support the leaf area resulting in stomatal closure during the late dry summers. Martín-Benito et al. (2010) observed that δ18Ocell was higher after thinning, and suggested that the increase was related to a hotter, drier environment after thinning, and was not related to changes in gs since δ13Ccell did not change with thinning. Thus, both thinning and fertilization have caused increases in δ18Ocell, but the speculated reasons for these increases were quite different.
Most of these long-term forest management experiments where isotopic analysis has been performed have only examined one management practice, such as different amounts of thinning or fertilization, but not a combination of treatments. Examining a combination of treatments might be particularly useful for helping to interpret δ18Ocell, since both canopy environment and plant water relations can change δ18Ocell values (Sternberg, 2009). In this study, we used the Shawnigan Lake fertilization and thinning experiment to further develop the dual isotope (δ13Ccell and δ18Ocell) approach under a wider range of management, where thinning, fertilization and the combination treatments were available. Shawnigan Lake has the added advantage in that measurements of soil moisture, leaf nitrogen, and foliage efficiency were made during the first 7 yr of the experiment, as well as many other measures to help interpret the stable isotope results.
- Top of page
- Materials and Methods
In the 5 yr before treatment (1966–70), the Douglas-fir trees where the plots were established were adding, on average, 950 mm2 in basal area yr–1. In the first 4 yr following treatment, fertilization and thinning increased tree growth by 69 and 53%, respectively, over pretreatment means, adding c. 600 mm2 yr–1 more (Fig. 1). The combination treatment tripled the growth rate, with the average tree gaining c. 1400 mm2 yr−1 at its peak. After the one-time treatment in 1971, BAI values for the fertilized (T0F2) and thinned trees (T2F0) were significantly greater than control trees until 1977 and 1978, respectively (Table 1). BAIs remained greater in the combined treatment trees (T2F2) through 1988 (end of the observation period). Most of the gain was a result of increased earlywood production; however, both thinned treatments also had significantly greater latewood growth relative to the controls in 1972–75.
Figure 1. Basal area increment (BAI) for trees (n = 6) in the four treatments: T0F0, control trees; T2F0, thinned trees; T0F2, fertilized trees; T2F2, combination. Values were normalized by subtracting the mean BAI for pretreatment years 1965–1970 for each tree from each year’s BAI. The arrow indicates the year that treatments were applied to the stand. Error bars are standard error of the mean. Statistical differences between treatments within a given year are shown in Table 1.
Download figure to PowerPoint
Table 1. Summary of significant statistical differences in basal area increment (Fig. 1) for the entire ring, latewood and earlywood (two-way repeated-measures ANOVA using a Holm–Sidak Multiple Comparisons test for differences between treatments and the control)
| ||Total basal area increment||Latewood basal area increment||Earlywood basal area increment|
Carbon isotope discrimination (Δ13Ccell) ranged from 16 to 21.7‰, a range of almost 6‰ among trees, treatments and years (Fig. 2). Earlywood Δ13Ccell values were more variable within a year than latewood values. The variation between years for latewood control Δ13Ccell values was highly correlated with mean annual precipitation (r = 0.85, P < 0.001) as well as other climate indicators of moisture (RH, PDSI, VPD) but to a lesser degree. Earlywood δ13Ccell values for controls were also correlated with mean annual precipitation (r = 0.58, P = 0.02) and other moisture indicators, but the correlations were much weaker than for latewood.
Figure 2. Changes in carbon isotope discrimination (Δ13C) over time for each treatment. Treatments: T0F0, control trees; T2F0, thinned trees; T0F2, fertilized trees; T2F2, combination. The arrow indicates the time at which the treatments were applied to the stands. Asterisks (*) indicate significant differences (α = 0.05) from control values in a given year. To normalize the data, the pretreatment means were subtracted from each tree’s Δ13C value in a particular year. Error bars are standard error of the mean.
Download figure to PowerPoint
Thinning alone had little effect on Δ13Ccell. Thinning (T2F0) decreased Δ13Ccell in 1972 for earlywood, and increased Δ13Ccell in 1971 for latewood (Fig. 2). Other than those two observations, Δ13Ccell values in thinned trees (T2F0) did not significantly differ from control values.
