Fine root biomass under light gap openings in an Amazon rain forest

Summary Belowground processes in light gap openings are poorly understood, particularly in tropical forests. Fine roots in three zones of light gap openings and adjacent intact forest were regularly measured in buried bags and surface litter envelopes for 2 years. Fine root biomass does not vary significantly within gaps for either buried bags or for surface litter envelopes. When entire gaps are compared without regard for within gap zones, root growth into both surface litter and buried bags is significantly different between gaps, with highest rates of fine root biomass accumulation in the smallest gap. These results suggest that the aboveground within-gap zones do not result in a congruent pattern of below-ground zonation. Gap size, decomposition of the fallen tree, and pre-gap fine root growth rates should be considered to determine fine root growth patterns following the formation of light gap openings.     

Artificial canopy gaps accelerate restoration within an exotic Pinus radiata plantation

We created small‐scale artificial canopy gaps to accelerate the growth of mature indigenous forest canopy species for restoration of an 18‐year‐old exotic Pinus radiata plantation forest, in the Marlborough Sounds, New Zealand. Small and large circular gaps were formed by felling. Seedlings of two indigenous forest canopy species, Podocarpus totara (Podocarpaceae) and Beilschmiedia tawa (Lauraceae), were planted within artificial gaps and undisturbed plantation canopy. Seedling height growth, mortality, and occurrence of animal browse were monitored at approximately 6‐month intervals over 17 months. Both P. totara and B. tawa differed significantly in height growth and in animal browse occurrence among artificial gap treatments. Growth of the light‐demanding P. totara was better under large canopy gaps, whereas growth of the shade‐tolerant B. tawa increased under gaps of any size but was most consistent under small gaps. For P. totara, any significant restoration benefit of gap formation on height growth was lost when browsed seedlings were taken into account. Animal browse significantly limited B. tawa height growth in large but not in small gaps. Small‐scale canopy gap creation is an effective method of modifying light transmission to the plantation understorey and accelerating seedling growth rates. Canopy gap size can be used to optimize understorey illumination according to species‐specific light requirements. The increased occurrence of animal browse in gaps requires consideration. Artificial canopy gaps within planted monocultures create structural heterogeneity that would otherwise take an extended period of time to develop. These results further support the role of plantations as indigenous forest restoration sites.     

Tree species distribution in canopy gaps and mature forest in an area of cloud forest of the Ibitipoca Range,south-eastern Brazil

The tree community of both canopy gaps and mature forest was surveyed in a 5 ha plot of cloud forest in the Ibitipoca Range, south-eastern Brazil, aiming at: (a) comparing the tree community structure of canopy gaps with that of three strata of the mature forest, and (b) relating the tree community structure of canopy gaps with environmental and biotic variables. All saplings of canopy trees with 1–5 m of height established in 31 canopy gaps found within the plot were identified and measured. Mature forest trees with dbh 3 cm were sampled in four 40×40 quadrats laid on the four soil sites recognised in the local soil catena. All surveyed trees were identified, measured and distributed into three forest strata: understorey (<5 m of height), sub-canopy (5.1–15 m) and canopy (15.1–30 m). The following variables were obtained for each gap: mode of formation, age, soil site, slope grade, size, canopy openness and abundance of bamboos and lianas. A detrended correspondence analysis indicated that the tree community structure of gaps in all soil sites was more similar to that of the mature forest understorey, suggesting that the bank of immatures plays an important role in rebuilding the forest canopy and that gap phases may be important for understorey shade-tolerant species. There was evidence of gap-dependence for establishment for only one canopy tree species. Both canonical correspondence analysis and correlation analysis demonstrated for a number of tree species that the distribution of their saplings in canopy gaps was significantly correlated with two variables: soil site and canopy openness. The future forest structure at each gap is probably highly influenced by both the present structure of the adjacent mature forest and the gap creation event.     

