Time constraints

4.6.1 Phenology: leafing, flowering and seed dispersal

The time at which various natural events, such as the first opening of oak tree buds or the flowering of bluebells, varies from year to year. This area of science is called phenology, the study of recurring natural phenomena, especially in relation to climatic conditions. Further consideration of phenology, including leafing dates and the effect of climate change, can be found in Section 11.3.2.

Deciduous trees of various species commence leafing and leaf fall at different times, so the lengths of the annual periods during which they photosynthe-size differ considerably. In Cantreyn Wood, Bridgnorth, Shropshire, England at the beginning of November 2003 ash had long since lost its leaves, and those of beech and hazel were following fast, while many of the oaks retained much of their canopy. The now yellowed leaves of the larch had yet to fall, while the dark green outlines of the Norway spruce were more prominent than ever, along with the ivy that grows in much of the wood. By the middle of the month many of the oaks and larch had lost a high proportion of their leaves, while the male catkins of hazel were prominently developed, though pollen discharge did not occur until mid January. The autumn of 2004 was very mild and some hazels were discharging pollen by 1 December, at which time a view over the Severn gorge at Bridgnorth showed that many deciduous trees, particularly oaks, had still retained a fair proportion of leaves though these were now falling.

Observations (JRP) of three English oaks at the southern margin of Cantreyn Wood, Shropshire, in the English Midlands reveal a similar story. A photograph taken on 4 May 2003 shows, as is usual, the western tree leafing first, followed by that to the east. Leaves of the central oak did not expand fully until the end of the month; some other oaks in the wood were even later. The same sequence occurred in both 2004 and 2005. Leaf fall does not necessarily follow the same order. In autumn 2003, after a very hot summer, the tree to the west retained the highest proportion of its leaves, while that in the centre had lost the most and the tree to the east was intermediate.

Phenology is also an important factor in tropical forests; indeed Richards (1996) has a whole chapter on the topic, devoting a considerable space to studies of leaf change. Tropical trees may be evergreen, develop their leaves continuously over the whole year, or expand them in conspicuous flushes. The total number of leaves in many evergreen species is considerably reduced before the new leaves flush. Deciduous trees may show marked lack of synchrony; in Ghana Ceiba pentandra may have expanded leaves on one half of the crown while the other is completely bare. The phenology of fungi is also interesting; in Malaya the fruit bodies of the larger fungi appear after dry spells and in normal years a succession develops over a 3-month period with each species having a season of a week or so.

A recent study by Hamann (2004) of the influence of climatic factors upon periods of flowering and fruiting in 5800 trees of the North Negros Forest Reserve on the island of Negros, Philippines only 10° 410N of the equator, is of considerable interest. The study area is a transition area between the lowland evergreen rain forest and lower montane forests. Elements of both are found here on volcanic soils at a height of 1000 m on the north-west slope of Mount Mandalagan. Though daily maximum and minimum temperatures remain relatively constant throughout the year, in which day length varies by only an hour, typhoons are frequent during the winter monsoon from July to November.

Figure 4.15 shows average monthly rainfall and daily maximum and minimum temperatures, which sometimes differed by well over 15 °C, at the study

1996 1997 1998 1999

1996 1997 1998 1999

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Figure 4.15 Average monthly rainfall (bars) and daily maximum and minimum temperatures (upper dashed and lower dotted lines, respectively) at 1000 m on the north-west slope on Mount Mandalagan, Island of Negros, Philippines. (From Hamann, 2004. Journal of Ecology 92, Blackwell Publishing.)

JFMAMJJASONDJFMAMJJASONDJFMAMJJASONDJFMAMJJASOND

Figure 4.15 Average monthly rainfall (bars) and daily maximum and minimum temperatures (upper dashed and lower dotted lines, respectively) at 1000 m on the north-west slope on Mount Mandalagan, Island of Negros, Philippines. (From Hamann, 2004. Journal of Ecology 92, Blackwell Publishing.)

