How Paleozoic vines and lianas got off the ground: On scrambling and climbing carboniferous-early permian pteridosperms

Late Paleozoic pteridosperms displayed various growth habits, including arborescent, leaning, and scrambling and/or climbing forms. This article reviews information gathered to date on vine- and liana-like forms among these plants, based on impression/compression material and cuticle preparations from the Upper Carboniferous and Lower Permian of Europe and North America. Vine- and liana-like pteridosperms used various modes of attachment for both anchorage and support. Such adaptations are very similar (and perhaps analogous) to those that exist in extant angiosperms and include hooks, leaflet tendrils, tendrils terminating in adhesive pads, and aerial adventitious roots. A number of morphological features of scrambling/climbing pteridosperms (e.g., tiny, deeply sunken stomata, marginal water pits, various types of secretory structures, and heterophylly) are considered as they relate to the autecological significance where they may be related to special physiological requirements necessary in the scrambling/climbing growth habit. We hypothesize that scrambling and/or climbing pteridosperms may have played an important role in some of the late Paleozoic coal-swamp forest ecosystems, perhaps even comparable to the role of angiospermous vines/lianas in tropical and subtropical forest ecosystems today.     

Life History Traits of Lianas During Tropical Forest Succession

Tropical secondary forests form an important part of the landscape. Understanding functional traits of species that colonize at different points in succession can provide insight into community assembly. Although studies on functional traits during forest succession have focused on trees, lianas (woody vines) also contribute strongly to forest biomass, species richness, and dynamics. We examined life history traits of lianas in a forest chronosequence in Costa Rica to determine which traits vary consistently over succession. We conducted 0.1 ha vegetation inventories in 30 sites. To examine the establishment of young individuals, we only included small lianas (0.5–1.5 cm diameter at 1.3 m height). For each species, we identified seed size, dispersal mode, climbing mode, and whether or not the seedling is self‐supporting. We found a strong axis of variation determined by seed size and seedling growth habit, with early successional communities dominated by small‐seeded species with abiotic dispersal and climbing seedlings, while large‐seeded, animal‐dispersed species with free‐standing seedlings increased in abundance with stand age. Contrary to previous research and theory, we found a decrease in the abundance of stem twiners and no decrease in the abundance of tendril‐climbers during succession. Seed size appears to be a better indicator of liana successional stage than climbing mode. Liana life history traits change predictably over succession, particularly traits related to seedling establishment. Identifying whether these trait differences persist into the growth strategies of mature lianas is an important research goal, with potential ramifications for understanding the impact of lianas during tropical forest succession.     

Geographical and environmental gradients of lianas and vines in China

Aim Climbing plants are characterized by long, wide vessel elements, which may be vulnerable to cold‐ or drought‐induced embolism. However, the difference in vulnerability between lianas (woody climbing plants) and vines (herbaceous climbing plants) has not yet been reported. Here we hypothesize that both lianas and vines are more sensitive to variations in water and temperature than are self‐supporting plants. Consequently, the proportions of lianas and vines in flora are expected to decline significantly along geographical and environmental gradients. Location China. Methods A unique dataset describing 82 floras in China was examined. The proportion of lianas in the flora (LPF) and the proportion of vines in the flora (VPF) were calculated independently. The proportion of the climbing plants in total spermatophyte flora (CPF) was also calculated. LPF and VPF were compared along latitudinal, mean annual rainfall (Rain), and mean January temperature (T₁) gradients. Local linear regression analyses showed the changing tendencies of LPF and VPF. Prediction models of LPF using geographical and environmental factors were studied in some subranges. Results (1) LPF was highest in the tropics (13.8% on average), decreased linearly with increasing latitude within the latitude < 42 °N subrange, and reached < 1% north of 42 °N in China. VPF fluctuated slightly from tropical (4.7%), to subtropical (4.2%) to warm temperate (4.5%) regions, but declined significantly in temperate (3.2%) and dry (1.5%) zones in China. (2) LPF decreased significantly with decreasing rainfall, and decreased significantly with decreasing T₁ in areas where T₁ > −10 °C. In contrast, VPF tended to be constant in areas where T₁ > −5 °C or Rain > 1000 mm, and declined under extreme water or temperature stresses. (3) Predictions of LPF using Rain and T₁ in areas where T₁ > −10 °C, and using latitude and altitude within the latitude < 42 °N subrange were both reliable. According to the geographical model for LPF and a constant VPF, tropical Asian forests would have a LPF as high as 25.4% and the highest CPF would be c. 30%. Conclusions We conclude that liana diversity is more sensitive to temperature and water availability than that of vines and other plants. Geographical and environmental gradients affected LPF but not VPF. Shorter life spans and underground nutrient storage may be effective strategies adopted by vines to avoid drought and cold stresses.     

