جستجوی مقالات مرتبط با کلیدواژه « time series » در نشریات گروه « جنگلداری »

For about two decades, the Zagros forests have been facing a declining issue, and the effects of this crisis are visible in various regions.

In the current study, the effect of climate change on the decline of Persian oak in the Zagros forest stands is probed using meteorological data and sample plots over time. To conduct this research, in order to collect field data, a regular random sampling model with fixed sample plots with two sampling periods in 1998 and 2018 was used. Furthermore, the trend of four climatic elements of precipitation, temperature, frost, and relative humidity in Gilan-e-Gharb station from 1988 to 2018 was studied.

The results showed that the frequency and stock of Persian oak species in the second period compared to the first period was decreased. The cross-sectional per hectare of Iranian oak was decreased from 5.2 in the first stage sampling to 4.4 in the second sampling. The results of comparing the quality and health conditions in two samples showed that in the first period, 11.9% of Persian oak trees were decayed and dried, while in the second period it was 71.55%. The oak decline rate in the second-period sampling was higher in health and quality grades 2 and 1 (41.9% and 28.5%, respectively) and was decreased in grades 4 and 5 (8.2% and 3/1%, respectively). However, the deterioration in higher degrees leads to the death of Persian oak trees. The Mann-Kendall analysis results of climatic elements showed that during the study period, the trend of precipitation, frost, and relative humidity was decreasing and the temperature was increasing. In the first decade and to some extent in the second decade, noticeable stability was observed in the studied indices, but in the third decade, there were many fluctuations.

According to the obtained results, it can be said that climate change and the occurrence of severe drought stresses cause oak trees to suffer from physiological weakness. By creating physiological weakness in the trees, the conditions for the invasion of pests and various diseases are provided to them and the oak tree goes to dryness and decay.

Urban vegetation monitoring can play a vital role in sustainable city management because of their diverse environmental, social, cultural, and economic functions. In this study, the vegetation trend was assessed using the Normalized Vegetation Difference Index (NDVI) obtained from the Landsat 5 and 8 at a maximum vegetation greenness time by applying a parametric approach (i.e. Ordinary Least Square Linear Regression) and a non-parametric approach (i.e. Theil-Sen and Mann-Kendall) over Tehran city during 2008 - 2019. The MOD13Q1 NDVI product was used as a complementary data due to the lack of appropriate Landsat data for 2011 and 2012. The data collected from four synoptic meteorological stations located in Tehran were used to extract the mean temperature and the total precipitation parameters. The relationship between NDVI variations and climatic characteristics, i.e. the mean temperature and total precipitation of one month before and one year before the maximum NDVI values were analyzed. The NDVI trend analysis showed a slight increase in Tehran vegetation cconditions during 12 years, however, the result of the Mann-Kendall test was not statistically significant (α = 0.05). Trend analysis for 22 individual districts showed a significant negative trend for five districts, and the remaining districts showed a non-significant trend. The NDVI was negatively correlated with temperature, and there was a positive correlation between NDVI and precipitation. The NDVI variations showed a more similar trend to the climate data of one month before NDVI data-set than one year.

Over time, environmental and human factors have caused positive and negative changes in the quantity and quality of vegetation, and these changes will continue. Vegetation greenness trend may be either increasing (Greening) or decreasing (Browning). Understanding the factors creating these changes and detection of trends in the past and future, would be useful for managers and decision makers. This study investigates the long-term changes in Kurdistan vegetation greenness by using the 16-day composite MODIS NDVI time series for 2000–2017, and 414 images with 250-m pixel size. The coordinate system of MODIS data was converted from the sinusoidal to the geographic map projection and a time series of data was created. After seasonality removal in the time series data, the long-term trend analyses were performed using parametric ordinary least squares (OLS) regression and non-parametric Theil-Sen and Mann-Kendall methods at pixel by pixel basis. Both OLS and Theil-Sen represented the similar result and indicated a slight increase in NDVI i.e., the poor greenness during the study period. According to the Mann-Kendall trend analysis, about 97 % of the province experienced a slight increasing and 2.46 % showed decreasing trend. However, based on the Kendall significance test, only 12 % of the province had a significant increasing or decreasing trend at 1 % confidence level and the rest of the area showed a slight and negligible increasing trend.

Trees respond to environmental and climatic changes during their biological life. Tree rings study is one way to discover these responses called dendrochronology. The most effective climatic factors which affect tree rings are temperature and precipitation. Present study aimed at investigating the influence of temperature and precipitation on annual growth of oriental beach (Fagus orientalis Lipsky) and oak (Querscus castaneifolia C.A. Mey), which clearly illustrate climatic factors on their tree rings. Trees were selected in three altitudes (450, 900 and 1380 a.s.l), from forest part 2 of Neka and Haft Khal in central Alborz mountains of Iran. Core samples were extracted from two sides of trees with an increment borer. Growth rings widths were measured by using LINTAB6 table with accuracy of 0.1 mm. The measurement results were analyzed using time series analysis software TSAPWin. Results showed that growth pattern of oak and beech in three altitudes were not similar. Altitudes affect mean growth of these species significantly. In each altitude, oak and beech have shown different responses to temperature and annual precipitation. These differences could be depending on species’ nature. In response to climatic factors, oak was more sensitive than beech. In high altitude, previous growth season temperature of affect oak chronology negatively while had positive effect on mid and low altitudes. In all altitudes, oak positively affected by precipitation of June (of current growth year). Beech of high altitude showed positive correlation with March and summer precipitation. However, no significand correlation was detected in beech in low altitude

The Hyrcanian forests are one of the most important global terrestrial ecosystems influenced by environmental factors. They have significant impacts on sustainable development of human communities. The forest monitoring based on phenological trends is essential for understanding the response of forest ecosystems to climate variability. The objective was thus to analyze long-term trends of vegetation activity using Mann Kendall test pixel by pixel on 756 biweekly NOAA NDVI time series in the forest areas of northern Iran during 1981-2012. Trends of temperature and precipitation and their regression relationships with NDVI were considered. The greening and browning trends appeared in 75.35% and 7.41% of these forests areas, respectively. The precipitation trend analysis exhibited no obvious significant trend in most pixels of the study area, however, trend analysis of temperature time series showed significant increasing trends (α = 0.01) in the whole region. Results of regression relationship between time series NDVI and temperature and precipitation demonstrated a weak relationship with changes in vegetation activities and climatological parameters during 1981-2012. The results of this research demonstrated that greening phenomenon in the Hyrcanian forests cannot be solely related to the climatological factors..

Fagus orientalis Lipsky accounted for 24 percent of stem number and 30 percent of total volume in northern forests of Iran. The aim of this research is finding the regularity in frequencies of diameter increment, determining the relationship between tree age and fluctuations in radial increment of beech trees. Several hypotheses including stationary, secular trend, cyclic and random variations in data series of the beech tree increment, were examined to find out the main effective factor on variations. Four sound beech trees with mean diameter of 10 cm and maximum age of 350 years were selected. One disk was prepared at breast height from each sample tree. Then rings width was measured and data processed with the Time Series analysis. Results showed that there is more than one trend (model) in radial increment diagram of the sample beech trees. Positive correlation was calculated between tree age and radial increment up to the age of 120 years, where as this correlation turns to negative after this age. The mean and distribution of radial increment differ in periods of tree life. Radial increment of the beech trees is fitted with cubic model along the two age periods. Volume trend of annual increment always is positive and is not inversed. Autocorrelation diagrams show one year periodicity for the fluctuations of radial increments. The other periodicities are irregular and the alterations are not statistically significant.