North American Agroforestry. Группа авторов
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Non‐solar energy inputs, MJ ha−1 (Mcal acre−1)
Note. SOM, soil organic matter; OM, organic matter; PAR, photosynthetically active radiation; NPP, net primary productivity.
4 Tree–Crop Interactions in Temperate Agroforestry
Eric J. Holzmueller
Modern agricultural practices have allowed a dramatic increase in crop and livestock production during the past several decades; however, it has come at the expense of many environmental challenges and the loss of long‐term agricultural sustainability (Foley et al., 2011; Funabashi, 2018; Poore & Nemecek, 2018; Tilman, Cassman, Matson, Naylor, & Polasky, 2002). Agroforestry, the intentional incorporation of trees, agricultural crops, and/or animals into a single land‐use system, is one way to reduce the negative impacts of modern agriculture (Sanchez, 2002). By combining multiple components in the same system, there is potential to increase nutrient use efficiency; control subsurface water levels; improve soil, water, and air quality; provide favorable habitats for plant, insect or animal species; and create a more sustainable agricultural production system (Garrett, McGraw, & Walter, 2009; Garrity, 2004; Jose, 2009; Jose & Dollinger, 2019).
The incorporation of multiple species in a single ecological system or ecosystem such as agroforestry brings about a unique set of ecological interactions among the different species. An ecological interaction refers to the influence that one or more components of a system has on the performance of another component of the system and of the overall system itself (Nair, 1993). If two species are competing for the same resources, and do so equally, both species will likely exhibit lower productivity compared with their potential for independent growth. If an agroforestry system can be designed so that the physiological needs for particular resources are spatially or temporally different for the individual species growing in the system, then there is a possibility that the system may be more productive than the cumulative production of those species if they were grown separately on equal land area (Cannell, van Noordwijk, & Ong, 1996; Wojtkowski, 1998). With time, this advantage may disappear as competitive vectors overtake complementary interactions, but management intervention (e.g., thinning of trees, pruning of branches, disking to reduce root interactions, etc.) may bring the yield advantage back (Figure 4–1).
An understanding of both the biophysical processes and the mechanisms involved in the allocation of resources is essential for the development of ecologically sound agroforestry systems that are sustainable, economically viable, and socially acceptable. In this chapter, we examine the complex biophysical interactions that are central to the ecological sustainability of temperate agroforestry systems. Although our focus is on tree–crop interactions, we also review those interactions involving animals when appropriate. Reviews on the topic include Garcia‐Barrios and Ong (2004), Jose, Gillespie, and Pallardy (2004), Thevathasan and Gordon (2004), Tsonkova, Böhm, Quinkenstein, and Freese (2012), and Atangana, Khasa, Chang, and Degrande (2014).
Fig. 4–1. The production possibility curves for two species, A and B: (a) Points A1 and B1 represent the maximum production potential if A and B were grown in monocultures, while Line A1–B1 represents the proportional yield of A and B when grown in mixtures, and the curve described by A+B represents overyielding (compared with either monoculture yield) of one possible mixture of A and B; (b) a hypothetical temporal production possibility surface for species A and B—as time progresses, overyielding gives away to underyielding, but a timely management intervention (e.g., root pruning of trees) alleviates competitive interactions, thereby resulting in overyielding again
(reprinted with permission from Jose et al., 2004).
Species Coexistence and Ecological Interactions
It is important to review the theoretical basis for species coexistence before discussing the biophysical interactions among them. The competitive exclusion principle (CEP), termed by Hardin (1960), also known as the competitive displacement principle, Grinnell’s axiom, the Volterra–Gause principle, or Gause’s law, has been a cornerstone in ecological thinking regarding species coexistence for decades. The CEP is based on Gause’s (1934) contention that two similar species competing for the same resources cannot stably coexist. Competition between species may lead to three outcomes: