Individual-based models (IBMs) of complex systems emerged in the 1960s and early 1970s, across diverse disciplines from astronomy to zoology. Ecological IBMs arose with seemingly independent origins out of the tradition of understanding the ecosystems dynamics of ecosystems from a 'bottom-up' accounting of the interactions of the parts. Individual trees are principal among the parts of forests. Because these models are computationally demanding, they have prospered as the power of digital computers has increased exponentially over the decades following the 1970s.This review will focus on a class of forest IBMs called gap models. Gap models simulate the changes in forests by simulating the birth, growth and death of each individual tree on a small plot of land. The summation of these plots comprise a forest (or set of sample plots on a forested landscape or region). Other, more aggregated forest IBMs have been used in global applications including cohort-based models, ecosystem demography models, etc. Gap models have been used to provide the parameters for these bulk models. Currently, gap models have grown from local-scale to continental-scale and even global-scale applications to assess the potential consequences of climate change on natural forests. Modifications to the models have enabled simulation of disturbances including fire, insect outbreak and harvest.Our objective in this review is to provide the reader with an overview of the history, motivation and applications, including theoretical applications, of these models. In a time of concern over global changes, gap models are essential tools to understand forest responses to climate change, modified disturbance regimes and other change agents. Development of forest surveys to provide the starting points for simulations and better estimates of the behavior of the diversity of tree species in response to the environment are continuing needs for improvement for these and other IBMs.

Structural changes in altitudinal vegetation zones along a 30 ° N parallel were studied based on vegetation data from 20 mountains in East Asia, from 85 ° E to 130 ° E longitude. The altitude of comparable vegetation zones showed a sharp increase of 1400-1900 m from east to west. Forest limit reached an altitude of 4400-4600 m in the eastern Tibetan Plateau, being the highest forest limit in the world. The limidng factor for the upper limit of a vegetation zone was different in the east and west. Low temperature in winter controlled the upward distribution of the evergreen broadleaf forest in the east, whereas the limiting factor was growing season warmth in the west. A close correlation was found between the climatic indices and annual range of monthly mean temperature (ART) at the upper limit of a vegetation zone.Component genera of each vegetation zone along the 30 ° N parallel were analyzed, and it was found that the alternation of component genera from east to west was much more apparent in cool-temperate forests, reflecting their response to macrotopography and air masses. The distribution of Fagus extended into winter-cold regions, whilst Tsuga occurred principally in oceanic and warm climates. The northern limit of Tsuga corresponded well to an ART isotherm of 23 °C and its southern limit coincided with that of Fagus. According to the distribution of Fagus and Tsuga, the cool-temperate forests in East Asia along the 30 ° N belt were divided into three types: deciduous broadleaf forest (represented by Fagus), mixed forest (dominated by Fagus, Tsuga and others), and mixed evergreen forest (consisting mainly of Tsuga and sclerophyll oaks).

In the newly cultivated oases of northwestern China, the soil properties of farmlands with different cultivation periods show a high degree of spatial heterogeneity on the field scale. However, the irrigation water allocation at the irrigation district scale was based mainly on cultivated area but soil conditions were not considered, which resulted in the shortage or super abound of the irrigation water in the farmlands with different soils. A deeper understanding of the effects of soils on crop IWP and irrigation water requirement is an essential prerequisite to accurately assessing regional irrigation water needs and water-saving potential. Therefore, measurements were taken of the yield, irrigation water productivity (IWP), and nitrogen (N) uptake of maize grown in sandy soils (S1, S2), loamy sand soil (S3), and sandy loam soils (S4, S5) and subjected to three irrigation treatments (full, medium, and low irrigation) in an arid oasis farming system in northwestern China. The results show that the highest yield was obtained under full irrigation in sandy loam. Medium and low irrigation reduced the maize yield by 12.5–21.8% and 13.5–20.6%, respectively, relative to full irrigation, with the greatest decrease in sandy loam. Maize IWP ranged from 1.06–1.20 kg m −3 for sand to 2.27–2.58 kg m −3 for sandy loam and was influenced by soil properties but not irrigation treatments. Soil properties also influenced crop N uptake, with sandy loam having a significantly higher such uptake than loamy sand or sand. Under a conventional flooding irrigation pattern, reduced irrigation does not appear to increase IWP in well-drained sandy soils. Crop irrigation water requirement and IWP were mainly influenced by soil texture and fertility. Soil management to improving water productivity should be addressed. In agricultural water management, reasonable irrigation water allocation based on soil conditions should be considered.