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摘要
林火影响着林木的更新、生长发育以及林分演替, 是森林生态系统的重要干扰因子。为了评估林火对不同龄级树木生长的影响, 该文研究了西藏林芝市朗县2005年林火前后高山松(Pinus densata)的树轮变异特点。在林分内, 选择62株过火林木, 进行树芯样品采集, 且依据胸径将样树分为幼树(胸径< 10 cm)和成年树(胸径≥10 cm)。树木年轮学交叉定年结果显示样本最大年龄为102年, 最小年龄为19年, 平均年龄为48年。研究结果表明: 过火前幼树径向生长与上一年11月平均最低气温显著负相关, 成年树径向生长与当年9月平均最低气温和平均气温显著正相关; 过火后幼树和成年树的径向生长均与当年1月平均气温和平均最高气温显著负相关。成年树对火干扰的抵抗力(过火年-过火前树轮宽度降低的百分比)和恢复力(过火后-过火前树轮宽度恢复的百分比)都显著高于幼树。过火后不同龄级的树木生长都加快。在地表火干扰中, 成年树比幼树更能抵抗火干扰的影响。研究结果可为全球变化背景下森林恢复及林火应用提供科学依据。
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Abstract
Forest fire plays a complex and important role in affecting forest regeneration, tree growth, and stand development. Despite the importance of forest fire in modulating forest dynamics, researches on the response patterns of trees of different age-classes to fire disturbances are scarce. This study was conducted to determine the growth patterns of surviving trees of different age-classes in an alpine pine (Pinus densata) forest in the southeastern Xizang Plateau, where a moderate surface fire occurred in 2005.
We collected tree-ring samples of P. densata in the Gong-Zi-Nong valley in Nang County, Xizang Autonomous Region, in western China. Based on the diameter at breast height (DBH), the sampling trees were divided into saplings (DBH < 10 cm) and mature trees (DBH ≥10 cm). The tree-ring samples were subsequently polished, measured, cross-dated, and detrended to produce sequences of tree-ring width indices. The detrended tree-ring sequences were averaged using bi-weight robust method to develop chronologies for saplings and mature trees separately. Pearson correlation coefficients and response coefficients between the sequences of tree-ring width indices and climate factors (air temperature and precipitation) were calculated for examination of the responses of tree-ring sequences to monthly mean air temperature and monthly total precipitation both before and after the fire event. Indices of growth resistance and growth recovery were calculated based on the relative changes of trees’ raw ring width before and after the fire event, for trees in different age-classes. These indices were then compared between the trees in different age classes to assess the impacts of fire on trees.
Before the fire event, the radial growth of saplings showed a significantly negative response to the monthly mean minimum temperature of the preceding November, whereas the radial growth of mature trees showed a significantly positive response to monthly mean minimum temperature and monthly mean temperature of current September; following the fire event, radial growth of both the saplings and the mature trees showed a significantly negative response to monthly mean temperature and monthly mean maximum temperature of January of the tree-ring formation year. Based on the ratios of mean tree-ring widths of 5 post-fire years to those of 5 pre-fire years, the mature trees were significantly more fire resistant than the saplings. Moreover, the mature trees also showed greater ability in post-fire recoveries than the burnt saplings. Our results demonstrated that moderate surface fire stimulated the radial growth of both saplings and mature trees, and that the mature trees better recovered from the fire event than the saplings. The changes in growth-climate relationships following the fire event may attribute to changes in understory vegetation and microenvironments.
Keywords：
自工业革命以来, 全球地表平均气温大约上升了0.85 ℃ (IPCC, 2013), 导致全球水循环加速(Trenberth et al., 2003), 干旱事件更严重、更广泛地发生(Sheffield et al., 2012; Dai, 2013; Luo & Chen, 2013)。随着全球变暖, 西藏高原气温显示出快速升高的特征。在1960-2012年间, 西藏高原气温的平均升温速率为每10年0.3-0.4 ℃, 大约是全球同期升温率的2倍 (IPCC, 2013)。在气候变暖和人类活动加强的背景下, 西藏高原自然灾害将趋于活跃, 森林火的发生几率有上升趋势(Keane et al., 1999; Kasischke, 2000; Dale et al., 2001; Westerling et al., 2006; 邬建国等, 2013)。一般按燃烧部位将林火简单划分为三类: 地表火、树冠火和地下火。据统计, 世界各国的林火均以地表火为最多, 占90%以上, 其次是树冠火, 最少是地下火(张思玉, 2006)。一方面, 适当强度和频度的火干扰可以有效地控制林分密度, 促进幼苗更新, 有助于放牧活动; 另一方面, 全球平均每年有将近1%的林地遭受火干扰的影响(Fraser & Li, 2002), 强度林火也会严重威胁人类的生命和财产安全(Bowman et al., 2011)。因此, 研究森林火及其对森林的影响具有重要的理论和现实意义。
在现有的森林火干扰文献中, 研究内容多集中于火干扰对更新、演替以及林分生长衰退的影响方面的问题。火干扰是影响森林更新和演替的主导因子之一, 其影响的方式和程度常与树种密切相关。在藏东南的针叶林区, 云杉(Picea asperata)、冷杉(Abies fabri)和铁杉(Tsuga chinensis)等对火干扰十分敏感, 在海拔3 000-4 000 m处的冷杉林和云杉林, 经过火灾或破坏, 冷杉、云杉会被落叶阔叶树白桦(Betula platyphylla)更替, 若遭反复火灾干扰, 则被灌木代替。在海拔较低的针叶混交林内, 有铁杉、高山松(Pinus densata)等针叶树种在遭受火灾或破坏后, 铁杉会消失, 形成高山松林; 若再遭破坏, 则向落叶阔叶林或灌木草本植物群落演替(胡海清, 2005)。不同类型和强度的火干扰会对森林产生不同的影响。地表火一般对成熟林木威胁较小, 但对幼苗、幼树危害很大, 火烧过后, 幼林常常成片死亡(李文华, 2013)。此外, 地表火会烧去一些可燃地被物, 减少林地可燃物的积累, 有利于降低高强度林火发生的几率。而高强度地表火严重影响树木生长, 有时造成大面积林木枯死(徐祥德等, 2002)。还有研究表明, 火干扰会导致森林生长发生不同的变化。一方面, 火干扰可能抑制成年树和幼树的生长, 过火森林将会衰退(Keeling & Sala, 2012); 另一方面, 火干扰可能促进成年树和幼树的生长, 过火森林将会加速生长(张瑶, 2009; 朱跃峰等, 2009)。此外, 不同龄级的树木对火干扰的响应可能不一致。火干扰可能促进成年树生长而抑制幼树生长(李文华, 2013); 火干扰也可能促进幼树生长而抑制成年树生长(周以良等, 1997)。以上研究表明, 火干扰对森林树木生长的影响是一个复杂的过程, 并且与树木龄级有关, 深入理解该过程需要在更广泛的典型林区开展定量研究。
在我国西藏地区, 天然林分布广泛, 漫长的干季是林火频发的季节。根据全国第八次森林资源连续清查结果(2012年公布), 西藏自治区森林面积为1471.56万hm2, 森林覆盖率为11.98%, 森林蓄积量为22.62亿m3, 其中天然林面积和蓄积量均占99%以上。在天然森林中, 面积较多的有云杉林、高山松林和冷杉林, 合计为341.96万hm2, 占全区天然乔木林面积的40.51%。在河谷侧坡上发育着以森林为代表的山地垂直带植被, 有亚热带温性常绿阔叶林、针阔叶混交林、寒温性针叶林。森林分布区气候以季风气候为主, 雨热同期, 季风季节(5-9月)温暖湿润, 季风间期气候寒冷干燥。西藏东南部的高山峡谷林区是森林火灾多发地区, 森林火险等级高, 森林防火期长达6个半月; 该地区森林火灾次数约占全国总数的0.2%, 平均每年发生林火40次, 平均每年过火面积为4 000 hm2, 占全国总过火面积的0.4%左右(胡海清, 2005)。据统计, 1992-2005年西藏共发生森林火灾223起, 其中一般森林火灾82起, 重大森林火灾2起, 火警139起; 这些森林火灾造成过火总面积8 041.46 hm2, 其中受害森林面积1 752.46 hm2, 97%的受害森林是天然林(田晓瑞等, 2007)。该地区森林火灾主要集中在干季(9月至翌年4月), 雨季一般不发生森林火灾。该区森林处于高山峡谷区, 交通不便, 给扑火带来极大困难。由于西藏森林对区域生态系统服务功能的维持(如水土保持、生物多样性保护等)具有重要作用(苏迅帆, 2008), 且遭受干扰和破坏后不易自然恢复(陈宫燕等, 2010), 探讨林火对森林生长的影响就成为一个值得关注的研究领域。
在本项研究中, 我们选取西藏自治区林芝市朗县的高山松纯林, 利用树木年轮生态学方法调查了过火迹地的存活树木, 分析了火干扰对成年树和幼树生长的影响差异, 拟回答以下问题: (1)成年树和幼树对火干扰的响应方式是否一致?抵抗力和恢复力是否相同?(2)火干扰是否导致过火前后气候-树木生长关系发生变化?这些研究结果将为森林恢复及林火应用提供科学依据。
研究区位于西藏东南部、雅鲁藏布江中下游的朗县(图1), 地处喜马拉雅山脉北麓, 属高原温带半湿润季风气候区。研究区东北与米林县相邻, 北与工布江达县靠近, 西临山南地区的加查县, 南与隆子县毗连。全县平均海拔3 200 m, 干湿季分明, 昼夜温差大, 雨水集中, 无霜期长, 日照充足, 年日照时间为1 500-2 500 h。年降水量在600 mm左右, 集中在5-9月, 多为夜雨。年平均气温11.2 ℃, 最热月平均气温约20 ℃, 极端最低气温在-13 ℃左右(白玛朗杰, 2006)。
图1 采样点和气象站示意图。
Fig. 1 Map showing the location of sampling site and the meteorological station in Lhünzê County, Xizang.
