• There are two theories about how allocation of metabolic products occurs. The allometric biomass partitioning theory (APT) suggests that all plants follow common allometric scaling rules. The ...optimal partitioning theory (OPT) predicts that plants allocate more biomass to the organ capturing the most limiting resource.
• Whole-plant harvests of mature and juvenile tropical deciduous trees, evergreen trees, and lianas and model simulations were used to address the following knowledge gaps: (1) Do mature lianas comply with the APT scaling laws or do they invest less biomass in stems compared to trees? (2) Do juveniles follow the same allocation patterns as mature individuals? (3) Is either leaf phenology or life form a predictor of rooting depth?
• It was found that: (1) mature lianas followed the same allometric scaling laws as trees; (2) juveniles and mature individuals do not follow the same allocation patterns; and (3) mature lianas had shallowest coarse roots and evergreen trees had the deepest.
• It was demonstrated that: (1) mature lianas invested proportionally similar biomass to stems as trees and not less, as expected; (2) lianas were not deeper-rooted than trees as had been previously proposed; and (3) evergreen trees had the deepest roots, which is necessary to maintain canopy during simulated dry seasons.
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1. Lianas compete intensely with trees, but few studies have examined long-term effects of liana infestation on tree growth and mortality. We quantified the effects of lianas in tree crowns (n = ...2907) and rooted within 2 m of trees (n = 1086) on growth and mortality of 30 tree species from 1995 to 2005 on Barro Colorado Island (BCI), Panama, documented liana infestation in tree crowns in 1996 and 2007 to determine the dynamics of liana infestation, and quantified liana infestation in the crowns of 3231 additional canopy trees (d.b.h. ≥20 cm) in 2007 to compare with the same metric determined by previous studies in 1967 and 1980. 2. Severe liana infestation increased tree mortality: 21% of liana-free trees in 1996 had died by 2007, whereas 42% of trees with more than 75% of the crown infested by lianas in 1996 had died by 2007. 3. Liana infestation of tree crowns significantly reduced tree growth, particularly on sun-exposed trees. The proximity of rooted lianas significantly reduced the growth of shaded trees. 4. Liana infestation was dynamic: 10.9% of trees with severe liana infestation in their crowns in 1996 had shed all of their lianas by 2007 and 5.3% of trees with no lianas in their crown in 1996 had severe liana infestation in 2007. 5. Liana infestation was common: lianas were present in 53% of trees of the 30 focal species. Including lianas rooted within 2 m of the tree increased this percentage to 78%. Using both above- and below-ground measures may provide a better estimate of liana competition than either measure alone. 6. Liana infestation is increasing on BCI. Lianas were present in the crowns of 73.6% of canopy trees (d.b.h. ≥20 cm). Liana canopy infestation was 57% higher than in 1980 and 65% higher than in 1967, which is consistent with reported increases in liana abundance, biomass, and leaf and flower production. 7.Synthesis. We used one of the largest studies ever conducted on lianas to confirm the negative effects of lianas on tree growth and survival over 10 years. Liana infestation of trees was widespread, dynamic and increasing on BCI.
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There is mounting empirical evidence that lianas affect the carbon cycle of tropical forests. However, no single vegetation model takes into account this growth form, although such efforts could ...greatly improve the predictions of carbon dynamics in tropical forests. In this study, we incorporated a novel mechanistic representation of lianas in a dynamic global vegetation model (the Ecosystem Demography Model). We developed a liana‐specific plant functional type and mechanisms representing liana–tree interactions (such as light competition, liana‐specific allometries, and attachment to host trees) and parameterized them according to a comprehensive literature meta‐analysis. We tested the model for an old‐growth forest (Paracou, French Guiana) and a secondary forest (Gigante Peninsula, Panama). The resulting model simulations captured many features of the two forests characterized by different levels of liana infestation as revealed by a systematic comparison of the model outputs with empirical data, including local census data from forest inventories, eddy flux tower data, and terrestrial laser scanner‐derived forest vertical structure. The inclusion of lianas in the simulations reduced the secondary forest net productivity by up to 0.46 tC ha−1 year−1, which corresponds to a limited relative reduction of 2.6% in comparison with a reference simulation without lianas. However, this resulted in significantly reduced accumulated above‐ground biomass after 70 years of regrowth by up to 20 tC/ha (19% of the reference simulation). Ultimately, the simulated negative impact of lianas on the total biomass was almost completely cancelled out when the forest reached an old‐growth successional stage. Our findings suggest that lianas negatively influence the forest potential carbon sink strength, especially for young, disturbed, liana‐rich sites. In light of the critical role that lianas play in the profound changes currently experienced by tropical forests, this new model provides a robust numerical tool to forecast the impact of lianas on tropical forest carbon sinks.
This is the first dynamic vegetation model that simulates lianas. We predict a strong impact on the carbon stocks, especially for younger (secondary) forests where liana density is higher.
