林业科学  2012, Vol. 48 Issue (8): 135-142   PDF    
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文章信息

李国雷, 刘勇, 祝燕, 蒋乐, 史文辉
Li Guolei, Liu Yong, Zhu Yan, Jiang Le, Shi Wenhui
国外容器苗质量调控技术研究进展
A Review on the Abroad Studies of Techniques in Regulating Quality of Container Seedling
林业科学, 2012, 48(8): 135-142.
Scientia Silvae Sinicae, 2012, 48(8): 135-142.

文章历史

收稿日期:2011-06-23
修回日期:2011-08-19

作者相关文章

李国雷
刘勇
祝燕
蒋乐
史文辉

国外容器苗质量调控技术研究进展
李国雷, 刘勇, 祝燕, 蒋乐, 史文辉    
北京林业大学省部共建森林培育与保护教育部重点实验室 北京 100083
摘要: 近年来,由于造林重点由宜林地向困难立地转移,对苗木质量的要求也随之增高。文章系统总结近年来国外容器苗的质量调控技术及其对于我国困难立地植被恢复的重要意义,从指数施肥、秋季施肥、底部灌溉、短日照处理和夏季造林、修根技术、容器规格、冻藏和解冻等研究进展进行了论述总结。指数施肥的每次施肥量与植物生长节律和养分需求同步,施肥效果较优。秋季施肥能避免苗木的养分稀释效应,已被应用于很多树种。实施底部灌溉的植物生长不亚于上部灌溉的植物,而底部灌溉用水量少,氮肥淋溶小。夏季造林时,将正处于速生期的苗木进行短日照处理,诱导顶芽形成,促进根系发育,能提高苗木抗性和造林效果。苗木冻藏可使空气中自由态的水凝固成结晶态的冰,苗木感染病菌几率降低,因此冻藏成为针叶树种贮藏的标准方法。在当前研究热点的基础上,论文就未来研究方向进行展望。
关键词:指数施肥    秋季施肥    底部灌溉    短日照    冻藏    苗木质量    
A Review on the Abroad Studies of Techniques in Regulating Quality of Container Seedling
Li Guolei, Liu Yong , Zhu Yan, Jiang Le, Shi Wenhui    
Key Laboratory for Silviculture and Conservation of Ministry of Education, Beijing Forestry University Beijing 100083
Abstract: In recent years, afforestation shifted from land suitable for afforestation to the harsh sites asks for a higher quality of seedlings to overcome the environment stress. It is essential to review the late decade abroad advances in intensive container seedlings culture for applying the techniques to forest restoration in stressful sites in China. This paper summarized current techniques such as exponential fertilization, fall fertilization, subirrigation, short-day treatments and hot-planting, root pruning, container size, as well as cold storage and thawing. Exponential fertilization has been approved to be more effective because fertilizer is raised progressively to match exponential growth and nutrient demand. Fall fertilization, applied in autumn, can avoid growth dilution of nutrients, which has been successfully applied in many tree species. Plants with subirrigation grow at least as well as their cohorts with overhead irrigation, besides, subirrigation needs less water inputs, and less nitrogen leaching. Field performance of a seedling can be improved by stopping shoot growth in summer with the seedling exposed to short-day treatments, by which the root system growth can be enhanced. Freezer storage of conifer seedlings has become a standard operating procedure for many commercial nurseries, because freezing converts all the free water to ice in the storage container, and thus the development of pathogenic fungi is retarded. This paper prospected future study trends on techniques to promote seedling quality.
Key words: exponential fertilization    fall fertilization    subirrigation    short-day    freeze storage    seedling quality    

苗木质量是在特定立地条件下能满足苗木成活和生长,并以最低成本来实现这种要求,是对造林目的的适合程度。随着造林工作的不断深入,植被恢复取得了巨大成绩,造林重点已向干旱、高寒、瘠薄等困难立地转移。困难立地特殊的立地条件要求高质量、强抗性的苗木,在苗圃培育苗木过程中应采取强化的调控措施以满足这一要求。文章围绕稳态营养加载、底部灌溉、短日照处理、根系调控、容器类型、苗木类型、贮藏方式等,就国外近年来苗木质量调控技术进行综述,以期为我国苗木培育提供借鉴。

