重庆酸雨区马尾松林凋落物特征及对干旱胁迫的响应

The litter-fall characteristics and their response to drought stress in the Masson pins forests damaged by acid rain at Chongqing,China

气候变化导致干旱等极端气象事件增多,这将和酸雨等环境胁迫一起影响森林健康。在重庆铁山坪于2010年监测了酸雨区受害马尾松林的凋落物数量和组分特征及其对土壤水分的响应。结果表明:马尾松林月凋落物量呈"双峰"模式,第1高峰出现在8—9月份伏旱期,第2高峰出现在冬季休眠的12月份;枯死针叶占年凋落物总量的48.42%,凋落物组分比例为针叶>树枝>阔叶>有机碎屑>树皮及球果;除阔叶凋落物外,其它组分凋落物在伏旱期均明显增加,与土壤湿度显著负相关,特别是30—40 cm和40—50 cm土层;此外,马尾松林凋落物量还与表征气象干旱的气象指标显著相关。和健康森林相比,酸雨区受害林木的细根数量变少和分布变浅导致马尾松林针叶的年形成量与凋落量减少,但枝凋落量及其占总凋落量的比例增大;凋落发生对干旱胁迫更加敏感,年凋落物量的时间分布更集中在伏旱期;酸雨和干旱的双重胁迫将进一步恶化研究地区的森林健康状况。

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 significantly negative relation with air humidity （P〈0.05）. All these relation with weather parameters reflected the dependence of litter-fall to drought stress. Compared with healthy forests, the fine roots of the studied Masson pine stands were decreased and shallower-distributed as the results of long-term acid rain impact, and the quantity of newly grown needles and their litter-fall was reduced, but the quantity of dead branches and their proportion in total litter-fall was increased. The variation of litter-fall of Masson pine forests in the acid rain region of Chongqing became quite sensitive to drought stress, and the litter-fall more distributed in the dry-summer period; this means that the double stresses of acidification and drought will further worsen the forest health condition in the studied region.