Cultivation of Non‐timber Forest Products Alters Understory Light Availability in a Humid Tropical Forest in Mexico1

The planting of non‐timber forest products (NTFPs) in the understory of tropical forests is promoted in many regions as a strategy to conserve forested lands and meet the economic needs of rural communities. While the forest canopy is left intact in most understory plantations, much of the midstory and understory vegetation is removed in order to increase light availability for cultivated species. We assessed the extent to which the removal of vegetation in understory plantations of Chamaedorea hooperiana Hodel (Arecaceae) alters understory light conditions. We also examined how any changes in light availability may be reflected by changes in the composition of canopy tree seedlings regenerating in understory plantations. We employed a blocked design consisting of four C. hooperiana plantation sites; each site was paired with an adjacent, unmanaged forest site. Hemispherical canopy photographs were taken and canopy tree seedlings were identified and measured within 12 3 × 2 m randomly placed plots in each site for a total of 96 plots (4 blocks × 2 sites × 12 plots). Plantation management did not affect canopy openness or direct light availability but understory plantations had a higher frequency of plots with greater total and diffuse light availability than unmanaged forest. Comparisons of canopy tree seedling composition between understory plantations and unmanaged forest sites were less conclusive but suggest that management practices have the potential to increase the proportion of shade‐intolerant species of tree seedlings establishing in plantations. Given the importance of advanced regeneration in gap‐phase forest dynamics, these changes may have implications for future patterns of succession in the areas of forest where NTFPs are cultivated.     

Irradiance heterogeneity within crown affects photosynthetic capacity and nitrogen distribution of leaves in Cedrela sinensis

Because light conditions in the forest understory are highly heterogeneous, photosynthetic acclimation to spatially variable irradiance within a crown is important for crown‐level carbon assimilation. The effect of variation in irradiance within the crown on leaf nitrogen content and photosynthetic rate was examined for pinnate compound leaves in saplings of Cedrela sinensis, a pioneer deciduous tree. Five shading treatments, in which 0, 25, 50, 75 and 100% of leaves were shaded, were established by artificial heavy shading using shade screen umbrellas with 25% transmittance. Although the nitrogen content of leaves was constant regardless of shading treatment, ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco) content and light‐saturated photosynthetic capacity were lower in shade leaves within partially shaded crowns than within fully shaded crowns. Shade leaves within partially shaded crowns contained higher amount of amino acids. Most shade leaves died in partially shaded crowns, whereas more than half of shade leaves survived in totally shaded crowns. Assumptions on photosynthetic acclimation to local light conditions cannot explain why shade leaves have different photosynthetic capacities and survival rates in between partially and totally shaded crowns. Irradiance heterogeneity within the crown causes a distinct variation in photosynthetic activity between sun and shaded leaves within the crown.     

Linking light attenuation,sunflecks, and canopy architecture in mesic shrub thickets

Expansion of shrubs into grasslands is often accompanied by a reduction in understory light and an associated reduction of shade-intolerant species. However, effects of specific canopy architectural characteristics on the light environment under shrub thickets are unknown. Our objective was to determine what characteristics of canopy architecture most influence understory light in monospecific shrub thickets. We quantified understory light and canopy architecture for five shrub species in the eastern United States that have a history of expansion, and we used multiple regression to determine which canopy characteristics best predicted light attenuation and relative contribution of sunflecks. Measurements included leaf angle, leaf azimuth, branch bifurcation ratio, leaf area index (LAI), canopy depth (the vertical distance from the bottommost leaf to the top of the canopy), and leaf area density (LAD) as well as understory photosynthetic photon flux density (PPFD). The best predictor of light attenuation and the occurrence of sunflecks for all species was canopy depth. Multiple leaf and plant-level traits were correlated with canopy depth but not with LAI or LAD. The invasive shrub Elaeagnus umbellata had the lowest understory light levels of the species examined although LAI values for Morella cerifera and Rhododendron maximum were higher. Branch bifurcation ratios for E. umbellata were significantly higher than for other species and this likely contributed to the differences in light attenuation and suppression of sunflecks. The potential of shrubs to intercept light is primarily dependent on vertical distribution of leaves in the canopy which is itself correlated with fine-scale, species-specific variations in leaf display.     

