The total United States (US) human-induced greenhouse gas emissions due to the burning of fossil fuels and land use changes reached 6870 million megagrams in 2014, representing a 7% increase since 1990 [1
]. These emissions are predicted to cause future warmer temperatures and changes in the precipitation patterns [2
]. In the southern US, annual temperatures are expected to increase between 2.5 and 3.5 °C by 2060, while precipitation forecasts are highly variable across the region [3
]. For example, the average precipitation change is expected to range between −50% and 25%, and −35% and 15%, in summertime and wintertime, respectively, for the period 2070–2090, compared to those levels of average precipitation between 1971–1999 [5
Climate change can significantly alter the productivity of southern forest ecosystems. Increased levels of carbon dioxide and warmer temperatures are expected to stimulate forest growth [6
]. Quinn Thomas and Ahlswede [8
] reported that the aboveground net primary production for a 25-year loblolly pine forest rotation would increase between 35%–60% under changing climatic conditions. However, the gains in forest growth can be offset if water becomes the limiting factor due to higher temperatures or prolonged droughts [9
]. For example, Sun et al. [10
] determined a 22% and 15% reduction of the productivity of mid-rotation loblolly pine plantations when precipitation was reduced by 30% and 15%, respectively. This reduction of forest productivity might be even more exacerbated with other climate change related effects such as fires, pest outbreaks, and other natural disturbances [11
]. With the increasing public interest in climate change, the recent calibration of forest simulation models that estimate forest productivity under different climate scenarios [13
] have become a powerful tool for forest landowners and managers to determine the optimal forest management regimes for commercial southern pines.
Southern forests cover around 100 million hectares [14
], and play a pivotal role in the economy of the region by: (i) leading the total growing stock removal with 227 million cubic meters in 2008 (63% of the Nation’s total removals) [14
]; (ii) supporting an industry that supplies 16% of the global industrial wood; and (iii) contributing 5.5% of the jobs and 7.5% of the industrial economic activity [15
]. These forests have the potential to sequester 23% of the regional greenhouse gas emissions [16
] and provide a clean source for aquatic habitat, drinking water, and groundwater recharge, supplying 34% of the regional water yield [9
]. Changing climatic conditions can severely impact the economic and ecological value of these forests.
Different climate models have projected that water yield (the difference between precipitation and evapotranspiration) across the South will decrease 10 mm per decade until 2060 [17
], driven by higher temperatures that increase evapotranspiration and decreased precipitation [18
]. Generally, a 1% increase (decrease) in precipitation is expected to increase (decrease) water yield by 2% [19
]. From a forest-based water production perspective, a 2 °C increase in temperatures can decrease water yield by 11%, and a 10% reduction of precipitation can lead to a 20% decline in water yield in loblolly pine forests [21
]. These impacts can be magnified by land use change and forest disturbances that reduce the forest estate. The reduction of water yield combined with increased population and land use changes may increase water stress by 10% by 2050 across the South [9
As such, forest practices such as thinnings, soil conservation, and targeted species selection are suggested as viable management strategies to increase water supply for downstream uses and for the forest itself [22
]. Forest thinnings reduce leaf area and forest transpiration, thus the evapotranspiration is reduced and the water yield is increased [21
]. Reported increased water yields after removal of basal area through thinnings range between 3%–64% depending on the forest species, age of the forest, location, and intensity of the thinnings [21
While there have been several studies assessing the impacts of forest stand management on water yield, the role of incentive payments in this context has been very limited. A notable exception is Susaeta et al. [25
], who analyzed the economic tradeoffs between timber benefits and water yield in slash pine forest plantations in Florida, and concluded that up to 33% higher economic profits for landowners were obtained with conservatively low prices for water yield, increased planting density, and heavily thinned slash pine forests. However, this study neglected to consider the impacts of climate change on the forest based water yield, and the use of other sites with different climatic and forest characteristics were not considered. Other studies that have included payments for water yield without considering the effects of climate variability on water production are Creedy and Wurzbacher [26
] who found that the value of forests in Australia increased, and the optimal harvest age can be lengthened (in some cases to infinity), with higher water values; and Bowes et al. [27
] who determined that managing forests in Colorado is not an attractive economic option given the high access costs to these forests, unless timber is managed in conjunction with watershed augmentation programs Considering the key role that southern forests play in the water supply for the region—around 49 million people in the region receive their drinking supply from state and private forests [28
], it is imperative that we determine the economic feasibility of forest management regimes that can meet the society’s demand for water production under changing climatic conditions.
