Recent warming (Wake 2005) and projected continued warming (Hayhoe et al. 2007) will have a significant impact on plants in natural and managed ecosystems. An advance in spring bloom date for several plant species in the NE has already been documented (Wolfe et al. 2005; Primack et al. 2004), and projections of spring bloom utilizing a lilac (Syringa spp.)/honeysuckle (Lonicera spp.) Spring Index model indicate the advance in spring bloom dates is likely to continue during the coming decades (Hayhoe et al. 2007).
Both positive and negative climate change effects on crop productivity may already be occurring in the region. For example, the rapid expansion and success of the European wine grape (V. vinifera) industry in upstate New York during the past 20 years may in part be attributed to less severe winters (reduced frequency of temperatures below −24°C) and reduced risk of vine and root damage (A. Lakso, Cornell University, personal communication). In contrast, an analysis of apple yields for Western New York (1971–1982) found that yields were lower in years when winters were warmer than average (based on accumulated degree days >5°C from January 1 to budbreak), possibly related to more variable fruit set following warmer winters (A. Lakso, personal communication).
An extended frost-free period as projected for the NE (Hayhoe et al. 2007; Frumhoff et al. 2006) will tend to benefit those attempting to produce crops requiring a relatively long growing season such as watermelon (Citrullus lanatus), tomatoes, peppers (Capsicum annuum), peaches (Prunus persica), and European red wine grape (V. vinifera) varieties. However, as discussed below, climate projections for the region also indicate an increase in summer heat stress, drought, and weed and pest pressure, which can have negative consequences for warm temperature-adapted crops as well as crops adapted to the historically cool climate of the region.
Many important grain crops, such as field corn, wheat, and oats tend to have lower yields when summer temperatures increase because the plant developmental cycle is speeded up and the duration of the grain-filling period is reduced (Rosenzweig and Hillel 1998; Mitchell et al. 1993). In addition, an increase in the frequency of day and/or night temperatures exceeding a high temperature threshold (varies with species, but typically between 27–35°C, Peet and Wolfe 2000) will negatively affect flowering, fruit set, and/or seed production of many crop species. Farmers in the NE may be able to adapt to these new climate constraints by switching to longer growing season or more heat tolerant varieties. But suitable new varieties may not always be available, or the market may not accept quality features of the new varieties (see Section 7 regarding farmer adaptation).
We compared projections of summer heat stress frequency (increase in number of days with maximum temperature exceeding 32°C) generated with IPCC future emissions scenarios A1fi (relatively higher) and B1 (lower). Figure 1 illustrates a subset of our results—the increase in number of heat stress days in the month of July at early-, mid-, and late-21st century based on the HadCM3 projections. At the higher emissions scenario, within just the next few decades (2010–2039), a 5–10 day increase in the number of July heat stress days is projected for the southern half of the region [i.e., much of Pennsylvania (PA), New Jersey (NJ), Delaware (DE), Connecticut (CT), and southern New York (NY)]. With a lower emissions scenario, the climate change impact does not become substantial until mid-century (2040–2069). By the end of century (2070–2099), with higher emissions, most days in July are projected to exceed the 32°C heat stress threshold for most of the NE. Even assuming relatively lower emissions, much of the NE is projected to have 10–15 more days of heat stress in July by end of century, except for some northern areas [e.g., northern Maine (ME) and Vermont (VT)], where the increase is in the range of 5–15 days.
The projected increase in summer heat stress will be particularly detrimental to many cool temperature-adapted crops (e.g., cabbage, potato, apples) that currently dominate the NE agricultural economy. For example, the sensitivity of potato to climate change was illustrated by Rosenzweig et al. (1996), who used a physiologically-based crop simulation model and predicted −12, −22, and −49% yield reductions for Buffalo, NY with an average warming of +1.5, +2.5, and + 5.0°C, respectively, due primarily to negative warm temperature effects on tuberization in late summer and fall. Some crops will be particularly sensitive to night temperature as opposed to daytime maximum temperature, such as common snap bean, which shows substantial yield reductions when night temperatures exceed 27°C (Rainey and Griffiths 2005). Even crop species generally considered well-adapted to warm temperatures and a long growing season, such as tomato, can have reduced yield and/or fruit quality when daytime maximum temperatures exceed 32°C for short periods during critical reproductive stages (Sato et al. 2001). For many high value horticultural crops, very short-term (hours or a few days) of moderate heat stress at critical growth stages can reduce grower profits by negatively affecting visual or flavor quality even when total tonnage is not reduced (Peet and Wolfe 2000).
