水杉人工林树冠结构及生物生产力的研究

研究了水杉人工林的树冠结构和林分生物生产力。结果表明,不同密度及林龄的林分树冠结构存在较大差异,随着树冠部位上升和林分密度增大,分枝角度逐渐减小;径阶大小与枝叶率成反比,与树冠重量成正比,径阶增大,树冠最大叶量层的集团上移,有效光合面积相对减少,树冠结构的变化直接影响到林分的生物量生产、分配分配和经济生物量,林分干、枝、叶的干物质累积趋势可用Ｒｉｃｈａｒｄ方程描述;林龄增大,分配到主干的生物量比例     

广西不同林龄杉木、马尾松人工林根系生物量及碳储量特征

为了解不同林龄杉木、马尾松人工林地地下根系生物量及碳储量特征,以广西杉木、马尾松主产区5个不同林龄阶段(幼龄林、中龄林、近熟林、成熟林、过熟林)的人工林为研究对象,采用全根挖掘法和土钻法获取标准木根系生物量、灌草根系生物量和林分细根生物量,并测定其碳含量,分析其不同林龄阶段地下根系生物量和碳储量分配特征。结果表明:杉木、马尾松林地下根系总生物量分别在9.06—31.40Mg/hm~2和7.91—53.40Mg/hm~2之间,各林龄阶段根系总生物量总体上呈现随林龄增加而增加的趋势,杉木林细根生物量随林龄的增加呈现出先减后增的趋势,马尾松呈现出逐渐减小的趋势;林分各层次根系碳含量表现为乔木灌木草本、细根;杉木、马尾松地下根系碳储量变化趋势与生物量变化趋势相同,杉木、马尾松林不同林龄阶段各层次根系和土壤细根总碳储量分别在7.56—21.97Mg/hm~2和8.86—29.95Mg/hm~2之间;地下根系碳储量总体上以乔木根系占优势,且随林龄的增大其比例呈增加的趋势。     

杉木林年龄序列地下碳分配变化

  森林地下碳分配在森林碳平衡和碳吸存中具有重要作用, 而揭示人工林生长过程中地下碳分配变化对于人工林碳汇估算和碳汇管理等有重要意义。通过采用年龄序列方法研究了杉木(Cunninghamia lanceolata)林生长过程中地下碳分配变化特点。年龄序列为福建省南平7 a生(幼龄林)、16 a生(中龄林)、21 a生(近熟林)、41 a生(成熟林)和88 a生(老龄林)的杉木林。细根净生产力测定采用连续土芯法, 根系呼吸测定采用壕沟法, 生物量增量测定采用异速生长方程, 地上年凋落物量采用凋落物收集框测定。结果表明: 杉木林细根净生产力在中龄林前没有显著差异, 维持在较高水平; 但此后则显著下降。细根净生产力/地上凋落物量比值随林龄增加而显著下降。老龄林的根系呼吸显著低于其它林龄林分, 根系呼吸与细根生物量间呈显著线性相关。中龄林和近成熟林的地下碳分配(Total belouground carbon allocation, TBCA)显著高于幼龄林和成熟林, 而老龄林的则最低。中龄林、近成熟林和成熟林的地上部分净生产力/TBCA比值显著高于幼龄林和老龄林, 而杉木林的根系碳利用效率(RCUE)则呈现出随林龄增加而降低的趋势。     

不同林龄樟子松人工林径向生长对气候及地下水位变化的响应

全球气候变化导致森林生态系统的结构与功能发生改变,甚至出现树木死亡与林分衰退的现象,研究林分生长对气候变化尤其是干旱事件的响应有助于预测未来气候变化下生态系统的稳定性。以辽宁章古台5个林龄的樟子松人工林为研究对象,分析了树木径向生长对气候因子与地下水位的响应,结果表明：秋季气温,尤其是最低气温显著影响樟子松林的生长（44年生林分除外）;低林龄樟子松林（36、39年）生长与当年夏季及生长季内的降水显著正相关,而高林龄樟子松林（52年）生长则与当年春季尤其是当年2月与5月降水显著正相关;36、39、52年生樟子松人工林年表与当年夏季的Palmer干旱指数（PDSI）显著正相关,44、58年生樟子松人工林年表则与地下水位显著正相关。应对早期干旱（即1997年）时,樟子松人工林表现为随林龄增加,其抵抗力增加而恢复力降低;在随后的两个干旱事件中,高林龄樟子松林的抵抗力不再明显高于低林龄,可能是由于地下水位显著降低影响根系吸水;受累积干旱的影响,所有林龄樟子松人工林对2007-2008干旱事件的弹性力均小于1,径向生长量明显降低。地下水位是影响不同林龄樟子松人工林生长及对干旱抵抗力的重要因子,考虑地下水位有助于进一步提升森林生态系统对气候变化响应研究的准确性。     