1 稳态营养加载技术 1.1 指数施肥营养加载技术

在苗圃育苗过程中,根据苗木生长的养分需求规律,每次施肥量呈指数增加,施肥量同步于苗木养分需求量,把肥料尽可能多地固定在苗木体内以形成养分库,造林后苗木将利用这一养分库促进根系生长和顶芽发育,从而提高造林效果(Timmer,1996Birge et al., 2006Oliet et al., 2009)。欧美地区已将此技术应用于栎属(Quercus)、云杉属(Picea)、松属(Pinus)主要造林树种容器苗的培育(Quoreshi et al., 2000Salifu et al., 2003a2006Dumroese et al., 2005Close et al., 2005Way et al., 2007Oliet et al., 2009)。为确定苗木最佳施肥量,需要研究者根据经验或研究资料设置多个施肥量,根据每个施肥量下的生物量、养分浓度、单株养分含量,模拟出生物量、养分浓度和单株养分含量对施肥量的响应曲线,通过寻找拐点的方法来确定出最佳施肥量。随施肥量增大,生物量增大速度增快,继续增大施肥量,生物量维持不变,而苗木体内氮浓度和单株含量一直增大;当施肥量持续增大到一定量时,基质溶液中氮浓度过高,胁迫效应出现,苗木生物量开始下降。苗木养分含量与生物量同时达到最大化时的施肥量称为最佳养分加载量。可见,制定生物量、养分浓度、单株养分含量对施肥量响应曲线时,施肥量不仅需设置足够多,而且涵盖亏缺、充分、过量等养分状态,因此苗木最佳施肥量的确定成为指数施肥研究的重点。目前北美红栎(Quercus rubra)、刺叶栎(Quercus ilex)、黑云杉(Picea mariana)等容器苗的最佳养分加载量均已探明,而松属还未找到(Salifu et al., 2003a2006Oliet et al., 2009)。

1.2 秋季施肥营养加载技术

在秋季,苗木进入木质化期,顶芽逐渐形成,高生长减慢,而苗木生物量特别是根系生物量继续增长,如果此时停止营养加载,苗木可获得的养分减少,体内养分浓度便会下降,不利于翌年造林效果。为避免苗木木质化期生物量增加引起的养分稀释效应,对木质化期的苗木进行适量养分加载,即称之为秋季营养加载。秋季营养加载由于操作简便、效果明显,被广泛应用于欧洲云杉(Picea abies)(Rikala et al., 2004)、黑云杉(Boivin et al., 20022004)、火炬松(Pinus taeda)(Switzer et al., 1963South et al., 2002VanderSchaaf et al., 2004)、湿地松(Pinus elliottii)(Duryea,1990Irwin et al., 1998)、脂松(Pinus resinosa)(Islam et al., 2009)、花旗松(Pseudotsuga menziesii)(van den Driessche,1985Margolis et al., 1986Birchler et al., 2001)、蓝桉(Eucalyptus globulus)(Fernández et al., 2007)等常绿树种。

在木质化期,苗木处于从生长到休眠的过渡阶段,秋季营养加载对苗木形态指标的调控呈现多样性,秋季营养加载有利于顶芽叶原基分化(Islam et al., 2009),对地径无显著影响(Rikala et al., 2004VanderSchaaf et al., 2004),对苗高的影响因树种而异(Irwin et al., 1998Rikala et al., 2004Islam et al., 2009),对生物量的影响受生长期施肥方式的制约(Boivin et al., 20022004)。秋季营养加载能否提高苗木抗寒性存在很大争议(Fernández et al., 2007Islam et al., 2009),比较一致的观点认为,秋季施肥能显著提高苗木氮的含量,从而提高造林效果,如秋季使用低氮、高氮营养加载的湿地松苗木造林后成活率分别提高12%,15%,苗高分别提高7%,15%(Irwin et al., 1998);火炬松苗木在秋季营养加载,造林6个月后生物量、苗高分别提高12%,24%(VanderSchaaf et al., 2004)。

