Is there a forest-water-nexus for Mexican temperate forests?

Abstract

For Mexican temperate forests (MTFs), understood as mountain forest ecosystems formed by pine, oak and fir forests, the water-forest-nexus is unknown. Because of their range of climatic, topographic, and overall ecological particularities, estimates for other temperate forests may not be transferred. A systematic literature review has been carried out to compile information on hydrologic variables in MTFs. Existing knowledge specific for MTFs remains scarce, as the limited amount of studies (20) shows, but suggests a water-forest-nexus (higher infiltration offsets evapotranspiration losses, compared to other ecosystems), in central Mexico and some evidence exists for such a nexus in the transition zones to semi-arid environments, for which it is generally assumed that they spend more water in evapotranspiration than they capture. For MTFs, infiltration is statistically significantly larger and faster than in other vegetation types and is associated with the presence of litter in the upper soil layers. The water-forest nexus cannot be expected in tropical dry forests and shrubs, as interception and evapotranspiration are much higher in semi-arid climates and don't leave enough water for deep infiltration. MTFs play a special role compared to other vegetation types because of their structure (especially the presence of litter), composition and function and their geographic setting. MTFs are located in high elevation areas so that they receive larger amounts of precipitation, but at the same time show a decreased potential evapotranspiration because of a decrease in temperature with elevation.

Introduction

It has long been known that forests and water share an intimate relationship and changes in the hydrological system have been observed as a consequence of forest loss or degradation since centuries (Hibbert 1967, Bosch and Hewlett 1982, Brown et al. 2005, van Dijk and Keenan 2007, Grant et al. 2013). Today, the crucial role forests play in the hydrological cycle, or also called the forest-water-nexus, is being increasingly recognized (e.g. Forests and Water Agenda (FAO), Expert Panel on Forests and Water of the International Union of Forest Research Organizations (IUFRO), World Water Council (WWC), green vs blue ecosystem services, etc.) and promoted to establish synergies between terrestrial and aquatic ecosystem conservation and management (FAO 2013, Gartner et al. 2013). Upstream forests play a critical role in supplying water downstream, since they intercept atmospheric moisture, contributing to cloud and rain formation. They regulate water flow to mitigate droughts and floods, purify water and stabilize soil, among others (FAO 2013, Grant et al. 2013, MacDonald and Stednick 2003), and thus have been coined as natural infrastructure for ecosystem services (Gartner et al. 2013). Forests cover 6% of the terrestrial surface but can retain up to 50% of rainfall (Myers 1997). About 75% of the world's freshwater for agriculture, industry and human settlements come from forests and demand for water will increase in the future (FAO 2013).

Other vegetation types also share some of the hydrological features of forests, but the latter possess a range of structural and functional characteristics that are closely related to the composition of woody plants. Trees are large plants that form a dense canopy, and together with other plants, form multiple layers of vegetation, so that falling rain is slowed down, and its erosive force reduced. Especially the thick layer of litter of the forest floor is an important component for both erosion control and infiltration. Trees have sturdy roots that help to retain soil, as well as providing deep ranging pathways for infiltration. This maximizes soil water content and minimizes overland flow, so that most rainfall moves to streams by subsurface flow where nutrient uptake, cycling, and contaminant sorption processes are fast (Galicia and Zarco-Arista 2014). Forest soils also play an important role in the water-forest-nexus, since their biological, chemical, and physical characteristics determine its structure with a high porosity, low bulk density, and highly saturated hydraulic conductivities and infiltration rates. This means that forests provide a triple effect to enhance infiltration and soil moisture: Root structure of trees and soil characteristics increase infiltration, while canopy density, stemflow and throughfall lead to a rain lag. The results are high quality water delivery to streams, peak flow moderation and base flow regulation (Neary et al. 2009; Galicia and Zarco-Arista 2014). preventing nutrient and contaminant delivery to streams (FAO 2013).

But forests are also considered major water users, which consume most of the precipitation through evapotranspiration (Grant et al. 2013). The relationship between forest and water is influenced by a large array of factors, primarily climate, topography, soil, forest structure and composition, as well as forest management (FAO 2013, Brown et al. 2005). Despite of the general recognition of the importance of forests for the water cycle, the relationships between forests and water in a particular context are little known (van Dijk and Keenan 2007). There are claims that this forest-water-nexus should not be assumed without testing this assumption with data (Chomitz and Kumari, 1998). Water yield (understood as the average amount of fresh water that runs off superficially) has been reported to decrease in reforested basins, because trees require more water as they grow (Brown et al. 2005, González Montiel 2013). But for tropical regions, reforestation has also been shown to be beneficial for infiltration (Ilstedt et al., 2007). A major obstacle to understanding this forest-water-nexus is the natural variability of water resources across and within watersheds of a given land cover type or management practice. Nevertheless, in the last years, scientific evidence that afforestation of agricultural land reduces stream flow has been found (van Dijk and Keenan 2007).

Mexican temperate forests (MTFs), understood as mountain forest ecosystems formed mainly by pine, oak and fir forests, are one of those cases, where the water-forest-nexus is unknown and no estimates of their role in the hydrological cycle are available (Muñoz-Villers and McDonnell 2013). MTFs have received little attention in empirical studies and theoretical reviews of forest ecology, ecosystem services and climate change (Galicia and Zarco-Arista 2014) and many aspects of forest hydrology are unknown (Cantú-Silva y González-Rodríguez 2001). Because of their range of climatic, topographic, and overall ecological particularities, estimates for other temperate forests, like e.g. in the US may not be transferred to the Mexican counterparts. MTFs are the world's center of diversity of pine and oak species with 50 and 140 species, respectively (Halffter et al. 2008). The in situ evolution of both genus is such that around 50% of pine and 33% of oak species worldwide are found in Mexico (Challenger and Soberón 2008), among which 24 pine species and 109 oak species are endemic to Mexico (Galicia et al. 2016). These ecosystems show a high floristic richness that constitute roughly 7000 species or 24% of the total plant species of Mexico (Rzedowski et al. 2006). Some of this diversity may represent direct modifications of hydrological flows compared to extra-tropical temperate forests, as in the case of tropical plants that inhabit MTFs, as epiphytes; they do not only modify throughfall and stemflow pattern (Carlyle-Moses et al. 2004), but also interception. Besides, one major difference in tropical climate that affects the hydrological role of vegetation is the much higher precipitation intensity in MTFs. On a yearly basis, precipitation events with > 50 mm are common (Návar 2011). MTFs cover a large magnitude of altitude and tropical mountain climate with a pronounced seasonal drought, as well as a large array of soils types (Galicia et al. 2016). So not only the biological, topographic, and climatic diversity contrasts with extra-tropical temperate forests, but also soil diversity since the latter are concentrated on molisols and aridisols on a global level (Palm et al. 2007).

In this study, we review the existing primary research and compile and synthetize studies reporting ecohydrological evidence, to evaluate the effect of MTFs characteristics on the forest-water-nexus. A forest-water-nexus for MTFs would exist, if larger infiltration offsets evapotranspiration losses, emphasizing the role of MTFs to provide crucial hydrological ecosystem services over a longer period of time. Based on this state-of-the-art review, we identify knowledge gaps and biases.