No strong evidence for increasing liana abundance in the Myristicaceae of a Neotropical aseasonal rain forest

Flower production decreases with warmer and more humid atmospheric conditions in a western Amazonian forest

Climate models predict that everwet western Amazonian forests will face warmer and wetter atmospheric conditions, and increased cloud cover. It remains unclear how these changes will impact plant reproductive performance, such as flowering, which plays a central role in sustaining food webs and forest regeneration. Warmer and wetter nights may cause reduced flower production, via increased dark respiration rates or alteration in the reliability of flowering cue-based processes. Additionally, more persistent cloud cover should reduce the amounts of solar irradiance, which could limit flower production. We tested whether interannual variation in flower production has changed in response to fluctuations in irradiance, rainfall, temperature, and relative humidity over 18 yrs in an everwet forest in Ecuador. Analyses of 184 plant species showed that flower production declined as nighttime temperature and relative humidity increased, suggesting that warmer nights and greater atmospheric water saturation negatively impacted reproduction. Species varied in their flowering responses to climatic variables but this variation was not explained by life form or phylogeny. Our results shed light on how plant communities will respond to climatic changes in this everwet region, in which the impacts of these changes have been poorly studied compared with more seasonal Neotropical areas.

Higher water and nutrient use efficiencies in savanna than in rainforest lianas result in no difference in photosynthesis

Differences in traits between lianas and trees in tropical forests have been studied extensively; however, few have compared the ecological strategies of lianas from different habitats. Here, we measured 25 leaf and stem traits concerning leaf anatomy, morphology, physiology and stem hydraulics for 17 liana species from a tropical seasonal rainforest and for 19 liana species from a valley savanna in south-west China. We found that savanna lianas had higher vessel density, wood density and lower hydraulically weighted vessel diameter and theoretical hydraulic conductivity than tropical seasonal rainforest lianas. Compared with tropical seasonal rainforest lianas, savanna lianas also showed higher leaf dry matter content, carbon isotope composition (δ13C), photosynthetic water use efficiency, ratio of nitrogen to phosphorus, photosynthetic phosphorus use efficiency and lower leaf size, stomatal conductance and nitrogen, phosphorus and potassium concentrations. Interestingly, no differences in light-saturated photosynthetic rate were found between savanna and tropical seasonal rainforest lianas either on mass or area basis. This is probably due to the higher water and nutrient use efficiencies of savanna lianas. A principal component analysis revealed that savanna and tropical seasonal rainforest lianas were significantly separated along the first axis, which was strongly associated with acquisitive or conservative resource use strategy. Leaf and stem functional traits were coordinated across lianas, but the coordination or trade-off was stronger in the savanna than in the tropical seasonal rainforest. In conclusion, a relatively conservative (slow) strategy concerning water and nutrient use may benefit the savanna lianas, while higher nutrient and water use efficiencies allow them to maintain similar photosynthesis as tropical seasonal rainforest species. Our results clearly showed divergences in functional traits between lianas from savanna and tropical seasonal rainforest, suggesting that enhanced water and nutrient use efficiencies might contribute to the distribution of lianas in savanna ecosystems.

Liana species decline in Congo basin contrasts with global patterns

Lianas, woody climbing plants, are increasing in many tropical forests, with cascading effects such as decreased forest productivity, carbon sequestration, and resilience. Possible causes are increasing forest fragmentation, CO₂ fertilization, and drought. Determining the primary changing species and their underlying vital rates help explain the liana trends. We monitored over 17,000 liana stems for 13 yr in 20 ha of old‐growth forest in the Congo Basin, and here we report changes and vital rates for the community and for the 87 most abundant species. The total liana abundance declined from 15,007 lianas in 1994 to 11,090 in 2001 to 9,978 in 2007. Over half (52%) of the evaluated species have significantly declining populations, showing that the community response is not the result of changes in a few dominant species only. Species density change (i.e., the change in number of individuals per hectare) decreased with mortality rate, tended to increase with recruitment rate, but was independent of growth rate. Species change was independent of functional characteristics important for plant responses to fragmentation, CO₂, and drought, such as lifetime light requirements, climbing and dispersal mechanism, and leaf size. These results indicate that in Congo lianas do not show the reputed global liana increase, but rather a decline, and that elements of the reputed drivers underlying global liana change do not apply to this DR Congo forest. We suggest warfare in the Congo Basin to have decimated the elephant population, leading to less disturbance, forest closure, and declining liana numbers. Our results imply that, in this tropical forest, local causes (i.e., disturbance) override more global causes of liana change resulting in liana decline, which sharply contrasts with the liana increase observed elsewhere.

Liana dynamics reflect land-use history and hurricane response in a Puerto Rican forest

We studied lianas in a subtropical wet forest in Puerto Rico to understand how hurricane impacts and past human land-uses interact to affect liana dynamics over a 14-year period. We compared a high-intensity land-use area, where the forest that had been cleared, and used for subsistence agriculture before being abandoned in 1934 then regrew to a low-intensity land-use area, in which there had been only some selective experimental logging by the USDA Forest Service in the 1940s. Prior to our study, both areas were strongly affected by Hurricane Hugo in 1989, and again damaged to a lesser degree by Hurricane Georges in 1998, increasing canopy openness and subsequently increasing tree stem densities. Between 2001 and 2015, changes in the light environment and the recovery of forest structure resulted in roughly a 50% reduction in tree stem densities in the high-intensity land-use area, as recruited saplings naturally thinned. In this area, liana abundance increased by 103%, liana biomass tripled, and occupancy of trees by lianas grew by nearly 50%. In the low-intensity land-use area, juvenile stem densities were stable, and resultantly liana abundance only increased by 33%, liana biomass rose 39%, and the occupancy of trees was constant. Liana flower and fruit production increased over the 14-year interval, and these increases were much greater in the high-intensity land-use quadrats. Results of this study do show how rapid forest tree successional dynamics coincide with liana increases, but the confounding of hurricane effects of disturbance at our site, prevent us from asserting that the increases in liana density and biomass can be attributed to the same causes as those in forests elsewhere in the Neotropics.