Bulk deposition of base cations in a subalpine natural forest landscape estimated by canopy budget model: A focus on the forest edge canopy

Calculations of base cation inputs of loads in forest edge canopies are rare, although forest edge canopies play a paradoxical role in the effective capture of atmospheric deposition. Throughfall deposition and canopy exchange of base cations were studied with a continuous throughfall investigation under a natural forest edge and a closed canopy in a subalpine forest over a period of 2 years. Compared with precipitation, the concentration of base cations in the throughfall of both canopies was enriched as expected, but the enrichment level in the forest edge was less than that in the closed canopy. Moreover, the throughfall deposition of base cation fluxes in the closed canopy (35.19 kg ha-1 y-1) was slightly higher than that in the forest edge canopy (33.50 kg ha-1 y-1). Seasonally, the base cation input in the rainy season was 2.32–2.70 times higher than that in the snowy season in throughfall in forest edge canopy and the closed canopy. Furthermore, the canopy budget model suggested that the direction and magnitude of canopy exchange and dry deposition controlled the net throughfall fluxes (NTF) of base cations, and obvious differences could be observed between the canopy and seasonal scales. Compared with other elements, K and Mg leached from the main canopy exchange process, while Ca was absorbed by both canopy types in the annual NTF. These results highlight the spatial variability of base cation chemical characteristics, enhance cognitive the deposition of nutrients and regulatory factors in different forest landscapes, preferably guide the formulation forest ecological management strategies.

Modelling the effects of forest management intensification on base cation concentrations in soil water and on tree growth in spruce forests in Sweden

The study investigated the effects of forest residue extraction on tree growth and base cations concentrations in soil water under different climatic conditions in Sweden. For this purpose, the dynamic model ForSAFE was used to compare the effects of whole-tree harvesting and stem harvesting on tree biomass and the soil solution over time at 6 different forest sites. The study confirmed the results from experimental sites showing a temporary reduction of base cation concentration in the soil solution for a period of 20–30 years after whole-tree harvesting. The model showed that this was mainly caused by the reduced inputs of organic material after residue extraction and thereby reduced nutrient mineralisation in the soil. The model results also showed that whole-tree harvesting can affect tree growth at nitrogen-poor forest sites, such as the ones in northern Sweden, due to the decrease of nitrogen availability after residue removal. Possible ways of reducing this impact could be to compensate the losses with fertilisation or extract residue without foliage in areas of Sweden with low nitrogen deposition. The study highlighted the need to better understand the medium- and long-term effects of whole-tree harvesting on tree growth, since the results suggested that reduced tree growth after whole-tree harvesting could be only temporary. However, these results do not account for prolonged extraction of forest residues that could progressively deplete nutrient pools and lead to permanent effects on tree growth.

Net Geochemical Release of Base Cations From 25 Forested Watersheds in the Catskill Region of New York

Chemical weathering of minerals is the principal mechanism by which base cations (Ca2+, Mg2+, K+, and Na+) are released and acidity is neutralized in soils, bedrock, and drainage waters. Quantifying the release of base cations from watershed soils is therefore crucial for the calculation of “critical loads” of atmospheric acidity to forest ecosystems. We used a mass-balance approach to estimate the rate of release of base cations in 25 headwater catchments in the Catskill region of New York, an area historically subject to high inputs of acid deposition. In 2010–2013, total net base cation release via geochemical processes averaged 1,704 eq ha–1 yr–1 (range: 928–2,622). Calcium accounted for 58% of this total, averaging 498 mol ha–1 yr–1 (range: 209–815). Mass balance estimates of net geochemical release of base cations were most strongly driven by stream export and biomass uptake fluxes, with only minor contributions from precipitation. Documented rates of base cation depletion from soil exchange sites in the region were also small relative to the net geochemical release rates. We observed a significant influence of bedrock type on net base cation release rates (P = 0.002), and a weak but significant negative correlation with watershed elevation (r = −0.51). Relationships with other geographic factors such as aspect and watershed size were not significant. Net base cation release was 4.5 times higher than precipitation inputs of SO42– and NO3–, suggesting that sources of acidity internal to the watershed are now more important drivers of weathering than acid deposition. Our data suggest that release of base cations from most Catskill forest soils is sufficient to neutralize existing inputs of acidity.