Drastic fluctuations in reproductive output are characteristic of numerous perennial plant populations. In forest trees, synchronized mast years impact higher trophic levels as well as plant population dynamics. In managed crops, fluctuations between high- and low-yielding years, or alternate-bearing, have serious implications for farmer livelihoods and agricultural markets. Ecological theory posits resource allocation tradeoffs as a driver of reproductive variability, but few experimental tests exist and results have differed across species. Here we explore the applicability of resource allocation models for mast-seeding to coffee (Coffea arabica), an alternate-bearing crop of global socio-ecological importance. Specifically, we investigated the role of resource tradeoffs in coffee’s alternate-bearing and their relation to shade-tree management and farm altitude. Observational plots were established on 33 randomly-selected active coffee farms (1500-2000 meters) in Santa María de Dota, Costa Rica. On each plot, elevation and shade canopy cover were recorded and 10 randomly-selected plants were sampled for reproductive effort and growth over the 2018 and 2019 fruiting cycles. Tradeoffs and alternate-bearing were assessed at three resolutions of reproduction on a branch: the number of reproductive nodes, the number of fruits per reproductive node, and the total number of fruits.
Results/Conclusions
The number of reproductive nodes exhibited a positive relationship between years (F=25.2, df=1, p<0.0001), indicating that more vigorous plants grow more from year to year. However, there was evidence for a tradeoff for both total fruits (F=2.94, df=1, p=0.086) and the number of fruits per reproductive node (F=3.4, df=1, p=0.064). To tease apart tradeoffs from vigor responses, alternate-bearing indices (ABI) were calculated to capture the magnitude of alternate-bearing at each resolution. Shade had an interactive effect with altitude on ABItotal fruit (F=5.7, df=2, p=0.003) and ABIfruits per node (F=12.5, df=2, p<0.0001): shade promoted stability at mid- and high-altitudes, but not at low altitude where plants exhibited less alternate-bearing overall. Shade also stabilized ABIreproductive nodes at all altitudes (F=19.3, df=1, p<0.0001). The results support the role of resource allocation tradeoffs in coffee’s alternate-bearing. The potential for shade management to promote stability is dependent on farm altitude. The study offers support for the resource budget model theory and highlights yield stability as a novel socio-economic benefit of diversified farms.