A Reply to Verbeeck and Kearsley: Addressing the Challenges of Including lianas in Global Vegetation Models

Verbeeck and Kearsley (1) rightfully point out that global vegetation models would greatly benefit from implicitly including the effects of lianas. Recent experimental evidence that lianas substantially reduce the capacity of tropical forests to uptake and store carbon is compelling (2, 3). Furthermore, lianas are increasing relative to trees rapidly in many neotropical forests (4), which will further change the way that forests uptake, cycle, and store carbon

Contribution of Lianas to Plant Area Index and Canopy Structure in A Panamanian Forest

Lianas are an important component of tropical forests, where they reduce tree growth, fecundity, and survival. Competition for light from lianas may be intense; however, the amount of light that lianas intercept is poorly understood. We used a large-scale liana-removal experiment to quantify light interception by lianas in a Panamanian secondary forest. We measured the change in plant area index (PAI) and forest structure before and after cutting lianas (for 4 yr) in eight 80 m × 80 m plots and eight control plots (16 plots total). We used ground-based LiDAR to measure the 3-dimensional canopy structure before cutting lianas, and then annually for 2 yr afterwards. Six weeks after cutting lianas, mean plot PAI was 20% higher in control vs. liana removal plots. One yr after cutting lianas, mean plot PAI was ~17% higher in control plots. The differences between treatments diminished significantly 2 yr after liana cutting and, after 4 yr, trees had fully compensated for liana removal. Ground-based LiDAR revealed that lianas attenuated light in the upper- and middle-forest canopy layers, and not only in the upper canopy as was previously suspected. Thus, lianas compete with trees by intercepting light in the upper- and mid-canopy of this forest

Fine roots, arbuscular mycorrhizal hyphae and soil nutrients in four neotropical rain forests: patterns across large geographic distances.

* It is commonly hypothesized that stand-level fine root biomass increases as soil fertility decreases both within and among tropical forests, but few data exist to test this prediction across broad geographic scales. This study investigated the relationships among fine roots, arbuscular mycorrhizal (AM) fungi and soil nutrients in four lowland, neotropical rainforests. * Within each forest, samples were collected from plots that differed in fertility and above-ground biomass, and fine roots, AM hyphae and total soil nutrients were measured. * Among sites, total fine root mass varied by a factor of three, from 237+/-19 g m-2 in Costa Rica to 800+/-116 g m-2 in Brazil (0-40 cm depth). Both root mass and length were negatively correlated to soil nitrogen and phosphorus, but AM hyphae were not related to nutrients, root properties or above-ground biomass. * These results suggest that understanding how soil fertility affects fine roots is an additional factor that may improve the representation of root functions in global biogeochemical models or biome-wide averages of root properties in tropical forests

Variation in small sapling density, understory cover, and resource availability in four neotropical forests

Even though many forest plants spend all or a significant portion of their lives in the forest understoty, few studies have compared understory composition, structure, and resource availability among forests. We used standardized transect-based methods to compare small sapling densities (10-50 cm tall), understory vegetation cover, canopy openness, and nutrient availability in non-gap portions of four lowland Neotropical forests: La Selva, Costa Rica (LS), Barro Colorado Island, Panama (BCI), Cocha Cashu, Peru (CC), and north of Manaus, Brazil (KM41). Sites differed significantly in all variables except canopy openness. LS had high palm and non-fern herb cover and low density of small saplings (0.7-1.6/m2) compared to other sites. CC had high fern cover, whereas BCI had low cover in all categories of understory vegetation (palms, ferns, and non-fern herbaceous plants). BCI, CC, and KM41 had similar small sapling densities, ranging from 4.8-7.5/m 2. Within each forest, cation (Ca, Mg, K, and Na) availability was usually higher on more fertile soil orders (Inceptisols, Alfisols, and Entisols) than on more weathered soil types (Ultisols and Oxisols). Extractable P was highest at LS and CC and lowest on BCI (no data for KM41). Spatial autocorrelation was present for some variables in some transects to distances beyond our detection ability (\u3e25 m). Understory palm cover was negatively correlated with small sapling density at fine (1 m2 quadrat) and coarse spatial scales (among forests), although across forests the effect of palms was due entirely to the difference between LS and the other three forests. These results provide cross-site support for the hypothesis that understory cover by palms decreases the density of small saplings that comprise the advance regeneration of the forest

Resilience of seed production to a severe El Niño‐induced drought across functional groups and dispersal types

More frequent and severe El Niño Southern Oscillations (ENSO) are causing episodic periods of decreased rainfall. Although the effects of these ENSO-induced droughts on tree growth and mortality have been well studied, the impacts on other demographic rates such as reproduction are less well known. We use a four-year seed rain dataset encompassing the most severe ENSO-induced drought in more than 30 years to assess the resilience (i.e., resistance and recovery) of the seed composition and abundance of three forest types in a tropical dry forest. We found that forest types showed distinct differences in the timing, duration, and intensity of drought during the ENSO event, which likely mediated seed composition shifts and resilience. Drought-deciduous species were particularly sensitive to the drought with overall poor resilience of seed production, whereby seed abundance of this functional group failed to recover to predrought levels even two years after the drought. Liana and wind-dispersed species were able to maintain seed production both during and after drought, suggesting that ENSO events promote early successional species or species with a colonization strategy. Combined, these results suggest that ENSO-induced drought mediates the establishment of functional groups and dispersal types suited for early successional conditions with more open canopies and reduced competition among plants. The effects of the ENSO-induced drought on seed composition and abundance were still evident two years after the event suggesting the recovery of seed production requires multiple years that may lead to shifts in forest composition and structure in the long term, with potential consequences for higher trophic levels like frugivores

