Since its inception in to the US-LTER Network in 1988, the Luquillo Long-Term Ecological Research Program (LUQ) conducted at the Luquillo Experimental Forest (LEF) has been centered on how tropical forests biota and biogeochemistry respond to disturbance. Our research has revealed that the LEF, like most tropical forests globally, is a mosaic of stands and reaches with different land use legacies from old growth communities to highly altered population and community structure, and associated ecosystem characteristics (Levin & Paine 1974, Waide & Lugo 1992, Lugo & Waide 1993, Lodge et al. 1994, Covich et al. 1991, Scatena et al 1993, Pringle et al. 1993, Silver et al. 1994, Crowl et al. 2001). Our conceptual framework explores the development of novel ecosystems resulting from the separate and combined effects of increased drought and hurricane frequency, and mediated by land use legacies (Figure 1). The LUQ-LTER is uniquely suited to address this critical question because it provides a key end member in terms of climate, net primary productivity, and biodiversity for the US LTER Network. LUQ provides a natural laboratory to understand ecosystem change in the tropics, coupling long-term monitoring with experiments and models that offer the opportunity to understand the link between forest and stream ecosystem dynamics and a changing precipitation regime. Using integrated theoretical, experimental, and observational approaches LUQ provides an innovative and comprehensive scientific framework for evaluating the management of tropical ecosystems confronting a changing climate. The program will continue to train numerous graduate and undergraduate students, especially members of underrepresented groups, producing a cadre of collaborative, multidisciplinary scientists who can link population, community, and ecosystem approaches to provide a predictive understanding of environmental change The following three interrelated questions form the basis for hypotheses designed to test ecological theory on disturbance, population and community assembly, and biogeochemical dynamics. Our hypotheses are also designed to explore broad-scale patterns in space and time and facilitate the development of models applicable to Puerto Rico, the Caribbean, and tropical forest ecosystems globally.
I. What are the short and long term effects of drought on biota and biogeochemical cycling in tropical forest?
Global circulation models (GCMs) indicate that precipitation will decline by as much as 50% in the Caribbean region over the remainder of the century (Neelin et al. 2008, Comarazamy & González 2011). Our models and empirical data suggest that rainfall is becoming more variable in the Luquillo Mountains (Scatena 1998, van der Molen 2010, Schaefer 2003, González et al. 2013). The longterm effect of these trends will be to increase the frequency of droughts as well as to produce an environment with more pronounced seasonality. The rate of these changes is more rapid than earlier believed (Hayhoe 2013). A Major question is how the biota and biogeochemistry in historically wet rainforests will respond to increased drought frequency.
II. What are the effects of increased frequency of intense hurricanes on tropical forest biota and biogeochemical cycling?
Hurricanes affect islands and coastal ecosystems on all continents except Antarctica. Hurricane disturbance is an important factor shaping the biotic and biogeochemical characteristics of the Luquillo Mountains (Waide & Lugo 1992, Walker & Willig 1999, Scatena et al. 2012, Brokaw et al. 2012). Our long-term data demonstrate that the effects of hurricanes are contingent on the initial state of the ecosystem (i.e., time since past disturbance) and the particular characteristics of the disturbance (i.e., canopy openness, debris deposition, rainfall intensity). For example, previous land use mediates the effects of hurricanes (Uriarte et al. 2009), and differences in intensity and duration of hurricanes produce correspondingly distinct effects on the biota and ecosystem processes (Beard et al. 2005). Climate models predict that the frequency and intensity of hurricane disturbance are changing globally (Royer et al. 1998, Emmanuel 2005, Trenberth 2005, Webster et al. 2005, Zhao & Held, 2012; Villarini & Vecchi, 2013). In addition, models based on our long-term data further predict that more frequent hurricanes will lead to changes in species composition (Doyle 1981, Uriarte et al. 2009) resulting in alterations in biogeochemistry and productivity (Sanford et al. 1991, Zimmerman et al. 1995).
III. How do changes in climate interact with hurricane disturbance, land cover and land use legacies to shape ecosystems of the future?
Changes in climate will lead to changes in biotic structure and biogeochemistry, and the rate of change will be exacerbated by more frequent disturbances (Doyle 1981, Sanford et al. 1991, Covich et al. 1991, 1996, O’Brien et al. 1992, Silver 1998, Wang et al. 2002a, b, Erickson & Ayala 2004). The combined effects of changes in climate and disturbance regimes will result in tropical forest ecosystems characterized by novel community compositions resulting in altered biogeochemistry, productivity, and population dynamics (Hobbs et al. 2006, Lugo et al. 2012, Willig et al. 2012).
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