Indirect effects of hiking trails on the community structure and diversity of trunk-epiphytic bryophytes in an old-growth fir forest

Abstract

Forest hiking trails may influence local microclimate and biodiversity, but the effects on community structure and diversity of epiphytic bryophytes on trees are currently unknown. Epiphytic bryophytes on 82 Abies faxoniana Rehder & Wilson tree trunks (41 along the hiking trail edge and 41 controls in the forest interior) were investigated at four heights from the ground (10, 50, 120, and 180 cm). At each site, air temperature and humidity were monitored for 1 year. The light radiation levels and air temperature were higher, and the canopy leaf area index and air humidity lower at the trail edge, indicating deterioration in microclimate, resulting from the trail establishment. The epiphytic bryophyte species richness, community cover, and mean cover of dendroid and pendent growth forms on trunks were significantly lower at the trail edge than the control site, suggesting that trail construction caused these reductions. One marked effect of the presence of the trail was the increase in some sun-loving species and decrease in shade-tolerant species. Moreover, the trail also slightly influenced species richness and epiphytic bryophyte cover at both community and species population levels along the height gradient. Comprehensive analyses showed that microclimate deterioration was mainly driven by the trail establishment, and that the change in micro-climate along the trail, rather than any host traits, played an important role in the declining epiphytic bryophyte community structure and diversity at the trail edge, confirming the initial hypothesis that the presence of a raised boardwalk (hiking trail) indirectly influences epiphytic bryophyte community and diversity by altering the microclimate.     

影响广东黑石顶树附生苔藓分布的环境因子

通过对广东省黑石顶自然保护区内5个2500 m2样地内树附生苔藓的调查及有关环境因子的测定,研究了树附生苔藓的分布格局及其与环境因子的关系.树附生苔藓在不同高度的分布存在一定梯度,20 cm高处树附生苔藓的种类数与盖度均大于60 cm及更高处,且其群落优势种的数量组成与后者存在较大差异.不同树种附生苔藓盖度和种数差异较大.基于附生苔藓植物的盖度进行DCA排序及聚类分析将树种分成四组,马尾松(Pinus massoniana)因具有两种特有的网藓(巴西网藓Syrrhopodon prolifer和鞘刺网藓S.armatus)单独一组,福建青冈(Quercus chungii)亦与其余各种的差异均较大,形成一组,其余阔叶树种根据其所处的森林类型分成两组,针阔叶混交林内的阔叶树种和次生阔叶林内的阔叶树种各形成一组.对环境因子及树皮含水量和pH的分析显示,垂直梯度上空气湿度的差异可能是造成附生苔藓在不同高度分布差异的主要影响因子之一,不同树种附生苔藓的差异在一定程度上受树皮pH的影响,而与树皮含水量无关.同一树种上树附生苔藓的分布又在一定程度上受森林类型的影响.     

浙江西天目山主要森林类型的苔藓多样性比较

苔藓是森林的重要组分, 是森林保护区的重要保护对象, 在物种资源和生态系统功能维护中有重要作用。该研究以浙江西天目山国家自然保护区内7种主要森林类型(落叶矮林、落叶阔叶林、常绿-落叶阔叶混交林、常绿阔叶林、针阔混交林、针叶林和竹林)内的苔藓植物为对象, 调查了32个10 m × 10 m的样地, 记录地面生苔藓植物盖度和树附生苔藓植物多度, 采用重要值、相似性系数、多样性指数分析了森林类型间的苔藓植物多样性差异。共采集969份标本, 隶属41科84属142种, 其中苔类植物13科18属33种, 藓类植物28科66属109种, 优势科为灰藓科、青藓科和羽藓科。2种混交林(常绿-落叶阔叶混交林和针阔混交林)的物种丰富度和多样性指数均高于其余5种森林, 其中物种丰富度以针阔混交林最高, 苔藓植物多样性则以常绿-落叶阔叶混交林最高, 竹林两者均为最低。海拔等环境因子较为接近的植被类型的苔藓植物多样性相似性较高, 常绿阔叶林与针叶林相似性最高, 而落叶矮林和竹林相似性最小。     

