Leaf Demography of Some Evergreen and Deciduous Tree and Shrub Species of Kumaun Himalaya, India

Leaf is the more sensitive organ, which reacts rapidly to environmental conditions and affects the growth and development of other organs [1]. Timiryazev [2] stressed the significance of leaf in the life of the plant and considered equivalent to a plant. Accurate and precise information with respect to amounts and distribution of foliar surface is requisite for reliable estimation of the primary productivity of forested ecosystems. Evans [3] laid emphasis on the study of leaf expansion and dry matter increase (due principally to accumulation of cellulose and lignin) with time. According to Brouwer and DeWit [4], the water stress may often check leaf expansion even before affecting the photosynthesis. Watson [5] emphasized that the yield of plants ultimately depends upon the photosynthetic efficiency and the extent of photosynthetic area. Evans and Huges [6] suggested that the environmental factors generally bring out large changes in specific leaf area.

highest at 400-900 m elevation and lowest at 1600-1700 m elevation. An increasing trend with elevation was found for soil organic matter content (r=0.67, P<0.05), clay (r=0.89, P<0.01) and water holding capacity (r=0.84, P<0.01). Soil pH ranging between 5.1 and 7.9. Among the soil nutrient the available Nitrogen concentration increases with increasing elevation (r=0.81, P<0.05). Available Phosphorus was maximum in the soil of high elevation sites, occupied by forest of oaks (Quercus spp.) and least in low elevation site occupied by Sal (Shorea robusta) forest. The soil of oak forests was markedly richer in available Potassium than those of other forest types such as Sal forest and Chir pine forest (Pinus roxburghii) [34].

Material and Methods
Species and its selection: The selection of evergreen and deciduous species was based on their natural co-occurrence. The species investigated were described in Table 1. To determine the demography of leaves for each of the 9 tree species, 10 average sized mature trees (dbh >31.5 cm), having a similar degree of crown development and 10 shrubs were selected within 1 ha permanent plot of each of the selected forest sites. In each of the selected individuals, 30 vegetative buds were marked in spring 2008. 15 new shoots (originating from marked buds) were tagged and observed for number of leaves randomly at monthly intervals. In this way it was possible to record the periodical changes in number of leaves precisely. Leaves for each of the tree and shrub species were collected randomly on each of the sampling date (at weekly intervals from bud-break to full leaf expansion, and at monthly intervals during the leaf senescence) from the marked twigs. All collected leaves of individual species were sketched on graph paper to measure the leaf area [35,36]. Mean leaf life-span was adapted from Negi [36] who has calculated it by considering the time interval (days) between peak leafing and peak leaf drop on a leaf population basis in a forest stand.

Results
The species investigated (Table 1) can be divided into three groups based on leaf habit: (i) evergreen-leaf fall completed only after substantial development (>80%) of new foliage, thus the trees never become leafless, (ii) semideciduous-as above, but leaf fall completed when new foliage development is limited (<20%), rendering some branches of a tree leafless for a few days or weeks, and (iii) Deciduous-with time gap between leafing and leaf fall, rendering the whole trees leafless for some time in an annual cycle. The periodicity of leafing and leaf drop for the tree and shrub species is depicted in Figure 3.

Leaf demography
Trees: In all the species leaf initiation occurred between mid of March to first half of May. However, leaf initiation was recorded earliest     The peak leaf area was found smallest in I. dipyrena (18.65 cm 2 ) and largest in S. robusta (179.14 cm 2 ). On an average the peak leaf area for DS (87.71) was greater than for the ES (33.67 cm 2 ). Accomplishment    In three DS leaf drop was confined to autumn and winter seasons (cool to cold, dry part of the year). The major period of leaf drop for seven ES was spring through summer season (warm-dry part of the year). In majority of species leaf drop period ranged from 5-8 weeks.

Trees Shrubs
Longest leaf drop period (9-10 weeks) was measured for three ES (D. salicifolia, M. paniculata and M. africana). The DS (V. cotinifolium) retained some leaves until late winter (January and February). This species showed lengthy leaf drop (average 10 weeks), and became leafless only for about one month in winter-spring season. The average leaf drop period in DS (7.2 weeks) is greater than ES (6.6 weeks) ( Table  3 and Figure 5).

