Spatial distribution of Hevea brasiliensis trunk phloem necrosis within a plot: aggregation but no evidence of constraint on cumulated growth
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P>Trunk phloem necrosis (TPN) is a major constraint in rubber (Hevea brasiliensis) plantations. Current knowledge supports a scenario of a multi-factor physiological 'disease' caused by an accumulation of endogenous and exogenous stresses. Without evidence of a pathogen, the apparent spread of the syndrome from tree to tree along the planting line, strongly suggested the implication of a major local soil constraint. The objectives of the study were to address, within a plot, three points of the above scenario: the short-distance aggregation of affected trees; their correspondence with local micro-areas facing the highest environmental constraints; and the relative weakness of TPN trees that had accumulated several stresses since planting. The study was conducted in a young rubber tree stand (clone RRIM 600) of Northeast Thailand, where the number of TPN affected individuals was 8% after 3 years of tapping. Trunk girth and trunk basal area were used as indicators of local environmental constraints. Spatial patterns were analysed using the framework of marked point processes and K functions. The results confirmed the short-distance aggregation of TPN affected trees. But, TPN trees were not located within micro-areas where trees had a lower trunk basal area than average. Hence, the clusters of TPN do not occur in the most constraining environmental micro-areas of the plot in terms of growth since planting. Moreover, from comparison of girth, current girth increment and height of bark consumption, we infer that TPN trees were not smaller and less vigorous than other trees before the syndrome outbreak. Finally, our conclusions mean the previous scenario driven by major soil constraints becomes a 'more complex scenario' in which specific soil conditions (not the most constraining in term of cumulated growth), possibly in interaction with inter-individual competition, induce a late and subtle stress favouring a necrotic reaction against tapping stress.
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