Abstract
Long-term studies of tree responses to drought stress help us to understand the capacity of species to adapt to their environment. In this study, we investigated how Eucalyptus obliqua adjusts physiological and morphological traits in response to seasonal and multi-year droughts. We monitored physiological and morphological traits over multiple years in undisturbed control and throughfall reduction plots in a eucalypt forest in south-eastern Australia. The throughfall reduction treatment did not induce significantly lower soil moisture in the throughfall reduction plots compared with the control plots. However, natural variability in precipitation and evaporative demand induced drought stress of varying intensity each summer in all plots. We observed a significant relationship between seasonal precipitation and leaf pre-dawn water potential (ΨPD), with less precipitation over summer, resulting in a decline in ΨPD and drought stress when ΨPD fell below −0.75 MPa. Eucalyptus obliqua responded to short-term summer drought through rapid leaf osmotic adjustment which lowered the leaf water potential at the turgor loss point beyond the minimum leaf water potential. Morphological adjustments, such as the reduction of leaf area to sapwood area (higher Huber Value) were moderate during the measurement period and only occurred under severe drought stress (pre-dawn water potential < −1.2 MPa). Overall, E. obliqua responded to short-term mild drought stress through physiological trait plasticity, while morphological adjustment only occurred under a more severe water deficit.
Highlights
Seasonal droughts are experienced by most tree species, and trees have evolved mechanisms to tolerate regular water deficits [1,2]
The ΨPD, ΨMD, ΨTLP, Ψsolute, leaf size (LA) and specific leaf area (SLA) showed no significant differences in seasonal trait expression between control and throughfall reduction treatments at any point in time
We observed no differences in ΨPD between treatment and control plots and except for the Huber value, no other traits changed in response to throughfall reduction
Summary
Seasonal droughts are experienced by most tree species, and trees have evolved mechanisms to tolerate regular water deficits [1,2]. To predict future changes in forest cover, productivity and species composition, a better understanding of the mechanisms driving how trees respond to drought is needed [5]. We have a limited understanding of how trees adjust to drier conditions over multiple years [6], how adjustments differ among species [7] and if these adjustments can improve drought resistance enabling the forest to recover [8,9]. Tree drought responses are often investigated in young seedlings in short-term controlled environment studies. There is a need to study the drought response of mature trees within the natural environment, which makes multi-year field studies extremely valuable. These studies are Forests 2020, 11, 1371; doi:10.3390/f11121371 www.mdpi.com/journal/forests
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