Values of Δ13Ccell decreased with fertilization (T0F2 and T2F2) relative to control values consistently for 3 yr after treatment in earlywood, and 4 yr for latewood (Fig. 2). Latewood Δ13Ccell values for both fertilized treatments were significantly lower than control Δ13Ccell values in other years through 1988 but not consistently. The years with significant Δ13Ccell differences tended to be years with low annual precipitation (< 1000 mm). While fertilization alone decreased discrimination by 1.5‰ in the first few years after treatment, the combined treatment decreased discrimination by over 2‰ in both late- and earlywood. These decreases in Δ13Ccell translated to increases in intrinsic water-use efficiency (A/gs) of 10–20 μmol mol−1 (Fig. 3). The average A/gs for controls was c. 70 μmol mol−1, and thus the observed increase was c. 15–20% greater intrinsic water-use efficiency for the first 3–4 yr following fertilization.
Figure 3. Changes in the ratio of photosynthesis to stomatal conductance (A/gs) over time. Values were estimated from Eqns 2–4. Data were normalized for pretreatment means per tree, and then normalized control averages were subtracted from treatment averages for each year. Treatments: T2F0, thinned trees (open triangles); T0F2, fertilized trees (closed circles); T2F2, combination (closed triangles). The arrow indicates the year that treatments were applied to the stand. Asterisks (*) indicate significant differences (α = 0.05) from control values in a given year. Error bars are standard error of the mean.
Download figure to PowerPoint
To understand if these gains in A/gs were related to increases in A, we compared these temporal increases with changes in leaf nitrogen and foliage efficiency (annual above-ground biomass production per unit foliage, kg kg−1) previously reported by Brix (1983, 1993). Control trees had a leaf nitrogen content of c. 1%, and produced on average 1 kg of biomass for every kg of foliage annually, while thinned and fertilized trees (T2F2) reached a maximum of 2.65 kg kg−1 (Brix, 1983) and nearly 2% leaf nitrogen content (Brix, 1993). The increases in both leaf nitrogen and foliage efficiency were also observed over the first 3–4 yr of treatments as they were for A/gs. For earlywood, the treatment increases in A/gs were linearly related to treatment increases in foliage efficiency (Fig. 4, R2adj = 0.73, F = 63.3). For latewood, only A/gs for the fertilized treatments (T0F2, T2F2) are linearly related to foliage efficiency (R2adj = 0.57, F = 20.9), while A/gs for thinned trees had no significant trend in foliage efficiency. The correlation with leaf nitrogen was much lower (R2adj = 0.38, F = 10.8), because leaf nitrogen peaked in the year after fertilization and declined rapidly afterwards, whereas foliage efficiency and A/gs peaked in the second year. Thinned trees had similar leaf nitrogen concentrations to controls over time (Brix, 1993).
Figure 4. The relationship between A/gs (Fig. 3 data) and foliage efficiency (measured as kg of above-ground biomass produced per kg of foliage in a year, Brix, 1983) or foliage nitrogen concentrations (Brix, 1993; Mitchell et al., 1996). All datasets were normalized by subtracting control means within a year from the treatment means. Lines are best-fit regression lines for each treatment. Treatments: T2F0, thinned trees (open triangles); T0F2, fertilized trees (closed circles); T2F2, combination (closed triangles).
Download figure to PowerPoint
Values for δ18Ocell ranged from 26 to 33‰ over time and with treatment (Fig. 5). Pretreatment values were very consistent between trees within a year, but varied considerably between years for both early and latewood. Earlywood values were generally greater than latewood values within the same year. The annual variation in earlywood δ18Ocell control values were correlated with climate variables, but the variation in latewood δ18Ocell control values were not. Earlywood δ18Ocell was highly correlated with spring (April–June) RH (r = −0.57, P < 0.001) and annual precipitation (r = −0.52, P < 0.001). However, latewood δ18Ocell variation in controls was not correlated with any combination of seasonal, annual or monthly temperature, PDSI, RH, VPD or precipitation.
Figure 5. Changes in oxygen isotope ratios (δ18O) over time for each treatment. T0F0, control trees; T2F0, thinned trees; T0F2, fertilized trees; T2F2, combination. The arrow indicates the time at which the treatments were applied to the stands. Asterisks (*) indicate significant differences (α = 0.05) from control values in a given year. Error bars are standard error of the mean. To normalize the data, the pretreatment means were subtracted from each tree’s δ18O value in a particular year.