Old trees contribute bio-available nitrogen through canopy bryophytes

Symbiotic cyanobacteria??bryophyte associations on the forest floor are shown to contribute significantly to stand-level nitrogen budgets through the process of biological nitrogen fixation (BNF), but few studies have considered the role of canopy bryophytes. Given the high biomass of epiphytic bryophytes in many tree species of the North American temperate rain forest, we suggest that canopy bryophytes may contribute substantially to stand-level N dynamics. We confirm the presence of cyanobacteria and measure rates of BNF at three heights (0, 15 and 30 m) in Sitka spruce trees across three watershed estuaries of Clayoquot Sound, British Columbia, Canada. This study is the first to report BNF by cyanobacteria associated with epiphytic and forest floor bryophytes in the coastal temperate rain forest of North America. Cyanobacteria density was significantly greater in epiphytic bryophytes compared to mosses on the forest floor, and rates of BNF were highest at 30 m in the canopy. The majority of total stand-level BNF (0.76 kg N · ha^-1 · yr^-1) occurs in the canopy, rather than on the forest floor (0.26 kg N · ha^-1 · yr^-1). We suggest that BNF by cyanobacterial-bryophyte associations in the canopy of coastal temperate rain forests is a unique source of ecosystem N, which is dependent on large, old trees with high epiphytic bryophyte biomass.     

Association between bryophytes and seedlings in Nothofagus sub-Antarctic forests

Bryophytes, which include mosses, liverworts and hornworts, indeed play a significant role in the natural dynamics of native forests. They contribute to fundamental ecological processes such as tree regeneration. In this context, the objective was to analyse the association of bryophytes with Nothofagus seedlings, as well as whether the substrates where bryophytes grew influenced bryophyte-tree seedling association, in sub-Antarctic forests at two contrasting landscapes (coastal and mountain) of Tierra del Fuego (Argentina), to better understand their impacts on the natural regeneration processes. We hypothesized that bryophytes act as a substrate for seed germination and initial regeneration growth in pure and mixed forests of N. pumilio and N. betuloides, resulting in widespread tree seedlings–bryophyte associations in all landscapes. In each forest and landscape location, 60 transects were established (10 m in length) to evaluate bryophyte cover (by point intercept method) and substrate type (bare soil, decaying wood and litter cover) where they were growing. An adaptation of the relative interaction index (RII) based on tree seedling cover associated with bryophyte species (%) and tree seedling cover in the absence of bryophytes (%) was calculated and analysed to assess competition or facilitation between bryophytes and tree seedlings. Nothofagus pumilio seedlings were less abundant in bryophytes compared to other substrates, suggesting an inhibitory effect on the germination and/or survival of N. pumilio. In contrast, the seedlings of N. betuloides, in both pure and mixed forests, exhibited higher abundance on bryophyte substrates, particularly in mosses at mountain landscape (RII = 0.83 ± 0.08 in litter and 0.62 ± 0.11 in decaying wood). These findings suggest that bryophytes play a facilitating role for N. betuloides seedlings during germination and initial phases of growth, mainly in the mountain. Therefore, their conservation promotes N. betuloides forest continuity, both in pure and mixed structures.     

Bole epiphytic bryophytes on <Emphasis Type="Italic">Lithocarpus xylocarpus</Emphasis> (Kurz) Markgr. in the Ailao Mountains,SW China

Epiphytic bryophytes growing on Lithocarpus xylocarpus (Kurz) Markgr. trunks of different diameter classes in primary (132 plots) and secondary (84 plots) Lithocarpus forests in the Ailao Mountains, SW China, were surveyed and analyzed to determine species composition and richness, and to identify environmental variables that may affect it. Among the 65 species (belonging to 32 genera, 19 families) found, 28 occurred in both forests, with Syrrhopodon gardneri (Hook.) Schwaegr. predominanting. Species richness and total coverage in primary forest were significantly higher than in secondary forest. We suggest that a period of perhaps much more than 110 years is necessary for the recovery of epiphytic bryoflora in montane forest of SW China. Fan, turf, and smooth mat are the most important life forms, with high occurrences in both forests. The life form composition of epiphytic bryophytes is determined mainly by microhabitat and host age. Tree age, the presence of primary forest, bark pH, and plot exposure are the environmental variables that have significantly influenced the composition of epiphytic bryophytes. Tree age explained most variations in epiphytic bryoflora. Bark pH is another important parameter that significantly influenced the epiphytic bryophyte community, but seemed indirectly correlated with tree age. Primary forest is a favorable habitat for epiphytes, due mainly to its diversified canopy structure and the presence of large diameter hosts. Moisture-laden southwest trade winds and forest structure could differentiate microclimate and impel a distinct composition of epiphytes in windward and leeward exposures.     