由于朗县和工布江达县没有气象观测站, 米林县、加查县的气象观测站观测资料的时间较短(从1991开始记录), 而朗县南部毗连的隆子县气象观测站观测资料时间较长, 故本文采用相邻隆子县气象观测站(92.47° E, 28.42° N, 海拔3 860 m) 1959- 2013年的观测资料分析树轮生长与气候的关系。气候因子包括月降水量(Pm)、月平均气温(Tm)、月平均最高气温(Tmax)和月平均最低气温(Tmin) (图2)。
图2 西藏隆子县气象站1959-2013年月平均气温(Tm)、月平均最高气温(Tmax)、月平均最低气温(Tmin)和月降水量(Pm)的变化。
Fig. 2 Mean monthly temperature (Tm), mean monthly maxi- mum temperature (Tmax), mean monthly minimum temperature (Tmin) and monthly precipitation (Pm) for the period 1959-2013 at the meteorological station of Lhünzê County, Xizang.
野外样品采集于2013年9月进行, 采样点位于朗县洞嘎镇达木村贡子弄沟阳坡处(93.20° E, 29.16° N), 海拔2 533-3 593 m。高山松是耐寒、耐旱、耐贫瘠、喜光、抗火性强的阳性树种, 天然更新良好, 特产于中国西南高山地区, 是西藏森林的主要建群种之一(四川植被协作组, 1980; 中国科学院青藏高原综合科学考察队, 1985; 《中国森林》编辑委员会, 1999; 陈定国和李春惠, 2009; 李文华, 2013)。根据林业部门记载, 该林分在2005年遭受中度地表火干扰, 过火迹地内有胸径不同的活树, 灌木较少; 该地区土壤为山地棕壤, 发育较好。调查林分远离人类活动区, 几乎不受人为干扰。在过火迹地上, 我们选择胸径不同的过火活树, 使用生长锥在胸径高度处采集树芯样本。取样方向为沿山坡的等高线方向, 每棵活树采集一根树芯样本, 共采集了62棵树的树芯样本。在本实验中, 以胸径10 cm作为阈值, 将树木分为幼树(胸径<10 cm)和成年树(胸径≥10 cm)两种类型。
在实验室内, 我们先将采集的树芯样本用白乳胶和细线固定在木质样槽中, 然后置于平坦处晾干, 晾干拆线后依次用280-600目的砂纸打磨树芯(Stokes & Smiley, 1968), 使得树芯样本上的树木年轮在显微镜下清晰可见。随后, 在显微镜下根据树芯样本间的年轮宽窄变化规律对树木年轮序列进行初步交叉定年。然后利用高分辨率树木年轮宽度测量仪(LinTab5, RINNTECH, Heidelberg, German)测量每个树芯样本的逐年树轮宽度, 测量精度为0.001 mm。将交叉定年结果用COFECHA程序(Holmes, 1983)进行质量检验, 更正因缺轮、伪轮和测量误差等带来的错误(Fritts, 1976), 最终使树芯样本上的每一个树轮都对应其实际生长年份, 得到每株树在胸径处的形成层年龄(年轮数), 本文中所指的年龄均为形成层年龄。
应用ARSTAN (Cook, 1985)程序, 通过负指数函数或者线性回归方法拟合, 以去掉树木由于年龄变化和小生境条件引起的生长趋势, 对去趋势序列进行双权韧性平均, 得到理论平均值为1、无量纲的树轮宽度标准年表(STD)。
为了分析火干扰事件对不同龄级高山松径向生长的影响, 本文选择了过火当年及其前后5年共11年的树轮宽度数据, 分析了火干扰事件对不同龄级高山松径向生长的影响。通过独立t检验比较过火前后幼树和成年树每株树原始树轮宽度的相对差异, 评估林火对幼树和成年树生长的影响。
我们利用过火当年、过火前1年、过火后1年的原始年轮宽度(raw tree-ring width, RW)与过火前5年和过火后5年的平均原始年轮宽度值的相对变化来评估不同龄级高山松对火干扰的响应是否一致。计算了过火当年高山松生长降低量(%)和过火后高山松生长恢复量(%), 并根据前人研究经验(Antos & Parish, 2002; 姜庆彪, 2012), 分别定义为高山松对火干扰的抵抗力(resistance, RS)和恢复力(recovery, RC), 计算公式为:
RS1 =[( RW0 - RW-1) / RW-1]× 100%
RS2 =[( RW0 - RW-5) / RW-5]× 100%
RC1 =[( RW+1 - RW0) / RW0]× 100%
RC2 =[( RW+5 - RW0) / RW0]× 100%
RC3 =[( RW+5 - RW-5) / RW-5]× 100%
式中, RS1、RS2分别代表与过火前1年比较的抵抗力、与过火前5年比较的抵抗力。RC1、RC2、RC3分别代表过火后1年的恢复力、过火后5年的恢复力、与过火前5年比较过火后5年的恢复力。RW0为过火年高山松树芯的原始年轮宽度, RW-1为过火前1年高山松树芯的原始年轮宽度, RW+1为过火后1年高山松树芯的原始年轮宽度, RW-5为过火前5年高山松树芯的平均原始年轮宽度, RW+5为过火后5年高山松树芯的平均原始年轮宽度。
为了研究树木生长-气候因子关系及其在森林过火前后是否发生变化, 我们利用DendroClim2002程序(Biondi & Waikul, 2004)分别分析过火前和过火后不同龄级高山松的树轮宽度标准年表与逐月气候因子的相关关系, 逐月气候因子包括上年10月到当年9月的气候指标, 即月降水量、月平均气温、月平均最高气温和月平均最低气温。
高山松的树轮序列交叉定年结果显示最大年龄为102年, 最小年龄为19年, 平均年龄为48年。幼树平均年龄为32年, 成年树平均年龄为59年。高山松幼树与成年树标准年表的统计特征显示(表1), 幼树和成年树的树间平均相关系数均在0.26左右, 表明幼树间及成年树间的轮宽变化均较为一致(图3)。幼树标准年表的标准差大于成年树标准年表的标准差, 表明幼树的年轮宽度波动幅度比成年树稍大。成年树的一阶自相关系数略大于幼树, 表明成年树当年生长受上一年气候条件的影响更大(Cook & Kairiukstis, 1990)。成年树的平均敏感度大于幼树, 表明成年树径向生长对外界环境变化响应敏感, 年表包含更多的环境年际波动信息(Cook & Kairiukstis, 1990; 吴祥定, 1990)。
不同龄级高山松的年轮宽度分析结果(表2)显示, 不同龄级高山松的平均原始年轮宽度差异显著(p < 0.05), 并且幼树的平均原始年轮宽度大于成年树。在不同龄级林木对火干扰的抵抗力的比较方面, 与过火前1年的树轮宽度比较, 成年树在过火当年轮宽降低的百分比略小于幼树, 差异未达到显著水平。与过火前5年比较, 过火当年幼树和成年树对火干扰的抵抗力存在显著差异, 幼树轮宽降低的百分比大于成年树, 表明成年树抵抗力更强。在不同龄级林木的过火恢复力比较方面, 与过火年比较, 过火后5年的成年树轮宽相对变化略大于幼树, 但是差异不显著, 但相对于过火前5年的平均径向生长情况, 幼树和成年树的恢复力则存在显著差异, 即成年树径向生长的恢复力显著强于幼树(表2)。
表1 高山松幼树与成年树标准年表的统计特征
Table 1 Statistics of the tree-ring width chronologies for the saplings and the mature trees of alpine pine (Pinus densata)
| 统计项 Statistics | 幼树 Saplings | 成年树 Mature trees |
|---|---|---|
| 样本量 Sample depth | 33 | 29 |
| 标准偏差 Standard deviation | 0.248 | 0.217 |
| 平均轮宽指数 Mean tree-ring width index | 0.999 | 0.994 |
| 平均敏感度 Mean sensitivity | 0.155 | 0.170 |
| 树间平均相关系数 Mean correlation coefficient between trees | 0.304 | 0.230 |
| 一阶自相关系数 First-order-autocorrelation coefficient | 0.532 | 0.583 |
图3 西藏朗县高山松幼树和成年树的标准年表以及对应的样本量。
Fig. 3 Tree-ring width chronologies and the corresponding number of samples for saplings and mature trees of alpine pine (Pinus densata) in Nang County, Xizang.
