Tree Water Use Patterns as Influenced by Phenology in a Dry Forest of Southern Ecuador.

Philipp Butz,Sophie Graefe,Eduardo Cueva,Dirk Hölscher

doi:10.3389/fpls.2018.00945

Philipp Butz, Sophie Graefe + Show 2 more

Open Access

https://doi.org/10.3389/fpls.2018.00945

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Abstract

Tropical dry forests are composed of tree species with different drought coping strategies and encompass heterogeneous site conditions. Actual water use will be controlled by soil moisture availability. In a premontane dry forest of southern Ecuador, tree water use patterns of four tree species of different phenologies were studied along an elevational gradient, in which soil moisture availability increases with altitude. Main interest was the influence of variation in soil moisture, vapor pressure deficit, species (representing phenology), elevation, and tree diameter on water use. Special emphasis was put on the stem succulent, deciduous Ceiba trichistandra, as high water use rates and drought coping involving stem succulence was to be expected. Tree water use rates increased linearly with diameter across species at high soil water content. However, when soil moisture declined, sap flux densities of the species responded differently. The stem succulent, deciduous Ceiba and other deciduous tree species reduced sap flux sensitively, whereas sap flux densities of the evergreen (broad leaved) Capparis scabrida were increasing. This was also reflected in diurnal hysteresis loops of sap flux vs. vapor pressure deficit (VPD) of the air. Under dry soil conditions, Ceiba and other deciduous tree species had much smaller areas in the hysteresis loop, whereas the area of Capparis was largely enhanced compared to wet conditions. The evergreen Capparis potentially had access to deeper soil water resources as water use patterns suggest that top soil drought was tolerated. The deciduous species followed a drought avoidance strategy by being leafless in the dry season. The stem succulent deciduous Ceiba flushed leaves at the end of the dry season before the rainy season began and also re-flushed early in the dry season after a rain event; however, water use rates at this occasion remained low. Ceiba was also ready for fast and strong response in water use when conditions were most favorable during the wet season. The study thus indicates a strong influence of species’ drought coping strategy on water use patterns in tropical dry forests.

Highlights

Drylands cover approximately 42% of the global land surface (Sorensen, 2009)
Based on leaf phenology they are classified as (i) deciduous species that shed their leaves at the beginning of the dry season and re-flush at the beginning of the wet season; (ii) deciduous, stem succulent species, which mainly differ from the former category by their big trunks and early leaf flush at the end of the dry season; (iii) brevi-deciduous tree species with a short leafless period of a couple of weeks during the dry season, during which all leaves are shed and replaced by new ones; and (iv) evergreen species (Borchert, 1996)
Relationships between sap flux density, diameter, and soil water content (SWC) were analyzed with linear regression, the relationship of sap flux density and vapor pressure deficit (VPD) by logarithmic regression

Summary

Introduction

Drylands cover approximately 42% of the global land surface (Sorensen, 2009). They receive low annual rainfall volumes with a pronounced seasonal distribution, and according to climate change scenarios they may further expand (Huang et al, 2016). Due to their vast occurrence and resulting high diversity the variety of mechanisms through which species in tropical dry forests avoid or tolerate drought are partly unknown (Delzon, 2015). Stomata of drought-deciduous species are more sensitive to changes in soil water availability than those of evergreen species, with deciduous species exhibiting greater declines in stomatal conductance than co-existing evergreen species during dry periods (Myers et al, 1997; Eamus and Prior, 2001) For the latter, water use during the dry season is explained by deep reaching roots with access to deep water reservoirs (Elliott et al, 2006)

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