Impact of light quality on leaf and shoot hydraulic properties: a case study in silver birch (Betula pendula)

Responses of leaf and shoot hydraulic conductance to light quality were examined on shoots of silver birch (Betula pendula), cut from lower (‘shade position’) and upper thirds of the crowns (‘sun position’) of trees growing in a natural temperate forest stand. Hydraulic conductances of leaf blades (K_lb), petioles (K_P) and branches (i.e. leafless stem; K_B) were determined using a high pressure flow meter in steady state mode. The shoots were exposed to photosynthetic photon flux density of 200–250 µmol m^?2 s^?1 using white, blue or red light. K_lb depended significantly on both light quality and canopy position (P < 0.001), K_B on canopy position (P < 0.001) and exposure time (P = 0.014), and none of the three factors had effect on K_P. The highest values of K_lb were recorded under the blue light (3.63 and 3.13 × 10^?4 kg m^?2 MPa^?1 s^?1 for the sun and shade leaves, respectively), intermediate values under white light (3.37 and 2.46 × 10^?4 kg m^?2 MPa^?1 s^?1, respectively) and lowest values under red light (2.83 and 2.02 × 10^?4 kg m^?2 MPa^?1 s^?1, respectively). Light quality has an important impact on leaf hydraulic properties, independently of light intensity or of total light energy, and the specific light receptors involved in this response require identification. Given that natural canopy shade depletes blue and red light, K_lb may be decreased both by reduced fluence and shifts in light spectra, indicating the need for studies of the natural heterogeneity of K_lb within and under canopies, and its impacts on gas exchange.     

白云山国家森林公园不同人为干扰强度的群落冠层结构和光照特征

为探究森林冠层结构与林下光照的变化规律及其相关性,在河南省白云山国家森林公园选取人工林、择伐林、皆伐林和老龄林建立四块面积为1 hm²（100 m×100 m）的固定监测样地,利用半球面影像技术获取冠层结构及林下光照数据。研究发现：1）随着人为干扰强度降低,冠层覆盖度与叶面积指数呈增加趋势,林下散射辐射与直接辐射呈减少趋势;2）择伐林冠层覆盖度与叶面积指数最大,平均叶倾角与透光比最小,林下光照（直接辐射、散射辐射）与冠层覆盖度和叶面积指数都呈显著负相关,林下散射辐射与冠层覆盖度和叶面积指数的负相关关系最强;3）处于不同干扰强度的群落由于冠层结构的差异,形成了不同群落内特定的光照环境。研究结果丰富了暖温带-亚热带生态过渡区森林冠层结构与林下光照动态变化研究资料,同时也为该区域森林的恢复与重建等提供科学的依据。     

Effects of overstory and understory vegetation on the understory light environment in mixed boreal forests

Abstract. The percentage of above-canopy Photosynthetic Photon Flux Density (%PPFD) was measured at 0, 50 and 100 cm above the forest floor and above the main understory vegetation in stands of (1) pure Betula papyrifera (White birch), (2) pure Populus tremuloides (Trembling aspen), (3) mixed broad-leaf-conifer, (4) shade-tolerant conifer and (5) pure Pinus banksiana (Jack pine) occurring on both clay and till soil types. %PPFD was measured instantaneously under overcast sky conditions (nine locations within each of 29 stands) and continuously for a full day under clear sky conditions (five locations within each of eight stands). The percentage cover of the understory layer was estimated at the same locations as light measurements. Mean %PPFD varied from 2% at the forest floor under Populus forests to 15% above the understory vegetation cover under Betula forests. Percent PPFD above the understory vegetation cover was significantly higher under shade intolerant tree species such as Populus, Betula and Pinus than under shade tolerant conifers. No significant differences were found in %PPFD above the understory vegetation cover under similar tree species between clay and till soil types. The coefficient of variation in %PPFD measured in the nine locations within each stand was significantly lower under deciduous dominated forests (mean of 19%) than under coniferous dominated forests (mean of 40%). %PPFD measured at the forest floor was positively correlated with %PPFD measured above the understory vegetation and negatively correlated with cumulative total percent cover of the understory vegetation (R² = 0.852). The proportion of sunflecks above 250 and 500 mmol m^–2 s^–1 was much lower and %PPFD in shade much higher under Populus and Betula forests than under the other forests. Differences in the mean, variability and nature of the light environment found among forest and soil types are discussed in relation to their possible influences on tree succession.     