为避免灼伤苗木,施用氮肥后应立即喷水。在秋季,苗木正处于木质化期,需要控制灌水量,这就限制了秋季施肥的次数;同时,苗木木质化期较生长期短,秋季施肥多采取3~5次等量施肥的方式进行(Rikala et al., 2004VanderSchaaf et al., 2004Islam et al., 2009)。尽管秋季施肥对苗木质量影响显著,但秋季施肥的肥料利用率、秋季施肥淋溶情况尚未见报道,秋季施肥对环境影响还有待研究。

1.3 养分加载与造林效果的关系

植苗造林后,苗木初期新根生长缓慢,从土壤中获取养分的能力较低,其成活和生长主要依赖于体内贮存养分的内转移和再分配(Timmer et al., 1987Trubat et al., 2010Millard et al., 2010)。如黑核桃(Juglans nigra)苗木造林后,顶梢生长所需氮的68%~83%来源于养分内转移,所施肥料43%被土壤固定,被苗木利用的仅9%(Salifu et al., 2009);通过养分内转移可满足黑云杉苗木造林初期生长所需氮的72%~80%(Boivin et al., 2004)。从光合产物方面也能解释矿质营养如何促进造林效果。叶片含氮量高的苗木造林后,光合作用较强,有利于碳水化合物的合成,进而诱导生根、促进茎生长(van den Driessche,1987)。因此,在苗水培育过程中对苗木进行充足施肥,使苗木体内贮藏大量养分,对于提高其造林效果具有重要作用。此外,苗木造林后进行施肥可促进苗木发育,同时也促进竞争物种的生长。而在苗圃培育苗木时,把肥料尽可能多地固定在苗木体内,造林后苗木就会利用这些养分库促进苗木快速发育,这样就避免造林施用的肥料被其他竞争物种吸收(Birge et al., 2006),指数施肥、秋季施肥等技术具有特殊意义。

在瘠薄(Folk et al., 2000)、杂草(Malik et al., 1998Imo et al., 2001)等困难造林地上,苗木能向生长点转移更多的养分,因此初始养分高的苗木造林效果较好,指数施肥和秋季施肥的作用得以充分体现。如秋季施氮肥200 kg·hm2的火炬松苗木,造林6个月后生物量和苗高分别较对照提高12%和24%(van den Schaaf et al., 2004)。在瘠薄土壤上,养分加载的北美红栎苗木120天生长量较普通苗木提高118%(Salifu et al., 2003a);在矿山废弃地上,普通苗木存活率仅为66%(Jacobs et al., 2004),而实施养分加载的北美红栎、白栎(Quercus alba)苗木成活率高达84%~93%(Salifu et al., 2008)。指数施肥相关研究已有30多年的历史,近年来,由于指数施肥营养加载的苗木在困难立地植被恢复效果较好,被视为困难立地造林苗木定向培育的关键技术。

矿质营养与造林效果间的关系受造林地制约。在好的立地,苗木本身提供的养分被土壤供给的养分所掩盖,稳态营养加载的效果则不明显(Gleason et al., 1990Rikala et al., 2004)。第1年对黑云杉容器苗进行指数施肥养分加载,第2年春季在温室进行沙培移栽试验,并将其中的一半苗木施用氮同位素,120天后发现,盆栽苗木在未施肥情况下(瘠薄土壤)氮内转移率为218%,而在施肥情况下(肥沃土壤)氮内转移率仅为23%(Salifu et al., 2003b)。

