Forest Gaps Alter the Total Phenol Dynamics in Decomposing Litter in an Alpine Fir Forest

Abstract

The total phenol content in decomposing litter not only acts as a crucial litter quality indicator, but is also closely related to litter humification due to its tight absorption to clay particles. However, limited attention has been focused on the total phenol dynamics in foliar litter in relation to forest gaps. Here, the foliar litter of six representative tree species was incubated on the forest floor from the gap center to the closed canopy of an alpine Minjiang fir (Abies faxoniana) forest in the upper reaches of the Yangtze River and eastern Tibetan Plateau. The dynamics of total phenol concentration in the incubated litter was measured from November 2012 to October 2014. Over two-year incubation, 78.22% to 94.06% of total phenols were lost from the foliar litter, but 52.08% to 86.41% of this occurred in the first year. Forest gaps accelerated the loss of total phenols in the foliar litter in the winter, although they inhibited the loss of total phenols during the growing season in the first year. In comparison with the effects of forest gaps, the variations of litter quality among different species were much stronger on the dynamics of total phenols in the second year. Overall, the loss of total phenols in the foliar litter was slightly higher in both the canopy gap and the expanded gap than in the gap center and under the closed canopy. The results suggest that the predicted decline in snow cover resulting from winter warming or vanishing gaps caused by forest regeneration will retard the loss of total phenol content in the foliar litter of alpine forest ecosystems, especially in the first decomposition year.

Fig 1. Daily mean air and soil surface temperatures of the four gap positions (gap center, canopy gap, expanded gap and closed gap) during two years of litter decomposition in an alpine Minjiang fir forest (November 17, 2012 to October 29, 2014) (n = 6).

Fig 2. Snow depth (cm) of the four gap positions (gap center, canopy gap, expanded gap and closed canopy) during two years of litter decomposition in an alpine Minjiang fir forest (n = 3).

Fig 3. Exponential regression of the litter remaining mass (g) from the six types of foliar litter (cypress, Minjiang fir, Masters larch, red birch, Kangding willow and Lapland azalea) in the four gap positions (gap center, canopy gap, expanded gap and closed canopy) at each sampling event over two years of decomposition in an alpine Minjiang fir forest (mean, n = 3).

Fig 4. Total phenol concentration (mg/g) of the six types of foliar litter (cypress, Minjiang fir, Masters larch, red birch, Kangding willow and Lapland azalea) in the four gap positions (gap center, canopy gap, expanded gap and closed canopy) at each sampling date over two years of decomposition in an alpine Minjiang fir forest.

Different lowercase letters indicate significant differences among gap positions at the same sampling time, P < 0.05 (mean ± SD, n = 3).

Fig 5. Total phenol loss (%) of the six types of foliar litter (cypress, Minjiang fir, Masters larch, red birch, Kangding willow and Lapland azalea) in the four gap positions (gap center, canopy gap, expanded gap and closed canopy) at the end of each period over two years of decomposition in an alpine Minjiang fir forest.

The first/second snow formation period (SF1/SF2), the first/second snow cover period (SC1/SC2), the first/second snow melt period (SM1/SM2), the first/second winter (W1/W2), the first/second growing season (G(S)1/G(S)2), the first/second year of decomposition (1st/2nd year), two years of litter decomposition (two years) Columns marked with different lowercase letters indicate significant differences among gap positions within the same period, P < 0.05 (mean ± 95% confidence intervals or mean ± SD, n = 3).