Response of fine roots to an experimental gap in a boreal Picea abies forest

We examined the initial response of the quantity and distribution of fine roots to the creation of an experimental canopy gap with a diameter of 50 m in a mature managed Norway spruce forest. Under the canopy, the fine root length densities of trees, shrubs, and grasses and herbs were 3207, 707 and 2738 m m^–2, respectively. The fine root biomass of trees, shrubs, and grasses and herbs were 182, 47 and 52 g m^–2, respectively. Two growing seasons after gap creation hardly any fine tree roots were found in the middle part of the gap. The living tree roots in the gap edge zone were mainly located within a 5-m distance from the standing edge trees. The indices developed here to show the influence of trees on fine root lenght density clearly revealed the effect of the vicinity of living trees on fine root lenght density. The root densities of grasses, herbs and dwarf shrubs did not show a clear response to gap creation despite the increase of their foliage. Our results suggest that in boreal spruce forests a gap disturbance creates a distinct tree root gap and that the gap edge trees do not extend their root systems rapidly into the formed root gap.     

Growth, Allocation and Water Relations of Shade-grownQuercus rubraL. Saplings Exposed to a Late-season Canopy Gap

For understory saplings to exploit canopy gaps successfully,carbon gain must increase in the gap environment. We predictedthat total biomass of shade-grown red oak saplings would increaseafter exposure to a late-season canopy gap, and that increasedwater and nutrient demand within the canopy gap would drivechanges in the allocation of this carbon. Shade-grown red oaksaplings acclimated to gaps by increasing biomass during theseason of gap formation and increasing the potential for carbongain in the following summer. Within-season carbon gain didnot result from greater production of leaf area, so it mostlikely arose from higher photosynthetic rates of existing shade-developedfoliage, which may be linked to accumulation of leaf nitrogen.During the season of gap formation, shade-gap plants increasedallocation to storage of total non-structural carbohydrates(TNC), and to root growth. The increase in TNC storage suggeststhat shade-developed saplings exposed to gaps were also primedfor fast growth and carbon gain in the following summer. Theincrease in root growth suggests that higher nutrient and waterdemand drove allocation shifts to enhance the capacity for nutrientand water uptake in the gap. Plant hydraulic conductivity (K_a)of shade-grown plants was limited upon exposure to the gap,possibly because of embolism formation resulting from the abruptincrease in water demand. Greater water potential gradientscompensated for limitations to K_a, allowing saplings to maintainhigh transpiration rates, suggesting that actual water uptakeof shade-gap plants was unaffected by gap exposure. Acclimation; canopy gaps; carbon allocation; hydraulic conductivity; nitrogen allocation; non-structural carbohydrates;Quercus rubra L.; red oak; stomatal conductance; transpiration; water-relations     

长苞铁杉幼苗在林窗不同位置的建立

通过2003年12月到2005年1月对福建省天宝岩国家级自然保护区长苞铁杉林内椭圆形林窗(面积118m2)中心、中部、边缘和林下样地进行种子埋藏及幼苗定位观测实验,研究了长苞铁杉幼苗在林窗不同位置中的建立。结果表明:在林窗内不同位置对长苞铁杉幼苗建立有显著影响。林窗中心样地、林窗中部样地、林窗边缘和林下样地内长苞铁杉幼苗发生率分别为10%,10.7%,6%和6%。从林冠下到林窗中心,长苞铁杉种子的幼苗发生率略有增高趋势。在林窗中心样地和林窗中部样地中雨水冲刷是幼苗死亡的最主要原因,而在林窗边缘样地和林下样地中昆虫的取食是幼苗死亡的最主要原因。林窗位置对幼苗的存活率有显著影响,林窗中部样地幼苗存活率最高(11.4%),林窗中心样地幼苗存活率次之(6.7%),而林窗边缘样地和林下样地幼苗则均全部死亡。种子营养消耗完后,在林窗中心、林窗中部、林窗边缘和林下等4个位置样地中,林窗中心样地幼苗平均高度最高。经过一个生长季后,林窗中心样地中幼苗的根生物量、茎生物量和总生物量均略高于林窗中部样地的幼苗,但差异并不显著。林窗中心样地幼苗叶生物量、叶重比、叶/地上等指标显著高于林窗中部样地的幼苗,而茎重比则低于林窗中部样地的幼苗。     

The role of canopy gaps in the regeneration of coastal dune forest

In regenerating coastal dune forest, the canopy consists almost exclusively of a single species, Acacia karroo. When these trees die, they create large canopy gaps. If this promotes the persistence of pioneer species to the detriment of other forest species, then the end goal of a restored coastal dune forest may be unobtainable. We wished to ascertain whether tree species composition and richness differed significantly between canopy gaps and intact canopy, and across a gradient of gap sizes. In three known‐age regenerating coastal dune forest sites, we measured 146 gaps, the species responsible for gap creation, the species most likely to reach the canopy and the composition of adults, seedlings and saplings. We paired each gap with an adjacent plot of the same area that was entirely under intact canopy and sampled in the same way. Most species (15 of 23) had higher abundance in canopy gaps. The probability of self‐replacement was low for A. karroo even in the largest gaps. Despite this predominance of shade‐intolerant species, regenerating dune forest appears to be in the first phase of succession with ‘forest pioneers’ replacing the dominant canopy species. The nature of these species should lead to successful regeneration of dune forest.     