Download figure to PowerPoint
Values of δ18Ocell showed significant responses to the treatments (Fig. 5). Once treatments were applied, δ18Ocell values for the thinning treatments increased relative to control values, while fertilization δ18Ocell values were similar to control values except for latewood in 1972, and earlywood in 1975. For earlywood, significant increases in δ18Ocell as a result of thinning were not noted every year after treatments were applied, but on alternate years (Fig. 6). These years tended to have drier spring seasons with low RH (r = −0.59, P = 0.05) and low PDSI scores, which indicated dry years (r = −0.58, P = 0.055). For latewood, the δ18Ocell increases related to thinning were more consistent year to year, with significant differences noted for the first 3 yr. After that, significant increases in δ18Ocell were only noted for the combined treatment and not thinning alone. The years with significant latewood differences did not have any particularly climate pattern. If anything, more humid climate was related to years with differences, but these were generally weak correlations.
Figure 6. Treatment changes in δ18O relative to control plots and climate patterns over time. Earlywood and latewood: T2F0, thinned trees (open triangles); T0F2, fertilized trees (closed circles); T2F2, combination (closed triangles). Lower panels: relative humidity (RH) and Palmer Drought Severity Index (PDSI) for earlywood comparisons are the mean monthly averages for April, May and June. RH (closed circles) and PDSI (open triangles) for latewood comparisons are the mean June RH and the PDSI averages for July and August. These climate variables had the highest correlation with the changes in δ18O. Asterisks (*) indicate significant treatment differences from controls within a particular year. Error bars are standard error of the mean.
Download figure to PowerPoint
- Top of page
- Materials and Methods
Trees responded dramatically to the thinning and fertilization treatments at Shawnigan Lake (Brix, 1983, 1993; Brix & Mitchell, 1983, 1986; Mitchell et al., 1996), and stable isotope analysis of the tree-ring cellulose has helped to illuminate the different mechanisms for response. Decreases in Δ13Ccell occurred in the fertilized trees and were short-lived, lasting only 3 or 4 yr for early- and latewood, respectively. These decreases indicate an increase in A/gs and were correlated with documented increases in leaf nitrogen and foliage efficiency (Brix, 1983, 1993). By contrast, increases in δ18Ocell mostly occurred in the thinned trees, and the response over time was more variable. The δ18Ocell values in thinned earlywood significantly increased over control values in drier spring seasons regardless of length of time since thinning occurred. Significant δ18Ocell increases in latewood were not correlated with climate, but were more associated with the time since thinning, where most significant differences from control values occurred soon after thinning.
The sharp decrease in Δ13Ccell and increase in A/gs following fertilization was likely the result of increased A. Both foliage nitrogen concentrations and foliage efficiency are related to A (Brix, 1981, 1983; Field & Mooney, 1986) and were both correlated with the A/gs dynamics observed in this experiment (Fig. 4). Foliage nitrogen increased from 1% to nearly 2% with fertilization in the first year after fertilization and then decreased rapidly such that N concentrations were similar to controls by year 4 (Brix, 1993). A/gs was more closely related to foliage efficiency than to leaf nitrogen (Fig. 4), indicating that leaf nitrogen alone did not drive the changes in A related to fertilization. Like A /gs, foliage efficiency peaked in year 2, but returned to control values in year 4 (Brix, 1983). We ruled out any isotopic effects of source CO2 from soil respiration since canopy foliage was at least 2 m from the ground (Buchmann et al., 2002). Decreases in Δ13Ccell could also be related to decreases in mesophyll conductance, not only increases in A /gs (Flexas et al., 2008; Seibt et al., 2008). However, Mitchell & Hinckley (1993) noted that mesophyll conductance increased in fertilized Douglas-fir trees, which would increase Δ13Ccell, not decrease it as we observed. If mesophyll conductance also increased in the fertilized Shawnigan Lake trees, then A /gs values cited in Fig. 3 would be too low, since estimates using Eqn 3 assume that mesophyll conductance was constant between treatments. In addition, δ18Ocell did not change with fertilization relative to the controls, indicating that gs was likely similar to control values as well. Therefore, we conclude that A /gs increased in fertilized trees because A increased from elevated foliar nitrogen and light exposure, while gs did not change.