过火前, 不同龄级高山松树木生长气候响应分析结果显示, 月降水因子与不同龄级高山松径向生长具有显著相关关系, 然而不同龄级的林木响应的月份不同。幼树的径向生长与当年5月降水量显著负相关, 成年树的径向生长与当年3月及8月的降水量显著正相关(p < 0.05)(图4)。在林木径向生长与温度因子的关系中, 幼树径向生长与当年9月平均最低气温显著正相关, 与上年11月平均最低气温显著负相关。成年树的径向生长主要与当年8-9月平均最低气温显著正相关。幼树与成年树径向生长对月平均最高气温和月平均气温的响应相对较弱。
表2 高山松幼树和成年树的平均树轮宽度与抵抗力和恢复力指数(平均值±标准偏差)
Table 2 The mean tree-ring widths, resistance and recovery indices for saplings and mature trees of alpine pine (Pinus densata) (mean ± SD)
| 龄级 Age class | 平均轮宽 Mean ring-width (mm) | RS1 (%) | RS2 (%) | RC1 (%) | RC2 (%) | RC3 (%) |
|---|---|---|---|---|---|---|
| 成年树 Mature trees | 0.86 ± 0.15a | -42.03 ± 9.24a | -31.98 ± 9.55a | 57.25 ± 18.32a | 172.07 ± 43.43a | 48.22 ± 21.07a |
| 幼树 Saplings | 1.38 ± 0.14b | -46.39 ± 5.06a | -51.65 ± 4.41b | 88.06 ± 14.36a | 146.76 ± 21.00a | 8.92 ± 13.40b |
图4 过火前(1970-2004) (A1, B1, C1, D1)与过火后(2006-2013) (A2, B2, C2, D2)不同龄级高山松标准年表轮宽指数与月降水量(A1, A2)、月平均最低气温(B1, B2)、月平均最高气温(C1, C2)、月平均气温(D1, D2)的相关关系, 星号表明二者的相关系数达到95%以上的置信水平。
Fig. 4 Correlation coefficients of the standard chronologies of saplings and mature trees of alpine pine (Pinus densata) with monthly precipitation (A1, A2), monthly minimum temperature (B1, B2), mean monthly maximum temperature (C1, C2), and mean monthly temperature (D1, D2) in the periods preceding the fire (1970-2004) (A1, B1, C1, D1) and following the fire (2006-2013) (A2, B2, C2, D2). The asterisks indicate the significance of correlation at the 95% confidence level.
过火后, 不同龄级高山松树木生长气候响应发生了变化(图4)。成年树和幼树径向生长与当年1月平均气温、平均最高气温达到极显著负相关(p < 0.01), 对平均最低气温及月降水量的响应相对较弱, 幼树和成年树的径向生长与当年1月平均气温、平均最高气温显著负相关(p < 0.05)。
研究结果表明, 相对于过火前5年的平均轮宽, 成年树在过火年的树轮宽度降低的比率显著低于幼树, 表明成年树具有更强的抗火性。这可能与不同龄级间树木的树皮厚度差异有关。高山松树干下部的树皮暗灰褐色, 深裂成厚块片, 上部树皮红色, 裂成薄片脱落(陈定国和李春惠, 2009)。树皮是热的不良导体, 一定程度上能起到阻隔热的作用, 保护树干的韧皮部和形成层免受火烧时的高温灼伤。树皮的抗火性不仅与树皮中大量的空细胞和栓皮的隔热作用有关, 而且与其结构、组成、密度和含水量有关。外皮中韧皮部所占比重愈小, 其抗火性越强(Hare, 1965)。树皮随着年龄的增加而增厚, 其结构也愈紧密, 成年树的树皮体积百分率可达原木材积的13% (胡淑宜, 1999)。树皮越厚, 结构越紧密, 导热系数越小, 抗火性越强, 树木形成层和韧皮部受害越轻。因此, 成年树抗火性要强于幼树(李俊清, 2006)。此外, 成年树和幼树在最大叶量层的垂直高度上存在显著差异, 也是影响其对地表火干扰抗性的因素之一。高山松幼树的最大叶量层在树冠中下部, 而高山松成年树的最大叶量层位于冠层中上部, 垂直高度高于幼树的最大叶量层的高度(徐凤翔, 1995)。高山松针形叶对火敏感, 对火的抵抗力低(胡海清, 2005), 过火后会发生颜色变化, 油脂损失, 形状扭曲, 甚至死亡脱落(姚树人和文定元, 2002)。相对于成年树而言, 幼树的树叶烧死烧伤更多, 光合效率降低更显著, 生长受到的影响更为明显(张思玉, 2006)。因此, 在过火当年, 不同龄级林木树轮宽度均降低, 但幼树树轮宽度降低的百分比要高于成年树。
在本研究中, 地表火干扰后, 幼树和成年树的生长都得到了促进, 且成年树恢复力强于幼树。类似的结果也在其他松属森林的火干扰研究中有所报道(Lageard et al., 2000)。火干扰会使部分树木死亡, 火烧减弱了树木间的竞争, 从而降低了群落密度, 减轻了树木的竞争压力, 从而促进了树木生长。此外, 火干扰会清除植被和部分或全部土壤表面的枯枝落叶层, 枯落物被烧毁, 枯落物中固化的养分被释放, 快速参与到地球化学循环和生物小循环, 为径向生长提供了大量的营养物质。土壤在地表裸露后直接接受日光照射, 接收更多光能(乌拉等, 2014), 其深色的表层吸收的热量也将增加, 从而导致土壤温度的升高(项凤武, 1990; 姜勇等, 2003)。随着土壤温度的升高, 土壤内的各种化学过程加快, 土壤动物、微生物的活动能力加强, 促进了土壤中营养元素的良性循环, 有效养分供给量增加, 提高了林地土壤肥力(Kozlowski et al., 1991), 从而使火干扰在一定程度上促进了耐火树种的生长(Mutch & Swetnam, 1995)。以上因素的变化会使存活木在过火后的几年内出现生长加快的现象(Sakulich & Taylor, 2007)。然而, 本研究发现不同龄级的树木个体的响应不同: 成年树恢复力要强于幼树。这可能与不同龄级间树木的叶量逐渐增长速率不同有关。研究表明, 高山松幼树(DBH小于10 cm时)叶量增长幅度不大, 而成年树叶量的增长较迅速(徐凤翔, 1995), 因而成年树的光合速率和生长恢复更快。此外, 与存活的成年树相比, 幼树根系较浅(Jacquart et al., 1992), 获得的水分和养分相对较少(Pichler & Oberhuber, 2007), 在土壤肥力提高中受益相对较少, 导致径向生长恢复力低于成年树。
相关分析结果表明, 气候因素对不同龄级高山松径向生长的影响差异较大。此外, 火干扰导致了树木径向生长-气候关系在过火前后发生了变化。过火前, 上年11月平均最低气温的升高, 使大气降水减少和土壤中水分蒸发散失较多, 这加剧了水分对树木生长的限制作用, 进而抑制了幼树翌年的径向生长(Fritts, 1976); 当年9月平均最低气温升高, 能提高光合作用有效酶的活性(盛浩等, 2007), 有利于提高光合作用效率, 积累更多的营养物质, 从而促进树木生长, 形成宽轮(王亚军等, 2001); 当年5月降水量增加到一定水平, 土壤水分条件极为充足时, 降水因子不再是树木生长的主要限制因子, 树木径向生长与降水不相关或负相关(Fritts & Dean, 1992; Wimmer & Grabner, 1997)。对于成年树而言, 当年8-9月平均最低气温的升高, 能提高光合作用有效酶的活性, 有利于光合作用, 提高光合速率, 积累更多的光合产物, 从而促进年轮的生长(王亚军等, 2001); 当年3月降水量的增加可以补给土壤水分, 缓解干旱胁迫, 加强根系的水分和营养物质的交换以及地上部分的活动, 促进树木的生长(吴祥定, 1990)。当年8月降水量的增加, 利于光合作用产物的积累和加速植物的后期生长, 从而促进了树木径向生长(Liang et al., 2001)。
火干扰除了对树木生长的影响外, 可能对高山松与气候因子之间的响应关系有一定影响。过火后, 幼树和成年树的气候响应关系趋于一致, 这种响应关系的变化可能与过火前后森林结构的变化有关。大量实地调查表明, 过火后, 森林生境多趋于干旱化(《中国森林》编辑委员会, 1999)。本研究中, 不同龄级高山松径向生长在过火后主要与当年1月平均气温和平均最高气温显著负相关。树木在冬季光合作用较弱, 而呼吸作用的养分消耗及蒸腾作用的水分消耗不容忽视。这时气温的升高会加快呼吸, 导致较多的有机物被消耗, 影响生长季初期树木的径向生长所需的物质供给, 从而抑制了树木的生长, 导致不同龄级高山松的轮宽指数与当年1月平均气温、平均最高气温显著负相关(邵雪梅和吴祥定, 1997)。此外, 火与气候因子二者之间亦存在密切的关系(Collins et al., 2006; Crimmins, 2006; Holden et al., 2007), 气候变暖所导致的极端干旱事件发生的频率和干旱程度都有增加的趋势(Meehl & Tebaldi, 2004; Trenberth et al., 2007), 甚至导致大范围的森林衰退(Williamson et al., 2000; Fensham & Fairfax, 2005; Voelker et al., 2008), 而干旱导致的火干扰则可能会加速森林衰退的过程。关于气候和火干扰的耦合作用的研究应该在更广泛的区域和气候背景下, 针对不同树种开展, 来提高我们对森林衰退的预防和治理能力。本研究中, 我们根据野外实地情况, 将高山松林木个体划分为成年树和幼树两个龄级进行取样分析, 建议在未来研究中将调查个体细分为幼树、中龄树、近熟树、成熟树、过熟树5个龄级, 从而进一步提高我们对不同龄级林木对火干扰响应规律的认识水平。
本文以西藏自治区朗县的高山松过火纯林为研究对象, 利用树木年轮生态学方法, 研究了不同龄级存活树木生长对地表火干扰的抵抗力和恢复力, 并分析了过火前后的气候响应特征。研究结果表明, 火干扰促进了不同龄级存活树木的生长, 但没有导致森林衰退。由于高山松成年树树皮厚, 叶冠层较高, 对地表火具有较强的抵抗力, 在火灾后也显示出较强的恢复力。此外, 火干扰可以造成部分树木死亡, 改变林下植被, 导致局地环境发生改变, 使得高山松生长-气候响应关系发生了变化。过火前幼树径向生长主要与上年11月平均最低气温显著负响应, 成年树径向生长主要与当年9月平均气温和平均最低气温显著正相关; 过火后幼树和成年树的径向生长均主要与当年1月平均气温和平均最高气温显著负相关, 这种响应方式的趋同性变化需要在更多的研究中检验。由于地表火是天然林火干扰最主要的形式, 具有重复发生的特征, 其发生频率在未来气候变化背景下可能会增加, 因此, 在更广泛的区域, 利用树木年轮学方法对多树种开展树木响应研究, 对森林保护和森林衰退预防具有重要作用。
