Growth, hydraulics and functional traits of Melia dubia across a steep climatic gradient in southern India
Doctoral thesis
Date of Examination:2022-09-27
Date of issue:2022-11-16
Advisor:Prof. Dr. Dirk Hölscher
Referee:Prof. Dr. Dirk Hölscher
Referee:Prof. Dr. Bernhard Schuldt
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Abstract
English
The global demand for wood has been continuously increasing and the world is also witnessing large-scale deforestation. This is largely contributing to global warming, land degradation and biodiversity loss. Quite a large number of countries in the world are importing wood and wood products for meeting domestic demands. In India, for example, the import value of wood and wood products increased from 0.04 billion USD in 1961 to 8.7 billion USD in 2019. Existing forest policy regimes and economic reforms are expected to create additional demand for wood and shortage of domestic supply in the future. To reduce the pressure on forests and to ensure wood supply, short rotation plantations with native and exotic tree species are being promoted in agroforestry and farm forestry systems. For augmentation of domestic wood production and to fetch early income to the farmers, Melia dubia Cav. (Meliaceae) has been identified as one of the potential Indian native species for plantations and agroforestry systems. To explore the ecology and production potential of the species an investigation was set up with following the objectives: (1) determination of factors controlling tree growth across a rainfall gradient and (2) exploration of variation in growth-determining hydraulic wood and leaf traits in M. dubia. In a first regional-scale study, we studied 186 M. dubia farmland woodlots, aged between one and nine years and covering a rainfall gradient from 420 to 2170 mm yr−1 located in the South Indian states of Andhra Pradesh, Karnataka and Tamil Nadu. All studied M. dubia woodlots were established on former agricultural land. To avoid early-stage failures of the woodlots, all interviewed farmers irrigated the seedlings for at least one growing season. Most farmers (66%) continued supplemental irrigation for more than one growing season, but with reduced irrigation frequencies. A total of 34% moved to exclusively rainfed cultivation after the initial irrigation period; the mean annual precipitation at all non-irrigated woodlots was higher than 670 mm yr−1. The computed power-law growth model predicts an average stand-level aboveground biomass of 93.8 Mg ha–1 for nine-year-old woodlots. The resulting average annual aboveground biomass increment over the length of the rotation cycle is 10.4 Mg ha–1 yr–1, which falls within the range reported aboveground biomass increment for other tropical tree plantations. The parameters of the growth model expressed as functions of management, climate and soil variables, explain 65% of the variance in aboveground biomass. The results indicate that water availability is the main driver of the growth of M. dubia. Compared to the effects of water availability, the effects of soil nutrients are 26% to 60% smaller. We conclude that because of its high biomass accumulation rates in farm forestry, M. dubia is a promising candidate for short-rotation plantations in South India and beyond. The second study investigated the intra-specific variability in growth performance, leaf traits, and wood anatomical and derived hydraulic properties in 31 M. dubia plantations along a steep rainfall gradient from 450 to 1700 mm yr−1, and addressed the role of different irrigation practices and soil nutrient availability. We found water supply and soil nutrient status to have a dominant influence on the aboveground biomass increment of M. dubia. aboveground biomass increment was higher at irrigated sites with a less negative climatological water balance (i.e. with lower rainfall) and sites with higher soil phosphorus availability. Stem wood anatomical traits associated with the water transport capacity mirrored the patterns in aboveground biomass increment. At sites with higher water and soil phosphorus availability, the xylem was composed of larger vessel diameters and lower vessel densities. However, both branch wood anatomical traits and leaf traits including the leaf-to-sapwood area ratio were largely independent of water availability and soil conditions. In conclusion, our study underscores that water availability is the main driver that controls the growth of M. dubia and that the effects of soil nutrients comparatively smaller. Further, the detected larger xylem vessel diameters and lower vessel densities and resultant high stem hydraulic efficiency, at sites with higher water and soil phosphorus availability highlights prerequisites for high aboveground productivity and growth. The results also indicate that the short-rotation plantations of M. dubia will clearly benefits from continuous irrigation in regions that receive annual precipitations substantially below 1000 mm yr−1. This also highlights the potential to increase yields in this important short-rotation forest species by improving irrigation and fertilization protocols.
Keywords: aboveground biomass; climatological water deficit; rainfall gradient; Hydraulic conductivity; Leaf morphology; Wood anatomy