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1. Light is thought to be the most limiting resource in tropical forests, and thus above-ground competition is commonly accepted as the mechanism that structures these communities. In many tropical ...forests, trees compete not only with other trees, but also with lianas, which compete aggressively for below-ground resources and thus may limit tree growth and regeneration. 2. Using a replicated experiment, we tested the relative strengths of above- and below-ground competition from lianas on tree saplings in a disturbed forest in Cote d'Ivoire with a heterogeneous canopy and relatively high light penetration. We planted seedlings of three tree species and subjected them to below-ground competition with lianas (BGC), above- and below-ground competition with lianas (ABGC), or a liana-free control treatment. After 2 years, we harvested the saplings and compared the amount of above-ground biomass and its relative allocation among the three experimental treatments and different tree species. 3. Lianas competed intensely with saplings in this tropical forest, substantially limiting sapling growth. Saplings grown in the ABGC and BGC treatments had only 18.5% and 16.8% of the above-ground dry biomass of those grown in the liana-free control treatment. 4. Sapling biomass did not differ significantly among the ABGC and BGC treatments, suggesting that below-ground competition was the driving force behind liana vs. tree competition in this forest. Above-ground competition with lianas, however, did affect the allocation of biomass in saplings, resulting in shorter, thicker stems and a poorly developed crown. 5. Collectively, our findings suggest that below-ground competition with lianas plays a substantial role in limiting the growth of saplings in disturbed and secondary tropical forests, and above-ground effects may be due to a combination of above-ground competition and mechanical stress. 6. Disentangling above- and below-ground competition between lianas and trees is critical for a comprehensive understanding of the dynamics of naturally regenerating tropical forests, as well as formulating successful management plans for sustainable timber harvest.
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Lianas in tropical forests compete intensely with trees for above- and belowground resources and limit tree growth and regeneration. Liana competition with adult canopy trees may be particularly ...strong, and, if lianas compete more intensely with some tree species than others, they may influence tree species composition. We performed the first systematic, large-scale liana removal experiment to assess the competitive effects of lianas on multiple tropical tree species by measuring sap velocity and growth in a lowland tropical forest in Panama. Tree sap velocity increased 60% soon after liana removal compared to control trees, and tree diameter growth increased 25% after one year. Although tree species varied in their response to lianas, this variation was not significant, suggesting that lianas competed similarly with all tree species examined. The effect of lianas on tree sap velocity was particularly strong during the dry season, when soil moisture was low, suggesting that lianas compete intensely with trees for water. Under the predicted global change scenario of increased temperature and drought intensity, competition from lianas may become more prevalent in seasonal tropical forests, which, according to our data, should have a negative effect on most tropical tree species.
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The seasonal growth advantage hypothesis posits that plant species that grow well during seasonal drought will increase in abundance in forests with increasing seasonality of rainfall both in ...absolute numbers and also relative to co-occurring plant species that grow poorly during seasonal drought. That is, seasonal drought will give some plant species a growth advantage that they lack in aseasonal forests, thus allowing them attain higher abundance. For tropical forest plants, the seasonal growth advantage hypothesis may explain the distribution of drought-adapted species across large-scale gradients of rainfall and seasonality. We tested the seasonal growth advantage hypothesis with lianas and trees in a seasonal tropical forest in central Panama. We measured the dry-season and wet-season diameter growth of 1,117 canopy trees and 648 canopy lianas from 2011 to 2016. We also evaluated how lianas and trees responded to the 2015–2016 El Niño, which was the third strongest el Niño drought on record in Panama. We found that liana growth rate was considerably higher during the dry-season months than the wet-season months in each of the five years. Lianas achieved one-half of their annual growth during the 4-month dry season. By contrast, trees grew far more during the wet season; they realized only one-quarter of their annual growth during the dry season. During the strong 2015–2016 El Niño dry season, trees essentially stopped growing, whereas lianas grew unimpeded and as well as during any of the previous four dry seasons. Our findings support the hypothesis that seasonal growth gives lianas a decided growth advantage over trees in seasonal forests compared to aseasonal forests, and may explain why lianas peak in both absolute and relative abundance in highly seasonal tropical forests. Furthermore, the ability of lianas to grow during a strong el Niño drought suggests that lianas will benefit from the predicted increasing drought severity, whereas trees will suffer, and thus lianas are predicted to increase in relative abundance in seasonal tropical forests.
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The positive relationship between species diversity (richness and evenness) and critical ecosystem functions, such as productivity, carbon storage, and nutrient cycling, is often used to predict the ...consequences of extinction. At regional scales, however, plant species richness is mostly increasing rather than decreasing because successful plant species introductions far outnumber extinctions. If these regional increases in richness lead to local increases in diversity, a reasonable prediction is that productivity, carbon storage, and nutrient cycling will increase following invasion, yet this prediction has rarely been tested empirically. We tested this prediction in novel forest communities dominated by introduced species (∼90% basal area) in lowland Hawaiian rain forests by comparing their functionality to that of native forests. We conducted our comparison along a natural gradient of increasing nitrogen availability, allowing for a more detailed examination of the role of plant functional trait differences (specifically, N
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fixation) in driving possible changes to ecosystem function. Hawaii is emblematic of regional patterns of species change; it has much higher regional plant richness than it did historically, due to >1000 plant species introductions and only ∼71 known plant extinctions, resulting in an ∼100% increase in richness. At local scales, we found that novel forests had significantly higher tree species richness and higher diversity of dominant tree species. We further found that aboveground biomass, productivity, nutrient turnover (as measured by soil-available and litter-cycled nitrogen and phosphorus), and belowground carbon storage either did not differ significantly or were significantly greater in novel relative to native forests. We found that the addition of introduced N
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-fixing tree species on N-limited substrates had the strongest effect on ecosystem function, a pattern found by previous empirical tests. Our results support empirical predictions of the functional effects of diversity, but they also suggest basic ecosystem processes will continue even after dramatic losses of native species diversity if simple functional roles are provided by introduced species. Because large portions of the Earth's surface are undergoing similar transitions from native to novel ecosystems, our results are likely to be broadly applicable.