致谢 感谢西藏自治区朗县林业局在野外考察工作中给予的帮助, 感谢中国科学院植物研究所树轮组野外考察队成员帮助采集树芯。
The authors have declared that no competing interests exist.
作者声明没有竞争性利益冲突.
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Humans and their ancestors are unique in being a fire-making species, but natural (i.e. independent of humans) fires have an ancient, geological history on Earth. Natural fires have influenced biological evolution and global biogeochemical cycles, making fire integral to the functioning of some biomes. Globally, debate rages about the impact on ecosystems of prehistoric human-set fires, with views ranging from catastrophic to negligible. Understanding of the diversity of human fire regimes on Earth in the past, present and future remains rudimentary. It remains uncertain how humans have caused a departure from natural background levels that vary with climate change. Available evidence shows that modern humans can increase or decrease background levels of natural fire activity by clearing forests, promoting grazing, dispersing plants, altering ignition patterns and actively suppressing fires, thereby causing substantial ecosystem changes and loss of biodiversity. Some of these contemporary fire regimes cause substantial economic disruptions owing to the destruction of infrastructure, degradation of ecosystem services, loss of life, and smoke-related health effects. These episodic disasters help frame negative public attitudes towards landscape fires, despite the need for burning to sustain some ecosystems. Greenhouse gas-induced warming and changes in the hydrological cycle may increase the occurrence of large, severe fires, with potentially significant feedbacks to the Earth system. Improved understanding of human fire regimes demands: (1) better data on past and current human influences on fire regimes to enable global comparative analyses, (2) a greater understanding of different cultural traditions of landscape burning and their positive and negative social, economic and ecological effects, and (3) more realistic representations of anthropogenic fire in global vegetation and climate change models. We provide an historical framework to promote understanding of the development and diversification of fire regimes, covering the pre-human period, human domestication of fire, and the subsequent transition from subsistence agriculture to industrial economies. All of these phases still occur on Earth, providing opportunities for comparative research.
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以林芝地区1986～2005年森林火灾统计数据为依据,分析森林火灾发生的年际变化特点. 结果表明,森林火灾总起数在80年代末90年代初发生频率较高,1995年以后总体呈下降趋势,受灾森林面积有2个高峰期,集中在1990～1992年. 林芝地区的火灾轮回期为18 008 a,火灾发生概率为0.000 056.对林芝地区森林火灾致灾原因进行分析,指出自然致灾因子包括相对湿度、风、温度、降水等气象条件是森林火灾发生的基础条件,人文致灾因子是森林火 灾的诱导因素.
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ABSTRACT
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Historical records of precipitation, streamflow and drought indices all show increased aridity since 1950 over many land areas(1,2). Analyses of model-simulated soil moisture(3,4), drought indices(1,5,6) and precipitation-minus-evaporation(7) suggest increased risk of drought in the twenty-first century. There are, however, large differences in the observed and model-simulated drying patterns(1,2,6). Reconciling these differences is necessary before the model predictions can be trusted. Previous studies(8-12) show that changes in sea surface temperatures have large influences on land precipitation and the inability of the coupled models to reproduce many observed regional precipitation changes is linked to the lack of the observed, largely natural change patterns in sea surface temperatures in coupled model simulations(13). Here I show that the models reproduce not only the influence of El Nino-Southern Oscillation on drought over land, but also the observed global mean aridity trend from 1923 to 2010. Regional differences in observed and model-simulated aridity changes result mainly from natural variations in tropical sea surface temperatures that are often not captured by the coupled models. The unforced natural variations vary among model runs owing to different initial conditions and thus are irreproducible. I conclude that the observed global aridity changes up to 2010 are consistent with model predictions, which suggest severe and widespread droughts in the next 30-90 years over many land areas resulting from either decreased precipitation and/or increased evaporation.
