Sybil G. Gotsch

Foggy days and dry nights determine crown‐level water balance in a seasonal tropical montane cloud forest

The ecophysiology of tropical montane cloud forest (TMCF) trees is influenced by crown-level microclimate factors including regular mist/fog water inputs, and large variations in evaporative demand, which in turn can significantly impact water balance. We investigated the effect of such microclimatic factors on canopy ecophysiology and branch-level water balance in the dry season of a seasonal TMCF in Veracruz, Mexico, by quantifying both water inputs (via foliar uptake, FU) and outputs (day- and night-time transpiration, NT). Measurements of sap flow, stomatal conductance, leaf water potential and pressure-volume relations were obtained in Quercus lanceifolia, a canopy-dominant tree species. Our results indicate that FU occurred 34% of the time and led to the recovery of 9% (24 ± 9.1 L) of all the dry-season water transpired from individual branches. Capacity for FU was independently verified for seven additional common tree species. NT accounted for approximately 17% (46 L) of dry-season water loss. There was a strong correlation between FU and the duration of leaf wetness events (fog and/or rain), as well as between NT and the night-time vapour pressure deficit. Our results show the clear importance of fog and NT for the canopy water relations of Q. lanceifolia.

Life in the treetops: ecophysiological strategies of canopy epiphytes in a tropical montane cloud forest

Tropical montane cloud forests (TMCFs) inhabit regions rich in biodiversity that play an important role in the local and regional water cycle. Canopy plants such as epiphytes and hemiepiphytes are an important component of the biodiversity in the TMCF and therefore play a significant role in the carbon, nutrient, and water cycles. With only partial or no access to resources on the ground, canopy plants may be vulnerable to changes in climate that increase canopy temperatures and decrease atmospheric humidity or precipitation inputs. Despite their importance in the TMCF, little is known about variation in functional strategies relating to drought avoidance or drought tolerance of canopy plants. In this study, we quantified variation in a number of functional traits in 11 species of epiphytes and hemiepiphytes in a Costa Rican TMCF. We also generated pressure–volume and xylem vulnerability curves that we used as indicators of drought tolerance. In addition, we hand-sectioned fresh leaves and examined cross ...

The functional roles of epiphytes and arboreal soils in tropical montane cloud forests

Epiphytes and their associated decomposing litter and arboreal soils (herein, epiphytic material, EM) are ubiquitous features of tropical montane cloud forests (TMCF) and play important roles in ecosystem function. EM intercepts water and nutrients from the atmosphere and from intercepted host tree sources, and may contribute significant inputs of these resources to the forest floor. Despite the importance of EM in the TMCF, a systematic review of the ecosystem roles of EM has not been compiled before. We have synthesized the literature that documents functions of EM in undisturbed TMCFs and discuss how these roles may be affected by disturbances, including changes in climate and land use. The range of EM biomass and water storage in the TMCF varies greatly across sites, with different amounts associated with stand age and microclimate. EM is important as habitat and food for birds and mammals, with over 200 species of birds documented as using EM in the Neotropics. Given its sensitivity to moisture, projected shifts in cloud base heights or precipitation due to changes in climate will likely have a large impact on this community and changes in EM diversity or abundance may have cascading impacts on the ecosystem function of the TMCF.

Variation in the resilience of cloud forest vascular epiphytes to severe drought

Epiphytes are common in tropical montane cloud forests (TMCFs) and play many important ecological roles, but the degree to which these unique plants will be affected by changes in climate is unknown. We investigated the drought responses of three vascular epiphyte communities bracketing the cloud base during a severe, El Niño-impacted dry season. Epiphytes were instrumented with sap flow probes in each site. Leaf water potential and pressure-volume curve parameters were also measured before and during the drought. We monitored the canopy microclimate in each site to determine the drivers of sap velocity across the sites. All plants greatly reduced their water use during the drought, but recovery occurred more quickly for plants in the lower and drier sites. Plants in drier sites also exhibited the greatest shifts in the osmotic potential at full saturation and the turgor loss point. Although all individuals survived this intense drought, epiphytes in the cloud forest experienced the slowest recovery, suggesting that plants in the TMCF are particularly sensitive to severe drought. Although vapor pressure deficit was an important driver of sap velocity in the highest elevation site, other factors, such as the volumetric water content of the canopy soil, were more important at lower (and warmer) sites.

