Tree growth resources and the efficiency of resource-use for biomass production determine the productivity of forest ecosystems. In nutrient-limited forests, nitrogen (N)-fertilization increases foliage [N], which may increase photosynthetic rates, leaf area index (L), and thus light interception (I C). The product of such changes is a higher gross primary production and higher net primary production (NPP). However, fertilization may also alter carbohydrate partitioning from below-to aboveground, increasing aboveground NPP (ANPP). We analyzed effects of long-term N-fertilization on NPP, and that of long-term carbon storing organs (NPP S) in a Pinus sylvestris forest on sandy soil, a wide-ranging forest type in the boreal region. We based our analyses on a combination of destructive harvesting, consecutive mensuration, and optical measurements of canopy openness. After eight-year fertilization with a total of 70 g N m-2 , ANPP was 27 ± 7% higher in the fertilized (F) relative to the reference (R) stand, but although L increased relative to its pre-fertilization values, I C was not greater than in R. On the seventh year after the treatment initiation, the increase of ANPP was matched by the decrease of belowground NPP (78 vs. 92 g C m-2 yr-1 ; ~17% of NPP) and, given the similarity of I C , suggests that the main effect of N-fertilization was changed carbon partitioning rather than increased canopy photosynthesis. Annual NPP S increased linearly with growing season temperature (T) in both treatments, with an upward shift of 70.2 g C m-2 yr-1 by fertilization, which also caused greater amount of unexplained variation (r 2 = 0.53 in R, 0.21 in F). Residuals of the NPP S-T relationship of F were related to growing season precipitation (P, r 2 = 0.48), indicating that T constrains productivity at this site regardless of fertility, while P is important in determining productivity where N-limitation is alleviated. We estimated that, in a growing season average T (11.5 ± 1.0 °C; 33-year-mean), NPP S response to N-fertilization will be nullified with P 31 mm less than the mean (325 ± 85 mm), and would double with P 109 mm greater than the mean. These results suggest that inter-annual H. Lim et al. 3 variation in climate, particularly in P, may help explaining the reported large variability in growth responses to fertilization of pine stands on sandy soils. Furthermore, forest management of long-rotation systems, such as those of boreal and northern temperate forests, must consider the efficiency of fertilization in terms of wood production in the context of changes in climate predicted for the region.
Trees are able to reduce their carbon (C) losses by refixing some of the CO2 diffusing out of their stems through corticular photosynthesis. Previous studies have shown that under ideal conditions the outflowing CO2 can be completely assimilated in metabolically active, young stem and branch tissues. Fewer studies have, however, been carried out on the older stem sections of large trees and, accordingly, the importance of refixation is still unclear under natural environmental conditions. We investigated the spatial and temporal variation in refixation in ~90-year-old boreal Scots pine (Pinus sylvestris L.) trees by utilizing month-long continuous measurements of stem CO2 efflux (Ec) made at four heights along the bole. Refixation rates were found to vary considerably along the bole, leading to a 28% reduction in long-term Ec in the upper stem compared with a negligible reduction at breast height. This vertical pattern correlated with variation in light availability, bark chlorophyll content and bark type. Analysis of the vertical and diurnal patterns in Ec further suggested that the influence of sap flow on the observed daytime reduction in Ec was small. The areal rates of corticular photosynthesis were much lower than previous estimates of photosynthetic rates per unit leaf area from the same trees, implying that the impact of refixation on tree-scale C uptake was small. However, upscaling of refixation indicated that 23-27% of the potential Ec was refixed by the bole and the branches, thereby significantly reducing the woody tissue C losses. Thus, our results suggest that refixation needs to be considered when evaluating the aboveground C cycling of mature P. sylvestris stands and that breast-height estimates should not be extrapolated to the whole tree.
Manipulating tree belowground carbon (C) transport enables investigation of the ecological and physiological roles of tree roots and their associated mycorrhizal fungi, as well as a range of other soil organisms and processes. Girdling remains the most reliable method for manipulating this flux and it has been used in numerous studies. However, girdling is destructive and irreversible. Belowground C transport is mediated by phloem tissue, pressurized through the high osmotic potential resulting from its high content of soluble sugars. We speculated that phloem transport may be reversibly blocked through the application of an external pressure on tree stems. Thus, we here introduce a technique based on compression of the phloem, which interrupts belowground flow of assimilates, but allows trees to recover when the external pressure is removed. Metal clamps were wrapped around the stems and tightened to achieve a pressure theoretically sufficient to collapse the phloem tissue, thereby aiming to block transport. The compression's performance was tested in two field experiments: a (13)C canopy labelling study conducted on small Scots pine (Pinus sylvestris L.) trees [2-3 m tall, 3-7 cm diameter at breast height (DBH)] and a larger study involving mature pines (∼15 m tall, 15-25 cm DBH) where stem respiration, phloem and root carbohydrate contents, and soil CO2 efflux were measured. The compression's effectiveness was demonstrated by the successful blockage of (13)C transport. Stem compression doubled stem respiration above treatment, reduced soil CO2 efflux by 34% and reduced phloem sucrose content by 50% compared with control trees. Stem respiration and soil CO2 efflux returned to normal within 3 weeks after pressure release, and (13)C labelling revealed recovery of phloem function the following year. Thus, we show that belowground phloem C transport can be reduced by compression, and we also demonstrate that trees recover after treatment, resuming C transport in the phloem.
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