Leaf Physiological Responses to Drought Stress and Community Assembly in an Asian Savanna

Thuy T Nguyen,Patrick J Baker,Stefan K Arndt

doi:10.3390/f10121119

Thuy T Nguyen, Patrick J Baker + Show 1 more

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https://doi.org/10.3390/f10121119

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Abstract

Deciduous dipterocarp forest (DDF) is the most extensive forest type in continental Southeast Asia, but across much of its range is functionally more similar to tropical savannas than tropical forests. We investigated water relations and drought responses of the four dominant tree species (two Shorea and two Dipterocarpus species) of the DDF in central Vietnam to determine how they responded to prolonged periods of drought stress. We quantified leaf water relations in nursery- and field-grown seedlings of the four species and conducted a dry-down experiment on 258 seedlings to study leaf water potential and morphological responses of the seedlings following the drought stress. The two Shorea and two Dipterocarpus species differed significantly in leaf water potential at turgor loss point and osmotic potential at full turgor, but they showed similar responses to drought stress. All species shed leaves and suffered from stem loss when exposed to water potentials lower than their turgor loss point (approximately −1.7 MPa for Dipterocarpus and −2.6 MPa for Shorea species). Upon rewatering, all species resprouted vigorously regardless of the degree of leaf or stem loss, resulting in only 2% whole-plant mortality rate. Our results suggest that none of the four deciduous dipterocarp species is drought tolerant in terms of their water relations; instead, they employ drought-adaptive strategies such as leaf shedding and vigorous resprouting. Given that all species showed similar drought avoidance and drought-adaptive strategies, it is unlikely that seasonal drought directly influences the patterns of species assembly in the DDF of Southeast Asia.

Highlights

Drought-induced tree mortality is a significant issue in tropical wet forests [1,2,3,4,5]
The exception was S. siamensis, where Ψtlp and πo were significantly more negative for seedlings outplanted in the field than those grown in the nursery (Figure 1a,b)
When we pooled the data from the field and nursery seedlings, we found that the Ψtlp of the two Shorea species (−2.69 MPa for S. obtusa and −2.53 MPa for S. siamensis) was significantly more negative than that of the two Dipterocarpus species (−1.78 MPa for D. tuberculatus and −1.75 MPa for D. obtusifolius)

Summary

Introduction

Drought-induced tree mortality is a significant issue in tropical wet forests [1,2,3,4,5]. Large areas of the tropics experience seasonal droughts each year and tree species in these areas have evolved mechanisms to cope with low water availability during the annual dry season. These mechanisms range from the molecular level such as the involvement of drought-tolerant genes and proteins [6] to cell functioning at low water availability [7] and leaf-level physiological responses such as stomatal closure [8,9] and osmotic adjustment [10]. Unlike the American and African seasonal tropics, vast expanses of the Asian seasonal tropics are dominated by a handful of closely related species, the deciduous dipterocarps [18,19,20,21]

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