林隙大小对不同林型天然红松混交林土壤有机碳和全氮的影响

在天然红松混交林3种不同林型(椴树红松混交林(TP)、云冷杉红松混交林(PAP)、枫桦红松混交林(BP))内,各选取小、中、大3个林隙,并分别以各自的郁闭林分作为对照,分析了2012年6—9月各林型不同大小林隙及其郁闭林分0~10 cm的土壤有机碳(SOC)和全氮(TN)含量,旨在阐明林隙大小对不同类型天然红松混交林土壤有机碳和全氮变化的影响,从而为小兴安岭林区天然红松混交林林隙更新和森林可持续经营提供基础数据。结果表明:在3种天然红松混交林内,林隙大小对SOC含量影响不显著;仅在PAP内,林隙大小对TN含量影响显著,在其他林型内均不显著;SOC、TN含量在3种林型内均呈现随月份不同差异显著的趋势(P0.05),而且相同月份不同林型之间的SOC和TN含量均差异显著(P0.05);PAP和BP林隙内SOC含量表现为大林隙小林隙中林隙,TN含量为大林隙中林隙小林隙,TP林隙内SOC、TN含量随林隙大小变化均为中林隙小林隙大林隙;SOC含量在3种林型内均表现为林隙内郁闭林分,TN含量在TP和BP内均为林隙内郁闭林分,PAP内却相反;SOC含量在不同林型林隙内的大小次序均为PAPTPBP,TN含量却为TPPAPBP;SOC、TN含量随月份变化大部分呈现单峰型曲线,在7或8月达到峰值,PAP林隙内TN含量在9月达到峰值;土壤碳氮比(C/N)在3个林型内不同大小林隙及郁闭林分之间均没有显著差异(P0.05)。     

小兴安岭原始红松混交林林隙土壤不同形态氮含量

以小兴安岭地区红松混交林(椴树红松混交林、云冷杉红松混交林和枫桦红松混交林)大、中、小3个林隙为研究对象,对林隙和郁闭林分土壤铵态氮(NH4+-N)、硝态氮(NO3--N)、可溶性全氮(TSN)、可溶性有机氮(SON)、微生物氮(MBN)和全氮(TN)含量进行分析,探讨不同混交林林隙中土壤氮形态特征。结果表明:林隙和郁闭林分土壤以有机氮为主,占TN 98%以上。林隙中土壤NO3--N含量高于NH4+-N含量;SON含量高于NH4+-N和NO3--N含量。红松混交林中土壤NH4+-N、NO3--N、SON和MBN含量在大、小林隙之间以及林隙与郁闭林分之间差异显著,尤其是MBN含量表现为郁闭林分小林隙中林隙大林隙,与林隙面积呈极显著负相关;3个林型中NO3--N/TN和MBN/TN随着林隙面积增大而减小;椴树红松混交林和云冷杉红松混交林林隙中NH4+-N/TN、TSN/TN和SON/TN随林隙面积增大而减小,枫桦红松混交林林隙中随林隙面积增大而增大。林隙和郁闭林分中土壤TSN与SON均呈现极显著正相关;林隙土壤NH4+-N与NO3--N和MBN呈显著相关,但在郁闭林中未表现出此关系。     

长白山云冷杉针阔混交林不同林隙下幼苗幼树密度及空间分布

2019年8月在云冷杉针阔混交林样地(0.36 hm²),对48个林隙及幼苗(0.2<更新高度RH<1 m)、幼树(RH≥1 m,胸径DBH<5 cm)进行调查,分析林隙大小(<20 m²,小;20～50 m²,中;50～120 m²,大;>120 m²,特大)对林隙内红松、鱼鳞云杉及冷杉幼苗幼树密度和生长指标(高、基径)的短期影响,并采用核密度估计法分析其空间分布规律。结果表明: 云冷杉更新的密度通常随林隙增大而降低,仅对幼树影响显著,小林隙下云冷杉幼树密度分别为0.34和1.74株·m^-2,红松密度不受林隙大小的影响。林隙大小对冷杉幼苗幼树生长指标的影响最大,对红松影响最小,平均最大值多出现在大林隙。红松和云杉幼树的基径和树高最大值均分布在小、中、大林隙东北部,在特大林隙中转移至冠空隙西北部。小林隙有助于幼苗的建立和萌发,可通过择伐冷杉创造小林隙,随后扩大林隙面积(>50 m²)促进幼树生长,需要持续监测来确定林隙大小对森林更新的长期影响。     