模拟喀斯特异质性小生境下三叶鬼针草地上地下协同生长对策

模拟在喀斯特异质生境下,通过随机区组实验,研究三叶鬼针草（Bidens pilosa L.）在两种土壤生境（浅而宽、深而窄）和3种水分处理（对照、减水50%、减水70%）下植物的地上和地下生长关系及生物量分配格局。结果显示：（1）两种生境中三叶鬼针草的地上生长（株高、地径、叶面积、叶生物量）与地下根系生长（根长、根表面积、根体积、根生物量）均随着施水量的减少而降低;叶面积比率随着施水量的减少而增加;根质量比在浅而宽土壤生境中呈先增后减的趋势,而在深而窄土壤生境中呈增加趋势。（2）两种生境中三叶鬼针草的地上生物量与地下根系生物量、叶面积与根长、叶面积与各层根系生长均呈显著正相关关系。但在浅而宽土壤生境中,三叶鬼针草的地上生物量与各土层根系生物量均呈显著正相关,而在深而窄土壤生境中,地上生物量仅与中上土层根系生物量呈显著正相关。研究表明三叶鬼针草在不同生境中均具有较好的地上地下协同生长对策,在增强对地下资源获取的同时也增强了对地上资源的获取。在浅而宽土壤生境中,三叶鬼针草通过协调根系的横向拓展能力与植物叶片的生长来应对快速的干旱缺水;在深而窄土壤生境中,植株能较好地协调根系向下拓展能力与地上叶面积的生长,更好地利用土壤深层的水分资源。     

近自然化改造对马尾松和杉木人工林生物量及其分配的影响

近自然化改造作为森林新增碳汇的最有希望的选择之一,将如何通过改变林分结构影响林分生物量和生产力进而影响林分固碳能力和潜力目前尚不清楚,因此,了解近自然化改造对人工林生物量及其分配的影响,对人工林生态系统碳管理具有重要意义。以马尾松近自然化改造林(P(CN))、马尾松未改造纯林(P(CK))、杉木近自然改造林(C(CN))和杉木未改造纯林(C(CK))4种人工林为研究对象,采用样方调查和生物量实测的方法,分析4种林分生物量差异,旨在揭示近自然化改造对马尾松和杉木人工林生物量及其分配的影响。结果表明:马尾松杉木人工林近自然化改造通过调整林分结构显著提升马尾松和杉木人工林生物量和生产力,8a后马尾松和杉木林分生物量分别增加46.71%和37.24%。乔木层生物量在林分生物量总量中占主导地位(95.48%-98.82%),并对林分生态系统总生物量变化起决定性作用。林分生物量和生产力的增加主要因为近自然化改造改变了林分群落结构,进而提高了乔木层生产力。研究结果表明,合理的经营措施不仅可以改善林分结构,提升林分生产力,并可为增强植被固碳能力创造有利条件。     

我国3个桉树人工林种植区生产力影响因素

研究影响桉树人工林生产力的主要因素,对于提高桉树人工林生产力以及经营水平和合理利用树种都具有重要的意义。收集我国3个桉树人工林种植区(广东省、广西壮族自治区、云南省)关于生物量与生产力研究中的相关数据,对海拔、林分密度、林龄、年均温和年降水量与乔木层生物量进行了主成分分析与混合模型分析,探究各因素对我国桉树人工林生物量与生产力的影响。结果显示:年均温、年降水量、林龄、海拔和林分密度均是影响广东、广西、云南这3个省(区)桉树人工林生物量的主要因素,其中年降水量对这3个地区桉树人工林的生物量影响极其显著(P0.01),林龄和林分密度对桉树人工林的生物量影响显著(P0.05)。这是因为这3个地区都位于亚热带,温度都属于桉树生长的适宜温度,而这3个地区存在明显的干湿季,降水量在季节和地域上分布不均匀,导致这3个地区年降水量存在较明显的差异。因此,年降水量成为影响这3个地区桉树人工林生物量的关键因素。     

