Oil palm yield varied substantially among the 12 commercial plantations in this study, with only minor effects of climatic variables, refuting our first hypothesis that the majority of explained variation in yield is due to climatic conditions. Nevertheless, we detected varied impacts of both temperature and rainfall on yield at time-lags corresponding to key stages of fruit development, with a greater effect of temperature than rainfall in our analyses of both raw and anomalised yield. In light of our findings, we discuss the expected yield gaps at our study sites, and the climatic drivers of oil palm yield that we detected. We briefly address the potential drivers of differences in yield among plantations, the implications of our findings for expected impacts of climate change on yield, and the potential for sustainable intensification of commercial oil palm production in Malaysia.
Current yield gaps
We found that differences among plantations were the primary source of variation in oil palm yield, with the mean annual yield of the least-productive plantation only half of that of the most-productive plantation. Hoffmann et al. (2014) estimated that the potential annual FFB yield of coastal areas in Malaysia was generally 36 t FFB/ha, which is over double the lowest plantation annual yield in this study, and suggests that only ~ 60% of the potential yield is currently achieved in the majority of plantations in this study. However, estimated potential yield varies substantially across Malaysia (9–48 t FFB/ha) (Hoffmann et al. 2014), so it is possible that the actual yield gap in the plantations in this study is considerably lower (or higher). This is in line with previous research suggesting that 44–63% of potential yield is achieved for the whole of Malaysia (depending on potential yield estimation) (Fischer et al. 2014). Nevertheless, Hoffman et al. (2017) estimated that four plantations in Malaysia and Indonesia achieved 67–89% of their potential yield, suggesting that the plantations in this study have large yield gaps. Overall, yield gaps for oil palm in Malaysia appear substantial. In combination with potentially weak expected impacts of climate change on average oil palm yield, given the minor role of climate in determining yield that we identified (see section below “Implications for expected changes to yield in Malaysia under climate change”), our findings suggest that there is considerable potential to improve oil palm yield in Malaysia in existing plantations. As Malaysia has the highest national-level palm oil productivity of any country (FAO 2020b), yield gaps in other countries are likely to be even more substantial, highlighting a strong potential for oil palm yield improvements globally. Although we identified a number of effects of climate on yield in this study, we were unable to explain a large proportion of the variation in raw oil palm yield (R2 = 0.38), highlighting the need for further research into drivers of yield gaps both in Malaysia and elsewhere (such as the role of soil, pests, pathogens, pollination and oil palm cultivar: see section below “Variation in oil palm yield among plantations”).
Likely importance of solar radiation for oil palm yield
We found that Tmax was the most important climatic variable for raw oil palm yield, with positive correlations of Tmax and Tmin with raw yield at all time-lags. Tmax and solar radiation are closely correlated (Harris et al. 2020), so our findings are in line with existing literature, suggesting that solar radiation is the most important climatic variable for oil palm yield in Southeast Asia (Hoffmann et al. 2014; Woittiez et al. 2017). We also found a positive effect of Tmin anomaly during sex determination, which is a probable correlate of ‘useful radiation’, representing increasing capacity for photosynthesis, previously identified as having a positive effect at this developmental stage (Dufour et al. 1998). The importance of solar radiation in determining yield could have determined the shape of the relationships of Tmax, rainfall, and their interaction, with yield during inflorescence development, where the hottest, driest months appear to have the greatest yield (Additional file 1: Fig. S13).
Water limitation at our study sites
Our findings suggest that plant-water relations vary by stage of oil palm fruit development, because we detected both weak positive and negative effects of rainfall on yield, suggesting that water is limiting to yield at our study sites by affecting only certain developmental stages. We found positive relationships between rainfall and yield at time-lags corresponding to determination of the number of FFB produced (sex determination, at a 29-month lag prior to harvest, found for anomalies; and inflorescence abortion, at a 10-month lag prior to harvest, found for both raw variables and anomalies), previously identified as sensitive to water availability in Southeast Asia (Dufour et al. 1998; Legros et al. 2009a, b; Legros et al. 2009a, b). Thus, our findings support previous research suggesting that water stress reduces photosynthesis and thus the carbohydrates available for fruit development, triggering a high ratio of male inflorescence initiation and/or high abortion rates, possibly with selective abortion of female inflorescences (Corley and Tinker 2016, Sects. 220.127.116.11, 18.104.22.168, 22.214.171.124).