Vegetative Life History of a Dominant Rain Forest Canopy Tree1

Tetramerista glabra has a remarkable combination of life history traits. It is a dense-wooded, large, common canopy tree in primary peat swamp rain forest. Its seedlings, although shade tolerant, can grow rapidly in high light conditions, but frequently lack structural stability. Most juvenile stems (94% in the understory and 38% in canopy gaps) collapse under their own weight or from branchfalls. Fallen stems then ramify into vegetative sprouts, which in turn may collapse, perpetuating a vegetative juvenile cycle. Most recruitment is from sprouts rather than from seed. Structural analysis of stem dimensions shows that stems 2–8 cm DBH (diameter at breast height) are close to the theoretical buckling limit, especially for those dependent on neighboring vegetation to maintain vertical form. Trees > 4 cm DBH persisting as upright stems develop stilt root support and become structurally independent at ca 8 cm DBH. Eventually, as stems thicken, stilt roots anastamose and trees adopt the cylindrical growth form of mature canopy trees (up to 150 cm DBH). Thus, the vegetative life history strategy of the species is to: (i) regenerate a large “ramet bank” from the majority of juveniles that fail structurally while suppressed in the understoty, and (ii) to maximize height growth at the expense of diameter growth in high light conditions. The growth pattern and plastic form of T. glabra shows how a shade tolerant species may adapt to utilize the ephemeral light resource in canopy gaps. The growth strategy of this species allows it to circumvent the normal trade-off between shade tolerance and rapid growth in canopy gaps.     

紫耳箭竹克隆形态可塑性对典型冠层结构及光环境的响应

在重庆金佛山国家自然保护内,选择了3种典型群落类型(落叶阔叶林、常绿落叶阔叶混交林和常绿阔叶林),使用Hemiview数字植物冠层分析系统量化群落冠层结构和光环境特征,并对林下紫耳箭竹(Fargesia decurvata)的形态可塑性特征进行调查,分析冠层结构和光环境特征改变下紫耳箭竹形态可塑性的差异,并探讨它们之间的相互关系。结果表明:(1)随着落叶阔叶林"常绿落叶阔叶混交林"常绿阔叶林演替的进行,群落的冠层开度降低,叶面积指数增加,平均叶倾角变小,趋于水平化,冠层对光的截获能力提高,林下光照的强度降低(P0.05)。(2)随着光照强度的降低,紫耳箭竹分株矮小化,叶片变窄,生物量积累降低,但通过增大比茎长、叶面积率和比叶面积提高对光的利用效率,并增大分枝角度和比隔长有效适应弱光环境。(3)在光照条件差的常绿阔叶林下,紫耳箭竹降低对地下茎的投资,将较多的生物量用于秆的增高增长和叶片的生长;而在光照条件好的落叶阔叶林环境下,紫耳箭竹降低对枝、叶生物量的分配,则加大对地下茎的投资,可认为是克隆植物对水分资源所表现的一种觅食行为。研究表明,紫耳箭竹种群随着冠层结构的改变发生了明显的可塑性变化,这些可塑性变化是种群对冠层结构和光环境差异的适应性反应的结果,有利于增强种群对异质生境中光资源的获取和利用;群落内部可以通过调控冠层结构的改变协调和控制小径竹种群的发展。     

Differences in Survival and Growth Among Tropical Rain Forest Pioneer Tree Seedlings in Relation to Canopy Openness and Herbivory