Traits and ecological strategies of Australian tropical and temperate climbing plants

Aim Understanding large scale patterns in trait variation in climbing plants (lianas, vines, scramblers, twiners) is important for the development of a stronger theoretical understanding of climbing plant ecology and for more applied issues such as prediction of community assembly under changing climatic conditions. We compared values of five key functional traits for 388 species of climbing plant from tropical and temperate regions of Australia to quantify variation between these two biogeographic regions. Location Australia. Methods Data on dispersal mode, growth habit, leaf form, leaf size and seed mass were compiled from field measurements and published sources. Comparative analyses were performed in three ways: (1) across species where each species was treated as an independent data point, (2) using evolutionary divergence analyses for each trait, and (3) in multidimensional space using a matrix of similarities between species. Results Tropical climbing plants had 22‐fold greater seed mass and four times greater leaf size than did temperate species. Tropical climbers were more likely to be woody (63%) than were temperate species (40%). Surprisingly we found a similar proportion of animal‐dispersed seeds in the two regions, although we expected animal‐dispersed seeds to be more prevalent in the tropics. We also found similar proportions of simple‐ and compound‐leaved species between the two regions. All of our findings were consistent between cross‐species and phylogenetic analyses indicating that patterns in present‐day species are reflected in the evolutionary history of Australian climbers. Multivariate analyses suggested that there is a spectrum of variation among climbing plants, with tropical species having greater seed mass, leaf size and woody growth compared with temperate climbing plant species. Main conclusions Tropical and temperate climbers of Australia exhibit a mixture of similar and contrasting traits and ecological strategies. Understanding strategy variation along latitudinal gradients will be particularly informative for predicting ecosystem and community structure with climate change.     

Short and Long-Term Soil Moisture Effects of Liana Removal in a Seasonally Moist Tropical Forest

Lianas (woody vines) are particularly abundant in tropical forests, and their abundance is increasing in the neotropics. Lianas can compete intensely with trees for above- and belowground resources, including water. As tropical forests experience longer and more intense dry seasons, competition for water is likely to intensify. However, we lack an understanding of how liana abundance affects soil moisture and hence competition with trees for water in tropical forests. To address this critical knowledge gap, we conducted a large-scale liana removal experiment in a seasonal tropical moist forest in central Panama. We monitored shallow and deep soil moisture over the course of three years to assess the effects of lianas in eight 0.64 ha removal plots and eight control plots. Liana removal caused short-term effects in surface soils. Surface soils (10 cm depth) in removal plots dried more slowly during dry periods and accumulated water more slowly after rainfall events. These effects disappeared within four months of the removal treatment. In deeper soils (40 cm depth), liana removal resulted in a multi-year trend towards 5–25% higher soil moisture during the dry seasons with the largest significant effects occurring in the dry season of the third year following treatment. Liana removal did not affect surface soil temperature. Multiple and mutually occurring mechanisms may be responsible for the effects of liana removal on soil moisture, including competition with trees, and altered microclimate, and soil structure. These results indicate that lianas influence hydrologic processes, which may affect tree community dynamics and forest carbon cycling.     

