From glacial refugia to hydrological microrefugia: factors and processes driving the persistence of the climate relict tree Zelkova sicula

With only two tiny populations, the climate relict Zelkova sicula (Sicily,
Italy) is one of the rarest trees in the world. It also represents the
most marginal member of genus Zelkova that was widespread in the
broadleaved forests thriving in warm-temperate climates throughout Eurasia
until the Last Glacial Age. Occurring at the westernmost range of the
genus under typical Mediterranean climate, the micro-topographic settings
have always appeared crucial for the survival of this Sicilian relict.
However, the factors and processes actually involved in its persistence in
the current refugia, as well as the response of similar relict trees in
arid environments are poorly understood worldwide. In the aim to elucidate
these aspects, in the two sites hosting Z. sicula analyses of
topographical attributes were combined with investigations on soil
moisture dynamics. Additionally, plants’ growth and spatial distribution
patterns were analysed to detect fine-scale differences between
populations and assess the possible ecological amplitude of the species.
Results revealed that convergent topographies are basic determinants of
microrefugia in arid environments. Within the investigated sites,
underground moisture never decreases below 25%, buffering seasonal
rainfall fluctuations. Therefore, hydrological microrefugia play a
key-role in decoupling from regional climate, supporting the target
species in coping with an unsuitable climatic envelope. Additionally, the
inter-population variability of biometric attributes showed that
individual growth is site-dependent and the species retains a relative
ecological plasticity, whereas the strongly clumped spatial patterns
confirmed the exclusive vegetative propagation and the common clonal
growth. On one hand, deeply incised landforms have acted as effective
hydrologic microrefugia, on the other clonality coupled with triploidy
supposedly improved the resistance of Z. sicula to harsh environments,
though entailing inability to reproduce sexually. Most likely, sterility
and environmental/physical barriers prevent since millennia this relict
from leaving the last suitable microrefugia, resulting in the two current
rear-edge populations.