2 底部灌溉技术

渗灌系统最初主要应用于大田裸根苗农林作物的培育,是利用地下管道将灌溉水输入田间埋于地下一定深度的渗水管道内,借助土壤毛细管作用湿润土壤的灌水方法。底部灌溉(subirrigation)将这一理念应用于容器苗上,在压力泵的作用下,把储水箱的灌溉水经注水管注入施水槽,育苗基质通过毛细管作用从施水槽底部吸水供植物利用,未被苗木直接利用的水通过回流管返回储水箱。由于底部灌溉采用的是密闭系统,未被植物利用的水分和养分可循环再利用,较上部灌溉节省64%~86%的水,减少55%的养分淋溶量,减少苗木生产成本和环境污染(Ahmed et al., 2000Goodwin et al., 2003Coggeshall et al., 2003Dumroese et al., 2006);水从由容器底部向上运动,容器表层含水量小,苔藓类植物缺少赖以生长的环境而数量大大降低;叶片始终保持干燥,松属和云杉属苗木感染灰霉菌(Botrytis cinerea)叶部病害的概率下降(Oh et al., 1998);每个容器接受的灌溉面积、时间均等,灌溉量相对一致,苗木生长均匀整齐;底部灌溉提高基质阳离子交换能力,降低基质上部pH值,增强苗木吸收养分能力,从而提高苗木质量(Davis et al., 2008Bumgarner et al., 2008)。

2002年美国密苏里州大学将底部灌溉技术应用于难生根的阔叶树种扦插获得成功,2006年美国林务局林业研究所利用灌溉系统培育容器苗。作为新兴技术,底部灌溉目前仅在红栎、云杉等树种上有所研究;从技术应用的程度来看,底部灌溉还处于研究开发的初期阶段,尚存在很多问题急需解决。由于水由容器底部向上运动,长时间灌溉可能会在容器上部积累盐分,引起基质pH值上升,从而影响苗木生长,还需要从上部用水淋洗以减小盐分的累积(Landis et al., 2004)。尽管在一些树种上已证明底部灌溉降低了基质上部pH值(Davis et al., 2008Bumgarner et al., 2008),尚需在更多树种上进行验证。在生长初期需要对苗木加以上部灌溉;在炎热的夏季,也需进行上部灌溉以防止高温灼伤苗木;底部灌溉的苗木根系缺失空气修根,利用碳酸铜涂抹容器表面的化学修根法尚不知效果是否理想,碳酸铜淋溶后是否会污染水资源还不清楚(Davis et al., 2008)。采用底部灌溉系统时,预先把缓释肥放入基质,而在苗木生长过程中,如何将肥料溶入水中,通过底部灌溉系统实现随水施肥需深入研究;灌溉水多次循环利用易产生晚疫病(Phytophthora)和腐霉菌(Pythium)等水霉菌,需引进UV辐射设备,对循环水进行消毒(Dumroese et al., 2007)。从系统自动化程度看,目前主要依靠人的主观判断,决定苗木是否需要灌溉,今后在底部灌溉系统中能否采用水分感应器或传感器实现全自动或智能灌溉也值得期待。可见,底部灌溉尽管节能、减排、高效,加之系统生产和安装成本低(Coggeshall et al., 2003),在生产上规模化、工厂化应用还有很多困难需要克服。

3 短日照处理与夏季造林

在高海拔地区,春季温度较低,造林一般集中在夏季(Revel et al., 1990);春季降水少的地区,土壤有限的水分供给是造成困难立地造林失败的因素之一,造林时间应更多地向多雨的夏季转移。但此时,苗木正处于速生期,幼嫩的顶梢极易在起苗、运输、造林的过程中折断,尤其影响针叶树种苗木的造林效果。短日照处理能提高苗木顶梢木质化程度,加大碳水化合物向根系分配,促进根系发育,苗木造林后,新根萌发能力强,苗木抗旱能力高(Luoranen et al., 2007),夏季造林效果因此较好(Luoranen et al., 2006)。短日照处理如果开始太早,造林后苗木还有较长的生长期,木质化的顶梢会有二次生长(Kohmannn et al., 2007Luoranen et al., 2009),如欧洲云杉短日照处理需在7月后进行(Konttinen et al., 2003)。因此开展短日照处理的起始期、持续时间研究,对于建立短日照制度具有重要意义。短日照处理后苗木形成顶芽,夏季造林后顶芽是否会被正常的日照打破、顶芽萌发后是否在秋季及时木质化、是否受早霜影响等受到的关注较多;另一方面,短日照处理后,高生长停滞,光合产物更多地向根系分配,根系得以充分发育,造林后苗木抗旱能力提高,短日照处理后苗木抗性方面的研究是另一重点,短日照处理技术是否在困难立地造林有特殊应用将是研究方向之一。因此目前短日照处理与夏季造林的研究多集中在北欧和北美树种(Nilsson et al., 2010),急需对我国乡土树种开展短日照处理与夏季造林技术研究,特别是生长期更长的暖温带地区树种。