Diversity of climbing seed plants and their reproductive habit in a karst seasonal rain forest in Nonggang,Guangxi, China

Aims Diversity of climbing seed plants and their reproductive habits and characteristics are central for the understanding of community structure and dynamics of forests and hence are important for forest protection. However, little is known about the climbing seed plants in northern tropical karst seasonal rain forests. Here, using the data of the species diversity and reproductive habits of climbing seed plants in Nonggang, Guangxi, China, we aim to 1) explore the species diversity and distribution of climbing seed plants in northern tropical karst seasonal rain forests, 2) study the flowering and fruiting phenology and 3) the associations of reproductive characteristics to the environment. Methods Species composition, preferred habitat, flowering time, fruiting time and fruit types of climbing seed plants were surveyed. The seasonality of flowering and fruiting were analyzed by concentration ratio and circular distribution. Climbing seed plants were divided into three groups according to their growth forms and places in spatial forest structure: bush ropes, herbaceous vines and lianas. Monthly flowering ratios, fruiting ratios, fruit types and their ratios in different groups were determined. These relationships of flowering ratio, fruiting ratio, fruit type and its ratio to meteorological factors were investigated using Pearson correlation analysis. Important findings There were a total of 333 species of climbing seed plants in Nonggang karst seasonal rain forest, belonging to 145 genera and 56 families. Bush ropes, herbaceous vines and lianas contained 119, 88 and 126 species, respectively. At species level, herbaceous vines were more abundance in valleys, while bush ropes and lianas were more abundance on slopes. Flowering and fruiting of climbing seed plants occurred seasonally, with flowering peaking in April to September, while fruiting peaking in July to December. The seasonality of flowering and fruiting in bush ropes was weaker than in herbaceous vines and lianas. Flowering ratio was significantly positively correlated with rainfall and air temperature, which suggest that flowering peaks in monsoon season. Peak time for fruiting was about three months later than the peak time of flowering, around the end of monsoon season. The ratio of samara species to all fruiting species in lianas was significantly positively correlated with wind speed, but negatively correlated with rainfall and air temperature. It showed that samara in lianas tended to occur in dry season with high wind speed. In conclusion, species diversity and the seasonal features of reproduction of climbing seed plants in Nonggang karst seasonal rain forest were closely related to the spatial and temporal variations of habitat resources.     

Distribution and abundance of vines along the light gradient in a southern temperate rain forest

Question: Are vines light‐demanding species? Location: Temperate evergreen rain forest of southern Chile (40°39′S, 72°11′W). Methods: In 45 plots of 25 m² distributed in treefall canopy gaps, secondary forest stands and old‐growth forest (15 plots per light environment), all climbing and non‐supported vines were counted and identified to species level, and canopy openness was quantified using hemispherical photographs. Vine abundance and diversity (species richness and Simpson's index) were compared in the three light environments and similarity between vine communities was estimated using Jaccard's similarity coefficient. We also determined the relationship between light niche breadth and local dominance at the species level. Results: In total there were 2510 vine individuals of 14 species. Canopy openness was significantly different in the three light environments. Species richness, diversity, community composition and density of vines were similar in treefall gaps, secondary and old‐growth forest. Of the seven more common vine species, which accounted for 91% of all vines, three had even distribution, two were more abundant in the shaded understorey, and two had higher density in well‐lit sites. Local dominance of vine species and niche breadth were not significantly associated. Conclusions: Our study in a temperate rain forest questions the widespread notion of vines as pioneer‐like species, which may be a consequence of the abundance of some lianas in disturbed sites of tropical forests. Functional arguments are needed to justify a general hypothesis on light requirements of vines, which constitute a vast group of species.     