广东黑石顶森林苔藓群落特征初探

通过对5个面积为2500m2的样地中树附生和地面苔藓植物的定量调查,初步研究了广东黑石顶森林中苔藓植物多样性特征。本次调查共发现苔藓植物50种,包括30种苔类和20种藓类。样地中树附生苔藓植物40种,地面苔藓植物24种,前者远高于后者。各样地中地面苔藓植物的种类为8-13种,盖度0.59%-1.12%,坡度及小生境的多样性对地面苔藓植物物种多样性及盖度均有很大影响;树附生苔藓植物12-20种,盖度0.63%-1.63%。结果显示30a的次生林中苔藓植物的物种丰富度及盖度均可恢复到与成熟阔叶林接近的水平。相似性分析表明,成熟阔叶林内苔藓植物种类组成与针阔叶混交林及30a的次生阔叶林差异不明显,但前者的苔藓植物群落结构与后者的差异较大。     

Ectomycorrhizal fungal communities of native and non-native Pinus and Quercus species in a common garden of 35-year-old trees

Non-native tree species have been widely planted or have become naturalized in most forested landscapes. It is not clear if native trees species collectively differ in ectomycorrhizal fungal (EMF) diversity and communities from that of non-native tree species. Alternatively, EMF species community similarity may be more determined by host plant phylogeny than by whether the plant is native or non-native. We examined these unknowns by comparing two genera, native and non-native Quercus robur and Quercus rubra and native and non-native Pinus sylvestris and Pinus nigra in a 35-year-old common garden in Poland. Using molecular and morphological approaches, we identified EMF species from ectomycorrhizal root tips and sporocarps collected in the monoculture tree plots. A total of 69 EMF species were found, with 38 species collected only as sporocarps, 18 only as ectomycorrhizas, and 13 both as ectomycorrhizas and sporocarps. The EMF species observed were all native and commonly associated with a Holarctic range in distribution. We found that native Q. robur had ca. 120% higher total EMF species richness than the non-native Q. rubra, while native P. sylvestris had ca. 25% lower total EMF species richness than non-native P. nigra. Thus, across genera, there was no evidence that native species have higher EMF species diversity than exotic species. In addition, we found a higher similarity in EMF communities between the two Pinus species than between the two Quercus species. These results support the naturalization of non-native trees by means of mutualistic associations with cosmopolitan and novel fungi.     

The effects of stand structure on ground-floor bryophyte assemblages in temperate mixed forests

The effect of tree species composition, stand structure characteristics and substrate availability on ground-floor bryophyte assemblages was studied in mixed deciduous forests of Western Hungary. Species composition, species richness and cover of bryophytes occurring on the soil and logs were analysed as dependent variables. The whole assemblage and functional groups defined on the basis of substrate preference were investigated separately. Substrate availability (open soil, logs) was the most prominent factor in determining species composition, cover and diversity positively, while the litter of deciduous trees had a negative effect on the occurrence of forest floor bryophytes. Besides, bryophyte species richness increased with tree species and stand structural diversity, and for specialist epiphytic and epixylic species log volume was essential. Sapling density and light heterogeneity were influential on bryophyte cover, especially for the dominant terricolous species. Many variables of the forest floor bryophyte community can be estimated efficiently by examining stand structure in the studied region. Selective cutting increasing tree species diversity, stand structural heterogeneity and dead wood volume can maintain higher bryophyte diversity in this region than the shelter-wood system producing even-aged, monodominant, structurally homogenous stands.     

Tree age‐dependent changes among epiphytic bryophyte communities in Mediterranean environments. A case study from Sicily (Italy)

Abstract

The epiphytic bryophytes inhabiting trees of different size/age in a Quercus ilex wood from Madonie Mountains (northern Sicily, Italy) were studied with the purpose of describing the changes that take place in the bryophyte stratum during the tree lifespan. Results indicate an increase of bryophyte cover combined with a progressive decrease of epiphytic lichens and the existence of an active process of species and community substitution. The way these processes take place corresponds to a succession sequence, which is characterised by a high number of pioneer species on the youngest trees and a sharp decline of species number on middle‐aged and old trees due to the great spread of Leptodon smithii and a few other pleurocarpous mosses. The comparison of these traits to those from central Spain evidences differences in patterns of tree colonisation by epiphyte bryophytes between the two areas and suggests that climatic conditions play a chief role in the epiphytic bryophyte establishment and succession in Mediterranean areas.     