An increase in winter temperatures will also have a profound effect on the region’s plant life and agriculture. Mid-winter warming can lead to bud-burst or bloom of some perennial plants, resulting in frost damage when cold winter temperatures return. For crops requiring a prolonged “winter chilling” period to flower, yields will be negatively affected if the chilling requirement is not completely satisfied, even if spring and summer temperatures are optimum for growth. Many varieties of agricultural shrubs (e.g., blueberry, Vaccinum corymbosum), fruit trees (e.g., apples, grapes), and winter cereal grains (e.g., winter wheat, T. aestivum), have a winter chilling requirement of 200 to 2000 cumulative hours within a very narrow temperature range (typically 0–10°C with optimum chill-hour accumulation at 7.2°C, Westwood 1993). Temperatures below or above this range are usually ineffective in meeting the chill requirement, and in some cases warm temperatures (e.g., >15°C) can negate previously accumulated chill hours (Michaels and Amasino 2000).
We used the HadCM3 AOGCM and higher (A1fi) and lower (B1) emission scenarios to forecast the percentage of years within a 30-year period at early- (2010–2039), mid- (2040–2069) and late (2070–2099) when winter temperatures satisfied (stayed below) a chilling threshold of 7.2°C for 400, 1000, and 1800 cumulative hours. Currently, most of the NE satisfies even the highest chilling requirement (1800 h) in most years (based on 1961–1990 simulations, data not shown). Projection results for the medium chilling requirement (1000 h) are shown in Fig. 2. Only a slight diversion between emission scenarios is observed at mid-century. By late century, with higher emissions much of southern NE (all of PA, NJ, DE, CT, and most of MA and NY) has less than 50% of years meeting the 1000 h chilling requirement, while with lower emissions (B1) most of NY and MA still meet the chilling requirement in the majority of years. Projections for the high (1800 h) and low (400 h) chilling requirement (data not shown here, but available at http://www.northeastclimateimpacts.org) essentially bracket the results for the 1000 h requirement presented in Fig. 2. Compared to results for the 1000 h requirement, climate change impact occurs more quickly for the 1800 h requirement, where by mid-century significant diversion between emission scenarios is observed and much of southern NE has less than 50% of years meeting the requirement. The low chilling requirement threshold (400 h), in contrast, is projected to continue to be met for most of the region in most years through mid-century regardless of emissions scenario.
To summarize, our analyses (based on a simple <7.2°C threshold) indicate that a 400 h chilling requirement will continue to be met for most of the NE during this century regardless of emissions scenario. However, crops with prolonged cold requirements (1000 or more hours) could be negatively affected, particularly in southern sections of the NE and at the higher emissions scenario, where less than 50% of years satisfy the chill requirement by mid 21st century. The impact on crops will vary with species and variety. For example, native American grapes (V. labruscana) have a much longer chilling requirement than V. vinifera varieties (Westwood 1993). Chill requirements for apple range from 400–1800 h, with varieties Gala and Red Delicious at the low end of the scale, and MacIntosh and Empire at the high end of the scale. An important next step will be more detailed studies with crop-specific winter chilling models.
Rainfall and drought
Historical data for the NE reveal a trend for increased frequency of high-precipitation events (>5 cm in 48 h) (Wake 2005). This trend is expected to continue, with projections of a further increase in number of high precipitation events of 8% by mid-century and 12–13% by the end of the century (Frumhoff et al. 2006).
One economic consequence of excessive rainfall is delayed spring planting, which jeopardizes profits for farmers paid a premium for early season production of high value horticultural crops such as melon (Cucumis melo), sweet corn, and tomatoes (Lycopersicon esculentum). Field flooding during the growing season causes crop losses associated with anoxia, increases susceptibility to root diseases, increases soil compaction (due to use of heavy farm equipment on wet soils), and causes more runoff and leaching of nutrients and agricultural chemicals into ground- and surface-waters.