Discussion
As in most areas of the Indian subcontinent, the monsoon pattern of rainfall is characteristic climatic feature of this region. The period of mid-June to mid-September when about three-fourth of the annual rainfall occurs is the most favorable season for plant growth, for it is also warm. The majority of tree species presently studied were evergreen. A characteristics feature of trees was that the concentrated leaf drop activity immediately followed the spurt of leafing. In present study, the leaf initiation in tree species occurred between mid-March to first half of May, however, it was recorded earliest i.e. in

Species
Leaf area at full expansion (cm 2 ) Leaf expansion rate (cm 2 day -1 ) % leaf area after two weeks of expansion % leaf area after four weeks of expansion    October) and F. strobilifera were last to produced leaves in August-September. There existed a positive relationship (r=0.470, df=19, P<0.05) between leafing period (days) of the tree and shrub species and elevation in which they occur. In a majority of species including those which form most of the forests the bud-break occurred within a month from mid-March to mid-April, when temperature and day length began to rise (Figure 3). A significant correlation was found between the leaf initiation day and mean temperature ( o C) of particular day for tree (r=0.468, df=17, p<0.05) and shrub species (r=0.456, df=19, p<0.05) of two study years.
Studies [37,38] have implicated day length and air temperature [39] increase as the inducer of leaf flushing, which holds true for the present study area where peak activity of bud break and leafing takes place during March-April when photoperiod and temperatures are increasing [34]. In the study sites, though the soil moisture continues to be low from October to mid-June, the long dry spells are broken by isolated rain showers (average monthly rainfall is 14.  [40] reported that a storm of 20 mm rainfall could revive the water potential sufficiently to support growth in Tabebuia neochrysantha.
In B. variegata the bud-break occurred in May, i.e. long after, it occurred in other species. In May photoperiods were close to the maximum within the annual cycle and the mean temperature was about twice as high as in January and February. Young leaves of these species are likely to be more adapted to water stress than the majority of the species which produced leaves earlier. For, because of higher temperatures water stress must be more severe at the end of the drought period [36].
In most species, leaf drop also occurred during the dry seasons immediately after rainy season in deciduous species and towards the end of dry period (mid-March to mid-April) in evergreen species. While in case of deciduous species photoperiod and temperature were declining at the time of leaf drop, in case of evergreen species both were increasing. The short flushing behavior is characteristic to temperate trees [41,42]. Some of the species of lower elevations (e.g. S. robusta) showed longer leafing period, thus suggesting the retention of a. character, attributed to tropical species [39,43,44]. Earlier and rapid leaf expansion in deciduous species than in evergreen species is considered typical of northern temperate trees and shrubs [45][46][47][48]. Gill and Mahall [49] demonstrated that in a chaparral shrub community of California, the deciduous species (Salvia mellifera) did not initiate growth significantly earlier and at faster rate than the evergreen species (Ceanothus megacarpus) co-occurring at a set of same microsites. Though in general, a majority of the leaves were recruited during spring/summer season and additional recruitments, generally on terminal portion of shoots occurred during rainy season or/and autumn season. However, leaves produced in each of these seasons were shed at the same time, indicating that those recruited during rainy and autumn seasons had shorter life -spans than those recruited during spring/summer season.
Of special interest is the leaf phenology of evergreen species which dominate the forests of this region, out competing deciduous species in mature communities of most habitats [33]. Evergreen species retain leaves throughout the winter and exhibit simultaneous leaf fall an emergence in summer resembling the "leaf-exchanging type" species of tropical forests [50]. Retaining leaves throughout a year enables the Evergreen species to utilize the same unit of nutrients to support the new growth [51] and maintain some photosynthesis throughout the winter [52]. This strategy makes a tight circulation of nutrients in the ecosystem, a characteristic feature of late successional communities [53] occupied by the evergreen species in this region. Although the Deciduous species of this region has significantly greater area compared to Evergreen species (87 .71 vs 33.67 cm 2 , trees; and 20.52 vs 11.06 cm 2 , shrubs), and they are known to have higher photosynthetic efficiency [54,55].