Bryophytes display allelopathic interactions with tree species in native forest ecosystems

Bryophytes are widespread in terrestrial ecosystems but little is known about their influence on vascular species. Water‐soluble leachates (0%, 1%, 5%, 10% concentration) derived from 18 species of bryophytes (mosses 11 species; liverworts 7 species) were tested on the germination and seedling growth of Lactuca sativa and two common trees Melicytus ramiflorus (Violaceae) and Fuchsia excorticata (Onagraceae) in southern New Zealand forests. Bryophyte water soluble extracts (BWSE) have minor impact on seed germination of Lactuca, stimulatory effects on radical growth at low (1%) concentrations and inhibitory effects at higher concentrations (5–10%). For Melicytus the BWSE had variable effects, with evidence of strong stimulatory (Dendrohypopterygium filiculiforme) and inhibitory (Lepidozia concinna) effects on germination, but generally inhibited radical growth. BWSE at all test concentrations consistently inhibit both germination and seedling radicle growth in Fuchsia. The toxicity effect of water‐soluble leachates varies significantly between bryophyte species but not consistently between mosses and liverworts. Bryophyte species exhibiting strongest inhibition effects under control conditions were associated with significantly reduced densities of broadleaved tree seedlings in forest ecosystems. Our results demonstrate that some bryophyte species via allelopathic interactions can inhibit seedling establishment and growth of forest trees. This mechanism provides an additional factor constraining the spatial distribution of the regeneration niche in forest communities.     

Epiphytic ferns and bryophytes of Tasmanian tree‐ferns: A comparison of diversity and composition between two host species

Abstract Ferns, bryophytes and lichens are the most diverse groups of plants in wet forests in south‐eastern Australia. However, management of this diversity is limited by a lack of ecological knowledge of these groups and the difficulty in identifying species for non‐experts. These problems may be alleviated by the identification and characterization of suitable proxies for this diversity. Epiphytic substrates are potential proxies. To evaluate the significance of some epiphytic substrates, fern and bryophyte assemblages on a common tree‐fern species, Dicksonia antarctica (soft tree‐fern), were compared with those on a rare species, Cyathea cunninghamii (slender tree‐fern), in eastern Tasmania, Australia. A total of 97 fern and bryophyte species were recorded on D. antarctica from 120 trunks at 10 sites, and 64 species on C. cunninghamii from 39 trunks at four of these sites. The trunks of C. cunninghamii generally supported fewer species than D. antarctica, but two mosses (particularly Hymenodon pilifer) and one liverwort showed significant associations with this host. Several other bryophytes and epiphytic ferns showed an affinity for the trunks of D. antarctica. Species assemblages differed significantly between both sites and hosts, and the differences between hosts varied significantly among sites. The exceptionally high epiphytic diversity associated with D. antarctica suggests that it plays an important ecological role in Tasmanian forests. Evidently C. cunninghamii also supports a diverse suite of epiphytes, including at least one specialist species.     

生态恢复方式对喀斯特生境苔藓植物物种组成的影响

为探究生态恢复方式对苔藓植物的影响，研究了自然弃耕和退耕还林等生态恢复方式对重庆市中梁山喀斯特地区不同生境类型下苔藓植物物种组成特征的影响。结果表明，研究区有苔藓植物15科26属41种，其中藓类39种，苔类2种，美灰藓(Eurohypnum leptothollum)为主要优势物种；研究区石生藓类植物占比较大，生活型以平铺型和丛集型为主，各生境物种多样性及相似性指数整体偏低，但相比弃耕杂草地、灌木林和经济型林地，竹林和马尾松林中苔藓植物具有更高的物种多样性；冗余分析表明，郁闭度和光照强度是影响苔藓植物分布的关键环境因子，乔木林为苔藓植物的生长提供了更适宜的生境。因此，相比于自然恢复和经济性林地等恢复方式，退耕还林是喀斯特退化生境更为理想的生态恢复方式。     

