Effects of clear felling and residue management on nutrient pools, productivity and sustainability in a clonal eucalypt stand in South Africa

[摘要] The subtropical ecosystem of the Zululand coastal plain is prized by the South Africancommercial plantation forestry industry for its rapid clonal Eucalyptus growth, short rotations (6to 7 years) and high yields. This region is typified by sandy soils that are low in clay and organicmatter, have small nutrient reserves and are poorly buffered against nutrient loss. The subtropicalclimate induces rapid decomposition of residues and tree litter resulting in small litter nutrientpools and rapid nutrient release into the soil, particularly after clearfelling. A combination oflarge nutrient demands through rapid growth, rapid nutrient turnover and small soil nutrientreserves implies that sites in this region are sensitive and may be at risk of nutrient decline underintensive management. The work in this study set out to determine the risk of nutrient depletionthrough harvesting and residue management on a site within the Zululand region, to assessnutritional sustainability and the risk of yield decline in successive rotations. Some bulkbiogeochemical cycling processes of macro-nutrients nitrogen (N), phosphorus (P), potassium(K), calcium (Ca) and magnesium (Mg) were assessed, and assessments also included sodium(Na).An existing Eucalyptus stand was clearfelled and treatments were imposed on the residues afterbroadcasting to simulate various levels of nutrient loss through levels of harvesting intensity andresidue management. These included residue burning (Burn), residue retention (No-Burn),fertilisation (stem wood nutrient replacement), whole tree harvesting and residue doubling. Outerblocks of the stand were not felled, but included as replicates of an undisturbed standing croptreatment. Biogeochemical nutrient cycling processes were assessed primarily in the standingcrop, Burn and No-Burn treatments, in the assumption that these represented the furthestextremes of nutrient loss. Data collection commenced a year prior to clearfelling and continuedto two years and six months after planting with key data collection over a 20.1 month periodfrom clearfelling to canopy closure (one year after planting). Water related nutrient pools andfluxes were assessed as atmospheric deposition (bulk rainfall, throughfall and stemflow) andgravitational leaching to 1m soil depth. Drainage fluxes were predicted using the Hydrus modeland real-time soil moisture data. Zero tension lysimeters collected soil solution for chemicalanalysis. Sequential coring in the 0 to 30cm soil layer was used to determine in situ soil Nmineralisation. Soil chemical and physical properties were assessed over the first meter of soil at clearfelling and new crop canopy closure to determine soil nutrient pools sizes. Biomass nutrient fluxes were assessed from litterfall, residue and litter decomposition, and above ground accretioninto the tree biomass. Leaching and N mineralisation were monitored in the No-Burn, Burn andstanding crop treatments only.Atmospheric deposition, while variable, was shown to be responsible for large quantities ofnutrients added to the Eucalyptus stand. Nitrogen and K additions were relatively high, butwithin ranges reported in previous studies. Rapid tree canopy expansion and subsequent soilwater utilisation in the standing crop permitted little water to drain beyond 1m resulting in smallleaching losses despite a sandy well drained soil. Further leaching beyond this depth wasunlikely under the conditions during the study period. Mineralisation and immobilisation of Nalso remained low with net immobilisation occurring. The standing crop was shown to be arelatively stable system that, outside of extreme climatic events, had a relatively balanced orpositive nutrient budget (i.e. nutrient inputs minus outputs).Large quantities of nutrients were removed with stem-wood-only harvesting in the No-Burntreatment leaving substantial amounts on the soil surface in the harvest residues. Whole treeremoval increased losses of all nutrients resulting in the largest losses of P and base cationscompared to all other treatments. This was mostly due to high nutrient concentrations in theremoved bark. Loss of N in the Burn treatment exceeded whole tree N losses throughcombustion of N held in the harvest residues and litter layer. The majority of K leached from theresidues prior to burning and a relatively small fraction of the base cations were lost from thepartially decomposed residues during burning. Ash containing substantial amounts of Ca andrelatively large amounts of N and Mg remained after burning. Surface soil Ca and Mg wassignificantly increased by the ash which moved into the soil with rainfall directly after burning.Rapid soil moisture recharge occurred within a few months after clearfelling, increasing leachingfrom the upper 50cm of soil. Clearfelling increased net N mineralisation rates, increasing mobile