How the diversity,abundance, size and climbing mechanisms of woody lianas are related to biotic and abiotic factors in a subtropical secondary forest,Taiwan

Lianas are woody vines that play an important role in forest dynamics in tropical and subtropical areas. Their relationship to various biotic and abiotic conditions is, however, not yet wholly clear. We explored how the size, climbing mechanisms, diversity and abundance of woody lianas is related to host plant size, environmental factors and topography. Liana assemblages were examined in twenty 20 × 20 m plots in each of three topographic sites (valley, slope and ridge) in a subtropical secondary forest in southeastern Taiwan. The valley site had the highest abundance and species richness of lianas. The abiotic factors, soil pH and rock cover, were related to different topographic sites. Larger lianas were always found on larger host trees, while smaller lianas were found in smaller trees; no lianas with a DBH greater than 10 cm were found. Significantly more adhesive lianas were found on larger trees whereas twining and leaning-hook lianas were found in smaller trees. In conclusion, this study demonstrates that the species of liana is associated with the size and type of tree growing under different topographic conditions.     

Removal of Nonnative Vines and Post‐Hurricane Recruitment in Tropical Hardwood Forests of Florida1

In hardwood subtropical forests of southern Florida, nonnative vines have been hypothesized to be detrimental, as many species form dense “vine blankets” that shroud the forest. To investigate the effects of nonnative vines in post‐hurricane regeneration, we set up four large (two pairs of 30 X 60 m) study areas in each of three study sites. One of each pair was unmanaged and the other was managed by removal of nonnative plants, predominantly vines. Within these areas, we sampled vegetation in 5 X 5 m plots for stems 2 cm DBH (diameter at breast height) or greater and in 2 X 0.5 m plots for stems of all sizes. For five years, at annual censuses, we tagged and measured stems of vines, trees, shrubs and herbs in these plots. For each 5 X 5 m plot, we estimated percent coverage by individual vine species, using native and nonnative vines as classes. We investigated the hypotheses that: (1) plot coverage, occurrence and recruitment of nonnative vines were greater than that of native vines in unmanaged plots; (2) the management program was effective at reducing cover by nonnative vines; and (3) reduction of cover by nonnative vines improved recruitment of seedlings and saplings of native trees, shrubs, and herbs. In unmanaged plots, nonnative vines recruited more seedlings and had a significantly higher plot‐cover index, but not a higher frequency of occurrence. Management significantly reduced cover by nonnative vines and had a significant overall positive effect on recruitment of seedlings and saplings of native trees, shrubs and herbs. Management also affected the seedling community (which included vines, trees, shrubs, and herbs) in some unanticipated ways, favoring early successional species for a longer period of time. The vine species with the greatest potential to “strangle” gaps were those that rapidly formed dense cover, had shade tolerant seedling recruitment, and were animal‐dispersed. This suite of traits was more common in the nonnative vines than in the native vines. Our results suggest that some vines may alter the spatiotemporal pattern of recruitment sites in a forest ecosystem following a natural disturbance by creating many very shady spots very quickly.     

The importance of frictional interactions in maintaining the stability of the twining habit

The stability of twining vines under gravitational loads suggests an important role for friction. The coefficient of friction, μ, between vine stems and wood is high, often five times greater than between leather and wood, as determined by slip tests on an inclined plane. Stem trichomes function like ratchets to facilitate climbing upward (or to facilitate slipping if the stem is inverted). A mathematical model predicts large masses (kg) must be applied to the base of a twining vine to cause slipping. Vines slip as predicted when μ is low and arc length on the pole is short, and they break before slipping when μ is large or arc length is long. In contrast, twining vines are unstable in compression, collapsing when small masses (<10 g) are hung from the top of the vine. However, if the loads are applied below the uppermost gyre, the stabilizing tensional effect dominates. Therefore, in nature vines twining on a cylindrical support are stable under gravitational loads, unless these loads occur near the apex. A corollary is that a short apical coil can hold up large masses of maturing shoot.     

Leaf damage induces twining in a climbing plant

Successful climbing by vines not only prevents shading by neighbouring vegetation, but also may place the vines beyond ground herbivores. Here we tested the hypothesis that herbivory might enhance climbing in a vine species, and that such induced climbing should be greater in the shade. We assessed field herbivory in climbing and prostrate ramets of the twining vine Convolvulus arvensis. We evaluated plant climbing after mechanical damage in a glasshouse under both sun and shade conditions, and determined whether control and damaged plants differed in growth rate or photosynthetic capacity. Plants experienced greater herbivory when growing prostrate than when climbing onto companion plants, in both an open habitat and a shaded understorey. Experimental plants increased their twining rate on a stake after suffering leaf damage, in both high- and low-light conditions, and this induced climbing was not coupled to an increase in growth rate. Increased photosynthesis was associated with enhanced twining rate only in the shade. Herbivory may be an ecological factor promoting the evolution of a climbing habit in plants.     