热带季节雨林林窗边缘不同热力作用面热力效应的季节变化特征

通过对热带季节雨林雾凉季和湿热季昼间林窗区域不同热力作用面的热力效应初步分析,指出在西双版纳,不论是雾凉季还是湿热季,热带季节雨林林窗边缘壁面均具有不可忽视的热力作用,且由于受林缘树木的影响,热力效应较强的东侧,北侧林缘壁面最大区域出现位置高于次生林林窗,而强度小于次生林林窗,显示了林窗边缘壁面的热力效应除与太阳高度角,太阳辐射的时间长短和强度有关之外,林窗边缘树木高度也是不可忽视的因子,其结果可为进一步研究林窗小气候形成机制提供研究基础,为研究林窗更新及生物多样性问题提供科学参考。     

亚高山暗针叶林不同林冠环境下华西箭竹的克隆可塑性

以亚高山暗针叶林3种林冠环境中以及暗针叶林林缘的华西箭竹（Fargesia nitida）为对象,对其无性系数量特征、无性系根茎特征、分株生物量以及分株形态特征进行了对比研究。结果表明：（1）林冠环境的差异导致了不同种群的基株密度和每基株分株数的显著差异,但林冠环境差异不影响分株密度。林冠郁闭度愈大,每基株分株数愈少,分株分布愈均匀。（2）不同林冠环境间。分株生物量、分株构件生物量和分株构件的生物量分配百分率均有显著差异。开敞的林冠环境有利于华西箭竹的生长和生物量积累。（3）随着林冠郁闭度的增加,华西箭竹通过增大分枝角度、叶生物量分配百分率、比叶面积和叶面积率以提高光能利用效率,有效适应弱光环境。（4）隔离者长度、隔离者直径和分枝强度在林缘和林窗环境中要显著大于林内环境;同级隔离者分枝角度随林冠郁闭度的增加而最大,其值在林下显著大于林窗和林缘,而异级隔离者分枝角度的变化则正好相反。研究表明,华西箭竹种群在不同的林冠环境中发生了明显的可塑性变化,这些可塑性变化是种群对林冠郁闭度差异的适应性反应的结果,有利于增强种群对环境中有限光资源的利用。     

Effects of forest gap on hemicellulose dynamics during foliar litter decomposition in an subalpine forest

 亚高山森林林窗可能通过改变冬季雪被格局和生长季水热环境影响林窗内凋落物中半纤维素的分解动态, 但目前对此还缺乏研究。采用凋落物分解袋法, 以亚高山森林5种典型物种岷江冷杉(Abies faxoniana)、红桦(Betula albosinensis)、四川红杉(Larix mastersiana)、方枝柏(Sabina saltuaria)和高山杜鹃(Rhododendron lapponicum)凋落物为研究对象, 研究雪被形成期、雪被覆盖期、雪被融化期和生长季节从林窗中心、林冠林窗、扩展林窗到郁闭林下物种凋落物的半纤维素变化特征。经历一年分解后, 5种凋落物的半纤维素均呈现净累积现象。针、阔叶凋落物半纤维素分别在雪被覆盖期和融化期表现出相对较高的损失率。在雪被覆盖期和融化期, 凋落物半纤维素在林窗中心和林冠林窗具有相对较高的损失率; 而在生长季节, 林窗中心呈现相对较低的凋落物半纤维素累积率。统计分析结果表明凋落物分解过程中半纤维素损失率与环境因子和凋落物质量因子均显著相关。这些结果表明亚高山森林林窗对凋落物分解过程中半纤维素损失率具有显著影响, 分别促进了半纤维素在冬季的损失以及抑制了半纤维素在生长季节的累积, 意味着亚高山森林林窗的形成有利于凋落物半纤维素的降解。     