Moisture conditions and the presence of bryophytes determine fescue species abundance in a dry calcareous grassland

Festuca ovina is the abundant matrix-forming species and F. rubra a subordinate species in shallow-soil calcareous grasslands. F. pratensis is a transient species, occurring sparsely in this community. We hypothesised that the different abundances of these three species are primarily due to the differential effect of moisture conditions on their germination and early establishment, and that the effect of the pattern of rainfall intensity depends on the presence or absence of a bryophyte layer. We studied the dependence of the germination and establishment of the three fescue species on the moisture conditions both in the laboratory and in the patches of intact grassland community (microcosms). In a laboratory germination experiment, F. pratensis showed the highest, F. rubra , the intermediate and F. ovina, the lowest drought tolerance. In microcosms, the establishment of F. ovina was the highest. At the same time, the annual mortality of seedlings of F. ovina was the lowest. All three species responded positively to an increasing irrigation level. Differently from F. ovina, F. rubra showed a positive response only in plots from which the bryophyte layer had been removed, while F. pratensis responded positively to both irrigation and bryophyte removal. We conclude that moisture conditions have a differential effect on the three fescue species mainly in the seedling establishment, not in the germination phase. For the successful establishment of F. rubra and F. pratensis, the coincidence of high rainfall and local disturbance, removing bryophytes, is required. The presence or absence of bryophytes had no effect on establishment in dry years, while in rainy years the removal of bryophytes has a clear positive effect.     

辽东山地古石河冰缘地貌森林生态系统苔藓物种多样性研究

基于9个20 m×30 m森林群落样地的调查数据,采用物种丰富度、α和β多样性指数,对辽东山地古石河冰缘地貌不同林型石生、树生苔藓植物物种多样性进行定量研究,采用皮尔逊相关分析方法对其影响因素进行分析。结果显示,古石河冰缘地貌苔藓植物共有26科46属59种;不同林型石生、树生苔藓植物物种丰富度和α多样性指数均为:暗针叶林针阔混交林落叶阔叶林;石生苔藓植物β多样性指数最高为落叶阔叶林-针阔混交林间(0.44),最低为落叶阔叶林-暗针叶林间(0.33);树生苔藓植物β多样性指数最高为针阔混交林-暗针叶林间(0.40),最低为落叶阔叶林-暗针叶林间(0.25);分析表明,林冠层郁闭度、海拔高度是影响辽东山地古石河冰缘地貌森林生态系统苔藓物种多样性的重要因子。     

Response of epiphytic bryophytes to simulated N deposition in a subtropical montane cloud forest in southwestern China

A field manipulation experiment was conducted in a subtropical montane cloud forest in southwestern China to determine the possible responses of epiphytic bryophytes to increasing nitrogen (N) deposition from community to physiology level, and to find sensitive epiphytic bryophytes that may be used as indicators for assessing the degree of N pollution. N addition had significantly negative effects on species richness and cover of the epiphytic bryophyte community. Harmful effects of high N loads were recorded for chlorophyll, growth, and vitality of the species tested. The decline of some epiphytic bryophytes may result from detrimental effects on degradation to photosynthetic pigments. Bazzania himalayana (Mitt.) Schiffn., Bazzania ovistipula (Steph.) Mizut., and Homaliodendron flabellatum (Sm.) Fleisch. are candidates in atmospheric nitrogen monitoring. Epiphytic bryophytes in the montane cloud forest are very sensitive to increasing N deposition and often difficult to recover once they have been destroyed, providing early detection of enhanced N pollution for trees or even the whole forest ecosystem. The inference that increasing N pollution may lead to loss of biodiversity is a concern to the developing economy in western China, and should alert the government to the adverse impacts caused by increased industrial pollution during the process of China’s West Development.     

