Effects of clonal fragmentation on Pyrrosia nuda depend on growth stages in a rubber plantation.

Introduction: Clonal fragmentation helps to assess clonal plants' growth resilience to human and environmental disturbance. Although clonal integration in epiphytes in tropical rubber plantations is important to understand their role in enhancing biodiversity and ecosystem services, research on this subject is limited. These plantations are typically monospecific economic forests that face increased anthropogenic disturbances. Methods: In this study, we selected the clonal fern Pyrrosia nuda to study its survival status, biomass, maximum quantum yield of photosystem II (Fv/Fm), and frond length in response to the level of clonal fragmentation in a tropical rubber plantation. Results and discussion: The results showed that (1) clonal fragmentation significantly negatively affected the survival rate, biomass, and frond length of clonal plants, but with minimal effects on Fv/Fm at different growth stages; (2) the performance of a ramet (e.g., biomass or frond length) increased with ramet developmental ages and decreased with the number of ramets in a clonal fragment. The age-dependent impacts of clonal fragmentation provide insights into the biodiversity conservation of epiphytes and forest management in man-made plantations. Therefore, to better conserve the biodiversity in tropical forests, especially in environment-friendly rubber plantations, there is a need to reduce anthropogenic disturbances and alleviate the level of fragmentation.

Effects of topography and historical disturbance on canopy height structure of tropical forests in Menglun, Xishuangbanna, China.

With the combination of airborne Lidar and panchromatic images in 1981 and 2021, we investigated the canopy height structure of tropical forests in Menglun sub-reserve in the Xishuangbanna National Nature Reserve of Yunnan Province, and analyzed its relationship with environmental factors by using multiple regression tree (MRT) method. The results showed that forests in the Menglun sub-reserve could be clustered into seven types based on canopy height structures, with tropical rainforest, monsoon evergreen broad-leaved forest, secondary forest, and flood plain forest as the main types. The potential solar radiation, altitude, terrain profile curvature, slope and the brightness value of imageries in 1981 and 2021 were main factors that drove the classification. The tropical seasonal rainforest dominated by Pometia pinnata occupied the largest area in valley and low-land. The monsoon evergreen broad-leaved forest dominated by Castanopsis echinocarpa mainly distributed in the ridge and disturbed areas. The secondary forests had homogeneous canopy surface, which was significantly different from the primary forests. The activities of swidden agriculture about three decades ago had legacy impacts on the physiognomy of secondary forests.

Hemiepiphytic figs kill their host trees: acquiring phosphorus is a driving factor.

Hemiepiphytic figs killing their host trees is an ecological process unique to the tropics. Yet the benefits and adaptive strategies of their special life history remain poorly understood. We compared leaf phosphorus (P) content data of figs and palms worldwide, and functional traits and substrate P content of hemiepiphytic figs (Ficus tinctoria), their host palm and nonhemiepiphytic conspecifics at different growth stages in a common garden. We found that leaf P content of hemiepiphytic figs and their host palms significantly decreased when they were competing for soil resources, but that of hemiepiphytic figs recovered after host death. P availability in the canopy humus and soil decreased significantly with the growth of hemiepiphytic figs. Functional trait trade-offs of hemiepiphytic figs enabled them to adapt to the P shortage while competing with their hosts. From the common garden to a global scale, the P competition caused by high P demand of figs may be a general phenomenon. Our results suggest that P competition is an important factor causing host death, except for mechanically damaging and shading hosts. Killing hosts benefits hemiepiphytic figs by reducing interspecific P competition and better acquiring P resources in the P-deficient tropics, thereby linking the life history strategy of hemiepiphytic figs to the widespread P shortage in tropical soils.

Contamination and source-specific risk analysis of soil heavy metals in a typical coal industrial city, central China.