林隙对小兴安岭阔叶红松林树种更新及物种多样性的影响

研究了小兴安岭阔叶红松林不同林隙梯度(林隙中心、林隙近中心、林隙边缘)中主要树种的数量特征,以及林隙大小对树种更新的影响.结果表明:不同梯度林隙内灌木树种的密度均明显高于非林隙,同种灌木密度的比值在1.08～18.15之间;随林隙面积增加,乔木幼苗更新密度逐渐增大,幼树Ⅰ(高度H≥1 m,胸径DBH≤2 cm)和幼树Ⅱ(H≥1 m,2 cm＜DBH≤5 cm)的更新密度呈多峰曲线.林隙灌木总体更新密度主要随幼苗和幼树Ⅰ数量而变化.林隙内不同位置幼苗的平均树高、平均基径、种密度和个体密度有所差异.从林隙中心到非林隙,更新层乔木幼苗重要值的大小顺序为:林隙中心＞林隙近中心＞林隙边缘＞非林隙;物种均匀度呈高-低-高的变化,物种多样性的变化为早期林隙＞中期林隙＞晚期林隙.     

不同生态条件下松栎混交林3种优势乔木种群的更新规律

为探究秦岭林区松栎混交林群落乔木优势种的更新规律,通过典型样地调查,对油松(Pinus tabulifor-mis)、锐齿槲栎(Quercus aliena var.acuteserrata)和华山松(Pinus armandii)等3种优势乔木种群更新随林分密度、海拔、坡向、坡位等生态因子的变化规律进行了分析。结果表明:(1)林分密度、海拔、坡向、坡位等生态因子对3种乔木的天然更新均具显著影响;3种优势乔木树种幼苗密度均随林分密度增加而显著增加,林分密度从850株.hm-2增加到1 525株.hm-2时,3种幼树密度也逐渐增加,但林分密度为1 900株.hm-2时,3种幼树密度均显著降低;从海拔1 283m增至1 665m,锐齿槲栎和油松幼苗幼树密度均随海拔增加而显著增加,但海拔增至1 835m时,这两种幼苗幼树密度均显著降低。(2)华山松幼苗幼树密度一直随海拔升高而增加;从南偏西20°到75°,锐齿槲栎和华山松幼树密度逐渐降低,至北偏东40°(阴坡)时又显著增加,但3种乔木幼苗及油松幼树密度一直随坡向变化而增加;锐齿槲栎幼苗密度在下坡位显著高于中坡位和上坡位,但油松和华山松幼苗幼树随坡位由下到上逐渐增加。结果提示,处于中海拔、阴坡且密度适中的林分3种优势乔木更新最佳,对这类林分实施封禁将有助于松栎混交林优势乔木的天然更新。     

红松阔叶混交林林隙光量子通量密度的时空分布格局

以小兴安岭原始红松阔叶混交林林隙为对象,采用网格法布点,对生长季林隙内各样点光量子通量密度(photosynthetic photon flux density,PPFD)进行连续观测,利用基本统计学和地统计学方法分析其时空分布格局.结果表明:红松阔叶混交林林隙的PPFD高值区日变化明显,最大值出现在12:00,位于林隙北侧.林隙的PPFD 6月最高,7、8、9月依次递减,其中7月PPFD不同位置间的变异系数最大;各月均为中等变异.不同月份林隙PPFD空间异质性的强度和尺度不同,6月变程最大,7月基台值和结构比最大;各月林隙PPFD斑块复杂程度不同,最大值均位于林隙东北侧.郁闭林分和空旷地的月平均PPFD变化次序与林隙一致.各样点月平均PPFD为空旷地最高,林隙次之,郁闭林分最低.     

