Forest Gaps Inhibit Foliar Litter Pb and Cd Release in Winter and Inhibit Pb and Cd Accumulation in Growing Season in an Alpine Forest

Abstract

Aims: The release of heavy metals (such as Pb and Cd) from foliar litter play an important role in element cycling in alpine forest ecosystems. Although natural forest gaps could play important roles in the release of heavy metals from foliar litter by affecting the snow cover during the winter and solar irradiation during the growing season, few studies have examined these potential roles. The objectives of this study were to document changes in Pb and Cd dynamics during litter decomposition in the center of gaps and under closed canopies and to investigate the factors that controlled these changes during the winter and growing seasons.

Methods: Senesced foliar litter from six dominant species, including Kangding willow (Salix paraplesia), Masters larch (Larix mastersiana), Mingjiang fir (Abies faxoniana), Alpine azalea (Rhododendron lapponicum), Red birch (Betula albosinensis) and Mourning cypress (Sabina saltuaria), was placed in litterbags and incubated between the gap center and closed canopy conditions in an alpine forest in the eastern region of the Tibetan Plateau. The litterbags were sampled at the snow formation stage, snow coverage stage, snow melt stage and during the growing season. The Pb and Cd concentrations in the sampled foliar litter were determined by acid digestion (HNO3/HClO4).

Important findings: Over one year of decomposition, Pb accumulation and Cd release from the foliar litter occurred, regardless of the foliar litter species. However, Pb and Cd were both released from the foliar litter during the winter and accumulated during the growing season. Compared with the gap center and the canopy gap edge, the extended gap edge and the closed canopy showed higher Pb and Cd release rates in winter and higher Pb and Cd accumulation rates during the growing season, respectively. Statistical analyses indicate that the dynamics of Pb were significantly influenced by frequent freeze-thaw cycles in winter and appropriate hydrothermal conditions during the growing season, the dynamics of Cd were strongly influenced by species and the presence of a forest gap at different decomposition stages. These results show that forest gaps could inhibit Pb and Cd release from foliar litter in the alpine forest of western Sichuan. In addition, a decrease in the snow depth in the winter warming scenario would promote the release of Pb during foliar litter decomposition. There exist some difference that may be influenced by litter quality, microenvironment and microtopography during litter decomposition.

Fig 1. The dynamics of litter and air temperature under different snow cover conditions in alpine forest of western Sichuan.

GC, gap center; CG, canopy gap edge; EG, extended gap edge; CC, closed canopy. The shaded area shows the temperature dynamics in winter.

Fig 2. Thickness changes of snow cover in different stages (mean ± SD).

GC, gap center; CG, canopy gap; EG, extended gap.

Fig 3. Dynamics of Pb concentration in litter during decomposition from gap center to closed canopy (mean ± SD, n = 3).

IV, initial value; SF, snow formation stage; SC, snow coverage stage; SM, snow melt stage; GS, growing season. GC, gap center; CG, canopy gap; EG, extended gap; CC, closed canopy. Different lowercase letters indicate significant differences (p < 0.05) among different snow patches within the same stages.

Fig 4. Dynamics of Cd concentration in litter during decomposition from gap center to closed canopy (mean ± SD, n = 3).

See Fig 3 for notes.

Fig 5. Release rate of Pb from foliar litter from gap center to closed canopy during different stages (mean ± SD, n = 3).

FT, the freeze-thaw season; 1 Yr, one year. See Fig 3 for notes.

Fig 6. Release rate of Cd from foliar litter from gap center to closed canopy during different stages (mean ± SD, n = 3).

FT, the freeze-thaw season; 1Yr, one year. See Fig 3 for notes.