Summary: Tropical forests are increasingly being subjected to hotter, drier conditions as a result of global climate change. The effects of drought on forests along successional gradients remain poorly understood. We took advantage of the 2015–2016 El Niño event to test for differences in drought response along a successional gradient by measuring the sap flow in 76 trees, representing 42 different species, in 8‐, 25‐ and 80‐yr‐old secondary forests in the 15‐km²‘Agua Salud Project’ study area, located in central Panama. Average sap velocities and sapwood‐specific hydraulic conductivities were highest in the youngest forest. During the dry season drought, sap velocities increased significantly in the 80‐yr‐old forest as a result of higher evaporative demand, but not in younger forests. The main drivers of transpiration shifted from radiation to vapor pressure deficit with progressing forest succession. Soil volumetric water content was a limiting factor only in the youngest forest during the dry season, probably as a result of less root exploration in the soil. Trees in early‐successional forests displayed stronger signs of regulatory responses to the 2015–2016 El Niño drought, and the limiting physiological processes for transpiration shifted from operating at the plant–soil interface to the plant–atmosphere interface with progressing forest succession.

Forested catchments in Central Panama can produce more base flow during the dry season compared to pasture catchments—the so‐called “forest sponge effect.” During rainfall events, peak storm runoff rates and storm runoff coefficients can be lower for forested catchments than pasture catchments, even when they have similar topographic characteristics, underlying geology, and soil texture. The internal mechanism of these differences in hydrological response due to land use is yet to be fully understood. A distributed model explicitly simulating preferential flow paths (PFPs), which is referred to as “PFPMod,” is used to explain the hydrological response caused by land use using data from three catchments with distinct land covers in Central Panama. Input parameters of forest and pasture land covers were identified using field observations and literature values. Multiple satisfactory objective criteria demonstrate that the two end‐member land cover parameter sets are adequate to explain the observed difference in dry‐season base flow and storm runoff coefficients. Field measurements of matrix infiltrability using soil cores and plot‐scale infiltration capacity enabled estimating the number of vertical macropores that fully penetrate the root zone. Model simulation results demonstrate that fast drainage through lateral PFPs in the early wet season and high flow in vertical PFPs to recharge deep groundwater in the late wet season contribute to the observed differences in peak storm runoff and the “forest sponge effect” during the dry season. This study provided insights to the mechanism by which reforestation may help to restore ecosystem services and water security in tropical settings.

In tropical forests, both vegetation characteristics and soil properties are important not only for controlling energy, water, and gas exchanges directly but also determining the competition among species, successional dynamics, forest structure and composition. However, the joint effects of the two factors have received limited attention in Earth system model development. Here we use a vegetation demographic model, the Functionally Assembled Terrestrial Ecosystem Simulator (FATES) implemented in the Energy Exascale Earth System Model (E3SM) Land Model (ELM), ELM‐FATES, to explore how plant traits and soil properties affect tropical forest growth and composition concurrently. A large ensemble of simulations with perturbed vegetation and soil hydrological parameters is conducted at the Barro Colorado Island, Panama. The simulations are compared against observed carbon, energy, and water fluxes. We find that soil hydrological parameters, particularly the scaling exponent of the soil retention curve (B_sw), play crucial roles in controlling forest diversity, with higher B_sw values (>7) favoring late successional species in competition, and lower B_sw values (1 ∼ 7) promoting the coexistence of early and late successional plants. Considering the additional impact of soil properties resolves a systematic bias of FATES in simulating sensible/latent heat partitioning with repercussion on water budget and plant coexistence. A greater fraction of deeper tree roots can help maintain the dry‐season soil moisture and plant gas exchange. As soil properties are as important as vegetation parameters in predicting tropical forest dynamics, more efforts are needed to improve parameterizations of soil functions and belowground processes and their interactions with aboveground vegetation dynamics.

Aspen forests and woodlands are some of the most species‐rich forest communities in the northern hemisphere. Changing climate, altered disturbance regimes, land use, and increased herbivore pressure threaten these forests both in Eurasia and North America. In addition, rapid mortality dubbed “Sudden Aspen Decline” is a concern for aspen's long‐term presence in the western United States, especially Colorado and Utah. Yet it is still unclear whether aspen is persistent or declining at the landscape scale. We assessed aspen persistence at different spatial scales in the Colorado Front Range by resampling 89 plots containing aspen from among 305 vegetation plots sampled by Robert Peet during 1972–1973. We hypothesized that aspen density and basal area had decreased at the landscape scale, with notable variability in change depending on the forest community type, and that this overall decrease has been more pronounced at lower elevations. We also assessed elevational range shifts of the major species in these forests. Aspen were no longer present in 22 of the 89 plots and aspen density for stems less than 2.5 cm diameter at breast height (DBH) had declined significantly overall, although density of medium (2.5–10 cmDBH) and large (>10 cmDBH) trees, as well as basal area, had not changed significantly. A comparison between montane (<2700 m elevation) and subalpine (2700–3500 m elevation) plots revealed that the decrease was more pronounced at higher elevations and was mostly the result of substantial decreases of stems in the eleven plots that were part of Peet's aspen‐dominated “Populus tremuloidesseries.” In these plots, aspen basal area decreased significantly whereas basal area ofAbies bifolia,Picea engelmannii,Pinus contorta, andPseudotsuga menziesiiincreased substantially. Upslope shifts were observed for most species, especially on northeast facing slopes, suggesting climate‐related responses. In summary, aspen have been resilient in mixed forests and may be beneficiaries of recent bark beetle epidemics, but have decreased and been subject to successional transitions in previously aspen‐dominated stands. Our results confirm the importance of region‐specific, multiple‐scale assessments of species persistence to make best management recommendations.