长白山红松阔叶混交林森林天然更新条件的研究

研究长白山红松阔叶混交林森林天然更新的变化规律及其条件,红松阔叶混交林是长白山主要的森林类型,保存不多,对于研究以前森林经营有重要意义。研究的目的在于揭示红松天然更新规律及其与森林组成结构的关系。研究结果表明,红松阔叶混交林天然更新与森林群落类型,海拔,森林植物条件及人类活动等干扰极大关系,一般在陡坡或山脊上胡枝子作树红松林中,那里林冠郁闭度达到80％－90％或更大,林分中红松组成也更大,红松天然更新良好。每公顷有红松幼苗和小幼树万株以上,但是在郁闭的红松林冠下,很少能长大。在大部分的阔叶红松混次林中,除了林冠比较郁闭的地方外,针叶树包括红松在内的天然更新通常稀少,每公顷仅有幼苗幼树数千株,其中大部是阔叶树,随着海拔上升到一定范围,在更新中云冷杉的成分增加,老择伐迹地由于林下植被密,常常缺乏更新。74．2％耐荫树种的幼苗幼树是在林冠下观查到,它们70％以上分布于离立木2．5 m以内,这正好等于林木平均冠幅的半径,所以,红松和耐荫树种幼苗的更新最好是在郁闭的林分下。     

红松阔叶混交林不同大小林隙小气候特征

利用HOBO自动气象站对小兴安岭红松阔叶混交林不同大小林隙、郁闭林分和空旷地的小气候因子进行了为期一个生长季的测定.结果表明：红松阔叶混交林林隙与对照样地的光照、地面温度和气温的日变化及其在生长季的变化趋势均呈单峰型曲线.林隙和对照样地空气相对湿度的日变化均为早晨和傍晚较高,正午较低,呈高-低-高的变化趋势;生长季内呈单峰型曲线,其中郁闭林分相对湿度最大,其次为小、中、大3个林隙,空旷地最小.林隙和对照样地降水量及降雨次数随林冠层郁闭程度的增加呈递减趋势,大林隙降雨量约为小林隙的1.4倍.在生长季内,空旷地、大、中和小林隙以及郁闭林分最大风速分别为3.34、2.97、2.87、2.41和1.84 m·s^-1.     

湖南栎类天然次生林幼树更新特征及影响因子

以湖南典型栎类天然次生林为研究对象,基于51块样地的调查数据,采用k-means聚类分析划分林分类型,研究湖南不同栎类天然次生林幼树更新特征,分析了湖南不同栎类天然次生林幼树更新指标(幼树密度、幼树平均地径、平均高以及平均冠幅)与环境因子、林分因子的相关性,旨在阐明环境因子、林分因子对幼树更新的影响,以期为湖南不同栎类天然次生林的恢复与经营管理提供理论依据。结果表明:(1)利用聚类分析可将研究区内栎类天然次生林划分为5个类型,包括甜槠锥栗混交林(CC)、亮叶水青冈多脉青冈混交林(FC)、石栎樟树混交林(LC)、枹栎甜槠混交林(QC)、青冈栎混交林(CG)。(2)不同类型栎类天然次生林更新幼树优势种分化明显,物种丰富度差异显著(P0.05)。5种不同栎类次生林幼树密度均未超过500株/hm~2,更新情况较差;幼树数量差异显著(P0.05),为亮叶水青冈多脉青冈混交林石栎樟树混交林青冈栎混交林枹栎甜槠混交林甜槠锥栗混交林;生长情况差异显著(P0.05),为青冈栎混交林亮叶水青冈多脉青冈混交林枹栎甜槠混交林甜槠锥栗混交林石栎樟树混交林。(3)相关分析结果显示,不同类型次生林幼树更新的主要影响因子存在差异。甜槠锥栗混交林中幼树密度与腐殖质厚度呈显著负相关(P0.05);幼树平均高与灌木盖度呈显著正相关(P0.05);幼树平均地径与草本盖度、灌木盖度呈显著正相关(P0.05)。亮叶水青冈多脉青冈混交林中幼树密度与海拔、腐殖质厚度、枯落物厚度呈显著正相关(P0.05),与草本盖度呈极显著正相关(P0.01);幼树平均地径与郁闭度呈显著负相关(P0.05);幼树平均高、幼树平均冠幅与坡位呈显著正相关(P0.05)。石栎樟树混交林中幼树密度与坡向、土壤厚度呈显著正相关(P0.05),其余因子对幼树生长无显著影响。枹栎甜槠混交林中幼树密度与郁闭度、乔木密度呈极显著正相关(P0.01),与坡位呈显著负相关(P0.05);幼树平均冠幅与坡度呈显著负相关(P0.05)。青冈栎混交林中幼树平均地径与土壤厚度呈显著正相关(P0.05),与乔木密度呈极显著正相关(P0.01);幼树平均冠幅与灌木盖度呈显著正相关(P0.05)。     

