Drought Differentially Affects Growth, Transpiration, and Water Use Efficiency of Mixed and Monospecific Planted Forests

Katherine Sinacore,Jefferson S Hall,Heidi Asbjornsen,Virginia Hernandez-Santana

doi:10.3390/f10020153

Abstract

Drought conditions may have differential impacts on growth, transpiration, and water use efficiency (WUE) in mixed species and monospecific planted forests. Understanding the resistance (i.e., the capacity to maintain processes unchanged) of different tree species to drought, and how resistance is affected by complementary interactions within species mixtures, is particularly important in the seasonally dry tropics where projected increases in the frequency and severity of drought threaten tree planting efforts and water resources. Complementary interactions between species may lead to more resistant stands if complementarity leads to greater buffering capacity during drought. We examined growth, transpiration, and WUE of mixtures and monocultures of Terminalia amazonia (J.F. Gmel.) Exell and Dalbergia retusa Hemsl. before and during a prolonged drought using intensive measurements of tree sap flow and growth. Tree sapwood area growth was highest for T. amazonia in mixtures during normal (6.78 ± 4.08 mm2 yr−1) and drought (7.12 ± 4.85 mm2 yr−1) conditions compared to the other treatments. However, stand sapwood area growth was greatest for T. amazonia monocultures, followed by mixtures, and finally, D. retusa monocultures. There was a significant decrease in stand transpiration during drought for both mixtures and T. amazonia monocultures, while Dalbergia retusa monocultures were most water use efficient at both the tree and stand level. Treatments showed different levels of resistance to drought, with D. retusa monocultures being the most resistant, with non-significant changes of growth and transpiration before and during drought. Combining species with complementary traits and avoiding combinations where one species dominates the other, may maximize complementary interactions and reduce competitive interactions, leading to greater resistance to drought conditions.

Highlights

High global rates of tropical deforestation are being countered by the establishment of new forested areas through tree planting [1], which provides an opportunity to restore many critical ecosystem services [2,3]
While T. amazonia trees planted in mixtures and D. retusa trees planted in monocultures trended toward greater sapwood area (SA) growth during drought compared to a normal year, this trend was not significant
Our work shows that mixed species plantings do not necessarily overyield, or have higher transpiration and water use efficiency (WUE) when compared to their respective monocultures, suggesting that selecting species based on information about their complementary functional traits alone is not sufficient to ensure overyielding

Summary

Introduction

High global rates of tropical deforestation are being countered by the establishment of new forested areas through tree planting [1], which provides an opportunity to restore many critical ecosystem services [2,3]. Tree planting is an increasingly popular approach for restoring degraded landscapes where natural regeneration processes may be slow or arrested [4]. Because different tree species—and their combinations—can have different impacts on hydrological and biogeochemical cycles, careful selection and combining of appropriate species is crucial in order to return land to productivity. Forests 2019, 10, 153 productive than monocultures [5,6]. Plantations with fast growing tree species that allocate to aboveground biomass over belowground biomass may be less resilient to severe drought [11]

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Discussion

Conclusion