高山森林林窗对凋落叶分解的影响

林窗对降水和光照等环境条件的再分配以及分解者群落的影响可能深刻作用于森林凋落物分解过程,但有关高山森林林窗大小对凋落物分解的影响尚无研究报道。采用凋落物分解袋法,研究了川西高山森林不同大小林窗对非生长季节和生长季节红桦(Betula albo-sinensis)和岷江冷杉(Abies faxoniana)凋落叶质量损失的影响。结果显示,经过1a的分解,不同生境下红桦和岷江冷杉凋落叶分别分解了27.25%—30.12%和27.04%—27.96%,其中非生长季节占53.83%—60.18%和50.23%—59.09%。林窗对红桦和岷江冷杉凋落叶质量损失的影响因物种不同而呈现季节差异。总体上,林窗加快了岷江冷杉凋落叶的分解而延缓了红桦凋落叶的分解。与郁闭林下相比,林窗显著增加了2种凋落叶非生长季节的质量损失速率,显著降低了生长季节2种凋落叶的质量损失速率;2种凋落叶质量损失速率在非生长季节随林窗面积增大而加快,在生长季节随林窗面积增大而减慢。林窗显著影响了初冻期、深冻期和融化期岷江冷杉凋落叶的质量损失率,但对红桦凋落叶质量损失率影响不显著。可见,高山森林凋落物分解过程受到林窗的显著影响,并且阔叶和针叶凋落叶在非生长季节和生长季节对林窗的响应具有明显差异。     

Spatial scale and sampling resolution affect measures of gap disturbance in a lowland tropical forest: implications for understanding forest regeneration and carbon storage

Treefall gaps play an important role in tropical forest dynamics and in determining above-ground biomass (AGB). However, our understanding of gap disturbance regimes is largely based either on surveys of forest plots that are small relative to spatial variation in gap disturbance, or on satellite imagery, which cannot accurately detect small gaps. We used high-resolution light detection and ranging data from a 1500 ha forest in Panama to: (i) determine how gap disturbance parameters are influenced by study area size, and the criteria used to define gaps; and (ii) to evaluate how accurately previous ground-based canopy height sampling can determine the size and location of gaps. We found that plot-scale disturbance parameters frequently differed significantly from those measured at the landscape-level, and that canopy height thresholds used to define gaps strongly influenced the gap-size distribution, an important metric influencing AGB. Furthermore, simulated ground surveys of canopy height frequently misrepresented the true location of gaps, which may affect conclusions about how relatively small canopy gaps affect successional processes and contribute to the maintenance of diversity. Across site comparisons need to consider how gap definition, scale and spatial resolution affect characterizations of gap disturbance, and its inferred importance for carbon storage and community composition.     

The effects of treefall gaps on understory vegetation in New York State

Abstract. Treefalls are a common form of disturbance in northeastern United States forests. The resultant gaps contribute to a high degree of environmental heterogeneity in the understory of these forests. Plant density, plant cover, and species richness in understory plant communities were monitored for three years during the growing season, May - September. Differences between treefall gap and closed canopy vegetation were less pronounced early in the growing season for plant density and leaf cover. Species richness was significantly greater within treefall gaps during the entire growing season. Eight species were found in greater abundance within treefall gaps (i.e., gap-phase species), while one species was found more commonly under closed canopy. Ordination results suggest that time since gap creation and treefall gap size marginally affect the species composition of vegetation found within treefall gaps.     

中亚热带常绿阔叶林林隙及其自然干扰特征的研究

通过对福建万木林中亚热带常绿阔叶林中96个林隙的调查,研究了中亚热带常绿阔叶林的基本特征和自然干扰规律,结果表明,在中亚热带常绿阔叶林中,扩展林隙(EG)和冠空隙(CG)在中亚热带常绿阔叶林景观中的面积比例分别为50．86％和16．66％,每年干扰频率分别为0．85％·年^-1和0．28％·年^-1,林隙干扰的返回间隔期约为357年．林隙形成方式由树木折干形成的最为普遍,占形成木总数58．04％。其次是由于掘根风倒而形成的,占33．48％．林隙大多由两株树木形成,平均每个林隙拥有形成木2．33株．扩展林隙的大小多在100～300m^2之间,其中以200～300m^2者所占的面积比例最大,而以100～200m^2者所占的数量比例最大,冠空隙的大小多在100m^2以下,其中50m^2以下所占的数量比例和面积比例都是最大的．林隙形成木分布最多的径级在20～30cm之间。     