NO3-N ions in the soil surface layers. Nitrate concentration peaked and K concentration dippedin the upper soil layers of the Burn treatment directly after burning. Deep drainage and leaching(beyond 1m depth) over the 20.1 month period was, however, not significantly different betweenthe Burn and No-Burn treatments. Rapid soil moisture depletion and nutrient uptake with newcrop growth reduced leaching fluxes to levels similar to the standing crop by six months afterplanting. Taking the full rotation into account, clearfelling induced a short-lived spike in N andcation leaching compared with the low leaching losses in the undisturbed standing crop. Soil Nmineralisation over the 20.1 month period in the burnt treatment was half that of the No-Burntreatment.Growth and nutrient accumulation was significantly higher in the fertilised treatment than inother treatments up to 2.5 years of age. Growth in the Burn treatment was greatest compared to other treatments during the first few months, but slowed thereafter. No significant growthdifferences were found between all other treatments from a year to 2.5 years after planting. Earlygrowth was therefore apparently not limited by N supply despite large differences in Nmineralisation between Burn and No-Burn. Foliar vector analysis indicated that fertilisationimproved growth initially through increased foliar N and P at six months after planting followedby Mg and Ca at one year. The Burn treatment was not nutrient limited. These growth resultscontrasted with similar international research on sandy tropical sites where growth was reducedafter residue removal and increased after residue doubling. The combined nutrients released frompools in the litter layer or ash and soil in addition to atmospheric inputs were sufficient toprovide most nutrients required to maintain similar growth rates across all treatments. Thisdemonstrated the importance of residue derived nutrients to early growth nutrient supply.Reduced N mineralisation through a lack of substrate may limit N supply later in the rotationwhere residue had been removed.Construction of a nutrient budget for the system revealed that high levels of atmospheric inputshave the potential to partially replenish a large proportion N, K, and Ca lost during clearfelling,provided losses are constrained to stemwood removal only. However, loss of Mg that occurredprimarily through leaching may not be replaced under the low Mg inputs recorded in this study.Larger nutrient removals (i.e. stemwood plus other plant parts) placed a heavier reliance on thesmall soil nutrient pools at this site which can limit future productivity. More intense harvestingand residue management practices dramatically increased the risk of nutrient depletion. Losses ofspecific nutrients depended on a combination of clearfelling biomass removal, residue burningand subsequent leaching. Nitrogen losses due to harvesting and burning were more substantialthan those due to leaching. Mg and K losses depended most strongly on the time afterclearfelling before re-establishment of the new crop and rainfall patterns, while Ca and P lossesdepended directly on the amount of biomass removed. Depletion risk was the greatest for Mgand K through rapid leaching, even after stem wood only removal. Deep root uptake and deepdrainage with associated cation loss needs to be investigated further to quantify ecosystem lossesand recovery of cations displaced beyond 1m.Atmospheric deposition is one of major factors countering nutrient losses. However,atmospheric inputs may not be reliable as these may lessen in future through pollution controllegislation and climate change. Changes in growth rate under poor nutrient managementpractices are small and difficult to detect relative to the large impacts of changing weatherpatterns (drought), wildfire and pest and disease. This makes it difficult to prove nutrient relatedgrowth decline. It may be possible that improvements in genetics, silvicultural technologies and atmospheric inputs may also be masking site decline (in general) and in part explain the lack ofevidence of a growth reduction in the region.As the poorly buffered sandy soils on the Zululand Coast are at risk of nutrient depletion underthe short rotation, high productivity stands, it may be necessary to stipulate more conservativeharvesting and residue management practices. A more conservative stem-wood only harvestingregime is recommended, retaining all residues on site. Residue burning should be avoided if Nlosses become a concern. The length of the inter-rotation period must be kept short to reducecation leaching losses. Site nutrient pools need to be monitored and cations may eventually needto be replenished through application of fertilisers or ash residues from pulp mills. Managementpractices therefore need to be chosen based on the specific high risk nutrients in order tomaintain a sustainable nutrient supply to current and future plantation grown Eucalyptus.

[发布机构] Stellenbosch University