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Gidgee (Acacia cambagei) forms woodlands and forests on moderately fertile clay soils throughout semi-arid areas of north-eastern Australia. Analysis of soil data from Queensland indicates that gidgee鈥檚 environmental domain has a broad overlap with treeless Mitchell grassland although a preference for slightly coarser soils with higher phosphorus and lower potassium content. Mapping of woody cover change for an area of central Queensland suggests that gidgee thickening and spread has occurred between 1951 and 1994. However, this increase has been more than offset by loss of gidgee cover by clearing in other areas. Gidgee thickening and encroachment onto grassland is a problem for graziers because the trees suppress grass and are unpalatable except when very young. Landholder surveys and other information indicate that gidgee has recruited sporadically and that thickening occurred with a massive seedling recruitment in the 1950s with a smaller recruitment wave in the 1970s. These waves coincide with relatively wet times and the extreme rainfall abundance in the 1950s has a multi-century expected recurrence. There are historical records describing gidgee death during the extreme droughts in the first part of the 20th century. Young gidgee plants are fire sensitive but well-established gidgee stands are fire retardant, and the importance of fire for control of gidgee thickening is unknown. One scenario explaining gidgee dynamics emphasises the role of environment (mostly climate and soil), while another emphasises management (mostly fire). Research priorities are presented and may contribute to appropriate management of gidgee woodlands.
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Estimating fire-related parameters in boreal forest using SPOT VEGETATION. https://doi.org/10.1016/S0034-4257(02)00027-5 URL [本文引用: 1] 摘要
The majority of burning in the boreal forest zone consists of stand replacement fires larger than 10 km 2 occurring in remote, sparsely populated regions. Satellite remote sensing using coarse resolution (鈮1 km) sensors is thus well suited in documenting the spatial and temporal distribution of fires in this zone. The purpose of this study was to investigate the utility of the SPOT VEGETATION (VGT) sensor for estimating three key parameters related to boreal forest fire: burned area, postfire regeneration age, and aboveground biomass. Based on a sample of fires across Canada, the best overall discrimination of burned forest was provided by a normalized short-wave-based vegetation index (SWVI) that combines near-infrared (NIR) and short-wave infrared (SWIR) channels from VGT. Multitemporal differencing of this index from anniversary date VGT composites was combined synergistically with active fire locations from NOAA/AVHRR to map Canadian forest that burned during 1998 and 1999. National burned area estimates for both years were within 15% of those compiled by the Canadian Interagency Forest Fire Centre. The normalized index also was correlated ( R =.68) with the age of regenerating forests in Saskatchewan and Manitoba that burned between 1949 and 1998. An artificial neural network (ANN) model developed using temporal metrics computed from VGT could predict the age of these forests with an RMS error of 7 years ( R =.83). By contrast, forest biomass based on Canada's Forest Inventory (CanFI) was estimated with relatively poor accuracy (RMS=32 tons/ha) from VGT reflectance and terrestrial ecozone using a network model. We conclude that the VGT instrument is effective for mapping large boreal burns at the end of a fire season and approximating the age of regenerating burns less than about 30 years old. This information can be useful to supplement conventional ground-based data sets in remote areas where coverage may be incomplete.
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Dendrochronological modeling of the effects of climatic change on tree-ring width chronologies from the Chaco Canyon area, southwestern United States.
Hypotheses about the causes of the growth and decline of the Chacoan regional interaction system in the southwestern United States between A.D. 900 and 1200 are evaluated against tree-ring evidence and the results of an empirical model (PRECON) that computes the statistical relationships between climate and ring-width indices during the 20th century and applies the results to hypothesized precipitation or temperature changes. The statistical responses of 23 indexed conifer ring-width chronologies from New Mexico and Colorado to variations in monthly temperature and precipitation were calculated. Simulated decreases in prior autumn-winter precipitation markedly reduced ring widths, while decreased current summer precipitation was less effective, sometimes reducing ring width or having little effect. Decreased prior winter temperature slightly reduced ring width, while decreased growing season temperature usually increased or did not effect ring widths. Evaluated in terms of these results, the Chaco Canyon area tree-ring record (1) indicates that favorable climatic conditions in the 10th, 11th, and early 12th centuries fostered the growth of the Chacoan system, (2) shows that dry autumn-winter and summer conditions in the middle 1100s contributed to the downfall of the system, (3) does not support the proposition that centuries-long climatic fluctuations evident in southwestern Colorado affected Chaco Canyon, (4) does not support the idea of shifts from summer-to winter-dominant precipitation regimes, and (5) contributes little to assessing the role of anthropogenic environmental change in the collapse of the Chacoan system.
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| [19] |
Contribution of bark to fire resistance of southern trees.
Time required to kill cambium in situ with a standardized propane flame was a function of both thickness and thermal properties of the overlying bark. There were 14 species tested, including hardwoods and softwoods. Resistance was directly correlated with tree diameter. Within diameter classes, species differences in both thickness and insulating efficiency of barks accounted for resistance differences. Time to a lethal cambium temperature was exponentially related to bark thickness and was also influenced by temperature of the cambium before the flame was applied.
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| [20] |
Fire season precipitation variability influences fire extent and severity in a large southwestern wilderness area, United States. https://doi.org/10.1029/2007GL030804 URL [本文引用: 1] 摘要
Despite a widely noted increase in the severity of recent western wildfires, this trend has never been quantified. A twenty-year series of Landsat TM satellite imagery for all forest fires on the 1.4 million ha Gila National Forest suggests that an increases in area burned and area burned severely from 1984-2004 are well correlated with timing and intensity of rain events during the fire season. Winter precipitation was marginally correlated with burn severity, but only in high-elevation forest types. These results suggest the importance of within-season precipitation over snow pack in modulating recent wildfire size and severity in mid-elevation southwestern forests.
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| [21] |
Computer-assisted quality control in tree-ring dating and measurement. |
| [22] |
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| [23] |
Thermal analysis of wood bark. 树皮的热分析法研究 .https://doi.org/10.3321/j.issn:1000-6842.1999.z1.017 URL [本文引用: 1] 摘要
应用热分析法研究造纸厂树皮废渣的热分解过程。通过对热重曲线(TG)和差热曲线(DTA)的解析，可知树皮发热值和炭化物得率比木材高，树皮添加碑酸制取活性炭的活化过程的热化学变化动态与木材相似，为树皮的热化学加工的合理利用提供实验依据。
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| [24] |
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| [25] |
Spatial and temporal tree responses to water stress in an old-growth deciduous forest. https://doi.org/10.2307/2426331 URL [本文引用: 1] 摘要
No abstract available.
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| [26] |
Growth Response to Climate in Chinese Pine as a Function of Tree Diameter. Master degree dissertation, 不同径级油松径向生长对气候的响应研究 .