The main goal of this study was to analyze the impacts of climate change on water yield and optimal stand level management of loblolly pine (Pinus taeda
L.) forests in the southeastern (SE) US. Loblolly pine is the main commercial, fast growing species in the SE US, planted on more than 10 million hectares (ha) [29
]. Its native range extends from north Florida to south New Jersey and from east Texas to south Missouri [14
]. We employed the semi-process based simulation model 3-PG (Physiological Processes Predicting Growth, [31
] calibrated for loblolly pine [13
]) to assess forest productivity under different climatic scenarios in three states in the SE US: Florida (FL), Georgia (GA), and South Carolina (SC). We selected the forest management schemes that increased loblolly pine based water yield and assessed the economic implications of payments for increased water yield on their land expectation values and optimal harvest ages. The reminder of the paper is as follows. In Section 2
, we describe the stand level economic model of loblolly pine for timber and water production, the forest growth simulation model 3-PG, the climatic scenarios, forest management schemes, and economic parameters. We also describe the application of the model and the criteria to determine the feasibility of alternative loblolly pine management approaches for increased water yield. In Section 3
, we present the findings of our study, which are discussed in Section 4
. Finally, we offer concluding remarks.
Our results suggest that water yield would increase when planting fewer trees either under low site productivity conditions with moderate or extreme changing climatic conditions, or under high productivity conditions with moderate changing climatic conditions. On average, one hectare of low productivity loblolly pine could increase water yield by 13.7, 12.2, and 9.2 kL·yr−1
, respectively, under moderate climatic conditions in FL, GA, and SC (Table S2
). This represents an 8.3%, 9.0%, and 5.6% increase in water yield, respectively. Also, planting loblolly pine trees under high productivity conditions with moderate climatic conditions would results in lower savings of water. On average, one hectare of loblolly pine would increase water yield by 7.2 (3.1%), 2.8 (4.8%), and 4.2 (1%) kL·yr−1
). Our findings on increases in water yield are in line with those obtained by Sun et al. [21
] who reported an increase of water yield in loblolly pine between 3%–13% using other climate models. Our water yield estimates are far below of the values suggested by McLaughlin et al. [23
] (64%) and Edwards et al. [52
] (23%), who did not include the combined effects of climate and forest management.
Managing loblolly pine forests for water conservation under changing climatic conditions is timely, since it could help offset public and domestic water consumption. However, the increase in water savings is modest. In 2010, groundwater withdrawals in FL, GA, and SC for public supply and self-supplied domestic water consumption were roughly 2787, 337, and 158 bL, and, 296, 159, and 159 bL, respectively [53
]. Private loblolly pine forests in FL, GA, and SC cover around 0.5, 2.8, and 2 million ha, respectively [54
]. If 20% of the private forests in these states were managed for water production under low productivity conditions and moderate climatic conditions, they could increase effective water yield by around <1%, 2%, and 2% of the annual public supply water consumption in FL, GA, and SC. In the case of groundwater withdrawals for self-supplied domestic water consumption, yearly increased water savings yield could reach <1%, 4%, and 2%, respectively.
Under high productivity conditions, loblolly pine forests could increase water yield by <1% of the public supply of water consumption in FL and GA, and 1% of the same groundwater withdrawal in SC. In the case of self-supplied domestic water consumption, the increase in water savings would reach <1%, 1%, and 1%, respectively. Furthermore, changes in forest management such as thinnings and the reduction of tree planting density to increase water productions are also timely considering that around 60% of the loblolly pine forests are near or beyond their optimal harvest date [54
]. Our findings demonstrated the critical role that forests play in the sustainable provision of timber; while also illustrating their potential contribution to water availability. Given that water resources are likely to become a pressing issue given forest losses due to expanding urbanization, population increases, and higher temperatures expected in the region [4
], it is necessary to consider other coupled alternatives to increase water savings such as improving efforts for soil conservation, developing high efficiency water irrigation systems, and introducing economically attractive conservation easements for water management.