Trees flowering Bird dispersal Gravity or unknown vector dispersal

Trees flowering Bird dispersal Gravity or unknown vector dispersal

Figure 4.16 Number of trees flowering in the submontane rainforest at 1000 m on the north-west slope on Mount Mandalagan. The figure also shows the number of trees fruiting, partitioned by how the fruits/seeds are dispersed. Fruit production fluctuates almost twice as much between years as flowering. The proportion of trees with animal-spread fruits increases around February and September, and was lower during 1998 and 1999. Bat-dispersed species fruited throughout the year during 1996 and 1997, but was more seasonal in 1998 and 1999. Fruiting of wind- and gravity-dispersed species was restricted to July to October and did not appear to be influenced by climatic fluctuations (Fig. 4.15). (From Hamann, 2004, Journal of Ecology 92. Blackwell Publishing.)

Figure 4.16 Number of trees flowering in the submontane rainforest at 1000 m on the north-west slope on Mount Mandalagan. The figure also shows the number of trees fruiting, partitioned by how the fruits/seeds are dispersed. Fruit production fluctuates almost twice as much between years as flowering. The proportion of trees with animal-spread fruits increases around February and September, and was lower during 1998 and 1999. Bat-dispersed species fruited throughout the year during 1996 and 1997, but was more seasonal in 1998 and 1999. Fruiting of wind- and gravity-dispersed species was restricted to July to October and did not appear to be influenced by climatic fluctuations (Fig. 4.15). (From Hamann, 2004, Journal of Ecology 92. Blackwell Publishing.)

site for the 4-year period 1996-1999. This should be compared with Fig. 4.16 that gives the number of trees flowering and fruiting during the same time period, and the general phenological patterns displayed by trees whose fruits are dispersed by birds, gravity or an unknown vector, wind, fruit bats or other mammals. The general similarity between the year on year fruiting patterns was influenced by the major El Nino that peaked in November 1997, causing a warming of the equatorial eastern Pacific Ocean, and a subsequent La Nina climate anomaly (cooling of the waters) that developed throughout the summer of 1998 with a peak in September. Such ocean temperature anomalies influence the western Pacific with Philippine weather patterns showing several months delay. Rainfall from February to May 1998 was exceptionally low.

The typhoon season (July to November) coincided with the extended periods of fruiting of the wind- and gravity-dispersed species. Fleshy-fruited trees showed peaks matching those of solar irradiance, either flowering at the beginning and fruiting at the end of the first peak (April), or flowering during the first peak and fruiting during the second (September). Some 95% of the intraspecific variation (i.e. variation within a species) of flowering and fruiting dates during the El Nino of 1997 and the La Nina climate anomaly of 1998 was ascribed to the delayed or advanced flowering and fruiting of a limited number of species. Mast fruiting in dipterocarps could not be correlated with the climatic events of 1997-8.

4.6.2 Seasonal climates, seed banks and dormancy

Persistent seed banks can be found in the soil or in the tree canopy. The largest of these is the soil seed bank, created by seeds falling to the ground and becoming buried under litter and humus, or, in seeds with an elaiosome, being abandoned underground once the fatty body has been eaten. Salisbury (1961) emphasized the differences in germination patterns, dormancy and longevity of buried seeds. These features, together with the development of soil seed banks, are important in the reproduction of vascular plants. Figure 4.17 shows the four types of seed bank detected by Thompson and Grime (1979) by sampling the surface soil (0-3 cm depth) of ten ecologically contrasted sites in the Sheffield region. Sampling took place at 6-weekly intervals; the experiment was designed to detect both the transient accumulation of germinable seeds during the summer and persistent seed banks. Some species were in several types of habitat. The results suggested that seasonal variation in seed number is related to the species concerned rather than to the environment involved. Type IV seed banks are the most important in woodlands, where disturbance is likely to occur at long and very irregular intervals.