Source-specific risk apportionment for soil heavy metals (HMs) is crucial for pollution mitigation and risk control in coal-mining areas. The ecological and human health risks resulting from different sources were evaluated through an integrated method that combines risk assessments with positive matrix factorization (PMF) model. Thirty soil samples were collected from a typical coal-mining city in central China and analyzed for six HMs (Cu, Ni, Pb, Cd, As, and Hg). The results indicate that surface soil in the study area suffered from moderate HMs pollution, especially pollution by Cd and Hg. Four potential sources of soil HMs were identified and quantified in the study area, including natural source (27.7%), traffic emissions (33.4%), agricultural practices (16.2%), and industrial activities (22.7%). The ecological risk of the study area was at moderate level, and the leading contributions in urban and suburban areas were from industrial activities and agricultural practices, respectively. The non-carcinogenic risks for adults and children were lower than the risk threshold, while the carcinogenic risks ranged between 1E-06 and 1E-04, suggesting that carcinogenic risks and hazards to human health should not be neglected. Traffic emissions and natural sources mainly contributed to the non-carcinogenic and carcinogenic risks, due to the strong non-carcinogenicity and carcinogenicity of As and Ni. These findings highlight the ecological and health risks linked to potential sources of soil HMs contamination and provide valuable information on the reduction of corresponding risks for local environmental managers.

When facilitation meets clonal integration in forest canopies.

Few studies have explored how - within the same system - clonality and positive plant-plant interactions might interact to regulate plant community composition. Canopy-dwelling epiphytes in species-rich forests provide an ideal system for studying this because many epiphytic vascular plants undertake clonal growth and because vascular epiphytes colonize canopy habitats after the formation of nonvascular epiphyte (i.e. bryophyte and lichen) mats. We investigated how clonal integration of seven dominant vascular epiphytes influenced inter-specific interactions between vascular epiphytes and nonvascular epiphytes in a subtropical montane moist forest in southwest China. Both clonal integration and environmental buffering from nonvascular epiphytes increased survival and growth of vascular epiphytes. The benefits of clonal integration for vascular epiphytes were higher when nonvascular epiphytes were removed. Similarly, facilitation from nonvascular epiphytes played a more important role when clonal integration of vascular epiphytes was eliminated. Overall, clonal integration had greater benefits than inter-specific facilitation. This study provides novel evidence for interactive effects of clonality and facilitation between vascular and nonvascular species, and has implications for our understanding of a wide range of ecosystems where both high levels of clonality and facilitation are expected to occur.

Ecological stoichiometry of the epiphyte community in a subtropical forest canopy.

Epiphytes in tree canopies make a considerable contribution to the species diversity, aboveground biomass, and nutrient pools in forest ecosystems. However, the nutrient status of epiphytes and their possible adaptations to nutrient deficiencies in the forest canopy remain unclear. Therefore, we analyzed the stoichiometry of five macroelements (C, N, P, K, and Ca) in four taxonomic groups (lichens, bryophytes, ferns, and spermatophytes) to investigate this issue in a subtropical montane moist evergreen broad-leaved forest in Southwest China. We found that the interspecific variations in element concentrations and mass ratios were generally greater than the intraspecific variations. And there were significant stoichiometric differences among functional groups. Allometric relationships between N and P across the epiphyte community indicated that P might be in greater demand than N with an increase in nutrients. Although canopy nutrients were deficient, most epiphytes could still maintain high N and P concentrations and low N:P ratios. Moreover, ferns and spermatophytes allocated more limited nutrients to leaves than to stems and roots. To alleviate frequent drought stress in the forest canopy, vascular epiphytes maintained several times higher K concentrations in their leaves than in the tissues of lichens and bryophytes. Our results suggest that epiphytes may have evolved specific nutrient characteristics and adaptations, so that they can distribute in heterogeneous canopy habitats and maintain the stability of nutrient metabolism.

Epiphytic bryophytes as bio-indicators of atmospheric nitrogen deposition in a subtropical montane cloud forest: Response patterns, mechanism, and critical load.

