The litter-fall characteristics and their response to drought stress in the Masson pins forests damaged by acid rain at Chongqing, China
重庆酸雨区马尾松林凋落物特征及对干旱胁迫的响应

The health condition of forests especially those already damaged in the acid rain region will be increasingly influenced by the increasing extreme drought events caused by climate change. Understanding the forest health response to multiple stresses is a new challenge. The characteristics of litter-fall can be used as an important indicator to reflect forest health condition and its response to environmental stresses. The aim of this study is to detect how drought stress affect the litter-fall characteristics of forests for a better understanding and more precise prediction of the forest health decline in the acid rain region of south China. Masson pine (Pinus massoniana) is a native and widely distributed tree species in south China. it is also one of the most important tree species used for forestation and widely damaged by acid rain. The litter-fall quantity and its components (dead needles, green needles, twigs, debris, bark and cones, and broad leaves of other tree species) in the Masson pine forest stands were monitored at Tieshanping of Chongqing in 2010, and their responses to soil moisture was analyzed through Pearson correlation. The results showed that the monthly variation of litter-fall presented a "two-peak" pattern. The highest peak appeared in the dry-summer period of August-September with the lowest soil moisture and amounted to 2.30 t/hm2; the second peak appeared during the dormancy period of December and was only 0.65 t/hm2; the amount of litter-fall in the two peak periods accounted for 49.56% of the annual total (5.96 t/hm2) in 2010. The annual dead needle was 2.89 t/hm2; it accounted for 48.42% of the annual total of litter-fall. The order of weight percentage of the litter-fall components was: needles > twigs > broad leaves > debris > bark and cones. The variation of monthly dead needle amount well presented the variation of total litter-fall amount. The monthly proportion of dead needle to total litter-fall was almost the highest in the whole year and fluctuated from 32.8% to 95.7%. The monthly variation of broad leaves also presented a "two-peak" pattern, but the highest peak appeared in the period of April-May and the period of November-December respectively. The monthly proportion of the litter-fall components of green needles, bark and cones were always the lowest and reached maximum in August. The monthly proportion of debris reached to maximum in the period of April-May and during the dry-summer period of August respectively. The monthly proportion of twigs presented only one peak in September. On the whole, the amount of all components of litter-fall, but except the broad leaves, presented an obvious increase during the dry-summer period and significantly negatively related with soil moisture, especially in the soil layers of 30-40 cm and 40-50cm. In addition, the litter-fall amount presented a significantly positive relation with air temperature, potential evaporation and solar radiation (P<0.05), but a significan