The changes in an ecosystem that follow a disturbance are collectively called succession, which is a dynamic and continuous process, often occurring gradually over time [1] . The broadest distinction between trees in a succession are the pioneer trees that are more prominent in open areas and absent from the forests, and forest trees that contribute to the canopy of a mature forest [2] . Compared to forest species, pioneer species are prolific seeders, fast-growing, short-lived, generally shade intolerant and with high rates of mortality under resource deprivation. Later successional species are shade tolerant and generally grow slowly [1] [3] . Thus, after stand-scale disturbance of natural forests, the landscape changes into patchy vegetation with defined areas including pioneer species and different degree of successional forests. Resource availability and environmental conditions as well as biotic factors vary within the whole landscape [4] [5] . In open habitats occupied by pioneer species forming sparse communities, competition for water and nutrients is lower in comparison to forest habitats [6] . In addition, open habitats created by deforestation experience relatively high variation in light intensity, temperature and water availability as compared to forest habitats [7] [8] . In late succession forest, the environmental conditions tend to be less variable and extreme. In forest areas, solar radiation is received by the canopy and heat is dissipated efficiently by transpiration. Conversely, in open areas light is received by the soils and heat is dissipated by convection which is a less efficient process.

The behavior of pioneer species is characteristically opportunistic because resources are utilized in accordance with their availability in the environment [9] . This results in larger maximum photosynthesis and growth rates as compared to forest species [7] -[9] . Additionally, within-leaf N allocation differs between pioneer and late successional forest species, with a trade-off between investment in carboxylation and bioenergetics (higher in pioneer species) versus investment in light-harvesting complexes (higher in forest species) [10] . Whole tree plant hydraulic conductance as well as leaf water loss is larger in pioneer as compared to forest species [11] [12] . Thus, opportunistic pioneer species are also more efficient in conducting water to their leaves [13] [14] . However, these species exhibit less control over water use compared to forest species [14] [15] . Overall, pioneer species are characterized by high rates of water loss and hydraulic conductance as well as low leaf water use efficiency compared to forest species [14] . Species that occupy different niches in successional habitats may greatly differ in water relations characteristics such as leaf solute concentration and cell wall elasticity [16] . These characteristics are vital determinants of the turgor maintenance required for growth as available water declines during drought. Thus, a clear understanding of the underlying leaf tissue water relations is crucial to fully assessment of drought adaptations of pioneer and late successional forest species.

This study was carried out using pioneer and successional forest species in a lower montane area with seasonal rains. Water limitation is a common occurrence, and both pioneer and forests species experience some degree of water stress during the dry season. In several species, adjustments to total leaf area during periods of drought allow for the maintenance of the water supply and physiological functions of the remaining leaves [14] . It is currently unknown whether leaf tissue water relations are clearly differentiated between forest species and opportunistic pioneer species. In this study, we compared leaf blade water relations between pioneer and forest tree species in tropical montane habitats. A more complete understanding of the adaptations of leaf tissue to withstand drought can provide further information concerning the responses of trees to change water availability in their native habitats. Such information can be viewed in light of previous ecophysiological findings in these habitats and species [14] [17] to gain a more thoroughly understanding of the tree species inhabiting low montane areas in tropical environments.

Parameters concerning leaf tissue water relations obtained from the pressure-volume curves are shown in Table 2. The osmotic potential at full turgor (Ψ_π(100)) was significantly lower in the forest species (FS) F. mathewsii and the pioneer species (PS) C. lanata compared to O. vervesinoides (PS) and C. xanthochloros (FS). Leaf Ψ_π(100) and Ψ_π(0) have been highly correlated in a number of studies [22] . Thus, it is consistent that the osmotic potential at zero turgor (Ψ_π(0)) showed a similar pattern, with the highest values found in O. vervesinoides and C. xanthochloros (Table 2). These results demonstrate a high solute concentration (low Ψ_π) in leaf cells of F. mathewsii (FS) and C. lanata (PS) compared to the other two species (Figure 1). Species with lower Ψ_π had larger turgor pressure (Y_t) values at full hydration whereas Ψ_π(0) represents the Ψ_π and Y upon loss of turgor. Therefore, F mathewsii and C. lanata had the larger Ψ_t at zero Ψ, and maintained positive Ψ_t at lower Ψ (Figure 2) compared to O. verbesinoides (PS) and C. xanthochloros (FS). A large Ψ_t favors leaf photosynthesis and other physiological processes related to growth maintenance. Leaf Ψ_π varies widely due to differences in genotype, environment and physiological plasticity. The range of Ψ_π found in this study compared well to those values found under mesic environments [23] . The Ψ measured previously under field conditions during the dry season was within the range of turgor maintenance in F. mathewsii (FS) and C. lanata but not in the range observed for O. verbesinoides (PS) and C. xanthochloros (FS) (Table 1, Figure 2) [14] . The most likely explanation is that the mid- day Y drop measured in O. verbesinoides and C. xanthochloros in the field was overestimated in the dry season measurements when terminal branches were partially embolized and defoliated (Table 1) [14] . Accurate measurements of Ψ using a pressure chamber in these conditions is challenging because pressurization could induce additional xylem embolization leading to very low Ψ.