Although differences in canopy openness, herbivory and their interaction may promote species coexistence, how these factors affect pioneer tree species and potentially limit growth, and survival has been poorly studied, particularly in tropical South Asia. We monitored the effect of canopy openness and herbivore damage on seedling survival and growth of 960 individuals of six pioneer tree species: Dillenia triquetra, Macaranga indica, Macaranga peltata, Schumacheria castaneifolia, Trema orientalis, and Wendlandia bicuspidata. Seedlings were placed in four gap‐understory positions—center, outer gap edge, inner forest edge, and understory—in four large, natural gaps within the Sinharaja World Heritage Reserve, Sri Lanka. Canopy openness positively affected survival probability beyond the 550‐d experiment, while herbivory decreased survival and was highest in understory conditions. The relative order of species survival stayed fairly consistent between gap‐understory positions and followed their known shade tolerance rankings. When averaged across all experimental conditions, T. orientalis had the lowest survival probability estimate beyond the 550‐d experiment (0.05), but the greatest capacity for growth where it successfully established, while the species with highest averaged survival probability (0.79), D. triquetra, showed the lowest growth. One species, W. bicuspidata, responded positively to herbivory by re‐sprouting. Coexistence of D. triquetra, T. orientalis, and W. bicuspidata can be explained by a trade‐off among species in survival, growth, and response to herbivory. In addition to variation in canopy light environment, herbivory may be important in determining pioneer species distribution through fine‐scale niche partitioning and should be carefully considered in reforestation efforts.     

Effects of Light Gaps and Litter Removal on the Seedling Performance of Six African Timber Species1

Valuable timber tree species frequently show poor regeneration after selective logging in tropical forests. Small size of logging gaps, lack of soil disturbance, and limited seed availability have each been blamed for observed regeneration failures. We investigated seed germination and seedling performance using a split‐plot factorial design involving light availability and litter removal for six Central African timber tree species, hypothesizing that canopy gaps and litter removal would improve seedling establishment, and that less shade‐tolerant species would show stronger responses to both factors. Contrary to our expectations, significantly more germinants established on intact litter than on exposed mineral soil 3 mo after seeding. After 18 mo, seedling survival, height and diameter growth, leaf area, and rooting depth were all much higher in gap plots than in the understory for all species, with the exception of Gilbertiodendron dewevrei, a highly shade‐tolerant species whose survival was higher in the understory. Leaf production was negatively influenced by litter removal in the least shade‐tolerant species, Nauclea diderrichii, with weak or positive effects in other species. G. dewevrei, while displaying a low‐light threshold for growth, exhibited a surprisingly high growth response to increasing light comparable to more shade‐intolerant species, a response that may help explain its local competitive dominance in the region. Due to the rapid closure of small gaps, we suggest that shade‐intolerant species such as N. diderrichii, Khaya anthotheca, and Entandrophragma utile might benefit from more intensive silvicultural practices that create larger canopy gaps.     

Pattern and Performance of Understory Bamboos (Chusquea spp.) under Different Canopy Closures in Old-Growth Oak Forests in Costa Rica1

The effect of canopy closure on spatial and morphological patterns of three understory Chusquea species was studied in pristine upper montane oak forests. Plots with either Chusquea talamancenis Widmer & L.G. Clark or C. tomentosa Widmer & L.G. Clark were set out in gaps, under intermediate tree canopy and under closed canopy. The third species Chusquea foliosa L.G. Clark was only found in gap and intermediate canopy sites in the study area. Clumps of all bamboo species tend to be few and large under open conditions and numerous and small under more shady conditions. The larger size of clumps in gaps is reflected in the number of culms per clump, the diameter of the culms and, as a consequence, clump basal area, clump crown area, and biomass (index of plant volume) compared to intermediate and closed canopy. The smaller clump density in gaps compared to closed canopy implies that there is more intraspecific competition and density dependent mortality (self-thinning effect) when a bamboo species is dominant under favourable light conditions. Parameters of performance like culm length, number of nodes per culm, number and length of primary branches, and number of branch nodes seem not to be affected by the light regime, unlike total number of branches, total branch length, and the branching pattern. Species differ in their response to the light environment: C. tomentosa and C. foliosa have a higher degree of morphological plasticity than C. talamancensis, which in turn, appears to be more shade tolerant. No difference between species has been found regarding their contribution to vegetation, parameters reflecting abundance per unit area (site clump area, site crown area and site index of clump volume) were similar for all three species.     