Our results indicated that for most of the cases in this study, the collective impacts of climate change, modifications of forest management, and payments for increased water yield would increase the economic rents for landowners. Although a sensitivity analysis of different parameters of our model were not included in our study, it is rather intuitive that, for example, the LEV will increase with higher prices for increased water yield [25
], or decrease with lower timber prices and higher discount rates [32
]. Our results indicated that the average economic benefits of managing loblolly pine for water production (H) for all sites ranged between $1894.8 ha−1
(Scenario RCP8.5) and $2530.7 ha−1
(Scenario RCP4.5). Incorporating climatic variables resulted in greater economic estimates compared to other studies such as Susaeta et al. [25
] who found that the economic benefits of managing forests for water production could be up to $940 ha−1
Harvest ages for loblolly pine stands might also be extended, depending on the type of climatic scenario and the site location. Longer rotation ages are in general obtained when age increasing non-timber benefits are considered [32
]. However, other studies such as Susaeta et al. [25
] have suggested that, absent climate considerations, payments for water yield increases do not have an impact on the harvest age for loblolly pine. Despite the positive economic effects of extending the harvest age, other studies have also suggested that, from a water conservation perspective, older forests have greater evapotranspiration given their larger leaf area index which intercept more water—leading to decreases in water availability [9
]. However, the type of forests and management goal could also offset the positive effects of reduced harvest ages on water yield. For example, fast growing species for bioenergy production or carbon sequestration, or the use of pine plantations that consume more water than unmanaged pines can have negative consequences for water yield and water quality [9
There are several avenues for further research on the role of forests water yield. First, the use of several climatic models (see, for example, the Coupled Model Intercomparison Project CMPI5 models; [58
]) may give a wider spectrum of future precipitation and temperatures to more accurately estimate forest growth simulations and water yield. Second, we conducted our economic analysis using a value of $0.1 kL−1
for increased water yield, which reflected, a contribution of the economic benefits of managing the forests for water production of 38% and 30%, respectively, of the land expectation values for climatic scenarios RCP4.5 and RCP8.5. As we expect stronger markets for forest based water conservation in the future—coupled with higher rates of forest growth due to changing climatic conditions [6
]—it is likely that the price of water will also increase, generating more economic returns for forest landowners and thereby ensuring the sustainability of southern forestlands. The tradeoffs between ecosystem services [59
] (e.g., planting more trees may increase carbon sequestration but also increase water used by trees) and the role of disturbance [61
] are also subjects of further research in this context.
Our study analyzed the impact of climate change, thinning schedules, and different forest productivity conditions regimes on the water yield and land expectation values of loblolly pine stands in the SE US. Our findings suggested that climate change and thinnings would not increase water yield in loblolly pine forests. However, water yield would increase if in addition to thinnings, tree planting density were reduced for both climate scenarios. On average, under climatic scenario RCP4.5, water yield increased by 341 kL·ha−1 for all sites and productivity conditions—584 kL·ha−1 when SI = 20 m, and 97 kL·ha−1 when SI = 28 m. For climatic scenario RCP8.5, water yield only increased by 100 kL·ha−1 for SI = 20 m. For an economic perspective, and assuming a payment for increased water yield of $0.1 kL−1, 96% and 95% greater land expectation values were obtained under scenarios RCP4.5 ($6653.7 ha−1) and RCP8.5 ($6424.1 ha−1), respectively, compared to those obtained for unthinned loblolly pine forests under low productivity conditions and current climatic conditions. For both climatic scenarios the contributions of payments for increased water yield were 38% ($2530.1 ha−1) and 30% ($1894.8 ha−1), respectively. We also found that the optimal harvest decision did not show a clear trend with the inclusion of payments for increased water yield, and future temperatures were a significant driver of the economic returns, but precipitation was not. Our results suggest that managing loblolly pine forests for timber and water conservation would economically benefit forest landowners, but joint efforts are required to improve the sustainability of forestlands in terms of the provision of ecosystem services in the SE US.