Brooks & Coulombe (2009) found similar short-term dynamics in Δ13Ccell as a result of fertilization in Douglas-fir trees in Wind River, Washington. In that experiment, three different concentrations of nitrogen fertilization were used, and all three resulted in the same 1.5‰ decrease that was observed here in the fertilization-alone treatment. The highest N addition at Wind River was similar to the amount used in this experiment (471 kg N ha−1 at Wind River vs 448 kg N ha−1 at Shawnigan Lake). However, in this experiment the addition of thinning to fertilization had a larger effect on Δ13Ccell with a drop of 2‰. These results indicate that the addition of nitrogen alone can only increase A /gs by c. 10–15 μmol mol−1. Thinning would open the canopy, exposing more foliage to higher light intensities and causing an additional increase in A /gs to 20 μmol mol−1. Thus the increase in A /gs and foliage efficiency in the combined treatment was from higher leaf nitrogen and higher light intensities. Higher light intensities in the thinning-alone treatment did not increase A /gs, but did slightly increase foliage efficiency. In fact, A /gs decreased for latewood in the first year of treatment. We speculate that this may be a result of an increase in water resources from reduced competition with other trees. Brix & Mitchell (1986) noted that thinning increased soil water potential during the dry summer period within this stand.
Not all fertilization experiments have resulted in a short-term increase in A /gs (Balster et al., 2009; J. R. Brooks, unpublished). Site fertility and rapid degree of growth response to fertilization likely influence leaf A /gs during the first few years, and the rate of new foliage development. Both the Wind River and Shawnigan Lake experiments were on very low nutrient sites and had very dramatic responses to fertilization. Other fertilization trials have not shown such a dramatic response (Stegemoeller & Chappell, 1990; Hinckley et al., 1992; Balster & Marshall, 2000; Jokela et al., 2004). More isotopic retrospective analyses are needed of these long-term experiments to better understand leaf nutrient and leaf area interactions on Δ13Ccell.
In this experiment, Δ13Ccell did not respond to thinning alone, except for an increase in discrimination for latewood in the first year of treatment, and a decrease in the second year for earlywood. Martín-Benito et al. (2010) also found no change in Δ13Ccell in response to thinning of European black pine. However, ponderosa pine, both in the Pacific Northwest and in Arizona, increased Δ13Ccell after thinning at least for some period of time (McDowell et al., 2003, 2006). These differing results could be the result of two counteracting factors increasing with thinning: canopy light exposure and soil water supply. Increasing canopy light would increase A, while increasing soil moisture would increase gs. In the ponderosa pine studies, the locations are drought-prone with relatively open canopies, and thus the increase in water supply after thinning was speculated to cause the increase in discrimination, while light intensities did not really change as a result of thinning. Warren et al. (2001) did note an increase in Δ13Ccell with increasing predawn water potentials in two species of pine, and this effect was greater at lower stand densities. As mentioned earlier for Shawnigan Lake, soil water potential was observed to increase in thinned stands (Brix & Mitchell, 1986), but light exposure also increased. Evidently this interaction between light and water supply caused A /gs to remain stable in these thinned trees.
The increase in δ18Ocell from thinning was likely the result of changes in canopy microclimate, namely decreases in RH and/or increases in leaf temperature (Tleaf) and their effect on ea/ei in Eqn 5, and not through a decrease in gs and thus E in Eqn 7. If the mechanism was decreasing gs, we would have expected to see a response in the fertilized trees where total stand leaf area was the highest, thus having the highest water depletion rates and being the most likely to close their stomata in late summer (Brooks & Coulombe, 2009). Since significant δ18Ocell increases were mostly found in the thinned trees where water resources increased relative to controls (Brix & Mitchell, 1986) and the δ18Ocell increases occurred during dry springs (Fig. 6), changes in canopy microclimate between thinned and control stands seem most likely. Using Eqns 5–9, we estimated how much each of those three factors (RH, Tleaf and gs) would have to change in order to obtain the treatment differences we observed in δ18Ocell (Fig. 7). It is important to note that the degree of sensitivity for each variable to change δ18Ocell is dependent on the initial value of all parameters. For example, using the Péclet model, the δ18Ocell response to gs is much greater at lower RH, since E varies more in response to gs when RH is low. In Fig. 7, we used average midsummer daytime values as our initial conditions, as these represent latewood conditions. Using the Péclet model and keeping Tleaf and gs constant, RH in the thinned stands had to decrease by 14% relative to the control stands to account for the observed δ18Ocell range. However, increasing Tleaf as much as 6°C above control trees (same as air temperature = 20°C) could not increase the predicted Δ18Ocell enough to account for the observed range. Further increases in Tleaf did not increase Δ18Ocell relative to controls any more. Likewise, at an RH of 60% and a Tleaf of 20°C, decreasing gs from 0.14 to 0 mols m−2 s−1 also could not account for the observed differences.