Habitat associations with topography and canopy structure of tree species in a tropical montane forest on Mount Kinabalu,Borneo

Habitat associations with topography and canopy structure of 42 abundant tree species were studied in a 2.74-ha plot of tropical montane forest on Mount Kinabalu, Borneo. Many of these species belong to the same higher taxa including eight families and four genera. Analysis of intraspecific spatial distributions for stems ≥ 10 cm diameter revealed that 28 species (including all six species of Fagaceae) showed aggregated distributions at the 100-m² and/or 400-m² scales, and that 20 species showed habitat associations with topography by torus-translation tests; 17 species showed both characteristics. Species' associations with the local canopy structure were characterized by crown position index (CPI), which was defined relative to neighbour trees. The CPI differed greatly among individual stems at 10–40 cm diameter, and 19 species showed significantly different frequencies of crowns exposed vertically versus those shaded beneath the canopy. Mean growth rates at 10–40 cm diameter and size distributions of species were not related to topographic associations, but were explained by the associations with canopy structure; species with more exposed crowns grew faster and had less positively skewed distributions. Diversity in habitat associations was manifest between two genera (Syzygium and Tristaniopsis) in the family Myrtaceae and among species in these genera, but was less evident in other families and two genera (Garcinia and Lithocarpus). This revised version was published online in June 2006 with corrections to the Cover Date.     

伏牛山自然保护区森林冠层结构对林下植被特征的影响

在伏牛山自然保护区典型地段设立样方,测定了森林生态系统内几种典型群落类型的冠层结构、光环境特征,调查了林下植被的特征,分析了它们之间的相互关系.结果显示:各群落的冠层结构和光环境有一定的差异,单因素方差分析表明,部分群落间的差异性达到显著水平;各群落灌木层物种丰富度、多样性和均匀度均高于草本层,而优势度正相反;线性拟合的结果表明,草本层的物种丰富度、多样性与冠下光合量子通量密度间呈极显著负相关,优势度与冠下光合量子通量密度间呈显著正相关,灌木层各参数与冠层结构特征间相关性不显著.研究表明,冠层结构的变化对草本层(包括更新幼苗)的影响显著高于灌木层.林隙/林窗或林中空地的出现可能对草本物种或其他阳性及先锋物种具有促进作用,而对优势种幼苗的萌发和定植产生负效应.推测在典型的落叶阔叶林生态系统演替进程中,林下光照强度可能不是最主要的限制因素,优势种种子的扩散、萌发和定植限制可能更重要.     

Architecture of Iberian canopy tree species in relation to wood density,shade tolerance and climate

Tree architecture has important consequences for tree performance as it determines resource capture, mechanical stability and dominance over competitors. We analyzed architectural relationships between stem and crown dimensions for 13 dominant Iberian canopy tree species belonging to the Pinaceae (six Pinus species) and Fagaceae (six Quercus species and Fagus sylvatica) and related these architectural traits to wood density, shade tolerance and climatic factors. Fagaceae had, compared with Pinaceae, denser wood, saplings with wider crowns and adults with larger maximal crown size but smaller maximal height. In combination, these traits enhance light acquisition and persistence in shaded environments; thus, contributing to their shade tolerance. Pinaceae species, in contrast, had low-density wood, allocate more resources to the formation of the central trunk rather than to branches and attained taller maximal heights, allowing them to grow rapidly in height and compete for light following disturbances; thus, contributing to their high light requirements. Wood density had a strong relationship with tree architecture, with dense-wooded species having smaller maximum height and wider crowns, probably because of cheaper expansion costs for producing biomechanically stable branches. Species from arid environments had shorter stems and shallower crowns for a given stem diameter, probably to reduce hydraulic path length and assure water transport. Wood density is an important correlate of variation in tree architecture between species and the two dominant families, with potentially large implications for their resource foraging strategies and successional dynamics.     