Contribution of vines to the evapotranspiration of a secondary forest in eastern Amazonia

The contribution of vines to the evapotranspiration (ET) of a secondary forest in eastern Amazonia was estimated based on field measurements of vine and tree transpiration, and seasonal changes in soil water content to 12 meters depth. Transpiration of vines and trees was measured with sapflow gauges placed around stems or branches. Total ET of the secondary forest was estimated as the sum of rainfall and reductions in soil moisture measured using Time Domain Reflectometry sensors installed in the walls of soil shafts. Our results suggest that vines transpire more than trees with stems of similar diameter, and with similar leaf crown exposure to sunlight. Trees experienced a smaller reduction in transpiration from the wet to the dry season than did vines. During the dry season, vines represented 8% (0.4 mm d^–1) of total secondary forest ET (5.4 mm d^–1), but they represented only 5.5% (0.5 m² ha^–1) of total secondary forest basal area (9.6 m² ha^–1). Considering that transpiration corresponds to 66–90% of forest ET, vines may contribute 9–12% to the transpiration of the forest. Hence, vine cutting, which is a commonly recommended management practice to favor the growth of tropical timber trees, may result in a proportionally larger reduction in evapotranspiration than in forest basal area.     

The differential responses of lianas and vines to rainfall gradients in distribution and abundance in Qinling Mountains, China

To clarify the differential responses of lianas and vines to rainfall, we investigated the distribution and abundance of climbers along the rainfall gradient in the Qinling Mountains of China. In our study, 28 climbing species were investigated across a rainfall gradient of 500?C1,200?mm. Results indicated that most lianas and vines show restricted distribution along the rainfall gradient, which imply their adaptation to different water availabilities. A significant increase in the percentage of the total climbing species with rainfall was observed. The abundance and richness of lianas increased significantly with rainfall, whereas the vines remained constant. This difference implies that lianas have a higher sensitivity to water availability than vines. During drought, lianas may be vulnerable because of the drought-induced embolism, whereas vines are drought-resistant because of their shorter life spans and safer nutrient storage. Further research on the ecophysiological differences between lianas and vines is needed to provide a mechanistic explanation for the patterns described here.     

Long-Term Impacts of Fuelwood Extraction on a Tropical Montane Cloud Forest

Fuelwood extracted from natural forests serves as a principal energy source in rural regions of many tropical countries. Although fuelwood extraction (even low intensities) might strongly impact the structure and species composition of natural forests, long-term studies remain scarce. Here, we estimate the potential long-term impacts (over several hundred years) of such repeated harvesting of single trees on tropical montane cloud forest in central Veracruz, Mexico, by applying a process-based forest growth model. We simulate a wide range of possible harvesting scenarios differing in wood volume harvested and preferred tree species and sizes, and use a set of indicators to compare their impacts on forest size structure and community composition. Results showed that the overall impact on forest structure and community composition increased linearly with the amount of harvested wood volume. Even at low levels of harvesting, forest size structure became more homogeneous in the long term because large old trees disappeared from the forest, but these changes might take decades or even centuries. Although recruitment of harvested species benefited from harvesting, species composition shifted to tree species that are not used for fuelwood. Our results demonstrate that fuelwood extraction can have marked long-term impacts on tropical montane cloud forests. The results also offer the possibility to support the design of management strategies for the natural species-rich forests that achieve a balance between economic needs and ecological goals of the stakeholders. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The Effects of Established Trees on Woody Regeneration during Secondary Succession in Tropical Dry Forests

