Tag: defoliation
The legacy effects of rubber defoliation period on the refoliation phenology, leaf disease, and latex yield
The leaf phenology of trees has received particular attention for its crucial role in the global water and carbon balances, ecosystem, and species distribution. However, current studies on leaf phenology have mainly focused on temperate trees, while few studies including tropical trees. Little attention has been paid to globally extensive industrial plantations. Rubber plantations are important to both the local and global economies. In this study, we investigated the legacy effects of defoliation phenology on the following year’s leaf flushing, leaf disease, and also latex yield of rubber trees, an economically important tree to local people and the world. Results show that extended duration of defoliation increased the subsequent duration of refoliation and rates of infection by powdery mildew disease, but led to reduced latex yield in March. This legacy effect of rubber defoliation may relate to the carbohydrate reserved in the trees. A longer duration of defoliation would consume more reserved carbohydrates, reducing available reserves for disease defense and latex production. Extended duration of defoliation period was associated with either a lower temperature before the cessation of latex tapping in October–November and/or a higher temperature after the cessation of latex tapping in December–January. Leaf falling signals the end of photosynthetic activities in deciduous trees. Thus, the leaf falling phenology will impact ecological processes involving rubber trees. Our findings indicated that the inclusion of defoliation periods in future rubber trees’ research, will be crucial to furthering our understanding of leaf flushing, powdery mildew disease, and latex yield.
Variation in Onset of Leaf Unfolding and Wood Formation in a Central African Tropical Tree Species
A diversity of phenological strategies has been reported for tropical tree species. Defoliation and seasonal dormancy of cambial activity inform us on how trees cope with water stress during the dry season, or maximize the use of resources during the rainy season. Here, we study the matching between leaf phenology (unfolding and shedding) and cambial activity for Prioria balsamifera, a key timber species in the Democratic Republic of Congo. In particular, we (i) evaluated the seasonality of cambial activity and synchrony of phenology among trees in response to climate and (ii) identified the seasonality of leaf phenology and its relation with cambial phenology. The study was conducted in the Luki Man and Biosphere Reserve, located in the Mayombe forest at the southern margin of the Congo Basin. Historic defoliation data were collected every ten days using weekly crown observations whereas recent observations involved time-lapse cameras. Cambial pinning was performed on ten trees during 20 months and radius dendrometers were installed on three trees during 13 months. Tree rings were measured on cores from 13 trees and growth synchrony was evaluated. We found that P. balsamifera defoliates annually with a peak observed at the end of the dry season and the beginning of the rainy season. The new leaves unfolded shortly after shedding of the old leaves. The peak defoliation dates varied across years from September 12 to November 14 and the fraction of number of trees that defoliated at a given time was found to be negatively correlated with annual rainfall and temperature; during the dry season, when precipitation and temperatures are the lowest. Wood formation (radial growth), was found to be highly seasonal, with cambial dormancy occurring during the dry season and growth starting at the beginning of the rainy season. Individual ring-width series did not cross date well. The within species variability of leaf phenology and cambial rhythms provides indication about resistance of the population against climatic changes.
Contrasted effects of temperature during defoliation vs. refoliation periods on the infection of rubber powdery mildew (Oidium heveae) in Xishuangbanna, China
Rubber powdery mildew caused by the foliar fungi Oidium heveae is one of the main diseases affecting rubber plantations (Hevea brasiliensis) worldwide. It is particularly serious in sub-optimal growing areas, such as Xishuangbanna in SW China. To prevent and control this disease, fungicides causing serious environmental problems are widely used. Strong correlations between the infection level and the temperature variables were reported previously, but they were related to monthly data that did not allow unraveling the patterns during the entire sensitive period. We correlated the infection level of powdery mildew of rubber trees recorded over 2003–2011 with antecedent 365 days daily temperature variables using partial least squares (PLS) regression. Our PLS regression results showed that the infection level of powdery mildew responded differently to the temperature variables of the defoliation and refoliation periods. Further analysis with Kriging interpolation showed that the infection level increased by 20% and 11%, respectively, per 1 °C rise of the daily maximum and mean temperature in the defoliation season, while it decreased by 8% and 10%, respectively, per 1 °C rise of the daily maximum and temperature difference in the refoliation season. This pattern was likely linked to the effects of temperature on leaf phenology. It seems highly possible that the infection level of powdery mildew increases, as increasing trends of maximum temperature and mean temperature during the defoliation continue.