REPRODUCTIVE PHYSIOLOGY AND ECOLOGY OF HORSE - CHESTNUT (Aesculus hippocastanum L.)

Abstract

The horse-chestnut (Aesculus hippocastanum L.) is a deciduous tree native to the Balkans with physical, geographical allocation restricted to Greece and in a much smaller degree in Albania and in FYROM. It traditionally belongs to the family of Hippocastanaceae (which includes two genus: Aesculus with 13 deciduous type of trees and Billia with 2 evergreen type) recently molecular data (Angiosperm Phylogeny Group) has categorized the whole family of the Hippocastanaceae in the Sapindaceae.
The basic aim of this dissertation focuses on the study of the reproductive biology of this type. During the period of 2003-2006 floral matter was collected (fruit and seeds) from natural subpopulations in Greece, from 3 main natural areas of diffusion, (Grevena – mountain range N. Pindos, Karditsa – Mount Agrafa and Larissa – Mount Kissavos) and 1 technical subpopulation (a 45 year old plantation in Kissavos). The goals of this dissertation were researched with repeated experimentation and observation so much in the laboratory as in the field. The seed count (and fruit) of the 3 natural origination as well as the plantation from Kissavos during the time period 2003-2006 regarded: the mass of the fruit and seeds, the contribution percentage of fresh and dry mass of the seed parts and the water-content of the parts and the entire seed.
Also growth experiments were conducted for 3 years in the laboratory (Department of Botany, Faculty of Biology, N.K.U.A.) in a significant number of seeds (about 2500-3000 for each year) as much right after the collection as well as later, gradually, with manipulation in between by humid-cold layering (at 5-8 °C) simple storage of the seeds in polythene bags (at 5 °C). For the study of the gradual arsis of lethargy the growth experiments were carried out at temperatures of 5, 10, 15, 20, 25, 30 and 36 °C. Also a control of the viability of the seeds was carried out after partial dehydration at deferent levels of humidity, as well as seeds which were stored at 5 °C for a time period of 1 year and 18 months after their collection. The motion of the imbibition, the increase of the embryonic axis and the possibility of long term storage after partial dehydration, as well as the ability of seed growth in polythene bags (without imbibition) was also studied. At the area (Kissavos) during the time period of 2003-2006 there were established 10 experiments with n=50 seed samples each time, with origins from Kissavos. Seed growth was confirmed in spring (March) and in the young saplings their survival and phenological rate of growth during the 1st year was studied (March – October), meaning, from their germination up to the falling of their leaves. The survival of the saplings was also studied right after their natural regeneration in 2 natural subpopulations in Kissavos.
Also there were carried out for the time period of 3 consecutive years, phenological observations and measurements which concerned the reproductive potential (flowering, fruit set, maturation) of specific (marked) adult specimens in 3 natural areas and research was also done on the dispersal of the seeds at Kissavos as well as extraneous dangers and threats that this type faces in its natural subpopulation.
Supplementary goal of this dissertation was the exploration and mapping for the first time, of the detailed diffusion of this type in Greece. With on site visits during the time period of March 2005 – August 2006, the presence of the species was confirmed in the mountainous bulges of Epirus, W. Macedonia, Thessaly and Sterea Ellas. For each subpopulation the total of the adult specimens was counted and various nonbiotic and biotic parameters were noted: geographical positioning, geological substrate, altitude, slope, exposure, surface occupancy, habitat type, accompanying species, degree of natural regeneration and other. From the mathematical process of the data there emerged graphs and conclusions, pertinent to the allocation of the size of the subpopulation, the vertical distribution, the ecological characteristics of the habitat, the regenerative ability etc. Moreover, the various dangers and threats, the species is facing, were estimated. Finally, with the application of the RAMAS
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software, the conservation status of the horse-chestnut in Greece is evaluated according to the new criteria of the IUCN.
The basic conclusions of this dissertation are summarized as follows:
 The ripe seeds of the Aesculus hippocastanum have an average mass of 14,45 g (the heaviest of the Greek and European flora) while the average mass of the fruit is 42.14 g.
 The water content at seed dispersal is about 50%. The seeds of this species are unorthodox (recalcitrant), meaning they cannot tolerate dehydration.
 The viability of the seeds is nullified at humidity content levels smaller than the “crucial” (or “fatal”) values, which are at about 20%, whereas also the long term viability of the seeds in storage conditions is relatively low (shorter than 2 years).
 The embryonic axis of the seed increases in length by 15% during its storage at 5 °C.
 Right after the collection, the seeds are dormant. The growth is satisfactory (about 60%) only at 30 °C while at 20 °C and lower it is 0%.
 Dormancy release is accomplished with stratification (cold humid layering) or simply with storage (at 5 °C). With increasing durations of stratification or storage, germination is improved and the temperature window of germination is widened towards lower temperatures. The complete dormancy release results with about 16-17 weeks of stratification.