秦岭山地油松群落更新特征及影响因子

采用样方法,对秦岭山地油松次生林群落更新特性和相关环境因子进行了研究.结果表明:油松次生林更新层乔木树种共36种,占总种数的51.4%;优势种有短柄枹栎、锐齿栎和榛子等;幼苗库丰富,高度级及龄级较小;更新方式以实生为主,该森林群落处于演替中期、林分密度对林下乔木幼苗、幼树数量影响极显著(P<0.01),当林分密度从580株.hm-2增加到1500株.hm-2时,林下更新的幼苗、幼树密度呈增加趋势,随着林分密度的继续增加,其密度逐渐减少;坡向对幼苗、幼树密度的影响显著(P<0.05),当坡向由南偏西10°到南偏西40°,林下幼树密度逐渐减少,至阴坡(北偏东10°)幼树数量又逐渐增加.不同坡位林分更新特征各异,平坡林分中幼苗、幼树密度均较大;从坡下向上,幼苗密度逐渐减少,而幼树密度逐渐增大;海拔从1159 m增至1449 m,幼树密度逐渐增加,至1658 m时,幼树密度逐渐减小,而幼苗密度则一直呈现增加趋势.处于阴坡的中密度林分更新效果最佳.合理调整林分密度是加快林分更新的有效途径之一.     

Gap formation, microsite variation and the conifer seedling occurrence in a subalpine old-growth forest, central Japan

To evaluate the effects of canopy gaps and forest floor microsites (soil, fallen logs, root-mounds, buttresses and stumps) on regeneration of subalpine forests, the gap regeneration and seedling occurrence of conifers (Abies mariesii, Abies veitchii, Picea jezoensis var. hondoensis and Tsuga diversifolia) were studied in two stands of a subalpine old-growth forest, central Japan. The percentage of gap area to total surveyed area was 11.2–11.3% in the stands. Gap regeneration was not common for P. jezoensis var. hondoensis and T. diversifolia. In contrast, gap regeneration by advanced regeneration was relatively common for Abies. Seedling occurrence of P. jezoensis var. hondoensis and T. diversifolia was restricted on elevated surfaces such as stumps and root-mounds, while Abies seedlings could occur on soil as well as on elevated surfaces. Rotten stumps were the most favorable microsites for conifer seedling occurrence, which covered small area in the forest floor. Although canopy gaps were not always favorable for seedling occurrence, all conifer seedlings were larger under canopy gaps than under closed canopy. Canopy gaps and forest floor microsites clearly affected seedling occurrence and growth of conifers. This suggests that regeneration of conifers is related to the difference of growth advantage under canopy gaps and favorable microsites for seedling occurrence.     

