Ecophysiological characteristics of leaves and shoots of Quercus hondae and Q. gilva in relation to their habitats

Abstract

Quercus hondae, an endangered evergreen oak, mostly inhabits on a lower position of slopes in southeastern Kyushu, Japan, while Q. gilva, a common species of the same region, has a wider range of topographic distribution. We compared the ecophysiological plasticity of leaves and shoots of Q. hondae with that of Q. gilva in order to examine the reason of its limited distribution. We determined parameters of leaf gas exchange, water relations, and leaf and shoot morphology for those corrected from three growing habitats (sunny and shaded crown under a moist soil, and sunny crown under a xeric soil). Based on the results, we examine the two hypotheses for the habitat characteristics of lower slopes in comparison with upper slopes; 1) better light regime maintained by frequent natural disturbances, and 2) better soil moisture condition. The two species showed almost same trends in maximum net photosynthesis, dark respiration and the light compensation point in response to three habitats, indicating the similar shade tolerance. On the other hand, the water relation parameters demonstrated clear differences between the two species. Q. hondae showed quite low plasticity of its leaf water relations in terms of small variations through the three habitats for each of osmotic potential at full turgor (Ψs,sat), leaf water potential at the turgor loss point (Ψl,tlp), relative water content at the turgor loss point (RWCtlp), and amount of losable water per unit leaf area until the turgor loss point (Wtlp/LA). In contrast, Q. gilva had lower Ψs,sat, Ψl,tlp and higher Wtlp/LA under the environment where leaves are easier to loss water. Q. gilva also had a wider range of the ratio of branch basal area to total leaf area per shoot. These suggested a high plasticity of leaf water relations and shoot morphology of Q. gilva against water stress. We concluded that the low plasticity of leaf and shoot water relations of Q. hondae must be one of the reasons for its natural distribution limited to lower slopes.