 The temperature base Tb of the growth is about 19 °C and is reduced by about 1 °C every 6 days. After about 90-110 days the Tb reaches the levels of stratification or storage (5 – 8 °C).
 Both types of control, i.e. stratification and storage at 5 °C result in similar germination induction. After 150 days of seed storage at 5 °C, full germination was observed even at low temperatures (5, 10 and 15 °C).
 A considerable number of seeds (37% in our experiments) are able to grow without imbibition, in the storage bags at 5 °C after a time period of 115 days.
 The seeds (right after their ripening) remain dormant covered by leaf litter and snow (conditions which are simulated in the laboratory with the stratification treatment). The temperatures recorded (with a digital temperature recorder) at the site of the naturally occurring cold conditions of the seed do not fall below 0 °C.
 The germination and early growth of the seeds is “hypogeal” (i.e. the cotyledons remain on the ground and are not transformed into photosynthetic organs). It is manifested at the beginning of spring (March) and the final percentages of the relative emergence experiments ranged from 12-90%.
 The viability of the saplings during the 1st year is considerably decreased and affected by biotic and climatic effects.
 The number of flowers per inflorescence range from 23-37 and the ripening of the seeds (October) is completed about 130 days from the end of flowering.
 The fruit set is limited to the base of the infrutescence and is affected considerably by the climatic conditions (rain, frost, wind).
 The species displays the phenomenon of masting, i.e. an annual, considerable quantitative fluctuation of its reproductive effort, with a period of 2 years (masting) and indeed synchronized throughout its distribution range.
 The main factor of natural dispersal of seeds is primarily their weight (autochory – barochory) and secondarily water (hydrochory).
 Its leaves are affected by the insect Cameraria ohridella, with the result of the reduction of the photosynthetic surface, the premature falling of the leaves and a considerable reduction of the mass of the seeds, while it was observed that affected trees bloom for a second time (in Autumn).
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 A total of 1464 adult trees of horse-chestnut and 98 subpopulations in 14 prefectures of Greece were counted.
 A considerably marked fragmentation of the natural distribution of the species is observed along the 2 main mountain ranges of Central and Northern Greece, at the mountainous massifs of the geographical districts of Epirus, W. Macedonia, Thessaly and Sterea Ellas.
 Out of the total of 1464 adult trees of the horse-chestnut, the majority is recorded at the mountain range of Pindos (37.6%), followed by Kissavos (10.0%), Agrafa (9.9%), Vardoussia (8.7%) and Voion (8.3%).
 A considerable vertical distribution is observed: from 218 m (Kissavos) to 1485 m (Mikra Ondria, Kastoria), with a maximum at the class of 900-1100 m, both in regard to the total number of individuals (35%) and the total number of subpopulations (31%). In the altitude range of 500-1100 m: 1051 adult trees grow (72% of the total number, belonging to 64 subpopulations).
 The size of the subpopulation shows a considerable range (from 1 to 153 individuals) whereas the mathematical average population is comprised by 15 adult trees. 63% of the populations of the horse-chestnut belongs to the class of 1-10 trees.
 The horse-chestnut is considered a remnant of the last ice age where it survived at refugia sites where the presence of water mitigates the extreme temperatures. The growth of the species demands the constant or casual presence of water and air humidity: 62% of the total number of subpopulations is found in ravines with a constant flow of water, 11% in ravines with casual flow and 9% in dry ravines. The rest 18% is observed by 12% in the forest and 6% on the slopes of the road.
 Generally the species seems to grow and develop in mineralized ground and not in grounds with shaped areas and exhibits an average height of 10-14 m and chest high diameter of 25-40 cm.
 From the detailed recordings of the accompanying species in each subpopulation, the presence of the following species prevails: Abies borisii regis (at a percentage of 52% of the total of the subpopulation), Fraxinus ornus (41%), Fagus silvatica (37%), Platanus orientalis (12%), Salix alba (11%) and Juniperus communis (10%).
 A particularly worrying fact recorded is the absence of natural regeneration at 62% of the entire subpopulation while only 6% of the population satisfactory regeneration.
 The immediate threats that the species faces are: erratic winter frosts, drought, flooding, wildfires, seed predation by various animals (cattle and other farm and wild animals), logging and collection of seeds for pharmaceutical purposes. Moreover, the projected climatic changes are very likely to cause serious negative impacts.
 On the basis of the new criteria of IUCN (V3.1, 2001), it is concluded that the conservation status of the of the species is CRITICALLY ENDANGERED – criteria B2ab (iv,v)
 Management proposals for the Aesculus hippocastanum L.: inclusion of the species in the new edition of the Red Data Book of the Greek Flora, increased familiarization of the public with the species, cultivation of the species in Botanical Gardens and a its wider utilization as an ornamental tree