However, we also identified a negative relationship between rainfall and yield during inflorescence development (at a time-lag of 14 months prior to harvest, for both raw variables and anomalies), which has previously been identified in Malaysia (Chow 1992). We are not aware of an explanation for this negative relationship, which contrasts with the evidence for water-limited yield during sex determination and abortion. Like temperature, rainfall is a correlate of solar radiation, through increased cloud cover (and thus lower solar radiation) when rainfall is higher (Harris et al. 2020). Our results thus suggest that during inflorescence development, when the number of spikelets per inflorescence, and the number of flowers per spikelet are determined (i.e., corresponding to the 14-month time-lag, see Table 1), solar radiation is more strongly limiting than water availability. This also suggests that both soil moisture and air humidity were sufficiently high at our study sites to prevent stomatal closure and thus support inflorescence development (Corley and Tinker 2016, Sects. 126.96.36.199, 5.3.4; Henson and Harun 2005).
We did not detect some relationships between rainfall and yield that have been identified previously; for example, Hoong and Donough (1998) identified a negative relationship between yield and rainfall six months prior to harvest, attributed to negative impacts of rainfall on pollination. Overall, relationships between rainfall and yield appear complex and variable through time, highlighting the need for ongoing research examining the effects of water availability on yield at different stages of oil palm fruit development.
Role of seasonality in determining yield
The relationships between climate and yield anomalies were weaker overall than those of the raw variables, suggesting that the main influence of climate on yield at the plantations in this study is through regular seasonal variation, although it is not possible to determine whether the stronger correlations of raw variables were valid or spurious. The importance of Tmax with a time-lag of 14 months may be artificially inflated in our analyses of raw yield, because its seasonal peak coincides with 14 months prior to the seasonal peak in yield. In addition, previous findings suggest that the majority of FFB yield variation (both overall and seasonally) is determined by FFB number (Corley and Tinker 2016, Sects. 5.4, 5.4.7; Donough et al. 2009), whereas the 14-month time-lag of the most important climatic variables for raw yield (Tmax and rainfall) corresponds to oil palm inflorescence development, determining FFB weight (Table 1, Fig. 3). Moreover, the relative importance of different time-lags in the analyses of climate and yield anomalies (rather than the raw variables) was more in-line with previous research findings, because: (i) developmental stages that determine number of FFB were highly important for yield (sex determination and inflorescence abortion; Fig. 3); and (ii) climatic conditions during developmental stages affecting FFB weight were also important for yield (inflorescence development), but to a lesser degree (Corley and Tinker 2016, Sects. 5.4, 5.4.7; Donough et al. 2009). Thus, relationships between climate and yield anomalies may have been more accurate representations of the impacts of climatic variation on yield, and the importance of Tmax may have been artificially inflated in the raw data. Alternatively, it is possible that Tmax is the primary driver of seasonality in yield at our study sites, owing to high year-round rainfall but stronger seasonal variation in temperature, unlike more rainfall-driven seasonality in other tropical locations (Corley and Tinker 2016, Sect. 5.5.2). The relationships between climate and yield anomalies could have been weaker than those of raw variables because a single anomaly value can correspond to a range of raw climate or yield values, introducing noise into climate-yield relationships (Additional file 1: Fig. S3), and because we used fewer data points in the anomaly analyses than the raw variables.
Variation in oil palm yield among plantations
We found that the majority of variation in yield that we could explain was due to differences among plantations, but we were unable to examine the environmental and management factors that could have driven this variation. These factors could have included oil palm cultivar, effectiveness of plantation-level management, pests and pathogens, soil type and properties, local topography and pollination efficiency (Barcelos et al. 2015; Murphy 2014; Teo 2015; Woittiez et al. 2017). Previous studies have also identified management as the most important determinant of yield among plantations and/or fields, rather than environmental factors (Euler et al. 2016; Hoffmann et al. 2017). The plantations in this study would be expected to be subject to the same company-wide management directives, but it is possible that the application of these directives varied among plantations. Frequency of harvesting is a key determinant of yield, because long harvesting intervals reduce the total ripe FFB harvested by allowing some to rot, and labour available for harvesting is limited in Malaysia (Cock et al. 2016; Donough et al. 2009; Euler et al. 2016; Murphy 2014). The state of Sarawak, in East Malaysia, has reported 15% yield losses owing to rotting of unharvested FFB (Murphy 2014). However, we found that the most productive plantation had yields about twice those of the least productive plantation (Table 2), which is considerably greater than the state-wide yield loss of 15%. The differences in yield among plantations in this study therefore likely arose from a combination of management and environmental factors other than climate. Investigating the effects of a range of environmental and management factors on yield should be a key priority for future research.
