Savanna Tree Seedlings are Physiologically Tolerant to Nighttime Freeze Events.

Kimberly O’Keefe,Jesse B Nippert,Anthony M Swemmer

doi:10.3389/fpls.2016.00046

Kimberly O’Keefe, Jesse B Nippert + Show 1 more

Open Access

https://doi.org/10.3389/fpls.2016.00046

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Abstract

Freeze events can be important disturbances in savanna ecosystems, yet the interactive effect of freezing with other environmental drivers on plant functioning is unknown. Here, we investigated physiological responses of South African tree seedlings to interactions of water availability and freezing temperatures. We grew widely distributed South African tree species (Colophospermum mopane, Combretum apiculatum, Acacia nigrescens, and Cassia abbreviata) under well-watered and water-limited conditions and exposed individuals to nighttime freeze events. Of the four species studied here, C. mopane was the most tolerant of lower water availability. However, all species were similarly tolerant to nighttime freezing and recovered within one week following the last freezing event. We also show that water limitation somewhat increased freezing tolerance in one of the species (C. mopane). Therefore, water limitation, but not freezing temperatures, may restrict the distribution of these species, although the interactions of these stressors may have species-specific impacts on plant physiology. Ultimately, we show that unique physiologies can exist among dominant species within communities and that combined stresses may play a currently unidentified role in driving the function of certain species within southern Africa.

Highlights

Occasional freeze events can cause substantial die-back of dominant woody plants in tropical and subtropical savannas (Brando and Durigan, 2004; Holdo, 2005, 2006, 2007; Whitecross et al, 2012)
We addressed the following questions: (1) Does tolerance to water limitation vary among dominant savanna tree species? (2) Do these species differ in their sensitivity to freezing temperatures? (3) Does water availability modify physiological responses to freezing? Based on the distribution of these species in northeastern South Africa, we hypothesized that C. mopane seedlings will be more tolerant to water-limitation but more sensitive to freezing than species common in the cooler, wetter Acacia–Combretum savanna (C. apiculatum, C. abbreviata, and A. nigrescens)
These species are widely distributed within southern African savannas, the ranges of C. apiculatum, C. abbreviata, and A. nigrescens extend to lower latitudes and higher altitudes than that of C. mopane, including areas where freeze events are more likely (Palgrave, 2002)

Summary

Introduction

Occasional freeze events can cause substantial die-back of dominant woody plants in tropical and subtropical savannas (Brando and Durigan, 2004; Holdo, 2005, 2006, 2007; Whitecross et al, 2012). Freeze events are generally uncommon in the savanna biome and savanna woody plants are considered to be more sensitive to frost than temperate species. Regular frost may be an important factor that prevents savanna woody species from invading neighboring grasslands (Wakeling et al, 2012), if freeze events kill seedlings of these species. Frost may restrict the distribution of dominant woody species within the savanna biome by preventing the expansion of tropical and sub-tropical dominants into savannas of higher latitude or altitude, where freeze events occur (Whitecross et al, 2012). In cooler savannas, occasional freeze events may limit the abundance of many woody species by limiting recruitment or “topkilling” adults, preventing dominance by woody plants and contributing to the co-existence of trees and grasses that characterizes these systems

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