4 根系调控技术

受容器空间的限制,根系严重畸形一直是容器苗培育过程中的难题,尤其是根系发达的栎属、欧洲赤松(Pinus sylvestris)和美国黑松(Pinus contorta)(Tsakaldimi et al., 2005Campbell et al., 2006Nilsson et al., 2010)等。因此,空气、物理、化学等控根技术受到普遍关注。美国黑松起苗时,空气修根培育的苗木、容器内壁涂抹碳酸铜化学修根培育的苗木的苗高、地径无显著差异。造林2年后,化学修根苗木的苗高、地径、茎生物量、整株生物量均高于空气修根的苗木。化学修根有利于提高造林效果,且化学修根的苗木真菌侵染率在起苗较空气修根的苗木低,化学修根抑制菌根形成(Campbell et al., 2006)。此外,苗木培育时使用大量碳酸铜是否对环境产生负面影响尚不清楚(Davis et al., 2008),在今后研究中,不仅要重视化学修根对根系矫正效果,还需对环境影响作出评价。

5 容器类型

容器规格和质地影响苗木质量,从而影响其造林效果(Aphalo et al., 2003Close et al., 2006, 2010Tsakaldimi et al., 2005)。用大规格容器培育的蓝桉,造林1年后,苗高和地径生长量较快,但造林4年后,不同规格容器培育的苗木高度没有差异(Close et al., 2010)。大容器培育出的苗木能提高造林效果的原因可能在于发达的根系能使苗木造林后尽快从土壤中吸收养分和水分(Nelson,1996),苗木体内贮存的大量养分造林后可更多地向生长点转移,造林时受到的伤害小(Close et al., 2010)。较大规格容器的高度较大,使得造林穴深,苗木根系能吸收深层的土壤水(Close et al., 2006)。容器规格大,需要填充的基质多,单位育苗量和运苗量小,造林穴深,育苗和造林成本较大;而大容器培育出的苗木抗性强,造林后抚育投入较少,因此需结合造林地情况,选择合适的容器规格育苗。针叶树种如火炬松,苗期能耐一定的遮荫,可以在小的容器里培育,以提高育苗密度,降低育苗成本;相反,阔叶树种叶片因能截留更多的水分和养分,相互遮荫大,育苗密度应小些,需要较大规格的育苗容器(Landis et al., 1990)。

用纸杯培育的刺叶栎、铁橡栎(Quercus coccifera)苗高、地径、茎生物量、根系体积均较塑料营养钵培育的苗木大,造林成活率高达73.3%和73.6%,用塑料营养钵培育的苗木造林成活率分别为42.9%~50.9%,47.7%~47.9%(Tsakaldimi et al., 2005)。黑色的塑料杯较纸杯吸收更多的太阳辐射,基质温度较高,不利于根系生长(Whitcomb,1989Landis et al., 1990);而纸杯渗透性强,利于水分和养分在容器间横向移动,纸质营养钵培育的苗木因此较好。