More rainfall concentrated into high precipitation events, combined with stable to modest reductions in summer and fall rainfall and increased temperatures leads to a projection for more short- (1–3 month) and medium-term (3–6 month) droughts for the region, particularly in the north and eastern parts of the NE (Hayhoe et al. 2007; Frumhoff et al. 2006). Drought frequency is projected to be much greater at the higher (A1fi) compared to lower (B1) emissions scenario. By the end of century (2070–2099) and with higher emissions, short-term droughts are projected to occur as frequently as once per year for much of the NE, and occasional long-term droughts (>6 month) are projected for western upstate NY—a region where many high value fruit and vegetable crops of considerable economic importance are grown.
Increased drought will be occurring at a time when crop water requirements are also increasing due to warmer temperatures. All crops are negatively affected by water deficits, but the relative impact on various agriculture sectors of the NE economy will depend on existing capacity for irrigation. Grain and silage crops do not bring sufficient profit to warrant investment in irrigation equipment and thus are typically “rainfed” in the NE. These crops, and the farmers growing them, would be particularly vulnerable to an increase in drought frequency. While many producers of high value horticultural crops in the NE have some irrigation equipment, most have not invested in enough equipment to optimize irrigation scheduling and fully meet ET requirements of all of their acreage in below “normal” years (Wilks and Wolfe 1998). Many horticultural crops are susceptible to quality defects (e.g., blossom end rot in tomato; tip burn in cabbage) caused by very short term (hours to days) fluctuations in water availability. Thus, the entire agricultural industry of the NE is vulnerable to drought, although farmers with sufficient capital may adapt by investment in expanded and more sophisticated irrigation systems (see Section 7 on farmer adaptation).
Direct CO2 fertilization effects
Carbon dioxide, in addition to being a greenhouse gas, can also directly affect Earth’s plant life because plants take up CO2 during photosynthesis to produce sugars for growth. Atmospheric CO
is projected to increase from current levels of approximately 380 ppm to 550 and 970 ppm by 2100 with the B1 and A1fi scenarios, respectively (Nakićenović et al. 2000). The plant growth stimulation from increasing CO2 generally follows an asymptotic curve that begins to saturate above about 600–800 ppm for most species (Wolfe 1994), so the potential “CO2 fertilization effect” will diminish over time under both emission scenarios.
Early studies, conducted primarily in growth chambers and greenhouses, found that plants with the so-called “C3” photosynthetic pathway, which includes most NE crop species (with the notable exception of corn) and many weed species, can show productivity increases of 20 to 30% or more when grown at twice current CO2 levels and at optimal conditions (Cure and Acock 1986). However, a recent review (Long et al. 2006) of more modern open-air field studies using the “Free-Air CO2 Enrichment” technology (no chamber effect), suggests that yield benefits for soybean, wheat and other C3 crops may be only about half of what was reported in the earlier enclosure studies.
Maximum CO2 beneficial effects on crop yields will likely require more fertilizer (to support bigger plants), optimum temperatures, unrestricted root growth, and excellent control of weeds, insects, and disease (Wolfe 1994). Weeds will be more aggressive and difficult to control as CO2 continues to rise (see Section 4.2, below).
High CO2 can have another direct effect on plants—reducing leaf conductance to transpirational water loss by causing partial closure of the stomates (the small openings on the leaf surface). However, this water conserving response to high CO2 at the leaf scale is modulated by processes at the whole-plant and/or ecosystem scales [e.g., high CO2 can cause an increase in total leaf (transpirational surface) area]. As a result, ET and soil water use are much less affected by high CO2 than is conductance at the leaf scale (Field et al. 1995), and an increase in ET at elevated compared to current ambient CO2 is sometimes observed (e.g., Hui et al. 2001).
Studies with bean (Jifon and Wolfe 2005), potato (see Peet and Wolfe 2000), and winter wheat (Mitchell et al. 1993) have shown that increasing CO2 cannot compensate for yield losses associated with negative heat stress effects on flower, fruit, or seed development. Thus, for heat-sensitive crops, much of the potential CO2 beneficial effect on crop growth will not be realized if CO2 increase is concomitant with an increase in frequency of high temperature stress as projected for the NE (Fig. 1).