树木高生长限制的几个假说

树木生长到一定年龄后高生长停滞,对这一现象的解释存在很多争议.成熟假说认为树木顶端分生组织分裂活性下降导致树木高生长减慢.营养限制假说认为土壤中营养元素(特别是氮素)在植物活体或枯落物中积累使土壤中可利用的养分含量降低,细根生物量增加和叶片光合能力下降导致了地上部分生长的减缓.呼吸假说认为边材呼吸消耗随个体发育的增加使投入到高生长的碳减少.水力限制假说认为水分运输阻力随高度增加而增加导致了叶片总光合碳同化下降,分配到高生长的生物量减少.树木发展假说认为植物用多种调节机制克服随个体发育增加的水力阻力,包括叶片结构和生理特征的变化,叶/边材面积比降低,边材渗透性和树干储水能力的增加等.水力限制假说得到了较多的关注,对不同高度树木的叶比导率、光合特征和树干生长量等测定结果支持这一假说.但对这一假说也存在很多的争议,主要表现在:水力阻力是否确实随高度的增加而增加,水力阻力的分布,补偿机制的作用和生物量分配转变等.本文综述了树木高生长限制的4个假说以及争论的焦点,并总结了目前研究的热点问题和今后的研究方向.     

树木高生长限制的几个假说

树木生长到一定年龄后高生长停滞, 对这一现象的解释存在很多争议。成熟假说认为树木顶端分生组织分裂活性下降导致树木高生长减慢。营养限制假说认为土壤中营养元素(特别是氮素)在植物活体或枯落物中积累使土壤中可利用的养分含量降低, 细根生物量增加和叶片光合能力下降导致了地上部分生长的减缓。呼吸假说认为边材呼吸消耗随个体发育的增加使投入到高生长的碳减少。水力限制假说认为水分运输阻力随高度增加而增加导致了叶片总光合碳同化下降, 分配到高生长的生物量减少。树木发展假说认为植物用多种调节机制克服随个体发育增加的水力阻力, 包括叶片结构和生理特征的变化, 叶/边材面积比降低, 边材渗透性和树干储水能力的增加等。水力限制假说得到了较多的关注, 对不同高度树木的叶比导率、光合特征和树干生长量等测定结果支持这一假说。但对这一假说 也存在很多的争议, 主要表现在: 水力阻力是否确实随高度的增加而增加, 水力阻力的分布, 补偿机制的作用和生物量分配转变等。本文综述了树木高生长限制的4个假说以及争论的焦点, 并总结了目前研究的热点问题和今后的研究方向。     

中国南方3种主要人工林生物量和生产力的动态变化

基于中国南方杉木、马尾松、桉树3种主要人工林的幼龄林、中龄林、近熟林、成熟林、过熟林5个不同年龄各3块1000 m2样地(共计45块)的建立和调查,采用样木回归分析法(乔木层)和样方收获法(灌木层、草本层、地上凋落物)获取不同林型不同林龄径级样木和其它基本数据,探讨了3种人工林各组分各层次林分生物量和生产力的分配特征及随林龄的变化规律,结果表明,林分生物量和生产力与林龄密切相关,增长模型的拟合度均较高,相关显著;杉木、马尾松、桉树人工林的生物量随林龄的增长呈增加趋势,成熟林的生物量分别为192.30、191.53、105.77 Mg/hm2,其中活体植物分别占95.76%—98.39%、75.01%—99.14%、85.60%—97.61%;生物量的层次分配乔木层占绝对优势,并随年龄而增加,其它层次所占比例较小,总体趋势为凋落物草本层灌木层;乔木层的器官分配以干所占比例最高,杉木、马尾松、桉树分别占54.89%—75.97%、49.93%—83.10%、51.07%—98.48%,随年龄的增加而增加,根的比例次之,枝叶所占比例较小,随林龄而下降;灌木层器官分配以枝的相对生物量较大,草本层的地上和地下分配规律不明显;与其它森林类型相比,杉木和马尾松的生物量处于中上游水平,桉树的生物量较低,但3种人工林的生产力均很高,分别为12.37、8.98、21.10 Mg hm-2a-1,均是光合效率高、固碳潜力大的中国南方速生丰产优良造林树种。     

