THE MORPHOMETRIC STRUCTURE OF THE LARIX GMELLINII RECRUITMENT AT THE NORTHERN LIMIT OF ITS RANGE IN THE FOREST-TUNDRA ECOTONE

The goal of the research presented herein is the investigation of the morphometric and age parameters of the recruitment of forest stands formed by Larix gmellinii, as an indicator of trends in the dynamics of the northern/upper forest boundary in the Russian Arctic, in Northern Central Siberia, in the Taimyr State Biosphere Reserve (the Ary-Mas site), and in the buffer zone of the State Biosphere Reserve “Putorana Plateau.” The morphometric parameters clearly reflect the conditions of growth and regeneration of Larix gmellinii at the northern and upper limits of its range. Both sample sites have relatively harsh conditions for growth and survival. Despite coarse soils (high gravel content) of the Putorana slopes, their significant steepness, frequent landslides and creep, the conditions for Larix gmellinii growth are better than at the Ary-Mas site. This is also reflected in the rate of regeneration. Thus, at the comparable average height of the recruitment at the forest line, its age on the Putorana Plateau is almost half of that at the Ary-Mas site (9.7 and 17.3, respectively). However, the age of the recruitment at the tree line and at the forest line on the Putorana Plateau is practically the same, while at the Ary-Mas site, the recruitment age at the tree line is 1.5 lower than at the boundary of forest. These results could indicate a trend of Larix gmellinii expansion into the ecotone over the last 20–30 yrs., especially in the mountains of the Putorana Plateau.


INTRODUCTION
Studies on the effect of the recent climate change on ecosystem components are of interest from different perspectives: preservation of traditional forms of nature management of the northern indigenous peoples and biodiversity; extraction and use of resources; the thawing of permafrost; carbon balance; etc. This can be seen in a variety of topics and projects carried out within the framework of the International Polar Year (IPY). One of the issues addressed in the IPY projects PPS Arctic (Present Day Processes, Past changes, and Spatiotemporal Variability of Biotic, Abiotic and Socio-Environmental Conditions and Resource Components Along and Across the Arctic Delimitation Zone) and Benefits (Natural and Social Science Research Cooperation in Northern Russia and Norway for Mutual Benefits Across National and Scientific Borders) is the study of the dynamics of the northern/ upper limits of forests using data on the tree regeneration (tree individuals <2 m) as an indicator of this process.
These projects have developed metho dologies for evaluation of forest stands' conditions, i.e., recruitment, age-structure, and morphometric parameters. These data allow assessing trends in vegetation cover. Changes in climatic and edaphic conditions and/or economic activities affect the ability of forest forming species to regenerate [Benkova, 2006;Bulygin and Yarmishko, 2001;Shiyatov, 2000]. Therefore, the abundance, morphologic features, and agestructure of forest stand recruitment are useful indicators of the recent succession processes and dynamics of the boundaries of the natural zones and belts.

RESEARCH OBJECTIVES AND METHODS
The goal of the research presented herein is the investigation of the morphometric structure of the recruitment of forest stands formed by Larix gmellinii, as an indicator of trends in the dynamics (expansion or retreat) of the northern/upper forest boundary in the Russian Arctic. The research used methods in the PPS Arctic and Benefits Programs. The research incorporated the results of the comprehensive field studies in 2010 in Northern Central Siberia, in the Taimyr State Biosphere Reserve (the Ary-Mas site), and in the buffer zone of the State Biosphere Reserve "Putorana Plateau".
In order to implement this goal, the following tasks have been addressed: 1. Investigation of the age of the recruit ment of Larix gmellinii at the northern limit of its range (the Taimyr peninsula, Ary-Mas); 2. Investigation of the age of the regenera tion of Larix gmellinii at the upper limit (elevation) of its range (the Putorana Plateau); 3. Identification of the morphometric and age parameters of the Larix gmellinii recruitment in the forest-tundra ecotone.
Following the guidelines of the PPS Arctic Project protocol, the samples of the Larix gmellinii recruitment were studied within the forest-tundra ecotone at the forest line (i.e. the border of the forest zone or the forest belt) and at the tree line (i.e. the boundary of isolated trees) for three size-groups: height <15 cm, 15-15 cm, and >50 cm, respectively. This methodology was also used in the arctic and alpine forest-tundra ecotones on the Kola Peninsula (Murmansk Oblast) near Lake Kanentiavr and in the Tuliok Ridge, Khibiny Massif ( Fig. 1), respectively [Aune, Hofgaard, Söderström, 2011].