Dormancy of seeds stored in the soil can be caused by a variety of mechanisms including an impermeable seed coat (as in acacias and other legumes), a chemical inhibitor, a chemical lack, a need for winter chilling or (in a few woody species) a need for light. It can also be caused by immaturity of the embryo at the time of seed shedding. Fruit of the European ash Fraxinus excelsior falling in autumn is unable to germinate immediately as a period of embryo growth, in which its length almost doubles, is required. Embryo growth ideally needs a temperature of 18-20 °C. Only after this takes place

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Figure 4.17 Four types of seed bank described by Thompson and Grime (1979). Shaded areas: seeds capable of germinating immediately after removal to suitable laboratory conditions. Unshaded areas: seeds viable but not capable of immediate germination. Type I, annual and perennial grasses of dry or disturbed habitats. Type II, annual and perennial herbs colonizing vegetation gaps in early spring. Type III, species mainly germinating in the autumn but maintaining a small but persistent seed bank. Type IV, annual and perennial herbs and shrubs with large persistent seed banks. (From Grime et al., 1988. Comparative Plant Ecology. Unwin Hyman.)

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Month

Figure 4.17 Four types of seed bank described by Thompson and Grime (1979). Shaded areas: seeds capable of germinating immediately after removal to suitable laboratory conditions. Unshaded areas: seeds viable but not capable of immediate germination. Type I, annual and perennial grasses of dry or disturbed habitats. Type II, annual and perennial herbs colonizing vegetation gaps in early spring. Type III, species mainly germinating in the autumn but maintaining a small but persistent seed bank. Type IV, annual and perennial herbs and shrubs with large persistent seed banks. (From Grime et al., 1988. Comparative Plant Ecology. Unwin Hyman.)

can prolonged stratification (exposure to low temperature) at about 5 °C, overcome dormancy. Because of this, ash seeds usually germinate in April or early May 2 years after the flowering season in which they were formed. A very small proportion germinates after the first winter, and when foresters wish to germinate ash quickly seeds are picked green in August and sown immediately.

Seeds can remain dormant and viable for centuries but, as described in Section 9.2.2, the bank rarely includes tree species (although there are notable exceptions such as cherries Prunus spp.). The reason for this lack is probably due to the comparatively large size of many woody plant seeds: they are less likely to be missed by rooting herbivores and will also be more eagerly sought after as a worthwhile meal.

The most effective seed bank in trees is found in the canopy of more than 500 different species of tree growing around the world in fire-prone areas. In these canopy seed banks the seeds are stored in fireproof fruits that open after they are burnt or heated by a fire. This is referred to as serotiny. A number of pines in the northern hemisphere are serotinous (such as the lodgepole pine Pinus contorta and jack pine P. banksiana of North America, and the Aleppo pine P. halapensis and maritime pine P. pinaster of Europe - see also Section 3.7.1)

but the majority of serotinous trees are found in the southern hemisphere among evergreen hardwoods (such as the proteas, banksias and eucalypts of the Cape area of South Africa and particularly the shrublands (bush) of southwest Australia). The development and characteristics of such seed banks in post-fire stands of four species of pine growing in south-east Spain were studied by Tapias et al. (2001). Aleppo pine started to fruit at 5 years of age and more than 86% of the cones were serotinous (sealed by resin); they had opening temperatures between 49.3 and 51.3 °C. The canopy seed bank of this species was much greater than in maritime pine, in which the proportion of serotinous cones was lower (66.7%). Only one population of this species was studied, in which first fruiting was later (12 years) and cone-opening temperature was almost 5 °C lower than in the Aleppo pine.

Serotinous cones help Aleppo and maritime pines survive fire; early flowering is also essential in such species, which grow in areas where crown fires are frequent. It is suggested that the late flowering and lack of serotinous cones found in the black P. nigra and Mediterranean stone pines P. pinea, where flowering is insignificant even 15 years after fire, results from evolution in areas where ignition led only to low-intensity ground fires.

This chapter has examined the responses made by forest plants to the environments in which they live. It pays particular attention to their regenerative strategies, the influence of animals, time constraints and the role of seed banks. The strategic responses of all living components of the ecosystem are of particular relevance to forest change and disturbance (Chapter 9) and to climate change (Section 11.3).

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