Is there a temporal niche separation in the leaf phenology of savanna trees and grasses?

Authors

  • Steven I. Higgins,

    Corresponding author
    1. Institut für Physische Geographie, Goethe Universität Frankfurt a.M., Altenhöferallee 1, 60438 Frankfurt am Main, Germany
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  • Maria D. Delgado-Cartay,

    1. Biodiversity and Climate Research Centre (LOEWE BiK-F), Senckenberganlage 25, 60325 Frankfurt am Main, Germany
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  • Edmund C. February,

    1. Department of Botany, University of Cape Town, Private Bag Rondebosch 7701, Cape Town, South Africa
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  • Hendrik J. Combrink

    1. Department of Botany, University of Cape Town, Private Bag Rondebosch 7701, Cape Town, South Africa
    2. Scientific Services, Kruger National Park, Private Bag X402 Skukuza 1350, South Africa
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Steven I. Higgins, Institut für Physische Geographie, Goethe Universität Frankfurt a.M., Altenhöferallee 1, 60438 Frankfurt am Main, Germany.
E-mail: higgins@em.uni-frankfurt.de

Abstract

Aim  It has been proposed that, in tropical savannas, trees deploy their leaves earlier in the growing season and grasses deploy their leaves later. This hypothesis implies a mechanism that facilitates the coexistence of trees and grasses in savannas. If true, this hypothesis would also allow algorithms to use differences in the phenological timing of grass and tree leaves to partition the relative contribution of grasses and trees to net primary production. In this study we examine whether a temporal niche separation between grasses and trees exists in savanna.

Location  A semi-arid, subtropical savanna, Kruger National Park, South Africa.

Methods  We use a multi-spectral camera to track through an entire growing season the normalized difference vegetation index (NDVI) of individual canopies of grasses and trees at eight sites arranged along a precipitation and temperature gradient.

Results  Among trees, we identified two distinct phenological syndromes: an early flushing syndrome and a late-flushing syndrome. Leaf flush in the tree strategies appears to pre-empt rainfall, whereas grass leaf flush follows the rain. The growing season of trees is 20 (late-flushing trees) to 27 (early flushing trees) days longer than that of the grasses.

Main conclusions  We show that grasses and trees have different leaf deployment strategies. Trees deployed leaves at lower temperatures than grasses and retained them for longer at the end of the growing season. The timing of the increase in NDVI is, however, similar between grasses and late-flushing trees and this complicates the separation of grass and tree signals from multi-spectral satellite imagery.

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