Understanding the mechanisms controlling secondary succession in tropical dry forests is important for the conservation and restoration of this highly threatened biome. Canopy‐forming trees in tropical forests strongly influence later stages of succession through their effect on woody plant regeneration. In dry forests, this may be complex given the seasonal interplay of water and light limitations. We reviewed observational and experimental studies to assess (1) the relative importance of positive and negative effects of established trees on regeneration; (2) the mechanisms underlying these effects; and (3) to test the ‘stress gradient hypothesis’ in successional tropical dry forests. The effects of established trees on seed dispersal, seed survival, and seed germination—either through direct changes to moisture and temperature regimes or mediated by seed dispersers and predators—are mainly positive. The balance between positive and negative effects on seedling establishment is more complex and depends on the season and leaf phenology of both trees and seedlings. Seedling survival is generally enhanced by established trees mitigating dry conditions. Established trees have counteracting effects on water and light availability that influence seedling growth. The probability of a positive effect of established trees on seedling survival decreases with increased rainfall, which supports the stress gradient hypothesis. Priorities for future research are experiments to test for facilitation and competition and their underlying mechanisms, long‐term studies evaluating how these effects change with ontogeny, and studies focusing on the species‐specificity of interactions.     

Influence of Seasonal Variations in Soil Water Availability on Gas Exchange of Tropical Canopy Trees

Seasonal variations in environmental conditions influence the functioning of the whole ecosystem of tropical rain forests, but as yet little is known about how such variations directly influence the leaf gas exchange and transpiration of individual canopy tree species. We examined the influence of seasonal variations in relative extractable water in the upper soil layers on predawn leaf water potential, saturated net photosynthesis, leaf dark respiration, stomatal conductance, and tree transpiration of 13 tropical rain forest canopy trees (eight species) over 2 yr in French Guiana. The canopies were accessed by climbing ropes attached to the trees and to a tower. Our results indicate that a small proportion of the studied trees were unaffected by soil water depletion during seasonal dry periods, probably thanks to efficient deep root systems. The trees showing decreased tree water status (i.e., predawn leaf water potential) displayed a wide range of leaf gas exchange responses. Some trees strongly regulated photosynthesis and transpiration when relative extractable water decreased drastically. In contrast, other trees showed little variation, thus indicating good adaptation to soil drought conditions. These results have important applications to modeling approaches: indeed, precise evaluation and grouping of these response patterns are required before any tree‐based functional models can efficiently describe the response of tropical rain forest ecosystems to future changes in environmental conditions.     

广西弄岗五桠果叶木姜子群落木质藤本空间格局及其与树种间的关系

木质藤本是生物多样性的重要组成,木质藤本通过影响支持木进而影响群落的结构和功能,但在生物多样性丰富的北热带喀斯特森林中,木质藤本与支持木的关系鲜为人知。以喀斯特季节性雨林的五桠果叶木姜子（Litsea dilleniifolia）群落为研究对象,对木质藤本的密度、分布格局及其与主要树种的关系进行调查研究,分析木质藤本对树木的影响。结果显示：（1）五桠果叶木姜子群落内木质藤本平均密度为0.0913株/m²,木质藤本在0-20m空间尺度整体表现为聚集分布,且随着尺度增大,聚集强度逐渐减弱;不同径级木质藤本在不同尺度上的分布格局不同。（2）木质藤本对不同径级、不同种类、不同聚集强度的支持木选择表现以下体征：随着支持木径级增加,木质藤本攀附的比例和每木藤本数有增加趋势,且木质藤本胸径与支持木胸径呈极显著正相关;附藤率较高的支持木有紫葳科（Bignoniaceae）种类和东京桐（Deutzianthus tonkinensis）,单木附藤数量多的是南方紫金牛（Ardisia thyrsiflora）;物种的聚集强度与附藤率、附藤数量呈负相关。（3）木质藤本的密度与支持木死亡率关系不显著,而物种的附藤率与死亡率呈极显著负相关。以上结果表明,木质藤本密度在原生性喀斯特季节性雨林中并不高,且木质藤本对支持木具有选择性,但其对五桠果叶木姜子群落的死亡率并未产生显著影响。该研究可为喀斯特原生性季节性雨林的物种共存、极小植物种群保育提供理论依据,也可为石漠化区域的植被修复提供科学参考。     

