The Causes of Inherently Slow Growth in Alpine Plants: An Analysis Based on the Underlying Carbon Economies of Alpine and Lowland Poa Species

Abstract

To investigate the underlying causes of the inherently low relative growth rate (RGR) in alpine plants, we compared the carbon economy of six altitudinally contrasting Poa species. These included two alpine (P. fawcettiae and P. costiniana), one sub-alpine (P. alpina) and three temperate lowland (P. pratensis, P. compressa and P. trivialis) species. All species were grown hydroponically under identical controlled conditions. The RGRs of each of the selected species were: P. fawcettiae (111 mg g -1 day -1 ), P. costiniana (125 mgg -1 day -1 ), P. alpina (166 mg g -1 day -1 ), P. pratensis (179 mg g -1 day -1 ), P. compressa (188 mg g -1 day -1 ) and P. trivialis (255 mg g -1 day -1 ). The slow growth of the alpines could not be attributed to lower rates of photosynthesis per unit leaf area. Similarly, higher rates of shoot and root respiration rates per unit dry mass per se could not explain the slower growth exhibited by the alpine species. The differences in RGR were also not owing to differences in whole plant carbon concentration or the proportion of whole plant biomass allocated to the leaves, stems and roots. The only parameter that did explain the variation in RGR was specific leaf area (SLA, leaf area per leaf dry mass), with the alpine species exhibiting significantly lower SLAs than the sub-alpine and lowland species. Consequently, the rate of photosynthesis per unit leaf dry mass was substantially lower in the slow-growing alpine species. The alpine species also used a greater proportion of their daily fixed carbon in respiration than did the fast-growing lowland species. We conclude that low SLA is the primary cause of the inherently low growth rates of the selected alpine Poa species in our controlled environment. The possible causes of low RGR of field-grown alpine plants are discussed.