# Below versus above Ground Plant Sources of
## 1 Introduction
### Biochemical Mechanisms of Isohydric and Anisohydric Strategies  
Given its important role in inducing g\({}_{\rm s}\) reductions, ABA has been implicated in isohydric behavior. When leaves of isohydric angiosperms were exposed to reduced \(\Psi_{\rm I}\) by modifying external atmospheric pressure, foliar ABA levels rapidly increased. In contrast, when leaves of anisohydric non-flowering plants were exposed to the same reductions in \(\Psi_{\rm I}\), significant increases in foliar ABA were not detected [51]. Therefore, it can be hypothesized that isohydric plants show a higher sensitivity of g\({}_{\rm s}\) to decreases in \(\Psi_{\rm I}\), and a lower variation in \(\Psi_{\rm I}\) along the day due to foliar accumulation of ABA. In contrast, anisohydric plants may show a lower sensitivity of g\({}_{\rm s}\) to decreases in \(\Psi_{\rm I}\), and a higher variation in \(\Psi_{\rm I}\) along the day due to the lack of a strong foliar ABA accumulation. Thus, leaf turgor loss provides an endogenous signal which appears to trigger ABA accumulation and that the high diversity of tropical tree species and their corresponding ecological niches may differ greatly in the turgor loss thresholds that trigger ABA accumulation. For example in the tropics, the classic Neotropical pioneer genera _Vismia_ and _Cecropia_ dominate large rainforest disturbance gaps in the Amazon Basin [52] where they help accelerate the regeneration of secondary forests by influencing forest successional pathways [53; 54; 55]. Their success in secondary forests is related to their ability to maintain high values of stomatal conductance and high corresponding rates of net photosynthesis, transpiration, and growth under conditions of full sunlight, high leaf temperatures, and low nutrient availability, often characteristic of tropical landscapes impacted by natural [52] and human [56] disturbances. In addition to high values of stomatal conductance, relative to climax species in mature forests, pioneer species in disturbed forests often have hydraulic characteristics to facilitate water transport including low height, low wood specific gravity (WSG), and large xylem vessel size [57; 58]. A high diversity of hydraulic traits in the Amazon has been observed and associated with the large spatial scale of the Basin (7.5 million km\({}^{2}\)) which has a high environmental heterogeneity and range of plant traits and many hyper-diverse ecosystems with a recent estimate of 6727 tree species across the Basin [59]. Changes in plant hydraulic functional traits are highly associated with local variations in soil type, forest structure, and moisture availability. For example, in the central Amazon, valleys (\"baixios\") with predominant sandy soils are vertically close to the water table giving essentially unlimited access to water by roots [60]. In contrast, plateau tree roots may not have direct access to the water table which has been observed at more than 30 m depth [61]. In an effort to minimize water stress during the dry season, plateau trees have been documented to enhance surface water availability during the day by hydraulic redistribution at night where water is moved from moist to dry regions of the soil profile [62]. This leads to the hypothesis that trees in the valleys may be associated with anisohydric stomatal characteristics as they have been documented to have both higher soil water availability and facility of moving water from soils to leaves. One characteristic of isohydric species is the low sensitivity of stomatal conductance to increases in VPD. The higher efficient water transport systems in the valley trees are associated with lower heights [63] and wood specific gravity (WSG), larger xylem vessel diameter, and higher stem cross-sectional sapwood area relative to plateau species [60]. This increased water transport efficiency may come at the expense of hydraulic safety [60].