Age-related decline in stand productivity: the role of structural acclimation under hydraulic constraints

The decline in above-ground net primary productivity (P_a) that is usually observed in forest stands has been variously attributed to respiration, nutrient or hydraulic limitations. A novel model is proposed to explain the phenomenon and the co-occurring changes in the balance between foliage, conducting sapwood and fine roots. The model is based on the hypothesis that a functional homeostasis in water transport is maintained irrespective of age: hydraulic resistances through the plant must be finely tuned to transpiration rates so as to avoid extremely negative water potentials that could result in diffuse xylem embolism and foliage dieback, in agreement with experimental evidence. As the plant grows taller, allocation is predicted to shift from foliage to transport tissues, most notably to fine roots. Higher respiration and fine root turnover would result in the observed decline in P_a. The predictions of the model have been compared with experimental data from a chronosequence of Pinus sylvestris stands. The observed reduction in P_a is conveniently explained by concurrent modifications in leaf area index and plant structure. Changes in allometry and shoot hydraulic conductance with age are successfully predicted by the principle of functional homeostasis.     

Above- and Below-Ground Carbon Stocks in an Indigenous Tree (Mytilaria laosensis) Plantation Chronosequence in Subtropical China

More than 60% of the total area of tree plantations in China is in subtropical, and over 70% of subtropical plantations consist of pure stands of coniferous species. Because of the poor ecosystem services provided by pure coniferous plantations and the ecological instability of these stands, a movement is under way to promote indigenous broadleaf plantation cultivation as a promising alternative. However, little is known about the carbon (C) stocks in indigenous broadleaf plantations and their dependence on stand age. Thus, we studied above- and below-ground biomass and C stocks in a chronosequence of Mytilaria laosensis plantations in subtropical China; stands were 7, 10, 18, 23, 29 and 33 years old. Our assessments included tree, shrub, herb and litter layers. We used plot-level inventories and destructive tree sampling to determine vegetation C stocks. We also measured soil C stocks by analyses of soil profiles to 100 cm depth. C stocks in the tree layer dominated the above-ground ecosystem C pool across the chronosequence. C stocks increased with age from 7 to 29 years and plateaued thereafter due to a reduction in tree growth rates. Minor C stocks were found in the shrub and herb layers of all six plantations and their temporal fluctuations were relatively small. C stocks in the litter and soil layers increased with stand age. Total above-ground ecosystem C also increased with stand age. Most increases in C stocks in below-ground and total ecosystems were attributable to increases in soil C content and tree biomass. Therefore, considerations of C sequestration potential in indigenous broadleaf plantations must take stand age into account.     

The hydraulic limitation hypothesis revisited

We proposed the hydraulic limitation hypothesis (HLH) as a mechanism to explain universal patterns in tree height, and tree and stand biomass growth: height growth slows down as trees grow taller, maximum height is lower for trees of the same species on resource-poor sites and annual wood production declines after canopy closure for even-aged forests. Our review of 51 studies that measured one or more of the components necessary for testing the hypothesis showed that taller trees differ physiologically from shorter, younger trees. Stomatal conductance to water vapour (g(s)), photosynthesis (A) and leaf-specific hydraulic conductance (K L) are often, but not always, lower in taller trees. Additionally, leaf mass per area is often greater in taller trees, and leaf area:sapwood area ratio changes with tree height. We conclude that hydraulic limitation of gas exchange with increasing tree size is common, but not universal. Where hydraulic limitations to A do occur, no evidence supports the original expectation that hydraulic limitation of carbon assimilation is sufficient to explain observed declines in wood production. Any limit to height or height growth does not appear to be related to the so-called age-related decline in wood production of forests after canopy closure. Future work on this problem should explicitly link leaf or canopy gas exchange with tree and stand growth, and consider a more fundamental assumption: whether tree biomass growth is limited by carbon availability.     