在树轮学研究中通常使用去趋势 方法移除非气候因子的影响。因此,树轮宽度与气候的相关被认为不受树木径级的影响。然而大量生理学研究表明树木生理过程与径级密切相关,而气候因子正是通 过作用于树木的生理过程对树木生长产生影响。该文通过建立黑里河(HL)、克旗(KQ)、松山(SS)及宁夏(NX)四个样点的两个径级油松(Pinus tabulaeformis)年表,研究了不同径级油松敏感度的差异,建立了年表与气候因子的相关关系并探讨了干旱对不同径级油松径向生长的影响。油松气 候—生长相关关系是通过相关与响应分析建立的,干旱对油松径向生长的影响是通过Superposed Epoch Analysis (SEA)方法建立的。主要结果如下：1)随取样地点的不同,平均敏感度与胸径的相关关系不同。在HL与KQ样点,平均敏感度与胸径呈线性负相关 (P0.001),而在NX与SS样点二者无显著相关关系。同一地点不同径级油松年轮宽度年表均达到极显著相关(P0.01)。2)取样地点油松径向生长 与降水呈正相关而与夏季温度呈负相关。春夏季的气候是影响油松径向生长的主要因子。温暖湿润的春季及凉爽多雨的夏季会形成宽轮。降水对油松径向生长的影响 具有明显的“滞后效应”。3)在干旱年份,HL与KQ油松的径向生长量表现出显著的降低(P0.01),在干旱后1年径向生长量恢复正常。在HL,干旱年 小径级油松的生长降低量显著高于大径级油松(P0.01),且干旱后的生长恢复量显著高于大径级油松(P0.01)。在KQ,干旱年不同径级油松的径向生 长的降低量及干旱后的生长恢复量无显著差异。从结果可以得出：1)在HL与KQ,油松敏感度与胸径显著相关且小径级油松具有更高的敏感度,而在SS及NX 则不具有显著相关。2)油松对干旱响应敏感,在干旱年份油松径向生长显著降低,且在1年的时间恢复生长。在HL油松对干旱的响应具有显著差异而在KQ则无 差异。不同径级油松的敏感度及其对干旱的响应均受到当地水热条件的影响。油松的气候—生长关系受到径级的影响,因此在树轮学取样中应该考虑径级因素以得到 更全面的结果。
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| [27] |
Influences of vegetation burning on soil properties. URL 摘要
Soil properties can be affected by vegetation burning through input of heat and ash, and furthermore, the effect can lead to some changes in soil environment and microclimate. The changes can be attributed to subsequent modifications of vegetation and the activities of soil macro- and micro-organisms as well. This paper summarizes the potential influences of vegetation burning on nutrient concentrations,availability and mobility, activities of biomass and enzyme, and soil organic carbon pool in soil ecosystems. The main objective is to provide reference for soil management after forest fire or grassland burning.
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| [28] |
火烧对土壤性质的影响 . |
| [29] |
https://doi.org/10.1007/978-0-387-21629-4_2 URL [本文引用: 1] 摘要
The terrestrial ecosystems found in the boreal region cover a little less than 17% of the earth鈥檚 land surface, yet they contain more than 30% of all carbon present in the terrestrial biome (Table 2.1). For the purposes of this discussion, we divide the ecosystems found in this region into three broad categories: boreal forests, peatlands interspersed throughout the boreal forest, and tundra. Although there is room for debate as to the exact definition of these categories, they are used here for descriptive purposes only and are based on the criteria developed by Apps and colleagues (1993) to estimate the amount of carbon present in boreal forests and tundra. The percentages of total area and total terrestrial carbon were derived by using the estimates of Smith and co-workers (1993).
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| [30] |
Temporal patterns of ecosystem processes on simulated landscapes in Glacier National Park, Montana, USA. |
| [31] |
Changing growth response to wildfire in old-growth ponderosa pine trees in montane forests of north central Idaho. https://doi.org/10.1111/j.1365-2486.2011.02574.x URL [本文引用: 1] 摘要
ABSTRACT North American fire-adapted forests are experiencing changes in fire frequency and climate. These novel conditions may alter postwildfire responses of fire-adapted trees that survive fires, a topic that has received little attention. Historical, frequent, low-intensity wildfire in many fire-adapted forests is generally thought to have a positive effect on the growth and vigor of trees that survive fires. Whether such positive effects can persist under current and future climate conditions is not known. Here, we evaluate long-term responses to recurrent 20th-century fires in ponderosa pine, a fire-adapted tree species, in unlogged forests in north central Idaho. We also examine short-term responses to individual 20th-century fires and evaluate whether these responses have changed over time and whether potential variability relates to climate variables and time since last fire. Growth responses were assessed by comparing tree-ring measurements from trees in stands burned repeatedly during the 20th century at roughly the historical fire frequency with trees in paired control stands that had not burned for at least 70 years. Contrary to expectations, only one site showed significant increases in long-term growth responses in burned stands compared with control stands. Short-term responses showed a trend of increasing negative effects of wildfire (reduced diameter growth in the burned stand compared with the control stand) in recent years that had drier winters and springs. There was no effect of time since the previous fire on growth responses to fire. The possible relationships of novel climate conditions with negative tree growth responses in trees that survive fire are discussed. A trend of negative growth responses to wildfire in old-growth forests could have important ramifications for forest productivity and carbon balance under future climate scenarios.
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| [32] |
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| [33] |
Using fire scars and growth release in subfossil Scots pine to reconstruct prehistoric fires. https://doi.org/10.1016/S0031-0182(00)00177-2 URL 摘要
Fire scars indicating low- to moderate-intensity fires on peat deposits have been sampled from subfossil Scots pine (Pinus sylvestris L.) at sites in England, Wales and south-west Ireland. Analysis of ring-width responses to one fire event in 2800 BC illustrates its rejuvenating effect on Pinus sylvestris woodland, supporting a growing body of modern and palaeoecological data that illustrates the regenerative role played by fire in persistence of Pinus sylvestris woodland. Both the scale and timing of these fires suggest that infrequent low- to moderate-intensity fires are sufficient to stimulate Pinus sylvestris growth. This effect is shown by average increases in ring-width following the fire of between 0.62 and 1.16mm in non-scarred trees and between 0.92 and 2.74mm in fire-scarred individuals. Growth release in non-scarred trees may prove to be a more reliable method of detecting fire than using the relatively rare fire scars alone. Radii at time of scarring varied between 1.85 and 11.2cm, much smaller than is predicted to survive from modern studies.
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| [34] |
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| [35] |
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| [36] |
Dendroclimatic evaluation of climate-growth relationships of Meyer spruce (Picea meyeri) on a sandy substrate in semi-arid grassland, north China. |
| [37] |
Observations from old forests underestimate climate change effects on tree mortality. https://doi.org/10.1038/ncomms2681 URL PMID: 23552070 摘要
Understanding climate change-associated tree mortality is central to linking climate change impacts and forest structure and function. However, whether temporal increases in tree mortality are attributed to climate change or stand developmental processes remains uncertain. Furthermore, interpreting the climate change-associated tree mortality estimated from old forests for regional forests rests on an un-tested assumption that the effects of climate change are the same for young and old forests. Here we disentangle the effects of climate change and stand developmental processes on tree mortality. We show that both climate change and forest development processes influence temporal mortality increases, climate change-associated increases are significantly higher in young than old forests, and higher increases in younger forests are a result of their higher sensitivity to regional warming and drought. We anticipate our analysis to be a starting point for more comprehensive examinations of how forest ecosystems might respond to climate change.
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| [38] |
More intense, more frequent, and longer lasting heat waves in the 21st century. https://doi.org/10.1126/science.1098704 URL PMID: 15310900 [本文引用: 1] 摘要
A global coupled climate model shows that there is a distinct geographic pattern to future changes in heat waves. Model results for areas of Europe and North America, associated with the severe heat waves in Chicago in 1995 and Paris in 2003, show that future heat waves in these areas will become more intense, more frequent, and longer lasting in the second half of the 21st century. Observations and the model show that present-day heat waves over Europe and North America coincide with a specific atmospheric pattern that is intensified by ongoing increases in greenhouse gases, indicating that it will produce more severe heat waves in those regions in the future.
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| [39] |
https://doi.org/10.1007/BF00841068 URL |
| [40] |
Radial growth response of coniferous forest trees in an inner Alpine environment to heat-wave in 2003. https://doi.org/10.1016/j.foreco.2007.02.007 URL [本文引用: 1] 摘要
The record-breaking heat-wave in summer 2003 was expected to have a strong impact on tree growth, especially where trees occur at their ecological limits. We studied radial growth response of Scots pine ( Pinus sylvestris L.) and Norway spruce ( Picea abie s (L.) Karst.) exposed to dry inner Alpine climate (Tyrol, Austria) to extreme hot and dry conditions in 2003. Tree ring chronologies from two stands comprising different social status, i.e. dominant, co-dominant and suppressed trees, on a south- and north-facing slope, which represent xeric and dry-mesic site conditions, respectively, were analysed. Growth鈥揷limate relationships were explored using response function analysis and Pearson correlation coefficients. Major findings of our study were: (i) radial growth in 2003 was strongest reduced in suppressed P. sylvestris growing on a xeric site and in dominant Picea abies growing on the dry-mesic site, (ii) median reductions in annual increments reached 35% compared to previous years (1998鈥2002) and were caused by early stop of cambial activity as indicated by pronounced decrease in latewood width and (iii) April through June precipitation was the environmental factor most strongly associated with growth of both species. Ring width of P. sylvestris and Picea abies was additionally limited by hot late spring and hot late spring/summer months, respectively. The minor impact of the 2003 summer heat-wave on growth of drought exposed forest trees might find its explanation in strong dependency of radial stem growth on precipitation during late spring and preconditioning of tree vigor in previous years. Results demonstrate that impact of climate extremes on radial tree growth can vary within site and canopy position and strongly depend on species-specific response to climate factors.