STUDY AREAS
The site Ary-Mas (the Taimyr Peninsula, the State Biopshere Preserve "Taimyrsky") includes the world's northmost forest "island" and is situated at the southern boundary of the arctic tundra [www.byrranga.ru]. This unique site was first described by A.I. Tolmachev and then was studied in detail by L.N. Tyulyna [1937] and the Polar Integrated Expedition of the Botanical Institute of the Academy of Sciences of the USSR [Ary- Mas, 1978]. The forest island is 20 km long and 0.5-4 km wide and is located on the high terrace of the Novaya River. The rest of the territory at this site is occupied by spotty and pit-and-mound sedgy-shrub-moss tundras, polygonal bogs, and, less frequently, by dwarf-birch tundras, floodplain shrubs, nival meadows, and meadow communities of floodplains. Larch forests (or rather sparse forests) in the central part of the "island" are relatively dense (canopy closure 0.3-0.4); they are much sparser to the east and west (canopy closure 0.1-0.2). The tree height is 4-7 m; isolated old larch trees are 10 m high with the trunk diameters of up to 25-30 cm; the average stem diameter is 10-14 cm. Trees of forest fringes with the canopy closure of less than 0.1, along the highest positions of the terraces, are 2-5 m in height; there, they are often lop-sides and have "flag" or "skirt" shapes ( Fig. 2).

DISCUSSION OF THE RESULTS
The averaged morphometric parameters of the sampled Larix gmellinii individuals are presented per area in the table below ( Table 1). The analysis of the morphometric parameters of the Larix gmellinii recruitment allows identification of the trends in the forest stand development in this region presented below.
The age of the Larix gmellinii recruitment at the tree line and the forest line on the Putorana Plato is practically the same, while at the Ary-Mas site, the recruitment age at the tree line is 1.5 lower compared to the forest line. This fact suggests expansion of the northern border of forest and the trend of advancement of larch into the tundra plains over the last 20-30 yrs. At the Ary-Mas site, the recruitment of the third group dominates (the height of 50 to 200 cm). The rate of the Larix gmellinii recruitment decreases with age; in the second group (15-50 cm), the annual vertical growth is almost 15% of the total stem height, while in the third group (taller than 50 cm), the annual growth of the recruitment does not exceed 7% (Table 1; Fig. 4). There are differences in the growth of the recruitment of larch along the studied profile: the speed (intensity) of the vertical growth increases to the north.
A similar pattern of growth of the Larix gmellinii recruitment is observed on the Putorana Plateau. The young growth of the second group prevails quantitatively (the height 15 to 50 cm) ( Table 1; Fig. 5). The highest annual vertical growth of 10% is in recruitment of the first and second groups, while in the third group, it does not exceed 6%.  and 17.3, respectively). However, the age of the recruitment at the tree line and at the forest line on the Putorana Plateau is practically the same, while at the Ary-Mas site, the recruitment age at the tree line is 1.5 lower than at the boundary of forest.
These results could indicate a trend of Larix gmellinii expansion into the ecotone over the last 20-30 yrs., especially in the mountains of the Putorana Plateau and is likely primarily associated with the climate change because anthropogenic impact on the vegetation cover in the conditions of the Reserves' regime is practically absent. However, a firm conclusion can't be made, as the mortality rate within the three size groups of recruiting larch is unknown.