INTERACTIONS BETWEEN CROWN STRUCTURE AND LIGHT ENVIRONMENT IN FIVE RAIN FOREST PIPER SPECIES

Measurements of light variation among leaves within crowns of five Piper species were compared with estimates of spatial variation in light within understory, forest edge, and clearing habitats to estimate the extent to which crown structure contributes to variation in leaf light environment. Daily photon flux density (PFD) varied greatly within and among crowns. Coefficients of variation for daily PFD among sensors within a single crown ranged from 26 to 79%. Within a single crown located in a clearing, the range in daily PFD among leaves was nearly as great as the range over the entire sample of plants. In the understory, localized sunfleck activity contributed to a high degree of spatial variation in instantaneous and total PFD among leaves within individual crowns. Much of the microsite variation in sunfleck activity, however, reflected environmental conditions within the understory habitat. Within an array of sensors placed next to Piper crowns in the understory, correlations were poor for light sensors spaced only 0.2 m apart, and only 8% of the variance in light readings was explained by measurements made 0.5 m away. In the clearing habitats, microsite heterogeneity among leaves was more strongly influenced by leaf positions within crowns and leaf angles than by spatial heterogeneity within the habitat.     

Patch structure and ramet demography of the clonal tree, Asimina triloba, under gap and closed-canopy

Clonal understory trees develop into patches of interconnected and genetically identical ramets that have the potential to persist for decades or centuries. These patches develop beneath forest canopies that are structurally heterogeneous in space and time. Canopy heterogeneity, in turn, is responsible for the highly variable understory light environment that is typically associated with deciduous forests. We investigated what aspects of patch structure (density, size structure, and reproductive frequency of ramets) of the clonal understory tree, Asimina triloba, were correlated with forest canopy conditions. Specifically, we compared A. triloba patches located beneath closed canopies and canopy gaps. We also conducted a three-year demographic study of individual ramets within patches distributed across a light gradient. The closed canopy-gap comparison demonstrated that the patches of A. triloba had a higher frequency of large and flowering ramets in gaps compared to closed-canopy stands, but total ramet density was lower in gaps than in closed canopy stands. In the demographic study, individual ramet growth was positively correlated with light availability, although the pattern was not consistent for all years. Neither ramet recruitment nor mortality was correlated with light conditions. Our results indicate that the structure of A. triloba patches was influenced by canopy condition, but does not necessarily depend on the responses of ramets to current light conditions. The lack of differences in ramet recruitment and mortality under varying canopy conditions is likely to be a primary reason for the long-term expansion and persistence of the patches. The primary benefit of a positive growth response to increasing light is the transition of relatively small ramets into flowering ramets within a short period of time.     

Woodland canopy structure and the light response of juvenile Quercus lobata (Fagaceae)

I investigated competition for light between canopy plants and juvenile valley oaks (Quercus lobata Nee) in a mixed-broadleaf woodland of California's northern Coast Ranges. Canopy effects on understory light supply were separated among the overlying adult valley oak, the adult's woody understory, and neighboring trees and shrubs through a series of light sampling surveys and measurements of the number, size, and spatial distribution of neighboring plants. Light supply in the understory was primarily influenced by neighboring plants, with no detectable effect of the overlying adult valley oak. Light supply in the understory averaged 25% full sun due to a high frequency of canopy gaps and a typically open understory. Seedling response to understory light supply was investigated in an experimental sunfleck gradient (10%, 19%, and 100% full sun). Between 10% and 100% full sun, seedling growth increased by 90% and the shoot:∗∗∗root ratio changed from 1.561 to 0.607. Shade seedlings were also taller and produced fewer, larger, and thinner leaves than seedlings grown in full sun. A field survey of the spatial distribution and crown morphology of saplings and young adults found 1) the distance between young valley oaks and neighboring overstory trees to increase with neighbor size, and 2) crowns of the young oaks to be skewed away from neighbors. Although shading by the canopy was only moderate, canopy effects on understory light supply may restrict juvenile recruitment of valley oak in this woodland.     