Increasing trends of atmospheric nitrogen (N) deposition due to pollution and land-use changes are dramatically altering global biogeochemical cycles. Bryophytes, which are extremely vulnerable to N deposition, often play essential roles in these cycles by contributing to large nutrient pools in boreal and montane forest ecosystems. To interpret the sensitivity of epiphytic bryophytes for N deposition and to determine their critical load (CL) in a subtropical montane cloud forest, community-level, physiological and chemical responses of epiphytic bryophytes were tested in a 2-year field experiment of N additions. The results showed a significant decrease in the cover of the bryophyte communities at an N addition level of 7.4 kg ha-1 yr-1, which is consistent with declines in the biomass production, vitality, and net photosynthetic rate responses of two dominant bryophyte species. Given the background N deposition rate of 10.5 kg ha-1yr-1 for the study site, a CL of N deposition is therefore estimated as ca. 18 kg N ha-1 yr-1. A disordered cellular carbon (C) metabolism, including photosynthesis inhibition and ensuing chlorophyll degradation, due to the leakage of magnesium and potassium and corresponding downstream effects, along with direct toxic effects of excessive N additions is suggested as the main mechanism driving the decline of epiphytic bryophytes. Our results confirmed the process of C metabolism and the chemical stability of epiphytic bryophytes are strongly influenced by N addition levels; when coupled to the strong correlations found with the loss of bryophytes, this study provides important and timely evidence on the response mechanisms of bryophytes in an increasingly N-polluted world. In addition, this study underlines a general decline in community heterogeneity and biomass production of epiphytic bryophytes induced by increasing N deposition.

Organic nitrogen uptake is a significant contributor to nitrogen economy of subtropical epiphytic bryophytes.

Without any root contact with the soil, epiphytic bryophytes must experience and explore poor, patchy, and heterogeneous habitats; while, the nitrogen (N) uptake and use strategies of these organisms remain uncharacterized, which obscures their roles in the N cycle. To investigate the N sources, N preferences, and responses to enhanced N deposition in epiphytic bryophytes, we carried out an in situ manipulation experiment via the (15)N labelling technique in an Asian cloud forest. Epiphytic bryophytes obtained more N from air deposition than from the bark, but the contribution of N from the bark was non-negligible. Glycine accounted for 28.4% to 44.5% of the total N in bryophyte tissue, which implies that organic N might serve as an important N source. Increased N deposition increased the total N uptake, but did not alter the N preference of the epiphytic bryophytes. This study provides sound evidence that epiphytic bryophytes could take up N from the bark and wet deposition in both organic and inorganic N forms. It is thus important to consider organic N and bark N sources, which were usually neglected, when estimating the role of epiphytic bryophytes in N cycling and the impacts of N deposition on epiphytic bryophytes in cloud forests.

Survival and Growth of Epiphytic Ferns Depend on Resource Sharing.

Locally available resources can be shared within clonal plant systems through physiological integration, thus enhancing their survival and growth. Most epiphytes exhibit clonal growth habit, but few studies have tested effects of physiological integration (resource sharing) on survival and growth of epiphytes and whether such effects vary with species. We conducted two experiments, one on individuals (single ramets) and another on groups (several ramets within a plot), with severed and intact rhizome treatments (without and with physiological integration) on two dominant epiphytic ferns (Polypodiodes subamoena and Lepisorus scolopendrium) in a subtropical montane moist forest in Southwest China. Rhizome severing (preventing integration) significantly reduced ramet survival in the individual experiment and number of surviving ramets in the group experiment, and it also decreased biomass of both species in both experiments. However, the magnitude of such integration effects did not vary significantly between the two species. We conclude that resource sharing may be a general strategy for clonal epiphytes to adapt to forest canopies where resources are limited and heterogeneously distributed in space and time.

Higher clonal integration in the facultative epiphytic fern Selliguea griffithiana growing in the forest canopy compared with the forest understorey.