Shedding of older needle age classes does not necessarily reduce photosynthetic primary production of Norway spruce

The hypothesis that the frequently observed shedding of older needle age classes in stands of Norway spruce suffering from atmospheric pollutants has minimal effect on photosynthetic primary production was assessed. Using structural parameters of young Norway spruce [Picea abies (L.) Karst.] trees, an existing 3-dimensional canopy photosynthesis model was adapted and validated for conifer canopies. This model was employed to quantitatively demonstrate that, depending on the stand density, the loss in photosynthetic active foliage area may be compensated by higher photosynthetic rates of the remaining younger foliage age classes due to higher levels of photosynthetically active radiation (PAR) within the thinned tree crowns. Under certain circumstances, beneficial effects on the tree's water use efficiency may be expected. Extrapolating the model results to mature tree stands is addressed. The potential effects of higher PAR levels on abundance and species diversity of the epiphytic and understory vegetation, as well as the general importance of the older foliage age classes for spruce trees on a long term scale, are discussed.     

Photosynthetic capacity of Eucalyptus globulus is higher when grown in mixture with Acacia mearnsii

Mixed species plantations of Eucalyptus and N₂-fixing species can be significantly more productive than monocultures. The aim of this study was to determine whether the improved growth resulted from increases in photosynthesis, light absorption and light-use efficiency, in addition to previously measured increases in leaf area, water-use efficiency and higher ratios of annual above-ground net primary production per unit of total annual below-ground carbon allocation in 1:1 mixtures near Cann River, Victoria, Australia. Light-saturated photosynthetic rate (A _max), electron transport (J), stomatal conductance (g _s) and foliar nitrogen concentrations were higher for Eucalyptus globulus trees growing in mixtures than those in monocultures. Similar increases in maximum rates of carboxylation (V _cmax), Rubisco, chlorophyll, and phosphorus concentrations were not significant. In contrast, A _max, V _cmax and J did not vary between mixtures and monocultures for A. mearnsii, whose growth was negligible by age 15 years. Mixtures also absorbed 24 and 41% more light than E. globulus and A. mearnsii., respectively, and were 38 and 154% more light-use efficient in the mixtures compared to monocultures. The increased nutrient availability in mixtures appeared to increase productivity of E. globulus by increasing the photosynthetic capacity of the foliage, as well as the leaf area, light absorption and light-use efficiency of the canopy.     

Structure and Function of an Age Series of Poplar Plantations in Central Himalaya: I Dry Matter Dynamics

The biomass and net primary productivity (NPP) of 5- to 8-year-oldpoplar (Populus deltoides Marsh, Clone D₁₂₁) plantations growingin the Tarai belt (low-lying plains with high water table adjacentto foothills of central Himalaya) were estimated. Allometricequations for all the above-ground and below-ground componentsof trees and shrubs were developed for each stand. Understorey,forest floor biomass, and litter fall were also estimated fromstands. The biomass of plantation, forest floor litter mass,tree litter fall and net primary productivity (NPP) of treesand shrubs increased with increase in plantation age, whereasherb biomass and NPP significantly (P < 0·01) decreasedwith increasing plantation age. The total plantation biomassincreased from 84·0 in the 5-year-old to 170·0t ha^-1 in the 8-year-old plantation and NPP from 16·8t ha^-1 year^-1 in the 5- and 6-year-old to 21·8 t ha^-1year^-1 in the 8-year-old plantation. The biomass accumulationratio (biomass: net production, BAR) for different tree componentsincreased with the age of plantation increase. The BAR ratioranged from 4·9 in the 5-year-old to 7·7 in the8-year-old plantation.Copyright 1995, 1999 Academic Press Populus deltoides plantations (Clone D₁₂₁), biomass, dry matter turnover, net primary productivity, Tarai belt of Central Himalaya     

