Dead or alive? Testing the use of C:N ratios and chlorophyll fluorescence in vertical shoot profiles to determine depth of vitality and point of senescence in populations of bryophytes

© 2015 The Authors. This is an Open Access article. Traditionally, senescence in plants is defined as internally programmed degeneration or age-related declines in survival and reproduction, leading to death or variation in mortality patterns ��oach 200��. �lants �ith indeterminate gro�th, such as clonal plants or bryophytes, may escape determinacy of death or aging by continuous gro�th �ith production of ne� meristems, thereby becoming potentially immortal and escaping �hole organism senescence �Watkinson and White 1986�. While the life span of clonal plants can be reflected in birth and death rates of its specific modules �i.e. repeated, structural and functional subunits of plants, de Kroon et al. 2005�, it is rare that bryophytes �ith indeterminate gro�th exhibit clearly identifiable modules or age segments, and it is also difficult to kno� if translocation of resources takes place or not bet�een such modules. One exception is Hylocomium splendens �Hed�.� Schimp. �here most shoots express sympodial or monopodial gro�th patterns �ith clearly distinct modules �Økland 1995�. Clonal bryophytes often exhibits different active zones vertically along their shoots or turfs. For example, in environments �ith little physical disturbance it is possible to find intact moss shoots that are 5–15 cm long, �ith green active upperparts, �hile the lo�est parts disintegrate into bro�n humus. Bryophyte shoots can be vertically divided into different regions of physiologically active zones, �here the youngest and apical shoot parts are the most physiologically active �ith a clear fresh green color. The physiological activity usually declines vertically in the shoot profile depth �ith senescence, aging and final decomposition �Bates 1979, Økland 1995, Tobias and Niinemets 2010�. Thus, bryophytes continues to gro� up�ards from the apical regions concurrently �ith being decomposed from underneath �Glime 2007�. Obviously, the gro�th of the shoot apex must be greater than the decomposition rate in order for the bryophyte shoot to continue to stay alive, photosynthesize, and avoid burial by the surrounding shoots �Økland 2000�. Three different physiological zones in the shoot profile depth of clonal bryophytes can be described: 1� Active apical zone In the first and uppermost zone of the moss shoot, the green moss apex is physiologically active �ith high amounts of chlorophylls and nitrogen �Tobias and Niinemets 2010�, and lo� carbon:nitrogen ratio, i.e. C/N �Nakatsubo 1990�. Observations by Bates �1979� and Tobias and Niinemets �2010� from single populations in UK and Estonia, respectively, confirms that physiological activity �in e.g. phytosynthesis and chlorophyll content� in the uppermost 2 cm of the perenDead or alive? Testing the use of C:N ratios and chlorophyll fluorescence in vertical shoot profiles to determine depth of vitality and point of senescence in populations of bryophytes.