Seedling and Sapling Dynamics of Treefall Pits in Puerto Rico1

Seedling and sapling dynamics in a Puerto Rican rain forest were compared between forest understory and soil pits created by the uprooting of 27 trees during Hurricane Hugo. Soil N and P, organic matter, and soil moisture were lower and bulk densities were higher in the disturbed mineral soils of the pits than in undisturbed forest soils ten months after the hurricane. No differences in N and P levels were found in pit or forest soils under two trees with N–fixing symbionts (Inga laurina and Ormosia krugii) compared to soils under a tree species without N–fixing sym–bionts (Casearia arborea), but other soil variables (Al, Fe, K) did vary by tree species. Forest plots had greater species richness of seedlings (<10 cm tall) and saplings (10–100 cm tall) than plots in the soil pits (and greater sapling densities), but seedling densities were similar between plot types. Species richness and seedling densities did not vary among plots associated with the three tree species, but some saplings were more abundant under trees of the same species. Pit size did not affect species richness or seedling and sapling densities. Recruitment of young Cecropia schreberiana trees (>5 m tall) 45 months after the hurricane was entirely from the soil pits, with no tree recruitment from forest plots. Larger soil pits had more tree recruitment than smaller pits. Defoliation of the forest by the hurricane created a large but temporary increase in light availability. Recruitment of C. schreberiana to the canopy occurred in gaps created by the treefall pits that had lower soil nutrients but provided a longer–term increase in light availability. Treefall pits also significantly altered the recruitment and mortality of many understory species in the Puerto Rican rain forest but did not alter species richness.     

Factors influencing sapling composition in canopy gaps of a temperate deciduous forest

To detect the factors that affect sapling species composition in gaps, we investigated 55 gaps in an old-growth temperate deciduous forest in Ogawa Forest Reserve, central Japan. Gap size, gap age, gap maker species, topographic location, adult tree composition around gaps, and saplings of tree species growing in the gaps were censused. For gaps 5 m², mean gap size was 70 m² and the maximum was 330 m². Estimated ages of gaps had a tendency to be concentrated in particular periods relating to strong wind records in the past. The sapling composition in gaps was highly and significantly correlated to that under closed canopy, indicating the importance of advance regeneration in this forest. However, some species showed significant occurrence biases in gaps or under closed canopy, suggesting differences in shade tolerance. The result of MANOVA showed that gap size and topography were important factors in determining the sapling composition in gaps. Species of gap makers affected the sapling composition indirectly by influencing gap size. The existence of parent trees around gaps had effects on sapling densities of several species. Gap age did not have clear influences on sapling composition. Variations in gap size and topography were considered as important factors that contribute to maintenance of species diversity in this forest.     

Understorey gaps influence regeneration dynamics in subtropical coastal dune forest

Dominant understorey species influence forest dynamics by preventing tree regeneration at the seedling stage. We examined factors driving the spatial distribution of the monocarpic species Isoglossa woodii, a dominant understorey herb in coastal dune forests, and the effect that its cover has on forest regeneration. We used line transects to quantify the area of the forest understorey with I. woodii cover and with gaps in the cover. Paired experimental plots were established in semi-permanent understorey gaps with I. woodii naturally absent and in adjacent areas with I. woodii present to compare plant community composition, soil, and light availability between the two habitats. Isoglossa woodii was widespread, covering 65–95% of the understorey, while gaps covered the remaining 5–35% of the area. The spatial distribution of this species was strongly related to tree canopy structure, with I.␣woodii excluded from sites with dense tree cover. Seedling establishment was inhibited by low light availability (<1% of PAR) beneath I.␣woodii. When present, I. woodii reduced the density and species richness of tree seedlings. The tree seedling community beneath I. woodii represented a subset of the seedling community in gaps. Some species that were found in gaps did not occur beneath I. woodii at all. There were no significant differences between the sapling and canopy tree communities in areas with I. woodii gaps and cover. In the coastal dune forest system, seedling survival under I. woodii is dependent on a species’ shade tolerance, its ability to grow quickly during I. woodii dieback, and/or the capacity to regenerate by re-sprouting and multi-stemming. We propose a general conceptual model of forest regeneration dynamics in which the abundant understorey species, I. woodii, limits local tree seedling establishment and survival but gaps in the understorey maintain tree species diversity on a landscape scale.     