Comparison of the physiology, morphology, and leaf demography of tropical saplings with different crown shapes

Branch architecture, leaf photosynthetic traits, and leaf demography were investigated in saplings of two woody species, Homolanthus caloneurus and Macaranga rostulata, co-occurring in the understory of a tropical mountain forest. M. rostulata saplings have cylindrical crowns, whereas H. caloneurus saplings have flat crowns. Saplings of the two species were found not to differ in area-based photosynthetic traits and in average light conditions in the understory of the studied site, but they do differ in internode length, leaf emergence rate, leaf lifespan, and total leaf area. Displayed leaf area of H. caloneurus saplings, which have the more rapid leaf emergence, was smaller than that of M. rostulata saplings, which have a longer leaf lifespan and larger total leaf area, although M. rostulata saplings showed a higher degree of leaf overlap. Short leaf lifespan and consequent small total leaf area would be linked to leaf overlap avoidance in the densely packed flat H. caloneurus crown. In contrast, M. rostulata saplings maintained a large total leaf area by producing leaves with a long leaf lifespan. In these understory saplings with a different crown architecture, we observed two contrasting adaptation strategies to shade which are achieved by adjusting a suite of morphological and leaf demographic characters. Each understory species has a suite of morphological traits and leaf demography specific to its architecture, thus attaining leaf overlap avoidance or large total leaf area.     

Altering Light Availability to Restore Invaded Forest: The Predictive Role of Plant Traits

Many studies have demonstrated that reduced light availability, which can be manipulated at local scales by planting or seeding canopy species, can curtail the growth of invasive species and promote the growth of native species. Species differences in functional traits, such as light use and stress tolerance, may be used to determine how native and invasive species will respond to these resource manipulations. We altered light availability in a mesic Hawaiian forest understory and found that low light levels reduced the biomass and growth of two invasive grasses (Pennisetum clandestinum and Ehrharta stipoides) relative to two native shrubs (Pipturus albidus and Coprosma rhynchocarpa) and two native canopy species (Metrosideros polymorpha and Acacia koa). Native species generally displayed traits associated with shade tolerance (high quantum yield, chlorophyll content, and leaf mass per area), whereas the two invasive grasses displayed traits associated with shade intolerance (high photosynthetic rate and growth rate). Several key traits pertaining to light acquisition and shade tolerance (quantum yield, chlorophyll content, and leaf mass per area) predicted seedling survival in low‐light treatments. Our data suggest that differences in light use among native and invasive species can help to determine the utility of resource manipulation as a restoration tool and, more specifically, to predict which native species will be optimal for restoration efforts that manipulate light availability.     

山西灵空山天然松栎混交林群落特征与冠层结构

以2011年建设的山西灵空山4 hm²天然松栎混交林森林动态监测样地为研究平台,以400个10 m×10 m样方为测量单元,于2016年进行群落特征研究,采用半球面影像法(DHP)分析冠层结构和林下光照特征.结果表明: 样地内共有乔木5558株,共计25种,分属于10科15属.冠层开阔度(CO)集中在15.0%～25.0%,叶面积指数(LAI)集中在1.5～2.5,林下光环境参数集中在10.0%～30.0%.建群种在样地内的分布对冠层结构和林下光环境影响显著;冠层结构对林下光环境所有参数的影响方向一致,其中采用叶面积指数评价冠层结构动态的效果更佳;冠层开阔度和叶面积指数对林下光环境产生相反的影响,且均对散射光入射率影响程度最大.温性松栎混交林的林冠层整体较为均匀,林下光分布较为集中,林分树种组成与冠层结构对林下光照影响显著.