| Woody debris (WD), including coarse woody debris (CWD) and fine woody debris (FWD), is an essential structural and functional component of many ecosystems, particularly in montane forests. CWD is considered to be the major part in forest woody debris and it is primarily composed of logs, snags, stumps and large branches, while FWD is mainly consisted of small twigs and is much less ecologically functional as compared to CWD. CWD plays critical roles in maintaining the integrity of forest ecosystems and influences the biological and non-biological process within the forest ecosystem, as they not only provide habitats for organisms such as amphibians, arthropods, birds, small mammals, fungi and plants, but also serves as the carbon and nutrient pool. Furthermore, CWD could prevent the soil erosion in forest ecosystem's hydrology.In this paper, the biomass and species composition of woody debris in primary forest (natural montane evergreen broad-leaved forest) and secondary forests in Ailao Mountains were studied; besides, the carbon and nutrient contents, ecological functions such as biodiversity maintenance in the primary forest were also analyzed. The results showed the total stock of woody debris in the primary forest in Ailao Mountains was 74.78 Mg ha-1, ranking the top in the forest type of subtropical evergreen broad-leaved forest. This was mainly caused by the decay-resistance of the dominant CWD source species (Fagaceae), low temperature, high humidity and the absence of anthropogenic disturbance in this area. But in secondary forests, which were caused by different degrees of anthropogenic disturbance, the WD stock was much less as compared to their primary conterparts, showing a trend to increase with progressive successions.WD is the important nutrients pool in the forest stand. Because of this high stock in WD, the primary forest hold considerable carbon and nutrients; the total carbon stored in WD was 36.33 Mg ha?1, which was about 6.5 times as much as that of carbon stored in litters on the ground. The total storage of N, P, Ca and other 7 nutrients in WD was 548.08kg ha?1, in which CWD shared 90.4% (495.55 kg ha?1), and being equal to the 80.0% of the litter fall nutrients.Based on the decrease of CWD density, the mean decomposition constant of dominant species was calculated 0.019 (0.010-0.030). This value was significantly lower than tropical and other subtropical forest, even lower than some temperate forests. Respiration rates of CWD varied between seasons, with 0.073μg C g?1 C min?1 (0.007-0.506μg C g?1 C min?1) in dry season and 0.170μg C g?1 C min?1 (0.023-0.892μg C g?1 C min?1) in rainy season. There was significant difference among different tree species (P<0.05) so far as respiration is concerned. It was estimated that the carbon flux of CWD into the atmosphere was 0.752-1.039 Mg ha-1 yr-1, and this figure was lower than the flux in the tropical forests.Some ecological functions of the woody debris were identified in the primary forest in this research. Beside the nutrient holding capacity, they could maintain high epiphytes diversity, as totally 58 species of epiphytes (52 bryophytes, 3 pteridophytes and 3 flowering plants), of which logs hold the 44 species. What is more, CWD exhibited huge potential in water storage. Its water holding capacity in dry season was 34.12 Mg ha-1 and in rainy season was 87.08 Mg ha-1, the latter figure exceeded its own dry weight.Snow is an essential way of disturbance in forest ecosystems. One snowfall as a case study in this paper showed to have increased the amount of CWD and FWD biomass by 2.26 Mg ha-1 and 0.88 Mg ha-1 respectively. According to the estimation based on previous climatic data, the snowfall periods is about 8-10 years, thus the input rate of CWD and FWD in the study area could be calculated about 0.23-0.28 Mg ha-1 yr-1 and 0.09-0.11 Mg ha-1 yr-1 respectively.Furthermore, the input of logs mainly come from the mortality of old and cavity trees and competition in the canopy and snowfall and wind spell were the common disturbance factors contributing the large branches components.
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