6 苗木类型

尽管容器苗(plug)培育周期短,造林季节不受限制,在干旱、瘠薄等困难立地优势明显,但裸根苗因其培育周期长、苗木规格大,抗象鼻虫(Hylobius abietis)危害能力强(Örlander et al., 1999),尤其是在肥沃、杂草衍生、象鼻虫危害大的立地,裸根苗造林更受青睐(Nilsson et al., 2010)。此外,裸根苗造林后,缓苗期较容器苗长,萌芽较晚,受春季晚霜危害因此较小,这也是晚霜高发地区选择裸根苗造林的主要原因(Langvall et al., 2000)。

为减少育苗和造林成本,芬兰、瑞典、加拿大采用将培育7~10周的微型容器苗(miniplugs,MP)进行造林,尤其是欧洲赤松、欧洲云杉等苗木研究相对较多(Lindström et al., 2005Murphy et al., 2006Johansson et al., 2007)。微型苗木挥发物柠檬烯能阻止象鼻虫,因此较1年生容器苗P+1受到的象鼻虫危害小(Danielsson et al., 2008)。尽管微型苗木生长速度较快,但对造林地杂草敏感,需要加强控草(Johansson et al., 2007),在肥沃、湿润等易生杂草立地的抚育成本较大,利用微型苗木MP造林受到限制。

白杨(Populus tremuloides)苗木冻藏时,1年生容器苗P1+0根系碳水化合物下降速度较裸根苗1+0、1年生容器苗再大田移栽培育1年的P1+1苗木快,造林2年后,1+0苗木没有发生回枯现象(shoot dieback),苗高和地径生长速度是另外2种苗型的2倍(Martens et al., 2007)。

美国和加拿大在1月或2月播种,利用温室进行冬季育苗,5月或6月初对苗木进行短日照处理,7月进行造林(hot-planting)(Grossnick et al., 2003),或不进行短日照处理,8月初直接进行造林(Page-Dumroese et al., 2008)。常规育苗是春季播种,秋季起苗,经冬季贮藏,第2年再进行造林。与常规育苗相比,这些冬季育苗、夏季造林未经过贮藏的苗木(non-stored seedling或hot-planted seedling)育苗周期大大缩短。尽管造林1年后,这些苗木与常规苗木的成活率并无显著差异,但造林5年后,前者造林成活率高达92%,后者为84%(Page-Dumroese et al., 2008)。冬季育苗需要加温,消耗能量,成本大,这一技术并未在北欧应用(Nilsson et al., 2010)。

7 贮藏方式

如果冷藏(1~2 ℃)时间超过2个月,苗木体内碳水化合物会迅速下降,需要将苗木进行冻藏(-2~-4 ℃)以保持其活力(Rithie,2004);更为重要的是,冻藏温度能将空气中的自由水变为结晶态,苗木感染真菌几率下降(Trotter et al., 1992),因此冻藏成为常绿树种特别是内陆、高海拔地区苗木贮藏的标准方法,而落叶树种休眠特性较强,抗冻和耐失水能力也高于常绿树种,海岸、热带、亚热带地区的树种生长期长,无真正意义的休眠,适宜的气候可使造林时间有充足的选择,因此也无需选择冻藏(Kooistra,2004Landis et al., 2010)。容器苗可直接放装入纸箱、放进冷藏室进行冻藏,箱内相对湿度为100%;裸根苗的根系则需包裹湿润的泥炭以避免根系直接暴露在空气中而失水(Martens et al., 2007);苗木呼吸产生热量,箱内的温度高于贮藏环境的温度,由于贮藏温度一般指的是箱内的温度,因此环境设置的温度要低于箱内目标温度1~2 ℃,并经常对环境和箱内温度进行检测,美国已建立主要造林树种苗木包装、贮藏方式(Landis et al., 2010)。

冻藏过程中,根系生长潜力先升高后下降,白杨苗木冻藏75天时,根系生长潜力最大,冻藏时间过长,根系生长潜力下降,造林后,茎梢枯(shoot dieback)、根系生长受抑(Martens et al., 2007)。不同贮藏方式的苗木造林效果受整地方式和调查时间影响。造林1年后,无论造林地是否去除表层有机质,冷藏和冻藏的黑松苗高、地径、造林成活率差异均不显著;造林5年后,在地表层去除有机质的造林地上,冻藏苗木造林成活率显著高于冷藏苗木,而在保留地表有机质的造林地上,2种贮藏方式的苗木造林成活率则无明显差异(Page-Dumroese et al., 2008)。