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| [41] |
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| [42] |
Fire regimes and forest structure in a sky island mixed conifer forest, Guadalupe Mountains National Park, Texas, USA. https://doi.org/10.1016/j.foreco.2006.12.029 URL [本文引用: 1] 摘要
Fire is a key disturbance agent in the fire-prone mixed conifer and ponderosa pine forests of the southwestern United States. Human activities (i.e., livestock grazing, logging, and fire suppression) have resulted in the exclusion of fire from these forests for the past century and fire exclusion has caused changes in forest structure and composition. This study quantifies spatial and temporal variability in fire regimes and forest change in a 1000-ha area of mixed conifer forest in Guadalupe Mountains National Park (GMNP), an area with an uncommon history of grazing and fire suppression. Dendroecological methods were used to quantify fire frequency, season, severity, and extent, as well as forest structural and compositional change. The mean composite fire return interval (CFI) for the study area was 4 years. Widespread fires were less frequent. The mean CFI for fires recorded in at least 10% of the samples collected was 9.2 years, and mean CFI for fires scarring at least 25% of samples was 16.3 years. Many of these widespread fires occurred in the 19th century. The mean point fire return interval (PFI) was longer at 24 years. Fire scars were primarily formed in the earliest portion of earlywood in annual rings, indicating that fires burned mainly in the spring, at the beginning of the growing season. The onset of grazing in the 1920s dramatically reduced fire frequency. An increase in tree density and a compositional shift from southwestern white pine ( Pinus strobiformis Engelm.) to Douglas-fir ( Pseudotsuga menziesii [Mirb.] Franco) coincides with the grazing era. In addition, the pre-ranching era was characterized by low-severity fires, while structural changes have resulted in a contemporary forest that is prone to high severity fire, as evidenced by two stand-replacing wildfires in GMNP in the 1990s.
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| [43] |
Reconstruction of climate change on Changbai Mountain, Northeast China using tree-ring data. 利用树轮资料重建长白山区过去气候变化 .
<p>本文以树轮气候学中常用的树轮宽指数资料作为过去气候的代用资料，重建了长白山区1655年以来1～4月月平均最高气温的变化。重建中的校准方程稳定性较好，并可解释重建变量方差的57．4％。在1833年以来重建可靠性较高的时段中，1974～1979年为低温期，1951～1963年为高温期。而结束于1861和结束于1897年的两个高温期持续时间最长。本文的研究结果展示了利用树轮资料重建我国长白山区过去气候变化的巨大潜力。</p>
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| [44] |
Little change in global drought over the past 60 years. https://doi.org/10.1038/nature11575 URL PMID: 23151587 摘要
Drought is expected to increase in frequency and severity in the future as a result of climate change, mainly as a consequence of decreases in regional precipitation but also because of increasing evaporation driven by global warming. Previous assessments of historic changes in drought over the late twentieth and early twenty-first centuries indicate that this may already be happening globally. In particular, calculations of the Palmer Drought Severity Index (PDSI) show a decrease in moisture globally since the 1970s with a commensurate increase in the area in drought that is attributed, in part, to global warming. The simplicity of the PDSI, which is calculated from a simple water-balance model forced by monthly precipitation and temperature data, makes it an attractive tool in large-scale drought assessments, but may give biased results in the context of climate change. Here we show that the previously reported increase in global drought is overestimated because the PDSI uses a simplified model of potential evaporation that responds only to changes in temperature and thus responds incorrectly to global warming in recent decades. More realistic calculations, based on the underlying physical principles that take into account changes in available energy, humidity and wind speed, suggest that there has been little change in drought over the past 60 years. The results have implications for how we interpret the impact of global warming on the hydrological cycle and its extremes, and may help to explain why palaeoclimate drought reconstructions based on tree-ring data diverge from the PDSI-based drought record in recent years.
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| [45] |
The dynamic response of plant root respiration to increasing temperature and global warming. 植物根呼吸对升温的响应 .
植物根呼吸碳释放量高达18 Pg/a，约为全球化石燃料燃烧碳排放量(6.5 Pg/a)的2.8倍。了解根呼吸对升温的响应对于构建陆地生态系统碳动态模型、评价地下碳库碳收支具有重要作用。短期升温能明显提高根呼吸速率，但在近乎恒定的温度梯度下，根呼吸速率可能逐渐恢复到温度变化前的水平。根呼吸的温度敏感性与植物种和测定的温度范围有关，其<EM>Q</EM><SUB>10</SUB>值介于1.1~10之间。在野外条件下，根呼吸的温度敏感性还会受到土壤湿度、养分状况、呼吸底物有效性、太阳辐射、光合产物的地下分配模式和天气状况等影响。通常根呼吸的温度敏感性比土壤微生物呼吸的温度敏感性高，但室内控制温度下和野外环割(girdling)实验中并未观测到类似现象。根呼吸是否具有温度适应性仍是一个尚未解决的重大科学问题。有关根呼吸对升温的适应机理仍不清楚，可能是碳循环研究存在不确定性的重要来源。今后的研究方向应集中在以下几方面：（1）深入探讨根呼吸的温度适应性；（2）扩大对成年植物种的研究；（3）扩大对环境因子交互影响和模拟研究；（4）扩大对植物根呼吸测定和升温新技术的研究。
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| [46] |
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| [47] |
Evaluation on Forest Ecosystem Service Value of Linzhi in Tibet. Master degree dissertation, Northwest A&F University, 西藏林芝地区森林生态系统服务价值评估研究 .
森林是全球维持生态平衡的主题和人类赖以生存的重要自然资源，其 服务价值的评估是一个研究热点。为增强人们对青藏高原森林生态系统服务价值的认识，本文以林芝地区森林为研究对象，依据国内外对森林生态系统服务价值评估 的研究成果，结合该地区地区森林生态系统服务功能的特点，探讨了评估指标的筛选原则和方法。构建了一套可用于林芝地区森林生态系统服务评价的指标体系。此 外，依据构建评价指标，运用生态学、生态经济学、资源经济学、环境经济学等多学科知识，对该地区森林八个方面的服务功能进行评估。主要结果如下： 1、依据科学性原则、全面性原则、...
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| [48] |
Study on the spatial and temporal distribution of forest fire in Tibet. 西藏森林火灾时空分布规律研究 .https://doi.org/10.3969/j.issn.1004-5309.2007.01.002 URL 摘要
根据1992-2005年西藏 藏族自治区森林火灾统计,分析了森林火灾发生的时间和空间规律。森林火灾的年际间波动较大,2001以来火灾次数呈明显的上升趋势,但主要是火警次数增 多。西藏的森林火灾主要发生在春季和冬季,特别是12月-翌年5月,2月和3月份的森林火灾最为严重。从森林火灾的空间分布来看,森林火灾主要发生在藏东 南地区,特别是芒康、察隅、林芝和米林等县的森林火灾较多。引起森林火灾的火源主要是由生产用火和生活用火引起的。文中还对西藏森林火灾发生的火环境进行 了分析,气候特点和可燃物分布特征决定了西藏的森林防火期为冬春季,火灾多发生在海拔2800 m-3600 m的阳坡针叶林和阔叶林中,以地表火最多,大多火场面积少于10 hm2。根据西藏森林火灾的发生特点,建议今后在防火基础建设、火源管理、可燃物管理、林火预防和扑救等方面加强工作,提高森林防火的能力。
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| [49] |
The changing character of precipitation. |
| [50] |
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| [51] |
Individual tree and stand level influences on the growth, vigor, and de- cline of red oaks in the Ozarks. |
| [52] |
Study on response relationship between tree-ring and climate factors and climate reconstruction in middle region of Qilianshan Mountains. 祁连山中部树木年轮宽度与气候因子的响应关系与气候重建 .
