*3.1. Humus*

The initial humus content (alkali-extractable substances) was 8–13% in the foliar litters and increased to 16–21% across all species in the control plots after four years of humification (Figure 2). The humus content was stable but greatly increased beginning in the second growing season. Snow manipulation did not significantly impact the humus content (*F* = 0.02, *p* = 0.90; Table 3), although its interactions with both sampling time (*F* = 2.2, *p* = 0.015) and litter species (*F* = 2.9, *p* = 0.022) were significant. During the first three years of humification, lower humus contents (*p* < 0.05) were observed in the reduced snow cover plots for larch, birch, willow and azalea litters, except at the second snow cover stage for willow litter (Figure 2d). In the fourth year, higher humus contents (*p* < 0.05) were observed in the reduced snow cover plots, except at the fourth snowmelt stage for larch litter (Figure 2b). At the end of this experiment, higher values (*p* < 0.05) were observed in the reduced snow cover plots for larch, birch and willow litter (Figure 2b–d). When all litter species were combined, the humus content significantly increased (*R*<sup>2</sup> = 0.40, *p* < 0.001; Figure 3a) with decreases in the remaining litter mass. Manganese (Mn) was the most significant factor that impacted the humus content, and the effect of acid-unhydrolyzable residue (AUR) was greater than that of labile carbon, although neither was significant and presented VIP values below 1 (Figure 4).

**Figure 2.** Humus contents (±SE, *n* = 6) in the five foliar litters. (**a**) Cypress, (**b**) larch, (**c**) birch, (**d**) willow and (**e**) azalea. SF: snow formation stage, SC: snow cover stage, SM: snowmelt stage, GS: growing season. \* *p* < 0.05, \*\* *p* < 0.01, \*\*\* *p* < 0.001.


**Table 3.** Results of three-way ANOVA testing for the effects of time, litter species and snow manipulation. Bold values are significant (*p* < 0.05).

**Figure 3.** Relationships between the remaining mass and humus contents (**a**), humuc acid contents (**b**) and fulvic acid contents (**c**). Shaded areas are 95% confidence intervals. Adjusted *R*<sup>2</sup> and *p*-values from linear or exponential regressions are shown in each panel (all *n* = 78).

**Figure 4.** Partial least squares (PLS) analysis results for testing factors that impacted the humus content. (**a**) PLS coefficient. (**b**) Variable importance of projection (VIP). Error bars represent standard errors. VIP values greater than 1 (above the dashed line) were significant. MT: mean temperature, LC: labile carbon, WSS: water-soluble substances, OSS: organic-soluble substances, ASS: acid-soluble substances, AUR: acid-unhydrolyzable residue, C: carbon: N: nitrogen, P: phosphorus, K: potassium, Ca: calcium, Na: sodium, Mg: magnesium, Mn: manganese, Al: aluminum, Cu: copper, Fe: iron, Zn: zinc, Pb: lead, Cd: cadmium, Cr: chromium. All these factors were monitored in our longterm experiment.

## *3.2. Humic Acid*

The initial humus content (alkali-extractable substances) was 1.9–2.4% in the foliar litter and increased to 4.3–8.9% across all species in the control plots after four years of humification (Figure 5). The humic acid content was stable but greatly increased in the first growing season. Snow manipulation significantly (*F* = 6.0, *p* = 0.015; Table 3) impacted the humic acid content. During four years of humification, lower humic acid contents (*p* < 0.05) were observed in the reduced snow cover plots for cypress (Figure 5a), willow (Figure 5d) and azalea (Figure 5e) litter, as well as in the control plots for larch (Figure 5b) and birch (Figure 5c) litter. At the end of this experiment, higher humic acid content was observed in the reduced snow cover plots for both larch and birch litters (both *p* < 0.01). When all litter species were combined, the humic acid content greatly increased as the remaining mass decreased until it reached 50–60% (*R*<sup>2</sup> = 0.65, *p* < 0.001; Figure 3b).

## *3.3. Fulvic Acid*

The initial fulvic acid contents were 5.6–10.4% in the foliar litter and increased to 10.6–12.4% across all species in the control plots after four years of humification (Figure 6). The fulvic acid content was stable during the first winter but greatly decreased during the first growing season and then increased in the third year. Snow manipulation did not significantly impact the fulvic acid content (*F* = 2.1, *p* = 0.14; Table 3), although its interactions with both sampling time (*F* = 3.1, *p* < 0.001) and litter species (*F* = 5.3, *p* < 0.001) were significant. At the end of this experiment, higher fulvic acid contents were observed in the reduced snow cover plot for willow litter (*p* < 0.01; Figure 6d), whereas lower values were observed in the reduced snow cover plot for azalea litter (*p* < 0.05; Figure 6e). When all litter species were combined, the fulvic acid content decreased when the remaining litter mass was approximately >70% but then greatly increased (*R*<sup>2</sup> = 0.19, *p* < 0.001; Figure 3c).

**Figure 5.** Humic acid contents (±SE, *n* = 6) in the five foliar litters. (**a**) Cypress, (**b**) larch, (**c**) birch, (**d**) willow and (**e**) azalea. SF: snow formation stage, SC: snow cover stage, SM: snowmelt stage, GS: growing season. \* *p* < 0.05, \*\* *p* < 0.01, \*\*\* *p* < 0.001.

**Figure 6.** Fulvic acid contents (±SE, *n* = 6) in the five foliar litters. (**a**) Cypress, (**b**) larch, (**c**) birch, (**d**) willow and (**e**) azalea. SF: snow formation stage, SC: snow cover stage, SM: snowmelt stage, GS: growing season. \* *p* < 0.05, \*\* *p* < 0.01, \*\*\* *p* < 0.001.