BACKGROUND AND AIMS: The advantage of clonal integration (resource sharing between connected ramets of clonal plants) varies and a higher degree of integration is expected in more stressful and/or more heterogeneous habitats. Clonal facultative epiphytes occur in both forest canopies (epiphytic habitats) and forest understories (terrestrial habitats). Because environmental conditions, especially water and nutrients, are more stressful and heterogeneous in the canopy than in the understorey, this study hypothesizes that clonal integration is more important for facultative epiphytes in epiphytic habitats than in terrestrial habitats. METHODS: In a field experiment, an examination was made of the effects of rhizome connection (connected vs. disconnected, i.e. with vs. without clonal integration) on survival and growth of single ramets, both young and old, of the facultative epiphytic rhizomatous fern Selliguea griffithiana (Polypodiaceae) in both epiphytic and terrestrial habitats. In another field experiment, the effects of rhizome connection on performance of ramets were tested in small (10 × 10 cm(2)) and large (20 × 20 cm(2)) plots in both epiphytic and terrestrial habitats. KEY RESULTS: Rhizome disconnection significantly decreased survival and growth of S. griffithiana in both experiments. The effects of rhizome disconnection on survival of single ramets and on ramet number and growth in plots were greater in epiphytic habitats than in terrestrial habitats. CONCLUSIONS: Clonal integration contributes greatly to performance of facultative epiphytic ferns, and the effects were more important in forest canopies than in forest understories. The results therefore support the hypothesis that natural selection favours genotypes with a higher degree of integration in more stressful and heterogeneous environments.

Water relations and gas exchange of fan bryophytes and their adaptations to microhabitats in an Asian subtropical montane cloud forest.

Fan life forms are bryophytes with shoots rising from vertical substratum that branch repeatedly in the horizontal plane to form flattened photosynthetic surfaces, which are well suited for intercepting water from moving air. However, detailed water relations, gas exchange characteristics of fan bryophytes and their adaptations to particular microhabitats remain poorly understood. In this study, we measured and analyzed microclimatic data, as well as water release curves, pressure-volume relationships and photosynthetic water and light response curves for three common fan bryophytes in an Asian subtropical montane cloud forest (SMCF). Results demonstrate high relative humidity but low light levels and temperatures in the understory, and a strong effect of fog on water availability for bryophytes in the SMCF. The facts that fan bryophytes in dry air lose most of their free water within 1 h, and a strong dependence of net photosynthesis rates on water content, imply that the transition from a hydrated, photosynthetically active state to a dry, inactive state is rapid. In addition, fan bryophytes developed relatively high cell wall elasticity and the osmoregulatory capacity to tolerate desiccation. These fan bryophytes had low light saturation and compensation point of photosynthesis, indicating shade tolerance. It is likely that fan bryophytes can flourish on tree trunks in the SMCF because of substantial annual precipitation, average relative humidity, and frequent and persistent fog, which can provide continual water sources for them to intercept. Nevertheless, the low water retention capacity and strong dependence of net photosynthesis on water content of fan bryophytes indicate a high risk of unbalanced carbon budget if the frequency and severity of drought increase in the future as predicted.

[Seasonal variation of soil respiration and its components in tropical rain forest and rubber plantation in Xishuangbanna, Yunnan].

By using trenching method and infrared gas analyzer, this paper studied the seasonal variation of soil respiration (SR), including root respiration (RR) and heterotrophic respiration (HR), in tropical seasonal rain forest (RF) and rubber (Hevea brasiliensis) plantation (RP) in Xishuangbanna of Yunnan, China. The results showed that the SR and HR rates were significantly higher in RF than in RP (P < 0.01), while the RR rate had less difference between the two forests. Soil temperature and moisture were the key factors affecting the SR, RR and HR. The SR and HR rates in the two forests were rainy season > dry-hot season > foggy season, but the RR rate was rainy season > foggy season > dry-hot season in RF, and foggy season > rainy season > dry-hot season in RP. The contribution of RR to SR in RF (29%) was much lower than that in RP (42%, P < 0.01), while the contribution of HR to SR was 71% in RF and 58% in RP. When the soil temperature at 5 cm depth varied from 12 degrees C to 32 degrees C, the Q10 values for SR, HR, and RR rates were higher in RF than in RP. HR had the highest Q10 value, while RR had the lowest one.