Figure 7. Changes in controlling variables necessary in the models (Eqns 5–9) to cause the observed treatment differences in Δ18Ocell from the control values. The arrows along the x-axis indicate values used in the models for the control treatments. Modeled responses of Δ18Ocell are shown with the Péclet effect (solid line, Eqns 6, 7) and without it (dashed line, Eqn 8). The gray area represents the range of δ18Ocell differences from controls observed in this experiment. Observations above the horizontal line at 0.8‰ were significantly greater than control values.
Download figure to PowerPoint
The model predicting leaf water enrichment using the Péclet effect greatly dampens the effect of RH and Tleaf compared with earlier models which do not include the Péclet effect (Roden et al., 2000). In addition, the Péclet equation (Eqn 7) requires estimates of effective path length (L), which is difficult to determine and might or might not be contant during an experiment such as this (Ferrio et al., 2009; Kahmen et al., 2009). Using Eqn 8 instead of the Péclet effect (Eqns 6, 7) decreased the amount by which RH and Tleaf would need to change in order to account for the observed treatment changes in δ18Ocell (Fig. 7, dashed lines). However, this model excludes E, and thus limits the effects of gs on δ18Ocell to εk in Eqn 5. Using this model, RH would have to decrease from 60% in the control stands to as much as 49% in the thinned stands, and Tleaf would have to be a maximum of 3.7°C above leaf temperature in the control tree to account for these treatment changes in δ18Ocell. These microclimate differences seem more realistic. More likely, both of these variables changed simultaneously, which would decrease the necessary range for each variable even more. Decreases in RH and increases in temperature as a result of thinning have been noted in other studies within the range found here (Riegel et al., 1992; Ma et al., 2010). Therefore, we conclude that changes in canopy microclimate were responsible for the changes in δ18Ocell as a result of canopy thinning.
Stomatal conductance could not account for the full range of δ18Ocell variation using either model at this RH. Also, the lack of δ18Ocell response to fertilization here differed from that found at Wind River, where fertilization dramatically increased latewood δ18Ocell, and the authors related the increase to gs. In the Wind River fertilization experiment, Brooks & Coulombe (2009) estimated that gs in fertlized trees decreased by as much as 50% in late summer to cause the range of variation they observed. One important difference between these experiments is that average midsummer RH was much higher at Shawnigan Lake (60% vs 33% at Wind River) because of the closer proximity of the ocean. A higher RH not only reduces the sensitivity of δ18Ocell to gs, but would also reduce total E for the site, making it much less likely for the fertilized trees with greatest stand leaf area to deplete soil water and close stomata relative to the controls. In the Wind River experiment, Brooks & Coulombe (2009) ruled out RH and Tleaf as possible drivers for the δ18Ocell changes because fertilization increased leaf area, and thus increased canopy shading and the canopy boundary layer, decoupling the canopy from the ambient condition. If anything, these structural changes would increase RH within the canopy, and likely decrease Tleaf through shading, which would decrease δ18Ocell rather than increase it. Decreases in gs might cause increases in Tleaf sufficient to increase δ18Ocell, but latent heat exchange effects on Tleaf are not included in current δ18O models.
In conclusion, we successfully used stable isotopes to examine the physiological mechanisms driving the growth responses to fertilization and thinning in the Shawnigan Lake thinning and fertilization experiment. δ13Ccell was the most reliable indicator of physiological processes, while δ18Ocell was largely responding to microclimate differences between stands. These isotope results concurred with the previous studies on the physiological mechanisms behind fertilization and thinning growth responses at Shawnigan Lake (Brix, 1993). This study continues to demonstrate that stable isotopes contained within tree rings can be used for retrospective analysis of physiological responses to management spanning decades, particularly if a nearby stand not subjected to management treatments can act as a control for separating climate effects. Future studies should obtain these long-term isotopic records from a range of forests with different site and climate conditions in order to understand how the basic physiology behind forest carbon gain changes with management over timescales of decades.