研究了祁连山中部不同海拔高度青海云杉的树轮宽度对气候因子的响应,重建了祁连山中部 230 a以来春季3~5月的降水和 170 a以来夏季6~8月的气温序列。结果分析发现,不同高度的云杉树轮生长对春季降水极为敏感,呈现显著正相关;对夏季气温的响应程度,各海拔高度却不相同,夏季气温对上、下限云杉生长有显著影响,但对于森林中部云杉作用并不明显,总体表现为负相关,夏季高温对树木生长不利。气候重建结果发现,祁连山中部的春季230 a以来经历了大幅度长阶段的干湿变化,存在明显的 69 a和 21a周期;170 a以来夏季气温变化频繁,存在明显的2~4a周期。目前,祁连山中部正处于相对干旱和温暖时期,呈现出向暖干方向发展的趋势。
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| [53] |
Warming and earlier spring increase western U.S. forest wildfire activity. https://doi.org/10.1126/science.1128834 URL PMID: 16825536 [本文引用: 1] 摘要
Western United States forest wildfire activity is widely thought to have increased in recent decades, yet neither the extent of recent changes nor the degree to which climate may be driving regional changes in wildfire has been systematically documented. Much of the public and scientific discussion of changes in western United States wildfire has focused instead on the effects of 19th- and 20th-century land-use history. We compiled a comprehensive database of large wildfires in western United States forests since 1970 and compared it with hydroclimatic and land-surface data. Here, we show that large wildfire activity increased suddenly and markedly in the mid-1980s, with higher large-wildfire frequency, longer wildfire durations, and longer wildfire seasons. The greatest increases occurred in mid-elevation, Northern Rockies forests, where land-use histories have relatively little effect on fire risks and are strongly associated with increased spring and summer temperatures and an earlier spring snowmelt.
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| [54] |
Amazonian tree mortality during the 1997 El Nino drought. https://doi.org/10.1046/j.1523-1739.2000.99298.x URL 摘要
In 1997, the Amazon Basin experienced an exceptionally severe El Ni09o drought. We assessed effects of this rare event on mortality rates of trees in intact rain forest in the central Amazon (80 km north of Manaus, Brazil) based on data from permanent plots. Long-term (5- to 13-year) mortality rates averaged only 1.12% per year prior to the drought. During the drought year, annual mortality jumped to 1.91% but abruptly fell back to 1.23% in the year following El Ni09o. Trees dying during the drought did not differ significantly in size or species composition from those that died previously, and there was no detectable effect of soil texture on mortality rates. These results suggest that intact Amazonian rainforests are relatively resistant to severe El Ni09o events.
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Effects of climate on vertical resin duct density and radial growth of Norway spruce [Picea abies (L.) Karst.]. |
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The study of factors influencing the natural regeneration of forests. 森林自然更新影响因子的探讨 .https://doi.org/10.13601/j.issn.1005-5215.2014.08.041 URL [本文引用: 1] 摘要
森林自然更新是对幼树进行补充的一个自然再生过程，是森林群落动 态变化的重要组成部分。文章综述了土壤种子库、林窗、林火、凋落物、立地条件等生态因子对森林自然更新的影响，为进一步认识自然更新动态和群落演替方向提 供参考，旨在为我国森林保护、植被恢复和持续经营提供科学依据和理论基础。
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The influence on forest soil’s nature and tree’s regeneration for forest fire in the big Xing’an maient. Journal of Jilin Forestry Institute ,大兴安岭北部林火对森林土壤的性质及林木更新的影响 .URL 摘要
本文对大兴安岭北部火烧迹地森 林土壤的性质及林木更新进行了调查、分析、研究。着重从林火、土壤、林木更新三者入手,将三者有机地结合起来,分别不同的火烧强度和不同的火烧时间,对火 烧迹地森林土壤的性质变化和林木更新情况,设置标准地进行测定、采样、分析并与未火烧的相对比,找出火烧与森林土壤的性质及林木更新的关系,为火烧迹地的 土壤改良,促进林木更新提供依据和措施:同时还可以找到适宜的火烧强度,将其做为该地区计划用火的指标,对森林实施人为控制定期火烧,清除林地枯枝落叶, 促进营养循环,提高林地土壤肥力,促进林木更新,并达到以火防火的目的。研究发现,重度火烧可烧毁林褥层,提高土壤温度,降低土壤水分含量,损失土壤
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The Effect of Forest Fires on Carbon Budget in Daxing’an Mountains. Master degree dissertation, 大兴安岭26年间林火对森林植被碳收支的影响 .
森林火灾是大兴安岭地区森林生态系统的重要干扰因子。森林火灾直接排放的含碳气体会影响大气碳平衡,进而对全球的气候变化产生影响。因此估算由于森林火灾 而释放的碳及植被恢复过程中固定的碳,对于量化森林火灾对碳平衡的影响具有重要的理论和实际意义,为揭示高纬度地区森林生态系统对全球变化的影响及其响应 问题提供科学依据。 本文充分利用现有的多年大兴安岭地区一类森林资源连续清查资料和黑龙江省火警火灾登记表,结合地理信息系统技术,对大兴安岭地区1980至2005年间发 生的615次林火进行统计,并对不同等级林火中森林的碳释放和火后植被的碳固定进行了估算,进而获得大兴安岭地区26年间林火引起的森林碳收支情况。野外 调查火后不同时期的落叶松林及其对照林分,钻取样木生长芯,经分析计算得到落叶松林火后恢复时期的净初级生产力(Net Primary Productivity,NPP),并建立26年间森林火后NPP的动态变化,分析林火对森林生产力的影响。 研究结果表明,1980至2005年期间,大兴安岭地区共发生火警火灾858起,其中林火615起,过火林地共129.12万hm~2。森林火警、轻度火 灾、中度火灾和重度火灾的过火的林地面积分别为117.87 hm~2、2 598.96 hm~2、7 950 hm~2和1 280 839.30 hm~2。 1980至2005年的林火期间,生物量损失共计约为31.94～39.60 Tg。其中落叶松在各场林火中的生物量损失约占总生物量损失的56%,白桦的生物量损失约占总生物量损失的33%。26年间,共计释放碳约 13.44～16.58 Tg。其中,1987年林火中碳释放量约为1.97～2.47 Tg,占26年间总的碳释放量的14.7%。1980～2005年期间,森林在火后恢复过程中共计固定碳约6.83～8.83 Tg,单位面积的碳固定量为5.29～6.84 t/hm~2。 26年间大兴安岭林区在林火中的碳收支为-7.75～-6.61 Tg。表明在此期间,大兴安岭地区的森林在林火的影响下,向大气的释放的碳大于其生长恢复过程中所固定的碳。与其他等级林火的火烧迹地相比,中度林火过后 的火烧迹地内碳收支为正值。由此可见,中度林火干扰能够促进森林生态系统对碳的固定,有利于增强森林生态系统的碳汇功能。 在火后26年期间NPP动态变化的研究中发现,火烧迹地的NPP随火后恢复时间的增加而增加;火后20年时,火烧迹地内的NPP值增加缓慢,林分生产力水 平基本稳定;在火后大约23～24年时,几乎与对照样地的NPP值相等,并且之后有继续上升的趋势,即表明火后23～24年时,乔木生产力已经基本恢复, 达到了对照样地的NPP水平。
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The influence of fire disturbance on the DBH growth of remained woods of Larix sibirica Ldb. 火干扰对西伯利亚落叶松保留木胸径生长的影响 .https://doi.org/10.3969/j.issn.1007-8614.2009.03.001 URL 摘要
以树轮年代学年轮指数为指标, 采用突变分析和对比分析的方法,分析了火干扰对新疆喀纳斯泰加林主要组成树种西伯利亚落叶松胸径生长的影响。结果表明,西伯利亚落叶松受到火干扰后如果得 以保留,其胸径生长过程会发生显著变化,主要表现为存在一个明显的胸径加速生长期。说明火干扰对西伯利亚落叶松不只有破坏作用的一面,而且对其保留木还有 促进生长的生态作用。
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