Ψ_t is regulated by Ψ_π and also by cell wall elasticity, which depends on the biomechanical properties of the

Pressure-volume parameters: osmotic potential at full (Y_π(100)) and zero turgor (Y_π(0)), relative water content at zero turgor (RWC_o), volumetric elastic modulus (e), apoplastic (A) and symplasmic (S) water content and A to S ratio (A/S). Measurements were taken of the pioneer species C. lanata and O. verbesinoides and the forest species C. xanthochloros and F. mathewsii from tropical montane habitats. Each value is the mean ± standard error (SE) of measurements in six to nine samples taken from three trees per species. For each parameter, the mean followed by the same letter were not statistically different, and those followed for different letters were statistically different at P < 0.05.

leaf cells [24] [25] . Assessment of the change in Ψ_t per unit of change in cell volume or relative water content (RWC) provides the volumetric elastic modulus (ε). Thus, leaves with low ε have more elastic cells that allow for larger cell volume reduction and still maintain turgor. By contrast, leaves with cells of low elasticity (high ε) lose turgor with a correspondingly small reduction in cell volume. In general, leaf cells of high ε possess thicker cell walls leading to less elastic cells [26] . The decline of cell volume with turgor maintenance in elastic cells (low ε) or avoiding large changes in cell volume within the range of turgor maintenance in rigid cells (high ε) may both be adaptive responses in certain environments. Increase in ε and a decline in Ψ_π aids in the maintenance of the Ψ gradient for water uptake with small changes in cell water content [27] . The ability to generate large Ψ gradients allows plants to maximize gas exchange, enhancing carbon gain and growth. Declines in ε facilitate turgor maintenance over a wide range of water contents and Ψ values by effectively accumulating solutes through reduction of cell volume [24] . Here, we found that the species characterized by more elastic leaf tissue and whose Ψ_π was low (C. lanata; PS) or high (O. verbesinoides and C. xanthochloros) also exhibited some degree of leaf shedding during the dry season (Table 1, Table 2 and Figure 1). Partial defoliation leads to maintenance of the physiological functions of the remaining leaves [14] . Consistent with this observation, the species O. verbesinoides (PS) with the lowest ε (4.7MPa) tended to lose turgor at lower RWC compared to the other species (Figure 1). Conversely, F. mathewsii (FS) has the most rigid leaf tissue (ε = 13.8 MPa) and lost turgor at significantly higher RWC (Figure 1, Table 2). The responses of C. lanata (PS) and C. xanthochloros (FS) were intermediate between these two extremes. The species F. mathewsii (FS), exhibiting both the highest leaf solute concentration as well as the lowest elasticity, is notable for not shedding leaves during the dry season. It has been reported that avoidance of extreme reductions in cell volume during drought in leaves of low elasticity may be important for the protection of membrane integrity and macromolecules as well as the maintenance of metabolic activity [28] . This may be true in F. mathewsii (PS), whose leaves were not shed during the dry season (Table 1). In tropical dry forests as well, evergreen species possess leaves with high solute concentrations and low elasticity compared to drought-deciduous species [29] .

The apoplasm (A) water content was statistically the highest in F. mathewsii (45%) which also had the thickest leaves among the trees in this study. We speculate that F. mathewsii leaf cells correspondingly have thick cell walls contributing to large apoplasm spaces (Table 1 and Table 2). Values of A in the other three species bearing thinner leaves were comparatively lower between about 17% and 21% (Table 1 and Table 2). Conversely, F. mathewsii had the significantly lowest symplasm (S) water content of about 55% as compared to 78% - 83% in the other species (Table 2). With respect to A to S ratios (A/V), the lowest value of about 0.21 corresponded to the PS O. verbesinoides and the highest value of about 0.87 to the FS F. mathewsii (Table 2). In general, plants possess higher A/S under unfavorable water conditions over space and time, and therefore it has been considered as an important indicator of plant drought resistance [30] . Leaves with lower A/S are more susceptible to drought as compared to those with higher values. Importantly, this was the case in this study, where the relative large A/S value was found in the FS F. mathewsii which possessed more drought resistant leaves.

This study compared the water relations of two pioneer and two forest species in tropical montane habitats. Although the pioneer and forest species occupied contrasting habitats and both groups were clearly differentiated with respect to their water transport capability and water use efficiency, their leaf tissue water relations showed clear differences across species but not between the groups. However, all the species had different physiological and ecological attributes in order to adapt to drought by avoiding water loss by leaf area reduction or enduring drought with more resistant leaf tissue. The leaf tissue water relations characteristic of each species were associated with these differing capacities of the species to tolerate periods of water deficit. The pioneering species C. lanata and O. verbesinoides as well as the forest species C. xanthochloros undergo leaf shedding and accumulate xylem embolisms during the dry season. Leaves of O. verbesinoides (PS) and of C. xanthochloros (PS) exhibit low levels of solute concentration (high Ψ_π) and lost turgor at higher Y as compared to C. lanata (PS). Leaves of these three species with high cell elasticity (low ε) presented large cell volume changes with loss of turgor (low RWC) compare to F. mathewsii. The forest species F. mathewsii retained its leaves and did not experience xylem embolisms during the dry season. The rigid leaves (high ε) of F. mathewsii were able to undergo lower leaf Ψ with only small changes in RWC. The small changes in cell volume during drought may aid to preserve leaf cell function, maintain a positive Ψ gradient between leaf and soil for water uptake, and consequently avoid accelerated leaf senescence and shedding during periods of water deficit during the dry season.

DID-USB-Fondo de Trabajo 1014-2015 provided partial financial support. Anonymous reviewers provided helpful suggestions to improve the manuscript.

M. A.Sobrado, (2015) Leaf Tissue Water Relations Are Associated with Drought-Induced Leaf Shedding in Tropical Montane Habitats. American Journal of Plant Sciences,06,2128-2135. doi: 10.4236/ajps.2015.613214