Estimating the age and population structure of encroaching shrubs in arid/semiarid grasslands using high spatial resolution remote sensing imagery

Many arid and semiarid grasslands around the world are suffering from shrub encroachment to a great extent. However, the process of shrub encroachment is still poorly understood. Particularly, there has been limited historical datasets and inadequate field samples about the encroachment species. Shrub age can serve as an important indicator of the shrub encroachment history; and the population structure based on the shrub age distribution can reflect the population dynamics of an encroachment species. Using a high spatial resolution (HSR) remote sensing data, this paper estimates the age and population structure of a typical shrub, the Caragana genus (hereafter shrubs) in the temperate arid and semiarid grasslands of Inner Mongolia, China. The primary methods include: (1) building a shrub age estimation model considering different shrub crown growth rates affected by different precipitation regimes in the Inner Mongolian grassland; (2) identifying shrub individuals from multiple HSR remote sensing images and estimating the age of shrub individuals using the developed shrub age estimation model; (3) establishing a shrub age histogram, fitting the population structure, and calculating the survival rate and regeneration count using a negative exponential function. The results indicate that this shrub age estimation model performs better than a simple statistical regression using only the shrub crown area. Further, employing the negative exponential function for research on shrub encroachment provides supporting evidence of its effective role in capturing the population structure. Result also identifies that the survival rate increases with precipitation.