如果土壤温度高于18 ℃,冻藏的苗木不需解冻可直接造林;在北欧或高海拔地区如果将造林季节选择在春季或秋季,土壤温度过低,冻藏的苗木需要先解冻才能造林(Camm et al., 1995Kooistra et al., 2002Helenius,2005)。解冻的一般方法是将苗木放入纸箱(cardboard box),在黑暗、3 ℃的环境下保持4天以使苗木缓慢解冻(Martens et al., 2007Nilsson et al., 2010)。也有研究将苗木放入水中加快解冻速度,苗木质量反而提高。与慢速解冻的苗木相比,欧洲云杉苗木放在水里快速解冻,苗木抗寒性强、碳水化合物含量高、萌芽时间晚(Flöistad et al., 2001)。

8 展望

苗木质量与造林效果取决于苗木体内最终的养分浓度,秋季营养加载只是苗木整个生长季营养加载的一部分,需建立春夏季的常规营养加载与秋季的营养加载的组合试验,在春季和夏季设置多个常规营养加载量,使苗木形成亏缺、充足、过量养分浓度,然后在秋季施用15N,设置多个秋季营养加载量,研究常规营养加载如何影响秋季营养加载的效果,探讨常规营养加载、秋季营养加载在苗木质量调控中的作用。

在资源危机和环境保护的双重压力下,在苗木培育过程中如何提高水肥利用效率尤为重要。底部灌溉采用的是密闭系统,水分可以循环再利用,极大减少了水资源浪费和肥料淋溶污染,苗木规格分化小,苗木质量优于上部灌溉方式的苗木,因此底部灌溉有强大的应用前景。盐分随灌溉水向上运移而在基质表层积累,可能使基质表层pH值、EC增大,影响苗木质量,需对基质性质动态监测,从泥炭类型选择与配比(Caron et al., 2005)、容器类型(Davis et al., 2008)等方面进行调控。由于这一新兴技术存在随水施肥、灌溉智能化等问题需要解决,今后也需加大研究,以使底部灌溉技术早日在生产中规模化应用。

1905—2004年,我国升温幅度约为0.5~0.8 ℃,近百年来降水呈减少的趋势,东北、华北地区尤为突出,干旱区范围在扩大(秦大河,2007),半干旱地区与半湿润地区的交界线向东南方向扩展了约300 km(马柱国,2011)。在气候变化的影响下,苗木光合和代谢将会发生变化,生长速度产生变异;春季萌芽时间提前,受晚霜影响是否加剧值得关注;造林地水分和温度也会变化,杂草滋生、病虫危害等干扰可能加剧。因此,从苗木角度如何选择树种、调控苗木质量应对气候变化需引起足够重视。

近年来,困难立地造林是国内外共同关注的问题,如何提高苗木本身的质量和抗性,使苗木造林后最大限度地适应干旱、瘠薄、高寒、矿山废弃地等立地条件成为研究热点,国际林业研究组织联盟(IUFRO)2011年9月在西班牙就困难立地造林苗木的质量调控技术进行专题研讨。第七次全国森林资源清查结果显示,我国营造森林难度越来越大,现有造林地质量好的仅占13%,质量差的高达52%,造林重点已向质量差的立地转移。我国困难立地面积广、类型多、难度大,需从苗木质量和造林技术2个方面进行攻关。以往的研究多从造林技术(客体)探讨提高困难立地造林的关键技术,现急需以乡土树种为对象,从苗木自身(主体)角度,研究在苗木培育过程中采用何种关键技术以提高苗木自身的质量和抗性,使苗木造林后能够适应困难立地,从而提高造林效果,保证“双增”目标的实现。

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