Diversity and distribution of physical dormant species in relation to ecosystem and life-forms

Impermeable seed/fruit coat, i.e. physical dormancy (PY) occurring only in several genera of 18 angiosperm families plays an important role in controlling seed persistence and germination timing. It has been theoretically speculated that PY is more prevalent in drylands than in moist vegetation zones, but unequivocal support for this assertion is currently unavailable. The broad objective of this contribution was to examine the distribution of PY on the various vegetation of tropics and temperate ecosystems using a data set of 13, 792 species. The number of species with PY in tropics (19%) is higher than the number of PY species in the temperate ecosystem (15%). However, in both tropics and temperate, there is a clear trend that PY is less common in moist and low-temperature vegetation zones compared with dry and high-temperature vegetation. In tropics, PY is more prevalent in dry woodlands (33%) and tropical deciduous forests (27.3%) compared with the evergreen rain forest (9%). Similarly, in the temperate zone, dry vegetation with seasonal rainfall such as Matorral (22.3) and deserts (19.5%) have a higher number of PY species compared with moist warm woodlands (8.1%) and deciduous forest (9%). Although PY is a trait found in various life-forms, it appears to be less common in trees, particularly of the temperate zone. We discuss the ecological adaptation of PY in the dry ecosystem and consider the mechanism of persistence and dormancy break in PY and physiological dormant (PD) species.