A test of the hydraulic limitation hypothesis in fast-growing Eucalyptus saligna

The hydraulic limitation hypothesis proposes that (1) reduced growth in taller trees is caused by decreased photosynthesis resulting from a decrease in hydraulic conductance promoted by a longer root‐to‐leaf flow path, and (2) this mechanism reduces stand productivity after canopy closure. This hypothesis was tested by comparing the physiology of 7 m (1 year) and 26 m (5 year) Eucalyptus saligna plantations where above‐ground productivity for the 26 m trees was approximately 69% of that for the 7 m trees, and water and nutrients were not limiting. The study compared whole tree physiology [water flux (Q_l), average crown conductance (G_T), crown hydraulic conductance per unit leaf area (K_L), carbon isotope discrimination (δ¹³C)] and leaf physiology under light saturation (leaf water potential at the canopy top (Ψ_LEAF), photosynthetic capacity (A_max), and photosynthesis (A) and stomatal conductance (g_s). K_L was 50% lower in the taller trees, but whole tree Q_l and G_T were the same for the 7 m and 26 m trees. Photosynthetic capacity was the same for leaves at the canopy top, but δ¹³C was ?1.8‰ lower for the 26 m trees. A and g_s were either lower in the taller trees or equal, depending on sampling date. The taller trees maintained 0.8 MPa lower Ψ_LEAF during the day and had 2.6‐times higher sapwood area per unit leaf area; these factors compensated for the effects of increased height and gravitational potential in the taller trees to maintain higher G_T. The hydraulic limitation hypothesis (as originally stated) failed to explain the sharp decline in net primary productivity after canopy closure in this study. The effects of increased height appear to be a universal hydraulic problem for trees, but compensation mitigated these effects and maintained Q_l and G_T in the present study. Compensation may induce other problems (such as lower Ψ_LEAF or higher respiratory costs) that could reduce carbon gain or shift carbon allocation, and future studies of hydraulic limitation should consider compensation and associated carbon costs. In this study, the combination of similar G_T and lower δ¹³C for the 26 m trees suggests that total crown photosynthesis was lower for the 26 m trees, perhaps a result of the lower Ψ_LEAF.     

Stem and leaf gas exchange and their responses to fire in a north Australian tropical savanna

We measured stem CO2 efflux and leaf gas exchange in a tropical savanna ecosystem in northern Australia, and assessed the impact of fire on these processes. Gas exchange of mature leaves that flushed after a fire showed only slight differences from that of mature leaves on unburned trees. Expanding leaves typically showed net losses of CO2 to the atmosphere in both burned and unburned trees, even under saturating irradiance. Fire caused stem CO2 efflux to decline in overstory trees, when measured 8 weeks post-fire. This decline was thought to have resulted from reduced availability of C substrate for respiration, due to reduced canopy photosynthesis caused by leaf scorching, and to priority allocation of fixed C towards reconstruction of a new canopy. At the ecosystem scale, we estimated the annual above-ground woody-tissue CO2 efflux to be 275 g C m(-2) ground area year(-1) in a non-fire year, or approximately 13% of the annual gross primary production. We contrasted the canopy physiology of two co-dominant overstory tree species, one of which has a smooth bark on its branches capable of photosynthetic re-fixation (Eucalyptus miniata), and the other of which has a thick, rough bark incapable of re-fixation (Eucalyptus tetrodonta). Eucalyptus miniata supported a larger branch sapwood cross-sectional area in the crown per unit subtending leaf area, and had higher leaf stomatal conductance and photosynthesis than E. tetrodonta. Re-fixation by photosynthetic bark reduces the C cost of delivering water to evaporative sites in leaves, because it reduces the net C cost of constructing and maintaining sapwood. We suggest that re-fixation allowed leaves of E. miniata to photosynthesize at higher rates than those of E. tetrodonta, while the two invested similar amounts of C in the maintenance of branch sapwood.     

沙棘对中国亚湿润干旱区杨树人工林生长与生产力的影响(英文)

通过将沙棘（Ｈｉｐｐｏｐｈａｅ ｒｈａｍｎｏｉｄｅｓ Ｌ．）与３种杨树品种（小黑杨（Ｐｏｐｕｌｕｓ ｃｖ．“Ｘｉａｏｈｅｉ”）昭林６号杨（Ｐ．ｃｖ．“Ｚｈａｏｌｉｎ０６”）和欧美杨６４号（Ｐ．ｅｕｒａｍｅｒｉｃａｎｅ ｃｖ．“Ｎ３０１６”））的人工林分别按株混两种方式进行混交实验,研究了固氮植物沙棘对亚湿润干旱区的杨树人工林生长和生产力的影响。研究结果表明：无论哪一种杨树品种或混交方式,沙棘与杨树