The disturbance regime of an old-growth forest in coastal California

This study deals with the disturbance regime of an old-growth, mixed-evergreen forest with a canopy composed of Arbutus menziesii, Lithocarpus densiflora, Pseudotsuga menziesii, Quercus chrysolepis, Q. wislizenii, and Sequoia sempervirens. 80 canopy gaps were randomly selected from throughout a 230-ha watershed. Of the land area sampled, 11.1 to 16.6 % was within gaps. Gap area had a mode of < 50m² and a range of 6–3437 m². Gaps were formed by snags, snaps, tips, and slope failures. Although < 10 % of the gaps sampled were due to slope failures, these accounted for 43 % of the total land area within gaps. All snags resulted from the death of a Pseudotsuga or Arbutus individual, the widely branched trunks of Arbutus individuals accounted for most of the irregularly shaped gaps, and larger gaps resulted from the death of Pseudotsuga and Sequoia individuals, averaging 119 and 111 m² respectively, than from the death of Lithocarpus or Arbutus, averaging 54 and 52 m². Gaps were more frequent over concave sections of slopes and large gaps were more frequent on north-facing slopes. The creation of a gap increased disturbance to the adjacent canopy, with half of the gaps formed through more than one disturbance. The relationship of disturbance regime to topography, the influence of canopy species biology on gap properties, and the repeated events involved in gap formation all indicate a fine scale variation in the pattern of gaps and their characteristics.     

热带季节雨林林窗辐射特征研究

利用不同季节热带季节雨林林窗、林内及旷地不同波长太阳辐射的实测资料,探讨了热带季节雨林林窗不同波长辐射特征。结果表明,热带季节雨林林窗中央和北侧林冠下的不同波长太阳辐射存在“突跃现象”,中午前后各辐射量值迅速达到最大后又急速下降;林窗内不同波长太阳辐射日总量值均大于林内,却小于旷地;林窗中央和北侧林冠下比较可见,在干热季、雨季和雨季后期,林窗中央的各辐射要素总量值高于北侧林冠下,但雾凉季时受浓雾和太阳高度以及方位的影响,各辐射要素总量值较北侧林冠下低;林窗中央的总辐射日总量在雨季太阳高度角最大时最高,雨季后期和干热季其次,雾凉季时最小;林窗中央和北侧林冠下的红外辐射及可见光在总辐射中所占份额随季节的变化而不同,充分显示了林窗辐射环境的异质性;与旷地和林内相比,林窗内各测点的红外辐射在总辐射中所占份额介于旷地和林内之间．高于旷地却低于林内;而可见光占总辐射的比值情况正好相反．另外,林窗可见光分配率的季节变化都大于红外辐射;热带季节雨林林窗育增加红外辐射、减少可见光的特征．     

Impacts of forest gaps on soil properties and processes in old growth northern hardwood-hemlock forests

We examined the influence of treefall gaps on soil properties and processes in old growth northern hardwood-hemlock forests in the upper Great Lakes region, USA. We found significantly greater solar radiation, soil moisture contents and soil temperatures in gaps compared to adjacent closed canopy plots. Gaps had significantly less exchangeable base cations (K, Ca, and Mg) compared to forest plots in the upper mineral soil (0–25 cm). Gaps also had significantly more dissolved organic N and extractable nitrate at depth (25–50 cm), indicating increased nutrient leaching in gaps. In-situ N mineralization was significantly greater in gaps and edge plots compared to forest plots. We found significantly greater potential N mineralization (measured in the laboratory at 25°C and 40% water holding capacity) in forest compared to gap plots. Microbial biomass N was significantly greater (ca. two-fold) in the gap edge compared to both gaps and closed forest. Using principal component analyses we found that edge plots were positively correlated with all principal components, indicating increased in-situ and potential N mineralization, microbial biomass N, soil NO₃⁻ and NH₄⁺, and soil organic matter. The gap edge may be a region of optimal microclimate and substrate to enhance microbial biomass and activity within these forest ecosystems. Responsible Editor: Bernard Nicolardet     