Implications for expected changes to yield in Malaysia under climate change
We found weak effects of climatic conditions on yield overall, so our ability to infer likely impacts of climate change on oil palm yield is limited. We briefly speculate on the implications of some climatic effects that we detected.
Our finding that raw yield increases with Tmax suggests that increasing temperatures will benefit yield, although our study only encompasses a limited range of Tmax (~ 27–34 °C), well below the heat stress threshold of ~ 38 °C (Corley and Tinker 2016, Sects. 3.1, 5.4.3). Paterson et al. (2015) estimated that much of western Peninsular Malaysia would exceed the oil palm heat stress threshold by 2100, although this will not be exceeded in central, eastern and southern Peninsular Malaysia, nor in Malaysian Borneo. Thus, the impacts of future temperature increase on oil palm yield in Malaysia do not seem substantial, but are highly uncertain, as future projected temperatures (particularly during heat waves) will be considerably greater than those currently experienced (Barros et al. 2014).
We found a negative relationship between Tmax and yield anomalies at the month of harvest, which could suggest impacts of heat stress on workers during harvesting, which would likely worsen with climate change. Oil palm FFB, which generally weigh 15–20 kg, are almost exclusively harvested by manual labour (Fig. 1; Donough et al. 2009; Murphy 2014), which is likely to be more difficult and less efficient if workers suffer heat stress at higher temperatures. We did not expect a negative impact of temperature at this time-lag based on our knowledge of oil palm fruit development, because oil palm fruit ripen until the point of harvest, and higher temperatures aid ripening (Hoong and Donough 1998; Corley and Tinker 2016, Sects. 3.1, 188.8.131.52), although it is possible that higher temperatures drive water loss from the FFB and thus reduce harvested weight. Thus, there may be a number of overlooked negative impacts of climate change on oil palm yield.
We detected evidence that yield is partially limited by water availability, through increased inflorescence abortion and a greater proportion of male inflorescences under lower rainfall, which suggests that future periods of low-rainfall, particularly drought events, will drive periods of low oil palm yield. El Niño Southern Oscillation (ENSO) droughts are expected to increase in frequency and intensity in Malaysia over the coming century, although mean annual precipitation is projected to undergo minimal change (Barros et al. 2014; Cai et al. 2014; Tangang et al. 2017). Thus the periodic reduction in oil palm yield corresponding to ENSO cycles (~ 2–7 years) is likely to be exacerbated under climate change (Caliman and Southworth 1998; Oettli et al. 2018; Tangang et al. 2017). Nevertheless, increasing atmospheric carbon dioxide concentration is projected to increase oil palm water-use efficiency (Corley and Tinker 2016, Sect. 17.3.1), so overall impacts of climate change on plant-water relations and oil palm yield are unclear (Wang et al. 2012).
Potential for sustainable intensification of oil palm in Malaysia
We identified large differences in yield among plantations, suggesting substantial yield gaps. Depending on the cause(s) of these differences, it is possible that yield could be improved considerably in many plantations in this study, potentially facilitating productivity increase without further land-use change. In theory, such yield improvements could help conserve rainforest in Southeast Asia and other tropical regions (Byerlee et al. 2014; Castiblanco et al. 2013; Greenpeace, 2012; Vijay et al. 2016; Wilcove et al. 2013). However, improving crop yields can lead to greater incentives for expansion, owing to higher returns from land-use change (Byerlee et al. 2014; Carrasco et al. 2014), particularly if markets are elastic (i.e., demands increase as the price decreases) (Hertel 2012). Given that global demand for vegetable oils is increasing (OECD/FAO 2019), effective governance and incentives to preserve natural habitat are essential for reducing land-use change driven by oil palm expansion, alongside improving productivity.