Non‐Native Grass Removal and Shade Increase Soil Moisture and Seedling Performance during Hawaiian Dry Forest Restoration

Invasive non‐native species can create especially problematic restoration barriers in subtropical and tropical dry forests. Native dry forests in Hawaii presently cover less than 10% of their original area. Many sites that historically supported dry forest are now completely dominated by non‐native species, particularly grasses. Within a grass‐dominated site in leeward Hawaii, we explored the mechanisms by which non‐native Pennisetum setaceum, African fountain grass, limits seedlings of native species. We planted 1,800 seedlings of five native trees, three native shrubs, and two native vines into a factorial field experiment to examine the effects of grass removal (bulldozed vs. clipped plus herbicide vs. control), shade (60% shade vs. full sun), and water (supplemental vs. ambient) on seedling survival, growth, and physiology. Both grass removal and shade independently increased survival and growth, as well as soil moisture. Seedling survival and relative growth rate were also significantly dependent on soil moisture. These results suggest that altering soil moisture may be one of the primary mechanisms by which grasses limit native seedlings. Grass removal increased foliar nitrogen content of seedlings, which resulted in an increase in leaf‐level photosynthesis and intrinsic water use efficiency. Thus in the absence of grasses, native species showed increased productivity and resource acquisition. We conclude that the combination of grass removal and shading may be an effective approach to the restoration of degraded tropical dry forests in Hawaii and other ecologically similar ecosystems.     

Modeling the Sensitivity of the Seasonal Cycle of GPP to Dynamic LAI and Soil Depths in Tropical Rainforests

The seasonality of pan-tropical wet forests has been highlighted by recent remote sensing and eddy flux measurements that have recorded both increased and sustained dry-season gross primary productivity (GPP). These observations suggest that wet tropical forests are primarily light limited and that the mechanisms for resilience to drought and projected climate change must be considered in ecosystem model development. Here we investigate two proposed mechanisms for drought resilience of tropical forests, deep soil water access and the seasonality of phenology, using the LPJmL Dynamic Global Vegetation Model. We parameterize a new seasonal phenology module for tropical evergreen trees using remotely sensed leaf area index (LAI) and incoming solar radiation data from the Terra Earth Observing System. Simulations are evaluated along a gradient of dry-season length (DSL) in South America against MODIS GPP estimates. We show that deep soil water access is critical for maintaining dry-season GPP, whereas implementing a seasonal LAI did not enhance simulated dry-season GPP. The Farquhar-Collatz photosynthesis scheme used in LPJmL optimizes leaf nitrogen allocation according to light conditions, causing maximum photosynthetic capacity in the dry season. High LAI, characteristic of tropical forests, also dampens the seasonal amplitude of the fraction of photosynthetically active radiation (FPAR). Given the relatively high uncertainty in tropical phenology observations and their corresponding proximate drivers, we recommend that ecosystem model development focus on belowground processes. An improved representation of soil depths and rooting distributions is necessary for modeling the dynamics of dry-season tropical forest functioning and may have important impacts for modeling tropical forest vulnerability to climate change. Author Contributions  BP conceived of the study, analyzed data, and wrote the paper. UH designed study and contributed new methods. WC designed study and contributed to paper.     

中国热带森林水文生态功能

中国热带森林水文生态功能周光益（中国林科院热带林业研究所,广州５１０５２０）Ｅｃｏ＿ｈｙｄｒｏｌｏｇｉｃａｌＦｕｎｃｔｉｏｎｓｏｆＴｒｏｐｉｃａｌＦｏｒｅｓｔ,Ｃｈｉｎａ．ＺｈｏｕＧｕａｎｇｙｉ（ＴｈｅＲｅｓｅａｒｃｈＩｎｓｔｉｔｕｔｅｏｆＴｒｏｐｉ...