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In tropical forest canopies, it is critical for upper shoots to efficiently provide water to leaves for physiological function while safely preventing loss of hydraulic conductivity due to cavitation ...during periods of soil water deficit or high evaporative demand. We compared hydraulic physiology of upper canopy trees and lianas in a seasonally dry tropical forest to test whether trade-offs between safety and efficiency of water transport shape differences in hydraulic function between these two major tropical woody growth forms. We found that lianas showed greater maximum stem-specific hydraulic conductivity than trees, but lost hydraulic conductivity at less negative water potentials than trees, resulting in a negative correlation and trade-off between safety and efficiency of water transport. Lianas also exhibited greater diurnal changes in leaf water potential than trees. The magnitude of diurnal water potential change was negatively correlated with sapwood capacitance, indicating that lianas are highly reliant on conducting capability to maintain leaf water status, whereas trees relied more on stored water in stems to maintain leaf water status. Leaf nitrogen concentration was related to maximum leaf-specific hydraulic conductivity only for lianas suggesting that greater water transport capacity is more tied to leaf processes in lianas compared to trees. Our results are consistent with a trade-off between safety and efficiency of water transport and may have implications for increasing liana abundance in neotropical forests.
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1. Plants compete with their neighbours for a finite set of limiting resources, and this decreases individual plant performance, growth and survival. However, neighbouring plants also affect each ...other in positive ways. 2. Positive facilitative effects can occur when neighbouring plants ameliorate harsh abiotic conditions (temperature, wind and high irradiation). Thus, when environmental conditions are severe, the importance of facilitation may increase. The co-occurrence and masking effects of competition and facilitation among neighbouring plants have made it difficult to tease them apart in the past. 3. We planted bur oak acorns (Quercus macrocarpa) into an experimental diversity gradient in a central MN grassland that provided a gradient in plant biomass. We predicted that greater biomass of neighbours would increase both competition and facilitation as measured by impacts on the minimum leaf water potential reached on any given day. Under moderate conditions, competition should predominate, but under hot/dry conditions, facilitation should become more important. We measured temperature, humidity and soil moisture in these plots for two growing seasons, as well as oak seedling leaf water potential across a range of daily conditions. 4. On cool/humid days, plant interactions were dominated by competition for soil water: leaf water potentials of juvenile oaks were lower in plots with greater herbaceous biomass (and higher diversity). Conversely, on hot/dry days, facilitation of the microclimate determined the net effect of plants on their neighbours: leaf water potentials of juvenile oaks were higher in plots with higher herbaceous diversity and biomass. 5. Synthesis. In terms of plant water status, plant interactions among neighbours can flip from net negative (competition) to net positive (facilitation) depending on daily abiotic conditions. The relative importance of both positive and negative interactions for plant water status may affect the overall performance of plants over time.
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Hydraulic traits are important for woody plant functioning and distribution. Associations among hydraulic traits, other leaf and stem traits, and species’ performance are relatively well understood ...for trees, but remain poorly studied for lianas. We evaluated the coordination among hydraulic efficiency (i.e., maximum hydraulic conductivity), hydraulic safety (i.e., cavitation resistance), a suite of eight morphological and physiological traits, and species’ abundances for saplings of 24 liana species and 27 tree species in wet tropical forests in Panama. Trees showed a strong trade-off between hydraulic efficiency and hydraulic safety, whereas efficiency and safety were decoupled in lianas. Hydraulic efficiency was strongly and similarly correlated with acquisitive traits for lianas and trees (e.g., positively with gas exchange rates and negatively with wood density). Hydraulic safety, however, showed no correlations with other traits in lianas, but with several in trees (e.g., positively with leaf dry matter content and wood density and negatively with gas exchange rates), indicating that in lianas hydraulic efficiency is an anchor trait because it is correlated with many other traits, while in trees both efficiency and safety are anchor traits. Traits related to shade tolerance (e.g., low specific leaf area and high wood density) were associated with high local tree sapling abundance, but not with liana abundance. Our results suggest that different, yet unknown mechanisms determine hydraulic safety and local-scale abundance for lianas compared to trees. For trees, the trade-off between efficiency and safety will provide less possibilities for ecological strategies. For lianas, however, the uncoupling of efficiency and safety could allow them to have high hydraulic efficiency, and hence high growth rates, without compromising resistance to cavitation under drought, thus allowing them to thrive and outperform trees under drier conditions.
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