玉龙雪山自然保护区丽江云杉林林窗特征研究

研究了玉龙雪山自然保护区云杉坪典型丽江云杉林林窗干扰的基本特征．结果表明,滇西北亚高山丽江云杉林林窗占林分面积比例冠层林窗和扩展林窗分别为28．8％和42．5％,干扰频率以林窗密度计算,林窗出现的平均密度为35个·hm^-2左右．林窗大小分布为面积大于100m^2的大林窗约占25％,面积为50～100m^2的中等林窗占41％,面积小于50m。的小林窗占34％．形成林窗最重要的方式是干基和干中折断,其次是枯立滇西北亚高山丽江云杉林林窗形成木80％以上是丽江云杉,通常直径为40～50cm、高度为15～25m．每个林窗形成木数量多为1～2株,约占68．8％,5株以上很少,最多为6株．随着林窗面积由大变小,林窗中更新苗木的密度逐渐变大,小林窗中更新木密度约为大林窗的5倍．     

Patterns of Fine Root Mass and Distribution along a Disturbance Gradient in a Tropical Montane Forest, Central Sulawesi (Indonesia)

Large parts of the remaining tropical moist forests of South-east Asia are encroached at their margins by selective logging, rattan harvesting and the establishment of small agroforest plantations under the rainforest canopy. These slight to heavy disturbances affect aboveground forest structure by reducing wood biomass and canopy cover; however, they may also have a profound impact on the belowground compartment. In a lower montane moist forest of Central Sulawesi, we studied the profile totals of fine root biomass (FRB_tot, roots <2 mm until 50 cm of soil depth) and of fine root necromass (FRN_tot), the vertical distribution of fine root mass, and the fine root live/dead ratio by root coring in 12 forest stands that represented a gradient in forest use (or disturbance) intensity (forest use type A: undisturbed natural forest, B and C: slightly or moderately disturbed forests with selective timber extraction, D: heavily disturbed cacao agroforest systems under a remaining rainforest cover; each forest types being replicated three times). FRB_tot decreased significantly from forest A to the disturbed B, C and D forests, and reached less than 60% of the FRB_tot value of A in the agroforest systems D. A similar decrease with increasing disturbance intensity was found for FRN_tot. Forest disturbance intensity had no significant influence on the vertical distribution of fine root biomass in the profiles. According to correlation and principal components analyses, fractional canopy cover was the most important factor influencing FRB_tot and FRN_tot, whereas diameter at breast height, stand basal area, stem density, soil pH and base saturation had only a minor or no influence on root mass. A reduction in canopy cover from 90% (forest type A) to 75% (types C and D) was associated with a reduction in FRB_tot by about 45% which indicates that timber extraction leads not only to canopy gaps but to corresponding ‘root gaps’ in the soil as well. We conclude that forest encroachment that is widespread in large parts of South-east Asia’s remaining rainforests significantly reduces tree fine root biomass and associated carbon sequestration, even if it is conducted at moderate intensities only.     

Influence of gap size and soil properties on microbial biomass in a subtropical humid forest of north-east India

We examined the effects of treefall gap size and soil properties on microbial biomass dynamics in an undisturbed mature-phase humid subtropical broadleaved forest in north-east India. Canopy gaps had low soil moisture and low microbial biomass suggesting that belowground dynamics accompanied changes in light resources after canopy opening. High rainfall in the region causes excessive erosion/leaching of top soil and eventually soil fertility declines in treefall gaps compared to understorey. Soil microbial population was less during periods when temperature and moisture conditions are low, while it peaked during rainy season when the litter decomposition rate is at its peak on the forest floor. Greater demand for nutrients by plants during rainy season (the peak vegetative growth period) limited the availability of nutrients to soil microbes and, therefore, low microbial C, N and P. Weak correlations were also obtained for the relationships between microbial C, N and P and soil physico–chemical properties. Gap size did influence the microbial nutrients and their contribution to soil organic carbon, total Kjeldhal nitrogen and available-P. Contribution of microbial C to soil organic carbon, microbial N to total nitrogen were similar in both treefall gaps and understorey plots, while the contribution of microbial P to soil available-P was lower in gap compared to the understorey. These results indicate that any fluctuation in microbial biomass related nutrient cycling processes in conjunction with the associated microclimate variation may affect the pattern of regeneration of tree seedlings in the gaps and hence be related with their size. This revised version was published online in June 2006 with corrections to the Cover Date.