Abstract
Agroforestry can contribute to an increase in tree cover in historically forested tropical landscapes with associated gains in biodiversity and ecosystem functioning, but only if established on open land instead of underneath a forest canopy. However, declines in yields with increasing shade are common across agroforestry crops, driving shade-tree removal in forest-derived agroforests and hindering tree regrowth in open-land-derived agroforests. To understand trajectories of change in tree cover in forest- and open-land-derived agroforests, and the impacts of tree cover on vanilla yields, we studied 209 vanilla agroforests along an 88-year chronosequence in Madagascar. Additionally, we used remotely sensed canopy cover data to investigate tree cover change in the agricultural landscape. We found yields to vary widely but independently of canopy cover and land-use history (forest- vs. open-land-derived), averaging at 154.6 kg ha−1 year−1 (SD = 186.9). Furthermore, we found that forest- and open-land-derived vanilla agroforests gained canopy cover over time, but that only open-land-derived agroforests gained canopy height. Canopy cover increased also at the landscape scale: areas in the agricultural landscape with medium initial canopy cover gained 6.4% canopy cover over 10 years, but canopy cover decreased in areas with high initial canopy cover. These opposing trends suggest tree cover rehabilitation across areas covered by vanilla agroforests, whereas remnant forest fragments in the agricultural landscape were transformed or degraded. Our results indicate that yield-neutral tree rehabilitation through open-land-derived agroforestry could, if coupled with effective forest protection, provide benefits for both ecosystem functions and agricultural production in a smallholder-dominated agricultural landscape.
Highlights
Rehabilitation of historically forested open land is widely advocated to re-establish connectivity and increase ecosystem functions in tropical rainforest landscapes (Bastin and others 2019; Chazdon 2003)
The difference in green vanilla yield per ha between the two years was small (2017: 158.8 kg ha-1 (SD = 200.1); 2018: 150.2 kg ha-1 (SD = 202.6)), and a Wilcoxon rank-sum test revealed no significant differences between years (W = 21,267, p = 0.642, N = 209 agroforests)
Across an 88-year chronosequence of 209 agroforests in the SAVA region of north-eastern Madagascar, we found vanilla yields to vary widely and to be positively affected by planting density and agroforest age, whereas land-use history, canopy cover, and precipitation had no effects on yields
Summary
Rehabilitation of historically forested open land is widely advocated to re-establish connectivity and increase ecosystem functions in tropical rainforest landscapes (Bastin and others 2019; Chazdon 2003). Governments and institutions have pledged to restore 140 million hectares of land in the tropics (Brancalion and others 2019) Realizing those pledges could jeopardize food security if tree cover restoration replaces cropland, casting doubt on their feasibility (Eitelberg and others 2016) and desirability (Holl and Brancalion 2020). Agroforests that are established on historically forested open land hold a large potential, because open-land-derived agroforests rehabilitate selected ecosystem functions like erosion control or carbon storage on open land (Martin and others 2020b) To describe this process, we use the word ‘tree rehabilitation’ based on Chazdon and others (2016), as the focus lies on the rehabilitation of ecosystem functions, without necessarily restoring ecological integrity. Agroforests planted under the canopy of existing forests typically contribute to forest degradation (Martin and others 2020a, b), hampering ecosystem functioning and ecological integrity (Coe and others 2013; McDowell and others 2020)
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