Short-term development of ambrosia and bark beetle assemblages following a windstorm in French broadleaved temperate forests

Abstract: In most temperate deciduous forests, windstorm is the main source of dead wood. However, the effects of this natural disturbance on ambrosia and bark beetle communities are poorly known. In managed oak‐hornbeam forests storm‐damaged in France in 1999, we sampled ambrosia (and second bark beetles) by ethanol‐baited window‐flight traps in 2001. By comparing uncleared gaps, undisturbed closed‐canopy controls and seedling‐sapling stands, we investigated the short‐term effects of gap formation, gap size and surrounding landscape to provide a snapshot of scolytid response. Contrary to expectations, neither the abundance nor the richness of ambrosia beetle species was significantly higher in gaps than in undisturbed stands. Few responses in abundance at the species level and only a slight difference in assemblage composition were detected between gaps and closed‐canopy controls. Gaps were more dissimilar from seedling‐sapling stands, than from closed‐canopy controls. More scolytid individuals and species were caught in gaps than in seedling‐sapling stands. Mean local and cumulative richness peaked in mid‐size gaps. Only mid‐size gaps differed from closed‐canopy controls in terms of species composition. We identified generalist gap species (Xyleborus saxesenii, X. cryptographus), but also species significantly more abundant in mid‐size gaps (Platypus cylindrus, Xyloterus signatus). The faunistic peculiarity of mid‐size gaps seemed to be partly related to a bias in oak density among gap size classes. Few landscape effects were observed. Only the scolytids on the whole and X. dispar were slightly favoured by an increasing density in fellings at the 78 ha scale. We did not find any correlation between scolytid abundance and the surrounding closed‐forest percentage area. We confirmed that temperate, deciduous, managed stands did not come under threat by ambrosia and bark beetle pests after the 1999 windstorm. Nonetheless, our data stressed the current expansion in Western Europe of two invasive species, X. peregrinus and especially X. germanus, now the predominant scolytid in the three oak forests studied.     

天保工程实施以来东北阔叶红松林的可持续经营

20世纪末以来有两个重要的变化影响着东北温带阔叶红松林资源的可持续性,一是天然林资源保护工程(下称天保工程)的实施,另一个是红松不再作为用材树种被采伐。但森林采伐后的更新造林模式并没有显著改变。以长白山腹地-露水河林业局的阔叶红松林为研究对象,利用森林资源清查(二类)和作业设计调查(三类)资料,评估了这两个重要的变化对当前的森林资源的产生影响。结果表明,天保工程的实施调减了采伐量,间接提高了森林的公顷蓄积量,整体上促进了森林资源的恢复。目前,紫椴,蒙古栎,水曲柳和春榆等占商品材总生产量94.7%(蓄积量)。与此同时,可供采伐的这4个树种年生长量却小于采伐量,而林下补植红松,却忽视阔叶树更新的经营模式势必会逐渐增加红松在阔叶红松林中的比例,使目前商品材资源不足的情况更加恶化。总的来说,天保工程促进了公益林区森林资源的可持续性,但降低了用于木材生产的商品林的面积;红松不采伐增加了红松在商品林中的比例,降低了商品林中用材树种比例和商品材的可持续生产能力,从长期看,必然导致可采伐林木资源的枯竭。据此,建议应根据森林培育的目标,选择合适的森林更新恢复模式。     

Monsoon rain forest seedling dynamics, northern Australia: contrasts with regeneration in eucalypt-dominated savannas