Vapor pressure deficit predicts epiphyte abundance across an elevational gradient in a tropical montane region

PREMISE OF THE STUDY Tropical Montane Cloud Forests (TMCFs) are important ecosystems to study and preserve because of their high biodiversity and critical roles in local and regional ecosystem processes. TMCFs may be particularly affected by changes in climate because of the narrow bands of microclimate they occupy and the vulnerability of TMCF species to projected increases in cloud base heights and drought. A comprehensive understanding of the structure and function of TMCFs is lacking and difficult to attain because of variation in topography within and across TMCF sites. This causes large differences in microclimate and forest structure at both large and small scales. METHODS In this study, we estimated the abundance of the entire epiphyte community in the canopy (bryophytes, herbaceous vascular plants, woody epiphytes, and canopy dead organic matter) in six sites. In each of the sites we installed a complete canopy weather station to link epiphyte abundance to a number of microclimatic parameters. KEY RESULTS We found significant differences in epiphyte abundance across the sites; epiphyte abundance increased with elevation and leaf wetness, but decreased as vapor pressure deficit (VPD) increased. Epiphyte abundance had the strongest relationship with VPD; there were differences in VPD that could not be explained by elevation alone. CONCLUSIONS By measuring this proxy of canopy VPD, TMCF researchers will better understand differences in microclimate and plant community composition across TMCF sites. Incorporating such information in comparative studies will allow for more meaningful comparisons across TMCFs and will further conservation and management efforts in this ecosystem.

Evaluating the effectiveness of urban trees to mitigate storm water runoff via transpiration and stemflow

Many cities in the Eastern United States are working to increase urban tree cover due to the hydrological services that trees provide, including the interception, storage and transpiration of water that would otherwise enter sewer systems. Despite the understanding that trees benefit urban ecosystems, there have been few studies that address the underlying physiology of different urban trees with regard to their capacity to take up water, particularly following rain events. We monitored the sap flow of nine species of trees in urban parks and linked sap flow to local microclimate. In addition, we measured throughfall, stemflow and crown architecture. Interspecific variation in sap flow was significant as were differences in time lags (i.e. difference in time between the increase of solar radiation versus sap flow) across species of large but not small trees. Interestingly the most important microclimatic drivers of sap flow were different in large versus small trees. Lastly, we found that trees with a large branch angle routed more rainwater to stemflow. In this study we found strong evidence that variation in urban tree physiology can impact important hydrological services that will influence the effectiveness of different trees as tools to manage stormwater runoff.

Foliar water uptake: processes, pathways, and integration into plant water budgets: Foliar Water Uptake

Nearly all plant families, represented across most major biomes, absorb water directly through their leaves. This phenomenon is commonly referred to as foliar water uptake. Recent studies have suggested that foliar water uptake provides a significant water subsidy that can influence both plant water and carbon balance across multiple spatial and temporal scales. Despite this, our mechanistic understanding of when, where, how, and to what end water is absorbed through leaf surfaces remains limited. We first review the evidence for the biophysical conditions necessary for foliar water uptake to occur, focusing on the plant and atmospheric water potentials necessary to create a gradient for water flow. We then consider the different pathways for uptake, as well as the potential fates of the water once inside the leaf. Given that one fate of water from foliar uptake is to increase leaf water potentials and contribute to the demands of transpiration, we also provide a quantitative synthesis of observed rates of change in leaf water potential and total fluxes of water into the leaf. Finally, we identify critical research themes that should be addressed to effectively incorporate foliar water uptake into traditional frameworks of plant water movement.