Vascular Plants Facilitated Bryophytes in a Grassland Experiment

In grassland communities vascular plants and bryophytes form two distinct layers. In order to understand the factors responsible for plant community structure, more information about interactions between these plant groups is needed. Often negative correlations between vascular plant and bryophyte covers have been reported, suggesting competition. Here we tested experimentally whether different grassland vascular plant species (Trifolium pratense, Festuca pratensis, Prunella vulgaris) had different influences on the cover of two bryophyte species (Rhytidiadelphus squarrosus, Brachythecium rutabulum). In a two-year garden pot experiment one bryophyte species and one vascular plant species were planted per pot. Bryophytes were planted at a constant density, vascular plants in four densities. The cover of both bryophyte species increased with increasing vascular plant cover, showing the facilitative effect of vascular plants through creating better microclimate, e.g., optimising temperature. Bryophyte responses to vascular plant species were species-specific. Festuca had significantly positive effects on both bryophyte species in the second year, and Trifolium on Brachythecium in both years, whereas Prunella had no significant effect on bryophytes. The facilitative effect of vascular plants was stronger at the second experimental year. In summary, the biotic effects between bryophytes and grassland vascular plants are species-specific and positive interactions are prevailing at low vascular plant densities.     

Effects of environmental heterogeneity on species diversity and composition of terrestrial bryophyte assemblages in tropical montane forests of southern Ecuador

Background: Most studies on tropical bryophytes deal with epiphytic species. This is the first ecological study of tropical forests that focuses specifically on terrestrial bryophytes.

Aim: To investigate the differences between slope and ridge environments in upper montane forests of southern Ecuador in terms of species diversity (richness, abundance), species composition and life forms of terrestrial bryophytes.

Methods: We used Non-metric Multidimensional Scaling (NMDS) to group bryophyte relevés by study location, habitat type and exposure class. Species indicator values were calculated and compared for different habitats.

Results: In total, 140 species were recorded, the majority being liverworts. NMDS analyses and Mantel correlations clearly separated between slope and ridge relevés, and between sunny and shaded microhabitats on ridges. Bryophyte life forms also showed different distribution patterns in slope and in ridge habitats. Mosses were more prominent in sunny than in shaded microhabitats.

Conclusions: Environmental differentiation between ridges and slopes, and small-scale variation in microclimatic conditions caused by differences in exposure, were stronger predictors of species richness and composition than geographical distance between study sites.     

Tree hollows can affect epiphyte species composition

Tree hollows often harbor animals and microorganisms, thereby storing nutritive resources derived from their biological activities. The outflows from tree hollows can create unique microenvironments, which may affect communities of epiphytic organisms on trunk surfaces below the hollows. In this study, we tested whether the species richness and composition of epiphytic bryophytes (liverworts and mosses) and lichens differ above and below tree hollows of Aria japonica and Cercidiphyllum japonicum in a Japanese temperate forest. The species richness of epiphytic bryophytes and lichens did not differ above and below hollows; however, the species composition of bryophytes differed significantly above and below hollows. Indicator species analyses showed that the moss species Anomodon tristis and the liverwort species Porella vernicosa were significantly more common below than above hollows, while the liverwort species Radula japonica and four lichen species, including Leptogium cyanescens, occurred more frequently above than below hollows. Our results highlight that tree hollows can produce unique microenvironments on trunk surfaces that potentially contribute to the maintenance of epiphytic diversity on a local scale.     

武汉市城区苔藓植物多样性和分布及与环境因子的关系

对武汉市7个中心城区和部分远郊城区共26个样点73个样地的苔藓群落进行调查,采集苔藓植物样本共431份。经鉴定调查区内共有苔藓植物20科35属91种,其中狭叶小羽藓（Haplocladium angustifolium（Hampe et C.Muell.）Broth.）和钝叶绢藓（Entodon obtusatus Broth.）等为优势种,分布于其中的24个样点。多样性指数分析结果显示,位于青龙山国家森林公园的α多样性Patrick和Shannon-Wiener指数最高,说明该样点苔藓植物群落的复杂程度高,群落所含信息量大。Patrick指数与环境因子的Pearson相关性分析结果显示,土壤含水率和与主要干道距离是显著影响地面生苔藓植物多样性的环境因子,而显著影响树附生苔藓植物多样性的环境因子仅与距主要干道距离相关。用典范对应分析法（CCA）研究26个样点中苔藓植物的盖度与主要环境因子的关系,结果发现人为干扰程度、草本盖度、与城市主要干道的距离等对地面生和树附生苔藓植物的分布都有显著影响。其中地面生苔藓植物的分布受人为干扰和草本盖度影响最为显著;而树附生苔藓植物的分布受附生树干胸径及与主要干道距离影响最为显著。研究结果表明武汉市区苔藓植物的多样性和分布受人为干扰较大,树附生苔藓对汽车尾气等因素更为敏感。