Aim To explore: (1) the relative influences of site conditions, especially moisture relations, on pathways and rates of monsoon rain forest seedling and sapling regeneration, especially of canopy dominants, in northern Australia; and (2) contrasts between regeneration syndromes of dominant woody taxa in savannas and monsoon rain forest. Location Four monsoon rain forest sites, representative of regional major habitat and vegetation types, in Kakadu National Park, northern Australia. Methods A decadal study involved: (1) initial assessment over 2.5 years to explore within‐year variability in seed rain, dormant seed banks and seedling (< 50 cm height) dynamics; and (2) thereafter, monitoring of seedling and sapling (50 cm height to 5 cm d.b.h.) dynamics undertaken annually in the late dry season. On the basis of observations from this and other studies, regeneration syndromes of dominant monsoon rain forest taxa are contrasted with comparable information for dominant woody savanna taxa, Eucalyptus and Corymbia especially. Results Key observations from the monsoon rain forest regeneration dynamics study component are that: (1) peak seed rain inputs of rain forest taxa were observed in the wet season at perennially moist sites, whereas inputs at seasonally dry sites extended into, or peaked in, the dry season; (2) dormant soil seed banks of woody rain forest taxa were dominated by pioneer taxa, especially figs; (3) longevity of dormant seed banks of woody monsoon rain forest taxa, including figs, was expended within 3 years; (4) seedling recruitment of monsoon rain forest woody taxa was derived mostly from wet season seed rain with limited inputs from soil seed banks; (5) at all sites rain forest seedling mortality occurred mostly in the dry season; (6) rain forest seedling and sapling densities were consistently greater at moist sites; (7) recruitment from clonal reproduction was negligible, even following unplanned low intensity fires. Main conclusions By comparison with dominant savanna eucalypts, dominant monsoon rain forest taxa recruit substantially greater stocks of seedlings, but exhibit slower aerial growth and development of resprouting capacity in early years, lack lignotubers in mesic species, and lack capacity for clonal reproduction. The reliance on sexual as opposed to vegetative reproduction places monsoon rain forest taxa at significant disadvantage, especially slower growing species on seasonally dry sites, given annual–biennial fires in many north Australian savannas.     

The regeneration niche of the grey mangrove (Avicennia marina): effects of salinity,light and sediment factors on establishment,growth and survival in the field

Field observations of seedlings and saplings of Avicennia marina showed patterns that correlated with salinity, light and sediment. Models that account for these observations were subsequently tested in a series of field experiments. Establishment varied within an estuary under controlled conditions but was not related to salinity or sediment type. Seedling survival was uniform over 3 years regardless of position in estuary and sediment type. Seedling densities and survival under canopies or in canopy gaps were not significantly different. However, seedling growth and density of saplings were greater in canopy gaps. Experiments involving manipulations of canopies showed no differences in seedling survival under canopies or in light gaps, but addition of slow-release fertilizer enhanced growth and survival in canopy gaps and under canopies. Long-term comparison of areas denuded of a canopy and with sediment disturbance showed enhanced establishment and survival when compared with areas with canopy gaps but with undisturbed sediments. Overall there appears to be no restriction to establishment of propagules within mangrove stands other than the supply of propagules and tidal or wave action. In contrast, recruitment to the sapling stage appears to be restricted by light and sediment resources. We suggest that propagules need to establish in a regeneration niche for seedling recruitment to the sapling stage. This differs from the view that seedlings in the under-storey are analogous to a seed pool in the soil.     

Effect of increased Populus cover on Abies regeneration in the Picea–feathermoss boreal forest

Question: Does the increase in Populus tremuloides cover within the Picea mariana–feathermoss domain enhance establishment and growth conditions for Abies balsamea regeneration? Location: Boreal forest of northwest Quebec, Canada. Method: To document the effect of Populus tremuloides on A. balsamea regeneration, mixed stands with a heterogeneous presence of P. tremuloides adjacent to Picea mariana‐dominated stands were selected. Abies balsamea regeneration, understorey environment and canopy composition were characterized from 531 sampling units distributed along transects covering the mixed–coniferous gradient. Abundance of understorey A. balsamea regeneration was described using three height groups: seedling (<30 cm), small sapling (30 to <100 cm) and tall sapling (100 to 300 cm). Growth characteristics were measured from 251 selected individuals of A. balsamea (<3 m). Results: Results showed that A. balsamea regeneration was generally more abundant when P. tremuloides was present in the canopy. Differences between seedling and sapling abundance along the mixed–coniferous gradient suggest that while establishment probably occurs over a wide range of substrates, the better growth conditions found under mixed stands ensure a higher survival rate for A. balsamea seedlings. Conclusions: The abundant A. balsamea regeneration observed within mixed stands of the Picea mariana–feathermoss domain suggests that the increase in P. tremuloides cover, favoured by intensive management practices and climatic change, could contribute to acceleration of the northward expansion of the A. balsamea–Betula papyrifera domain into the northern boreal forest dominated by Picea mariana.