Lianas reduce carbon accumulation and storage in tropical forests

Tropical forests store vast quantities of carbon, account for a third of the carbon fixed by photosynthesis, and are a major sink in the global carbon cycle. Recent evidence suggests that competition between lianas (woody vines) and trees may reduce forest-wide carbon uptake. However, estimates of the impact of lianas on carbon dynamics of tropical forests are crucially lacking. Here, we used a large-scale liana removal experiment and found that, three years after liana removal, lianas reduced net above-ground carbon uptake (growth and recruitment minus mortality) by ~76% per year, mostly by reducing tree growth. The loss of carbon uptake due to liana-induced mortality was 4-times greater in the control plots were lianas were present, but high variation among plots prevented a significant difference among the treatments. Lianas altered how aboveground carbon was stored. In forests where lianas are present, the partitioning of forest aboveground net primary production is dominated by leaves (53.2% compared to 39.2% in liana-free forests) at the expense of woody stems (from 28.9% compared to 43.9%), resulting in a more rapid return of fixed carbon to the atmosphere. After three years of experimental liana removal, our results clearly demonstrate large differences in carbon cycling between forests with and without lianas. Combined with the recently reported increases in liana abundance, these results indicate that lianas are an important and increasing agent of change in the carbon dynamics of tropical forests

Effect of lianas on forest-level tree carbon accumulation does not differ between seasons: Results from a liana removal experiment in Panama

1. Lianas are prevalent in Neotropical forests, where liana-tree competition can be intense, resulting in reduced tree growth and survival. The ability of lianas to grow relative to trees during the dry season suggests that liana-tree competition is also strongest in the dry season. If correct, the predicted intensification of the drying trend over large areas of the tropics in the future may therefore intensify liana-tree competition, resulting in a reduced carbon sink function of tropical forests. However, no study has established whether the liana effect on tree carbon accumulation is indeed stronger in the dry than in the wet season. 2. Using six years of data from a large-scale liana removal experiment in Panama, we provide the first experimental test of whether liana effects on tree carbon accumulation differ between seasons. We monitored tree and liana diameter increments at the beginning of the dry and wet season each year to assess seasonal differences in forest-level carbon accumulation between removal and control plots. 3. We found that median liana carbon accumulation was consistently higher in the dry (0.52 Mg C ha-1 yr-1) than the wet season (0.36 Mg C ha-1 yr-1), and significantly so in three of the years. Lianas reduced forest-level median tree carbon accumulation more severely in the wet (1.45 Mg C ha-1 yr-1) than the dry (1.05 Mg C ha-1 yr-1) season in all years. However, the relative effect of lianas was similar between the seasons, with lianas reducing forest-level tree carbon accumulation by 46.9% in the dry and 48.5% in the wet season. 4. Synthesis: Our results provide the first experimental demonstration that lianas do not have a stronger competitive effect on tree carbon accumulation during the dry season. Instead, lianas compete significantly with trees during both seasons, indicating a large negative effect of lianas on forest-level tree biomass increment regardless of seasonal water stress. Longer dry seasons are unlikely to impact liana-tree competition directly; however, the greater liana biomass increment during dry seasons may lead to further proliferation of liana biomass in tropical forests, with consequences for their ability to store and sequester carbon

Liana Competition with Tropical Trees Varies Seasonally but not with Tree Species Identity

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

Sex-Specific Associations Between Trauma Exposure, Pubertal Timing, and Anxiety in Black Children

Recent research has linked early life stress (ELS), such as trauma exposure, with early puberty. Early puberty has also been identified as a risk factor for poor mental health outcomes. However, these two paths have primarily been examined independently. In addition, more studies have examined these associations in girls than boys, and findings for boys remain mixed. We hypothesized that early puberty (relative to peers) would be positively associated with both prior trauma exposure and concurrent anxiety symptoms. We anticipated that these associations might differ by sex. We tested these hypotheses within a cross-sectional sample of 133 8- to 13-year-old Black girls and boys with trauma exposure. The association between trauma and accelerated pubertal timing was sex-specific: it was positive for girls and negative for boys. We stratified subsequent analyses by sex. Regression analyses indicated that early puberty relative to peers predicted more anxiety symptoms for girls but not boys, after accounting for trauma exposure. A statistical mediation analysis indicated that, for girls, the positive association between trauma exposure and anxiety was partially mediated by pubertal timing. These results indicate that trauma exposure may have sex-specific effects on pubertal timing and anxiety risk in Black children. We also found that, for girls, trauma may increase risk for adverse outcomes by prompting earlier puberty, which is linked to higher anxiety. These findings are consistent with cascading effects of trauma across development, and highlight the need for further study of sex-specific mechanisms